Process for the preparation of unsaturated esters

Abstract

The present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein R1 is a hydrogen atom, a C1-C4alkyl group, or a monocyclic or polycyclic aromatic hydrocarbon group, which may optionally be substituted, especially by one, or more groups R4; R2 is an aliphatic, or an alicyclic group; R3 denotes the aliphatic, or the alicyclic group R2, comprising one additional double bond; and R4 may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H, NO2, CO2H, a tert-butyl group and a phenyl group. The method enables conversion of a saturated alcohol into an unsaturated alcohol in high yield.

Description

[0001] Process for the preparation of unsaturated esters

[0002] Description

[0003] The present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein R1is a hydrogen atom, a Ci-C4alkyl group, or a monocyclic or polycyclic aromatic hydrocarbon group, which may optionally be substituted, especially by one, or more groups R4; R2is an aliphatic, or an alicyclic group; R3denotes the aliphatic, or the alicyclic group R2, comprising one additional double bond; and R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CChMe, SO3H, NO2, CO2H, a tert-butyl group and a phenyl group. The method enables conversion of a saturated alcohol into an unsaturated alcohol in high yield.

[0004] Prior Art

[0005] In order to produce bio-based citral, suitable raw materials from renewable sources are being sought. Isoprenol (3-methyl-3-buten-1-ol) plays a central role in citral synthesis and has so far been produced from the fossil raw materials, isobutene and formaldehyde.

[0006] Fusel oil is a mixture of different alcohols (C3 - C5), which is produced during alcoholic fermentation by metabolic processes of yeast strains. Isoamyl alcohol (3-methyl-1 -butanol) often accounts for the largest proportion by mass (up to 75 wt.%; J. Braz. Chem. Soc. 2023, 34, 2, 153-166). For the conversion of isoamyl alcohol to isoprenol, selective desaturation (oxidation) to the terminal olefin must take place without reacting the alcohol group. A chemical, direct (C-H) desaturation to olefin (one-step reaction) without the introduction of a protective, or activating group, respectively is not known. However, there are isolated microorganisms (e.g. algae) that can catalyze such a reaction (not isoamyl alcohol) in a substrate-specific manner (Chem Nat Compd 2014, 49, 1160-1161).

[0007] The review articles by Beller et al., Catal. Sci. Technol., 2020, 10, 3825 and Goldman et al., Chem. Rev. 2017, 117, 12357-12384 provide a detailed summary of thermal desaturations in the presence of transition metal complexes. Primary alcohols are converted to the corresponding aldehydes or esters under the described reaction conditions.

[0008] Due to the high bond energy of aliphatic C-H bonds, particularly strong H-atom transfer catalysts are required to activate the substrate. Besides others, light-stimulated benzophenones (N. Ishida et al., J. Am. Chem. Soc. 2019, 141 , 19611-19615), anthraquinones (M.-J. Zhou et al., J. Am. Chem. Soc. 2021 , 143, 16470-16485), or polyoxometalates (TBADT: [WioC>32]4'(n-Bu4N+)4; K. Yamada et al., Chem. Eur. J. 2017, 23, 8615 - 8618) have been shown to facilitate challenging H-atom abstractions. In the case of polyoxometalates, tungsten-oxygen clusters are often used to activate C-H bonds in aliphatics.

[0009] In order to close the catalysis cycle postulated in the literature, the reduced photocatalyst [W10O32]5" must be regenerated by single-electron oxidation. The combination of tetrabutylammonium decatungstate (TBADT) and cobaltoxime (COPC = Co(dmgH)2(py)CI) for acceptorless desaturation of aliphatics and esters was described (Sorensen et al., NATURE COMMUNICATIONS | 6:10093 | DOI: 10.1038 / ncomms10093 and ACS Catal. 2016, 6, 7174-7182 2015). According to Sorensen et al. isovaleric acid ethyl ester could only be desaturated by irradiation with a compact fluorescent lamp (CFL) in very small quantities and after a long reaction time:

[0010] TBADT (0.4 mol%) CFL lamp, CD3CN,

[0011] Isovaleriansaureethylester ^3 C, 48 h 6%

[0012] The dehydrogenation of secondary alcohols through cooperative catalysis results in ketones.

[0013] Z. Huang et al., J. Am. Chem. Soc. 2021 , 143, 16470-16485 published a catalyst system consisting of the photocatalyst 2-chloroanthraquinone and the hydrogen evolution catalyst Co(OAc)2*4H20 as well as a ligand dimethylglyoxime (dmgH2) for photoinduced desaturation of aliphatic compounds and side chains, respectively.

[0014] Kanai et al., Org. Lett. 2018, 20, 2042-204 reported that the desaturation of alkenes and tetrahydronaphthalene to the corresponding aromatic systems with the release of hydrogen is possible. For this purpose, a catalyst system consisting of acridinium salt (photocatalyst), thiophosphoric acid ester and Ni salt was used.

[0015] D. Leonori et al., Angew. Chem. Int. Ed. 2022, 61 , e202201870 reported a catalyst system consisting of iridium polypyridyl complex (photocatalyst), Co(dmgH)2(py)CI as hydrogen evolution catalyst and a secondary amine as organocatalyst. Desaturation occurs when exposed to blue LED light and with the release of hydrogen. The reported substrates fall into the group of cyclohexene carbaldehydes.

[0016] B. Kbnig et al., Adv. Synth. Catal. 2023, 365, 605 - 611 adapted a previously reported system involving tetrabutylammonium decatungstate (TBADT) as hydrogen atom transfer (HAT) agent and a cobaloxime co-catalyst for dihydrogen evolution for the dehydrogenative preparation of linear enamides and enecarbamates from saturated precursors.

[0017] In 2022 W. Jin and S. Yu, J. Org. Chem. 2022, 87, 14715-14722 published a photocatalytic method to form appropriately desaturated alcohols in the presence of Pd(PPh3)4 and blue LED light starting from N-alkoxypyridinium salts. The prior activation of the OH group of the alcohol by the conversion into the pyridinium salt is necessary.

[0018] Oxidation of organic substrates by air or in the presence of N2O and metal-doped ammonium decatungstate in MeCN and when exposed to visible light is described in CN 110624603, CN 110586180 and CN 105732244.

[0019] Selective oxidation of cyclohexene to cyclohexenone by photocatalyst decatungstate and by exposure to light in the presence of H2O2 is described in CN109694312. Oxidation of 5- hydroxymethylfurfural to furan-2,5-dicarbaldehyde and furan-2,5-dicarboxylic acid via decatungstate photocatalysis is described in CN 109107605. FR3000070 relates to a process for the preparation of functionalized polyorganosiloxanes by hydrosilylation of unsaturated compounds by polyoxometalate photocatalysis.

[0020] An object of the present invention is to provide a method for efficiently obtaining unsaturated esters.

[0021] It is a further object of the present invention to convert a saturated alcohol in an unsaturated alcohol in high yield and selectivity.

[0022] Surprisingly, it has been found that the above objects could be solved with the method according to the invention, in particular by converting a saturated alcohol in its ester, which is converted to an unsaturated ester by a photochemical process and then into an unsaturated alcohol in high yield and selectivity.

[0023] The method of the present invention is characterized by mild reaction conditions and yields unsaturated alcohols with high selectivity and in the absence of stoichiometric oxidant and without oxidizing the alcohol functionality.

[0024] Accordingly, the present invention relates to a method for the production of unsaturated esters of formula

[0025] 30R\ A 1

[0026] (I), comprising the step a) photochemical conversion of esters of formula

[0027] 20R\ JL 1

[0028] ^O^R

[0029] (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein

[0030] R1is a hydrogen atom, a Ci-C4alkyl group, or a monocyclic or polycyclic aromatic hydrocarbon group, which may optionally be substituted, especially by one, or more groups R4;

[0031] R2is an aliphatic, or an alicyclic group;

[0032] R3denotes the aliphatic, or the alicyclic group R2, comprising one additional double bond; and

[0033] R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CChMe, SO3H, NO2, CO2H, a tert-butyl group and a phenyl group.

[0034] In principle, the present invention relates to a method for introduction of a double bond into the aliphatic, or the alicyclic group R2of esters of formula (II). Aliphatic groups can be saturated, joined by single bonds (alkyl groups), or unsaturated, with double bonds (alkenyl groups).

[0035] An alicyclic group contains one or more all-carbon rings, which may be either saturated, or unsaturated, but do not have aromatic character. The alicyclic group may optionally be substituted, especially by one, or more Ci-C4alkyl groups.

[0036] In one embodiment the present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein

[0037] R1is defined above, or below,

[0038] R2is an acyclic alkenyl group,

[0039] R3denotes the acyclic alkenyl group R2, comprising one additional double bond; or two, or more different acyclic alkenyl groups R2, comprising one additional double bond.

[0040] The acyclic alkenyl group is, for example, a C2-C2salkenyl group, which may be linear, or branched.

[0041] Examples of esters of formula (II), which can be used as educt and contain one, or two double bonds are citronellyl acetate ( ) and geranyl acetate (

[0042] In another embodiment the present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein

[0043] R1is defined above, or below,

[0044] R2is a monocyclic, or polycyclic alkenyl group,

[0045] R3denotes the monocyclic, or polycyclic alkenyl group R2, comprising one additional double bond; or two, or more different monocyclic, or polycyclic alkenyl groups R2, comprising one additional double bond. A monocyclic alkenyl group is, for example, a Cs-C monocycloalkenyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0046] A polycyclic alkenyl group is, for example, a CyCispolycycloalkenyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0047] In a preferred embodiment the present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein

[0048] R1is defined above, or below,

[0049] R2is an alkyl group, or a cycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups;

[0050] R3denotes a single alkenyl group, or two, or more different alkenyl groups, or a single cycloalkenyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups, or two, or more different cycloalkenyl groups, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0051] The term “alkyl group” as used herein denotes acyclic alkyl groups which may optionally comprise monocyclic, or polycyclic units or mixtures thereof (having no side of unsaturation).

[0052] An acyclic alkyl group is, for example, a C2-C2salkyl group, which may be linear, or branched.

[0053] A monocyclic alkyl group is, for example, a Cs-C monocycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0054] A polycyclic alkyl group is, for example, a CyCispolycycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0055] The term “alkenyl group” as used herein denotes acyclic alkenyl groups, which may optionally comprise monocyclic, or polycyclic units or mixtures thereof, having at least a carbon-carbon double bond.

[0056] An acyclic alkenyl group is, for example, a C2-C2salkenyl group, which may be linear, or branched.

[0057] A monocyclic alkenyl group is, for example, a Cs-C monocycloalkenyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0058] A polycyclic alkenyl group is, for example, a CyCispolycycloalkenyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0059] In a preferred embodiment, the method of the present invention is characterized in that: R1is defined as defined as above or below, in particular as defined in one or more of the claims;

[0060] R2is a linear or branched C2-C2salkyl group, a linear or branched C2-C2salkenyl group, a C3- Ciomonocycloalkyl group, which may optionally be substituted by one or more Ci-C4alkyl groups, or a CyCispolycycloalkyl group, which may optionally be substituted by one or more Ci-C4alkyl groups; and

[0061] R3is defined as defined as above or below, in particular as defined in one or more of the claims; and if present, R4is defined as defined as above or below, in particular as defined in one or more of the claims.

[0062] In a preferred embodiment, the method of the present invention is characterized in that: R1is defined as defined as above or below, in particular as defined in one or more of the claims, preferably is a linear or branched Cs-C alkyl group, a linear or branched C3- C alkenyl group, in particular -CH3;

[0063] R2is a linear or branched Cs-C alkyl group, a linear or branched Cs-C alkenyl group, a C3- Ciomonocycloalkyl group, which may optionally be substituted by one or more Ci-C4alkyl groups, or a CyCispolycycloalkyl group, which may optionally be substituted by one or more Ci-C4alkyl groups; and

[0064] R3is defined as defined as above or below, in particular as defined in one or more of the claims; and if present, R4is defined as defined as above or below, in particular as defined in one or more of the claims.

[0065] In a preferred embodiment the present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein

[0066] R1is defined above, or below, in particular in one or more of the claims,

[0067] R2is a C2-C2salkyl group, or a Cs-C cycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups;

[0068] R3denotes a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; or a single Cs-C cycloalkenyl group, which may optionally be substituted by one, or more C1- C4alkyl groups, or two, or more different Cs-C cycloalkenyl groups, which may optionally be substituted by one, or more Ci-C4alkyl groups.

[0069] R1may be an unsubstituted or substituted mono- or polycyclic aromatic hydrocarbon group, preferably having 6 to 24 carbon atoms, more preferably having 6 to 20 carbon atoms, especially having 6 to 14 carbon atoms as ring members.

[0070] The unsubstituted or substituted mono- or polycyclic aromatic hydrocarbon group R1is preferably selected from unsubstituted or substituted phenyl, unsubstituted or substituted naphthyl, unsubstituted or substituted indenyl, unsubstituted or substituted fluorenyl, unsubstituted or substituted anthracenyl, unsubstituted or substituted phenanthrenyl, unsubstituted or substituted naphthacenyl, unsubstituted or substituted chrysenyl. The unsubstituted or substituted mono- or polycyclic aromatic hydrocarbon group R1is more preferably selected from unsubstituted or substituted phenyl and unsubstituted or substituted naphthyl.

[0071] The unsubstituted or substituted mono- or polycyclic aromatic hydrocarbon group R1is in particular selected from unsubstituted or substituted phenyl.

[0072] The unsubstituted or substituted mono- or polycyclic aromatic hydrocarbon group R1may optionally be substituted by one, or more groups R4.

[0073] R4may be the same, or different in each occurrence and is preferably selected from F, Cl, Br, I, CN, CO2Me, SO3H, NO2, CO2H, tBu and Ph.

[0074] In a more preferred embodiment, the present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein R1is a hydrogen atom, a Ci-C4alkyl group, a phenyl group, which may optionally be substituted by one, or more groups R4;

[0075] R2is a C2-C2salkyl group (CXH2X+I , x = 2 to 25), or a C3-C cycloalkyl group, which may optionally be substituted by one, or more groups Ci-C4alkyl groups;

[0076] R3denotes a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups (CXH2X.II , x = 2 to 25); or a single C3-C cycloalkenyl group, or two, or more different C3- C cycloalkenyl groups, which may optionally be substituted by one, or more groups Ci- C4alkyl groups; and

[0077] R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H and NO2.

[0078] The term "C3-Ciocycloalkyl" as used herein denotes a monocycloalkyl group which may optionally be substituted by one or more Ci-C4alkyl groups. Examples of C3-C cycloalkyl include but are not limited to cyclopentyl, cyclohexyl, methylcyclohexyl and dimethylcyclohexyl.

[0079] In a more preferred embodiment, the present invention relates to a method for the production of unsaturated esters of formula (I), comprising the step a) photochemical conversion of esters of formula (II) in neat form, or in a solvent in the presence of a catalyst system with light, wherein R1is a hydrogen atom, a Ci-C4alkyl group, a phenyl group, which may optionally be substituted by one, or more groups R4;

[0080] R2is a C2-C2salkyl group;

[0081] R3denotes a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; and R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H and NO2. In said embodiment the alkenyl group R3(CXH2X-I , x = 2 to 25) is derived from the alkyl group R2(CXH2X+I , x = 2 to 25) by the abstraction of hydrogen.

[0082] Preferably, R2is C2-Cis-alkyl, especially linear C2-Ci2-alkyl or branched C2-Ci2-alkyl. Examples for linear Ci-Ci2-alkyl are, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n- octyl, n-nonyl, n-decyl, n-undecyl or n-dodecyl. Examples for branched C2-Ci2-alkyl are isopropyl, sec-butyl, 2-methylpropyl (isobutyl), 1 -methylbutyl, 2-methylbutyl, 3-methylbutyl,

[0083] 1.1 -dimethylpropyl, 1 -ethylpropyl, 1 -methylpentyl, 2-methylpentyl, 3-methylpentyl, 4- methylpentyl, 1 ,1 -di methyl butyl, 2,2-dimethylbuty, 3,3-dimethylbutyl, 1 ,2-dimethylbuty, 1 ,3- dimethylbutyl, 2,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1 -methylhexyl, 2-methylhexyl, 3- methylhexyl, 4-methylhexyl, 5-methylhexyl, 1 ,1 -dimethylpentyl, 2,2-dimethylpentyl, 3,3- dimethylpentyl, 4,4-dimethylpentyl, 1 ,2-dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4- dimethylpentyl, 2,3-dimethylpentyl, 2,4-dimethylpentyl, 3,4-dimethylpentyl, 1 -ethylpentyl, 2- ethylpentyl, 3-ethyl pentyl, 1 -methylheptyl, 2-methylheptyl, 3-methylheptyl, 4-methylheptyl, 5- methylheptyl, 6-methylheptyl, 1 ,1 -di methyl hexyl, 2,2-dimethylhexyl, 3, 3-di methyl hexyl, 4,4- dimethylhexyl, 5, 5-di methyl hexyl, 1 ,2-dimethylhexyl, 1 , 3-di methyl hexyl, 1 ,4-dimethylhexyl,

[0084] 1.5-dimethylhexyl, 2,3-dimethylhexyl, 2,4-dimethylhexyl, 2,5-dimethylhexyl, 3,4- dimethylhexyl, 3, 5-di methyl hexyl, 1-ethylhexyl, 2-ethylhexyl, 3-ethylhexyl, 4-ethylhexyl and

[0085] 3.5.5-trimethylhexyl.

[0086] Preferred alkenyl groups R3are those, which are formed by the abstraction of hydrogen from the preferred alkyl groups R2. Tertiary hydrogens can be abstracted particularly easy.

[0087] In a preferred embodiment, the method of the present invention is characterized in that: R1is defined as defined as above or below, in particular as defined in one or more of the claims, preferably is a linear or branched Cs-C alkyl group or a linear or branched C3- C alkenyl group, in particular -CH3;

[0088] R2is selected from the group consisting of ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl or n-dodecyl, isopropyl, sec-butyl, 2-methylpropyl (isobutyl), 1 -methylbutyl, 2-methylbutyl, 3-methylbutyl, 1 ,1 -dimethylpropyl, 1 -ethylpropyl, 1- methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1 ,1 -di methyl butyl, 2,2- dimethylbutyl, 3,3-dimethylbutyl, 1 ,2-dimethylbutyl, 1 , 3-di methyl butyl, 2,3-dimethylbutyl, 1- ethylbutyl, 2-ethylbutyl, 1 -methylhexyl, 2-methylhexyl, 3-methylhexyl, 4-methylhexyl, 5- methylhexyl, 1 ,1 -dimethylpentyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl,

[0089] 1.2-dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4-dimethylpentyl, 2,3-dimethylpentyl, 2,4- dimethylpentyl, 3,4-dimethylpentyl, 1 -ethyl pentyl, 2-ethylpentyl, 3-ethyl pentyl, 1 -methylheptyl, 2-methylheptyl, 3-methylheptyl, 4-methylheptyl, 5-methylheptyl, 6-methylheptyl, 1 ,1- dimethylhexyl, 2,2-dimethylhexyl, 3, 3-di methyl hexyl, 4,4-dimethylhexyl, 5,5-dimethylhexyl,

[0090] 1.2-dimethylhexyl, 1 , 3-di methyl hexyl, 1 ,4-dimethylhexyl, 1 ,5-dimethylhexyl, 2,3- dimethylhexyl, 2,4-dimethylhexyl, 2,5-dimethylhexyl, 3,4-dimethylhexyl, 3, 5-di methyl hexyl, 1- ethylhexyl, 2-ethylhexyl, 3-ethylhexyl, 4-ethylhexyl, 3,5,5-trimethylhexyl, citronellyl, and geranyl; and

[0091] R3is defined as defined as above or below, in particular as defined in one or more of the claims; and if present, R4is defined as defined as above or below, in particular as defined in one or more of the claims.

[0092] In a preferred embodiment, the method of the present invention is characterized in any of the following features:

[0093] (a) an unsaturated ester of formula (I) having a chemical structure 1c1 or 1c2 or a mixture thereof as obtained from the step (a) of photochemical conversion of an ester of formula (II) having a chemical structure 2c:

[0094] 1c1 1c2

[0095] 2c wherein R1is selected from the group consisting of H, methyl, ethyl, n-propyl and phenyl; b) an unsaturated ester of formula (I) having the chemical structure as obtained from an ester of formula (II) having the chemical structure: c) an unsaturated ester of formula (I) having the chemical structure or or a mixture thereof as obtained from an ester of formula (II) having the chemical structure: d) an unsaturated ester of formula (I) having the chemical structure or or a mixture thereof as obtained from an ester of formula (II) having the chemical structure: e) an unsaturated ester of formula (I) having the chemical structure or a mixture of two or more thereof as obtained from an ester of formula (II) having the chemical structure: f) an unsaturated ester of formula (I) having the chemical structure as obtained from an ester of formula (II) having the chemical structure: g f formula (I) having the chemical structure as obtained from an ester of formula (II) having the chemical structure:

[0096] In a preferred embodiment, the method of the present invention is characterized in that: R1is a hydrogen atom, a Ci-C4alkyl group, a phenyl group, which may optionally be substituted by one, or more groups R4;

[0097] R2is a C2-C2salkyl group, or a Cs-C cycloalkyl group, which may optionally be substituted by one, or more groups Ci-C4alkyl groups; and

[0098] R3denotes a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; or a single Cs-C cycloalkenyl group, or two, or more different Cs-C cycloalkenyl groups, which may optionally be substituted by one, or more groups Ci-C4alkyl groups; and

[0099] R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H and NO2.

[0100] In a preferred embodiment of the present invention in step a) esters of formula (Ila) are photochemically converted (irradiated). n is 0 to 4, especially 1. m is 0, or 1 , especially 0.

[0101] R1is a hydrogen atom, a Ci-C4alkyl group, especially a methyl group, an ethyl group, a n- propyl group, or a tert-butyl group, a phenyl group, which may optionally be substituted by one, or more groups R4, especially a hydrogen atom, a methyl group, an ethyl group, or a phenyl group, very especially a methyl group.

[0102] R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H and NO2. R2’ and R2” are independently of each other a hydrogen atom, or a Ci-Csalkyl group.

[0103] In a more preferred embodiment of the present invention in step a) esters of formula (Ila’) are photochemically converted.

[0104] R1is a hydrogen atom, a Ci-C4alkyl group, especially a methyl group, an ethyl group, or a tert-butyl group, a phenyl group, which may optionally be substituted by one, or more groups R4, especially a hydrogen atom, a methyl group, an ethyl group, or a phenyl group, very especially a methyl group. n is 0 to 4, especially 1.

[0105] R2* is a hydrogen atom, or a Ci-Csalkyl group, especially CH3.

[0106] R2” is a hydrogen atom, or a Ci-Csalkyl group.

[0107] In a particularly preferred embodiment the present invention is directed to a method for the production of unsaturated esters of formula (Ia1) and (Ia2) and depending on the meaning of R2*and R2” further isomers, comprising the step a) photochemical conversion of esters of formula (Ila):

[0108] R1is a hydrogen atom, a Ci-C4alkyl group, especially a methyl group, an ethyl group, or a tert-butyl group, a phenyl group, which may optionally be substituted by one, or more groups R4, especially a hydrogen atom, a methyl group, an ethyl group, or a phenyl group, very especially a methyl group. n is 0 to 4, especially 1.

[0109] R2* is a hydrogen atom, or a Ci-Csalkyl group, especially CH3.

[0110] R2” is a hydrogen atom, or a Ci-Csalkyl group.

[0111] In a particularly preferred embodiment the obtained unsaturated esters of formula (I) are compounds of formulae 1c1 and 1c2 and the starting material used for their production is the ester of formula 2c shown below:

[0112] 2c

[0113] Examples of the obtained unsaturated esters of formula (I, i.e. cpd. 1a to 1f) and the starting materials used for their production, i.e. the esters of formula (II; i.e. cpd. 2a to 2f) are shown below:

[0114] R1is a hydrogen atom, a Ci-C4alkyl group, especially a methyl group, an ethyl group, a n- propyl group, or a tert-butyl group, a phenyl group, which may optionally be substituted by one, or more groups R4, especially a hydrogen atom, a methyl group, an ethyl group, or a phenyl group, very especially a methyl group.

[0115] If R1is a Ci-C4alkyl group it is preferably a methyl group, an ethyl group, or a tert-butyl group. If R1is, for example, a n-propyl group, the n-propyl group may be dehydrogenated to a prop- 1-en-1-yl group, a prop-2-en-1-yl group. Hence, a n-propyl group is less preferred.

[0116] If R1is a phenyl group the phenyl group may optionally be substituted by one, or more groups R4, wherein R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CChMe, SO3H and NO2. A phenyl group is preferred against an optionally substituted phenyl group.

[0117] R1is especially a hydrogen atom, a methyl group, an ethyl group, or a phenyl group, very especially a methyl group.

[0118] The esters of formula (II) are preferably obtained by esterification of alcohols of formula (III). The esterification is preferably carried out in the presence of one or more catalysts, for example using Bronsted or Lewis acids or bases or 4- dimethylaminopyridine (DMAP) as catalyst. Especially preferred is the esterification of the alcohol of formula (III) in the presence of an acid chloride, DMAP and Et3N in dichloromethane at low temperature. The esterification can be carried out under customary process conditions in typical esterification apparatuses known to the person skilled in the art. Accordingly, the present invention is directed to a method for the production of unsaturated esters of formula (I), comprising the steps

[0119] 0) esterification of alcohols of formula (III) to obtain esters of formula (II), and a) converting the esters of formula (II) photochemically in neat form, or in a solvent in the presence of a catalyst system with light, wherein R1, R2and R3are defined above, or bellow.

[0120] Alcohols from a natural source may be used. In said embodiment isoamyl alcohol from a natural source, such as, for example, “fusel oil”, is preferably be used, whereby bio-based isoprenol / prenol is obtained. A process for producing isoamyl alcohol by distillation of fusel oil is, for example, described in W02016 / 054706A1.

[0121] In a preferred embodiment, the ester of formula (II) is obtained from an alcohol obtained from a bio-based source, in particular bio-based fusel oil, and optionally an acid from a bio-based source, in particular acetic acid.

[0122] The term “fusel oil” as used herein refers to products that are formed as a by-product of alcoholic fermentation. Fusel oil is well known in the art and typically comprises a mixture of light alcohols, fatty esters, terpenes and furfural. The alcohols comprised in fusel oil are mainly propanol, butanol, amyl alcohol, isoamyl alcohols and hexanol and optionally heavier linear alcohols such as C? or Cs alcohols.

[0123] Fusel oils, occasionally referred to as “amyl oils” or “fusels”, have compositions which vary depending on their origin (potato, beet, wheat, barley, etc. musts).

[0124] Fusel oils form colorless or yellowish liquids, which have a characteristic odor. They have a density of about 0.83 g / mL. Their boiling point is far from constant, since they are complex mixtures of substances with a very variable boiling point. Boiling commences at about 80° C and rises to 130 to 134°C. Fusel oils insoluble in water and are usually washed with water and separated out by settling of the phases in order to reduce the amount of ethanol they contain by about 4% to 5%. It should be noted that fusel alcohols are natural alcohols directly produced via biotechnology in distilleries, without any intermediate chemical step.

[0125] Fusel oil may be obtained by several processes well known from the skilled person, e.g. by direct removal in the distillation column and cooling. The removed fraction can be purified e.g. by extraction and decantation. A liquid / liquid extraction by addition of water followed by a decantation leads to the formation of two phases. The upper phase comprises mainly amyl and butyl alcohols, slightly soluble in water. The various fractions of fusel oil may also be separated by using adsorbents, which are regenerated thereafter. Among the tested adsorbents, granulated vegetal activated charcoal is preferred since it is able to adsorb eight times its weight of fusel oil.

[0126] Fusel oil typically comprises 5 to 20% of water, 60 to 95% of alcohols mainly consisting of linear or branched alkanols containing from 2 to 5 carbon atoms, and impurities including furfurals, ethers and / or fatty acids.

[0127] In one embodiment, the composition of fusel oil is as follows: ethanol: 5 to 40%,

[0128] 1 -propanol: 1 to 8%,

[0129] 2-propanol: 0 to 1%,

[0130] 2-methylpropanol: 5 to 15%,

[0131] 1 -butanol: 0 to 1%,

[0132] 2-methyl-1-butanol: 10 to 30%,

[0133] 3-methyl-1 -butanol (isoamyl alcohol): 25 to 70%, the combination of alkanols representing 100%.

[0134] In an embodiment, the isoamyl alcohol, which is obtained from the fusel oil and contains the Cs-Cs primary alcohol, has a pMC greater than 90 when measured by a method as described in the ASTM norm D6866 (the current version is D6866-22), which defines the concept of “percent Modern Carbon” or pMC. Preferably the pMC is greater than 91 , preferably greater than 93, preferably greater than 95, preferably greater than 96, preferably greater than 97, more preferably greater than 98, even more preferably greater than 99, more preferably about 100.

[0135] In an embodiment, the verification that a feedstock was derived from renewable raw materials is possible according to ASTM D6866 via14C for example. A feedstock shall be regarded as “derived from renewable raw materials” for the purposes of this invention when the carbon-14 (14C) presence therein corresponds substantially (to within not more than 6%) to the ASTM D6866 content of14C in atmospheric CO2.

[0136] The14C content of a material may be determined by determining the decays of14C in this material by liquid scintillation. Such raw materials shall preferably be regarded as derived from renewable raw materials when they have a14C content displaying a radioactive decay of not less than 1.5 dpm / gC (decays per minute per gram of carbon), preferably 2 dpm / gC, more preferably 2.5 dpm / gC and yet more preferably 5 dpm / gC.

[0137] “Renewably-based” or “renewable” denote that the carbon content of a precursor and subsequent products is from a “new carbon” source as measured by ASTM test method D 6866-05, “Determining the Biobased Content of Natural Range Materials Using Radiocarbon and Isotope Ratio Mass Spectrometry Analysis”, incorporated herein by reference in its entirety. This test method measures the14C / 12C isotope ratio in a sample and compares it to the14C / 12C isotope ratio in a standard 100% biobased material to give percent biobased content of the sample. “Biobased materials” are organic materials in which the carbon comes from recently (on a human time scale) fixated CO2 present in the atmosphere using sunlight energy (photosynthesis). On land, this CO2 is captured or fixated by plant life (e.g., agricultural crops or forestry materials). In the oceans, the CO2 is captured or fixated by photosynthesizing bacteria or phytoplankton. For example, a biobased material has a14C / 12C isotope ratio greater than 0. Contrarily, a fossil-based material, has a14C / 12C isotope ratio of about 0. The term “renewable” with regard to compounds such as alcohols or hydrocarbons (linear or cyclic alkanes / olefins / alkynes, aromatic, etc.) refers to compounds prepared from biomass using thermochemical methods (e.g., Fischer-Tropsch catalysts), biocatalysts (e.g., fermentation), or other processes, for example as described herein.

[0138] The term “precursor” refers to an organic molecule in which all of the carbon contained within the molecule is derived from biomass and is thermochemically or biochemically converted from a feedstock into the precursor.

[0139] “Carbon of atmospheric origin” as used herein refers to carbon atoms from carbon dioxide molecules that have recently (e.g., in the last few decades) been free in the earth's atmosphere. Such carbon atoms are identifiable by the ratio of particular radioisotopes as described herein. “Green carbon”, “atmospheric carbon”, “environmentally friendly carbon”, “life-cycle carbon”, “non-fossil fuel based carbon”, “non-petroleum based carbon”, “carbon of atmospheric origin”, and “biobased carbon” are used synonymously herein.

[0140] The solvent used in step a) is preferably selected from acetone, ethylacetate, dimethylacetamide, dichloromethane, 1 ,2-dichloroethane, methyl-tert-butylether, acetonitrile, dimethylsulfoxide (DMSO), tert.-alcohols, water, N-methyl-2-pyrrolidone (NMP), tetra hydrofuran (THF) and mixtures thereof. More preferably the solvent is an aprotic solvent, which is selected from acetone, dimethylacetamide, dichloromethane, 1 ,2-dichloroethane, methyl-tert-butylether, acetonitrile, dimethylsulfoxide (DMSO) and mixtures thereof.

[0141] The solvent used in step a) is present in an amount of 99 to 0 % by weight, especially 95 to 20 % by weight based on the amount of solvent and esters of formula (II).

[0142] In an alternative embodiment the esters of formula (II) are photochemically converted in neat form, i.e. no solvent is added in step a).

[0143] Ethylacetate can function simultaneously as ester of formula (II) and solvent.

[0144] The catalyst system used in step a) comprises at least one photoredox catalyst and optionally at least one hydrogen evolution catalyst.

[0145] The photoredox catalyst may simultaneously function as photoredox catalyst and hydrogen evolution catalyst. The photoredox catalyst may be selected from benzophenone and derivatives thereof, such as for example, Michler’s Ketone, thioxanthen-9-one, xanthone, anthraquinone and derivatives thereof, 1 ,4-bis(butylamino)anthraquinone, 1 ,4-bis(methylamino)anthraquinone, 2-chloroanthraquinone, 2-ethylanthraquinone, benzo[1 ,2-c:4,5-c’]diacridine- 6,9,15,18(5H,14H)-tetrone, 9-anthrone, tetramethoxy-antracen-9-one, diacetyl-4,5- bis(carbazol-9-yl)-1 ,2-dicyanobenzene (2CzPN), 3,4,5,6-tetra(9H-carbazol-9-yl)phthalonitrile (4CzPN), 1 ,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzlPN), 4- methoxythioxanthone (4-MeOTX), 3-methoxythioxanthone (3-MeOTX), 2- methoxythioxanthone (2-MeOTX), 3-fluorothioxanthone (3-FTX), 2-fluorothioxanthone (2- FTX), 3,6-dimethoxythioxanthone (3,3’-MeOTX), 3,6-difluorothioxanthone (3,3‘-FTX), 2- methoxy,7-fluorothioxanthone (2-F,2‘-MeOTX), 2,7-dimethoxythioxanthone (2,2‘-MeOTX) and 9-Mesityl-10-methylacridinium tetrafluoroborate, mesityl-10-methylacridinium perchlorate, Eosin Y, Eosin B, 9,10-diphenylanthracene, 9,10-dicyanoanthracene, Rose Bengal, VAT Violet 13, Sudan blue and mixtures thereof.

[0146] In addition, the photoredox catalyst may be any suitable polyoxometalate (POM) or suitable salt thereof including polyoxometallates of Group 5 and Group 6 metals such as vanadium, niobium, tantalum, molybdenum, and tungsten. Preferably the photoredox catalyst is selected from tetraalkylammonium salts of decatungstate, ammonium salts of decatungstate, pyridinium salts of decatungstate, salts of decatungstate with an organic counterion, salts of decatungstate with an inorganic counterion and mixtures thereof.

[0147] The photoredox catalyst comprises preferably at least one compound of formula [W O32]X4, wherein X is Li+, Na+, K+, Rb+, Cs+, N(R7)4+or P(R7’)4+, wherein R7and R7’ may be the same, or different in each occurrence and are selected from a hydrogen atom, a Ci-Csalkyl group and a phenyl group and R7’ may be the same, or different in each occurrence and are selected from a Ci-Csalkyl group and a phenyl group.

[0148] Examples of particular suitable photoredox catalyst are tetrabutylammonium decatungstate (TBADT), sodium decatungstate ([W O32]Na4 (NaDT)), tetraethyl ammonium decatungstate, tetra-n-propylammonium decatungstate, tetrabenzyl ammonium decatungstate and mixtures thereof. The at the moment most preferred catalyst is NaDT.

[0149] In a preferred embodiment of the present invention the catalyst system comprises at least one hydrogen evolution catalyst.

[0150] Examples of the hydrogen development catalyst are Ni(acac)2, Ni(NTf2)2*xH2O (nickel(ll) bis(trifluoromethanesulfonimide) hydrate), Ni(BF4)2*6H2O, RuO2, RuO@graphene, nanoparticles (NP) consisting of Pt, Pd, Ag, Co, Ru, Ir, Cu, Ni, Au, Rh and alloys thereof, metal phosphides [CoP, C02P, FeP, NiP or Ni2P, MoP or M03P, CU3P] and alloys thereof.

[0151] The hydrogen evolution catalyst is preferably an organometallic complex on basis of palladium, nickel, cobalt, or ruthenium; especially an organometallic complex of formula

[0152] , or Co(dmgH)2R5R5(V) (a complex having cobalt as central atom in oxidation state II or III, which is planarly coordinated by 2 dimethylglyoxime (dmgH) ligands each other Cl, Br, or a group of formula , wherein R6is a hydrogen atom,

[0153] -CN, -OMe, -tBu, or -CChMe.

[0154] Most preferred, the hydrogen development catalyst is an organometallic complex of formula Co(dmgH)2R5R5(Va), wherein

[0155] R5and R5’ are Cl; or

[0156] R5and R5’ are Br; or

[0157] R5and R5’ are a group of formula . The at the moment most preferred catalyst is Co(dmgH2)2Ch.

[0158] The photochemical conversion (i.e. irradiation) is done with light in the wavelength range of from 300 to 800 nm, in particular in the wavelength range of from 320 to 580 nm. Examples of the light source for use in performing light irradiation are, although not limited to, high- pressure mercury lamps, xenon lamps, fluorescent lamps, incandescent lamps, electroluminescent lighting devices etc.

[0159] 'Light” in the proper sense is electromagnetic radiation with a wavelength (range) in the visible spectrum (380 to 780 nm). However, in terms of the present invention, unless specified otherwise, the term “light” also encompasses the directly adjacent wavelength spectrum, i.e. near IR (>780 nm to 1 pm) and near UV (300 to <380 nm). Monochromatic light consists of a (small) bandwidth of wavelengths. A filter isolates monochromatic light from a broadband light source; lasers or LEDs generate with light, preferably monochromatic light, directly.

[0160] The irradiation is preferably done with light, preferably monochromatic light, in the wavelength range of from 320 to 580 nm, in particular in the wavelength range of from 365 to 470 nm.

[0161] To perform the photodesaturation efficiently, the amount of photoredox catalyst and hydrogen evolution catalyst used in the reaction is at a concentration that ensures absorbance of light in the photoreactor. The amount of photoredox catalyst is in the range of 0.01 to 100 mM, preferably in the range of 1.0 to 10 mM, and most preferably in the range of 3.0 to 3.5 mM. The amount of hydrogen evolution catalyst is in the range of 0.01 to 50 mM, preferably in the range of 1.0 to 5 mM, and most preferably in the range of 1.5 to 1.7 mM. Further the amount of dmgFL added is in the range of 0.01 to 100 mM, preferably in the range of 1.0 to 10 mM and most preferably in the range of 6.0 to 7.0 mM.

[0162] “Absorbance” is defined as the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls).

[0163] In said embodiment “monochromatic light” as understood within this disclosure is preferably all radiation at least 90 % of its power and at most 100 % thereof being emitted in the range from 320 nm to 580 nm. The power of minor components of the monochromatic light being outside the given wavelength at most amounts up to 10 % depending on the filter-free electroluminescent lighting device employed, the nature and quantity of the catalyst system and the organic solvent. However, the great majority of embodiments of monochromatic light only contains small amounts of light portions beyond 350 nm to 490 nm. In one embodiment monochromatic light is understood to be an entity, at least 95 % of the power of said monochromatic light and at most 100 % of said power being emitted in the range from 350 nm to 490 nm. In yet another embodiment monochromatic light means, at least 98 % and further preferred at least 99 % of the power of said monochromatic light and at most 100 % of said power being emitted in the range from 365 nm to 470 nm. The amount of the monochromatic light is expressed in power since by doing so one is not urged to otherwise define the permissible amount of light in lumen Im or Wh or candela cd above and below the claimed wavelength range. Said amount, when not expressed in power, would vary as a function of the wavelength considered. In yet a further specified embodiment, monochromatic light, as understood within this disclosure, is all radiation at least 90 % of its power and at most 100 % thereof being emitted in the range from 365 nm to 470 nm and its monomodal emission spectrum exhibiting a halfwidth of + / - 10 to + / - 30 nm in relation to the wavelength of the emission maximum. Said defined halfwidth gives a highly structured lighting signal, which results in an improved yield of the unsaturated ester of the formula (I). The irradiation is preferably done with light, preferably monochromatic light, in the wavelength range of from 320 to 580 nm, in particular in the wavelength range of from 365 to 470 nm.

[0164] E1 . The method according to any of the preceding embodiments, where in step (a) esters of formula (II) are photochemically converted in neat form, or in a solvent in the presence of a catalyst system with light in the wavelength range of from 320 to 580 nm.

[0165] E2. The method according to embodiment E1 , where in step (a) the reaction mixture is irradiated with light in the wavelength range of from 365 to 470 nm.

[0166] E3. The method according to any of the preceding embodiments, where in step (a) the reaction mixture is irradiated with light, preferably monochromatic light.

[0167] E4. The method according to embodiment E3, where photochemical conversion in step (a) is carried out using a filter which provides monochromatic light from a broadband light source, wherein at least 90% of the monochromatic light is in the wavelength range of from 320 to 580 nm, especially in the range of from 365 to 470 nm, preferably a monochromatic light source, more preferably an electroluminescent lighting device emitting monochromatic light, where at least 90% of the light emitted by said monochromatic light source is in the wavelength range of from 320 to 580 nm, especially in the range of from 365 to 470 nm.

[0168] E5. The method according to embodiment E4, where photochemical conversion in step (a) is carried out using a monochromatic light source, preferably an electroluminescent lighting device emitting monochromatic light, where at least 90% of the light emitted by said monochromatic light source is in the wavelength range of from 365 to 470 nm.

[0169] E6. The method according to any of embodiments E4, or E5, where photochemical conversion in step (a) is carried out using an electroluminescent lighting device emitting light, preferably monochromatic light, where the electroluminescent lighting device consists of at least one LED.

[0170] The photochemical conversion with light is preferably accomplished by the use of a LED (light emitting diode), or laser.

[0171] The term “LED” used in this context can also refer to organic light emitting diodes (OLEDs), an active matrix organic light emitting diode (AMOLED), or any other diode based lighting source. Preferably, the LED refers to a high-power LED that can be used with 350 mW electrical power or more. Preferably, the LED is configured to be applied with an electrical power above 1 W.

[0172] The light emitted by the LED unit is chosen such that at least a part of the light can be used for the photochemical conversion. Preferably, the LED unit is adapted to emit light in the ultraviolet and (optionally) visible part of the electromagnetic spectrum, preferably light with a wavelength between 320 and 580 nm, more preferably between 365 and 470 nm.

[0173] A filter-free electroluminescent lighting device within this disclosure is any electroluminescent device emitting light, which does not comprise a filtering means. A filtering means can be a layer, a chemical compound or product applied onto the lighting device. A filtering means can also be a compound, which is immersed or solubilized in a solvent circulating, pumped or floating around the lighting device and adapted to absorb light in a distinct range but not to transfer energy emerging from said absorbed light onto esters of formula (II). The electroluminescent lighting device is required not to operate by means of any kind of chemically induced lighting like gas ionization or by means of heating. The filter-free electroluminescent lighting device is understood to provide light (photons) emerging from electrons supplementing holes or gaps in an electron-poor material with emission of electromagnetic radiation preferably in the form of visible light. Said filter-free electroluminescent lighting device is selected from the group of light emitting electrochemical cells, electroluminescent wires, field-induced electroluminescent polymers, light emitting diodes, organic light emitting diodes, polymer light emitting diodes, active-matrix organic light-emitting diodes (AMOLED’s), electroluminescent films especially based on inorganic luminescent materials, semiconductor lasers, diode lasers. The electroluminescent lighting device include chemical lasers, dye lasers, free-electron lasers, gas dynamic lasers, gas lasers, ion lasers, laser flashlights, metal-vapor lasers, non-linear optics quantum well lasers, ruby lasers, solid-state lasers etc.

[0174] In a further elaborated embodiment of the inventive process at least 90 % of the power of said light, preferably monochromatic light, and at most 100 % of said power being emitted in the range from 365 and 470 nm. Working at this somewhat smaller wavelength range still provides good conversion rates and high yields of the ester of formula (I), however, by means of a narrower wavelength spectrum thus expending less power and energy. Side reactions like e.g. isomerization into undesired compounds are even further suppressed or preferably completely avoided at this wavelength range. Narrowing of the wavelength range within the filter-free, electroluminescent lighting device is achieved by selectively controlling distinct electronic parts within said device.

[0175] The photochemical conversion may be carried out in a pumping circuit of a reaction vessel with back-mixing, in a continuously stirred reaction vessel, or in a continuous flow reactor. The lighting device and photochemical reactor described in WO2021 / 233951 and / or W02023 / 011951 A1 may be used in conducting step a) of the process of the present invention.

[0176] Step a) is preferably done in a temperature range of -20 to 100 ° C.

[0177] Step a) is preferably done in a pressure range from 50 mbar to 10 bar. In step a) the unsaturated esters of formula (I) may be continuously separated from the esters of formula (II), the solvent and the catalyst system so that the chemical equilibrium is shifted to the benefit of the product.

[0178] In a preferred embodiment the method of the present invention comprises the additional steps b1) separation of the unsaturated esters of formula (I), the esters of formula (II) and the solvent from the catalyst system, b2) separation of the solvent from the unsaturated esters of formula (I) and the esters of formula (II), b3) separation of the unsaturated esters of formula (I) from the esters of formula (II), and b4) recycling of the catalyst system, the solvent and the esters of formula (II) to step a).

[0179] The method of the present invention may comprise the additional step c) removal of the ester group of the unsaturated esters of formula (I) to obtain unsaturated alcohols of formula

[0180] (IV), wherein R3is defined above, or below.

[0181] It will be understood that an alcohol of formula (IV) that bears a double bond between carbon atoms Ci and C2 adjacent to the hydroxy group (i.e., ,..-CR=CR-OH, wherein R is H or another residue such as an organic residue, where ,..-CR=CR- in total fulfills the requirements of R3as defined herein) may undergo keto-enol-tautomerization and thus lead to the respective aldehyde or ketone (i.e., ,..-CHR-CR=O, wherein R is defined as aforementioned). Such reaction may occur immediately after generation of the respective alcohol. Thus, in this case, the method of the present invention may comprise the additional step c’) removal of the ester group of the unsaturated esters of formula (I) to obtain aldehydes or ketones of formula R3’=O (IV’), wherein R3’ is defined as R2above, or below (in other words, where R3’ corresponds to R3except that it does not contain a Ci=C2 double bond adjacent to the hydroxy group).

[0182] In other words, a preferred embodiment, the method of the present invention comprises removal of the ester group comprises cleavage of the ester group, preferably under basic conditions, and optionally isolating the alcohol of formula (IV) and / or the aldehyde or ketone of formula (IV’).

[0183] In a preferred embodiment, as used herein, an alcohol of formula (IV) does not comprise a Ci=C2double bond adjacent to the hydroxy group. Otherwise, this may undergo keto-enol- tautomerization to the respective aldehyde or ketone.

[0184] In a preferred embodiment, the method comprises the additional step c’) removal of the ester group of the unsaturated esters of formula (I) to obtain aldehydes or ketones of formula R3’=O (IV’), wherein R3’ is defined as R2as defined herein. In a preferred embodiment, removal of the ester group comprises cleavage of the ester group, preferably under basic conditions, and optionally isolating the alcohol of formula (IV) and / or the aldehyde and / or ketone of formula (IV’).

[0185] Accordingly, the present invention is directed to a method for the production of unsaturated esters of formula (I), comprising the steps

[0186] 0) esterification of alcohols of formula (III) to obtain esters of formula (II), a) photochemical conversion of esters of formula

[0187] (II) in neat form, or in a solvent in the presence of a catalyst system with light, b1) separation of the unsaturated esters of formula (I), the esters of formula (II) and the solvent from the catalyst system, b2) separation of the solvent from the unsaturated esters of formula (I) and the esters of formula (II), b3) separation of the unsaturated esters of formula (I) from the esters of formula (II), and b4) recycling of the catalyst system, the solvent and the esters of formula (II) to step a); and c) removal of the ester group of the unsaturated esters of formula (I) to obtain unsaturated alcohols of formula (IV) and / or the aldehyde or ketone of formula R3’=O

[0188] (IV’), wherein

[0189] R1is a hydrogen atom, a Ci-C4alkyl group, a phenyl group, which may optionally be substituted by one, or more groups R4;

[0190] R2is a C2-C2salkyl group, or a Cs-C cycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups;

[0191] R3denotes a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; or a single Cs-C cycloalkenyl group, or two, or more different Cs-C cycloalkenyl groups, which may optionally be substituted by one, or more Ci-C4alkyl groups;

[0192] R3’ denotes a single C2-C2salkyl group, or two, or more different C2-C2salkyl groups; a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; or a single C3- C cycloalkenyl group, or two, or more different Cs-C cycloalkenyl groups, which may optionally be substituted by one, or more Ci-C4alkyl groups; and R4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H and NO2.

[0193] The unsaturated esters (I) can be further purified by suitable separating and / or purification methods, in particular by distillation.

[0194] The solvent used in step a) is preferably selected from acetone, dimethylacetamide, dichloromethane, 1 ,2-dichloroethane, methyl-tert-butylether, acetonitrile, dimethylsulfoxide (DMSO), tert.-alcohols, water, N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF) and mixtures thereof. More preferably the solvent is an aprotic solvent, which is selected from acetone, dimethylacetamide, dichloromethane, 1 ,2-dichloroethane, methyl-tert-butylether, acetonitrile, dimethylsulfoxide (DMSO) and mixtures thereof.

[0195] The solvent used in step a) is present in an amount of 99 to 0 % by weight, especially 95 to 20 % by weight based on the amount of solvent and esters of formula (II).

[0196] In an alternative embodiment the esters of formula (II) are photochemically converted in neat form, i.e. no solvent is added in step a).

[0197] The catalyst system used in step a) comprises preferably at least one photoredox catalyst and at least one hydrogen evolution catalyst.

[0198] The photoredox catalyst comprises preferably at least one compound of formula [W O32]X4, wherein X is Li+, Na+, K+, Rb+, Cs+, N(R7)4+or P(R7’)4+, wherein R7and R7’ may be the same, or different in each occurrence and are selected from a hydrogen atom, a Ci-C3alkyl group and a phenyl group and R7’ may be the same, or different in each occurrence and are selected from a Ci-C3alkyl group and a phenyl group.

[0199] Examples of particular suitable photoredox catalyst are tetrabutylammonium decatungstate (TBADT), sodium decatungstate ([W O32]Na4 (NaDT)), tetraethyl ammonium decatungstate, tetra-n-propylammonium decatungstate, tetrabenzyl ammonium decatungstate and mixtures thereof. The at present most preferred catalyst is NaDT.

[0200] The hydrogen evolution catalyst is preferably an organometallic complex on basis of palladium, nickel, cobalt, or ruthenium; especially an organometallic complex of formula

[0201] , or Co(dmgH)2R5R5’ (V) (a complex having cobalt as central atom in oxidation state II or III, which is planarly coordinated by 2 dimethylglyoxime (dmgH) ligands each other Cl, Br, or a group of formula , wherein R6is a hydrogen atom,

[0202] -CN, -OMe, -tBu, or -CChMe.

[0203] Most preferred, the hydrogen development catalyst is an organometallic complex of formula

[0204] Co(dmgH)25R5(Va), wherein

[0205] R5and R5’ are Cl; or

[0206] R5and R5’ are Br; or

[0207] R5and R5’ are a group of formula . The at present most preferred catalyst is Co(dmgH2)2Ch.

[0208] The at present most preferred catalyst system comprises NaDT and Co(dmgH2)2Cl2.

[0209] Using excess dmgH2 in addition to the complex of formula (Va), especially Co(dmgH2)2Cl2 increases the stability of the cobalt complex and the product yield.

[0210] E1 . The method according to any of the preceding embodiments, where in step (a) esters of formula (II) are photochemically converted in neat form, or in a solvent in the presence of a catalyst system with light in the wavelength range of from 320 to 580 nm.

[0211] E2. The method according to embodiment E1 , where in step (a) the reaction mixture is irradiated with light in the wavelength range of from 365 to 470 nm.

[0212] E3. The method according to any of the preceding embodiments, where in step (a) the reaction mixture is irradiated with light, preferably monochromatic light.

[0213] E4. The method according to embodiment E3, where the photochemical conversion in step (a) is carried out using a filter which provides monochromatic light from a broadband light source, wherein at least 90% of the monochromatic light is in the wavelength range of from 320 to 580 nm, especially in range of from 365 to 470 nm, preferably a monochromatic light source, more preferably an electroluminescent lighting device emitting monochromatic light, where at least 90% of the light emitted by said monochromatic light source is in the wavelength range of from 320 to 580 nm, especially in range of from 365 to 470 nm.

[0214] E5. The method according to embodiment E4, where the photochemical conversion in step (a) is carried out using a with light source, preferably monochromatic light source, preferably an electroluminescent lighting device emitting monochromatic light, where at least 90% of the light emitted by said light source, preferably monochromatic light source is in the wavelength range of from 365 to 470 nm.

[0215] E6. The method according to any of embodiments E4, or E5, where the photochemical conversion in step (a) is carried out using an electroluminescent lighting device emitting light, preferably monochromatic light, where the electroluminescent lighting device consists of at least one LED.

[0216] The irradiation with light is preferably accomplished by the use of a LED (light emitting diode), or laser.

[0217] Step a) is preferably done in a temperature range of -20 to 100 ° C.

[0218] Step a) is preferably done in a pressure range from 50 mbar to 10 bar.

[0219] In step a) the unsaturated esters of formula (I) may be continuously separated from the esters of formula (II), the solvent and the catalyst system so that the chemical equilibrium is shifted to the benefit of the product.

[0220] In a particularly preferred embodiment the obtained unsaturated esters of formula (I) are compounds of formulae 1c1 and 1c2 and the starting material used for their production is the ester of formula 2c:

[0221] 1c1 1c2

[0222] 2c

[0223] R1is especially a hydrogen atom, a methyl group, an ethyl group, or a phenyl group, very especially a methyl group.

[0224] The ester 2c is obtained from isoamyl alcohol. The unsaturated esters 1c1 and 1c2 can be purified by suitable separating and / or purification methods, in particular by distillation, and be at least largely freed from undesired impurities or by-products, and are converted to prenol and isoprenol.

[0225] Additional aspects of the present invention relate to the use of the prenol (3-methyl-2-buten-1- ol) / isoprenol (3-methylbut-3-en-1-ol), obtained according to the method (process) described above and / or obtained according to the method of any of claims 1 to 24, preferably from sources of renewable raw materials, as starting material in the synthesis of Citral (3,7- dimethylocta-2,6-dienal), Linalool (3,7-dimethyl-1 ,6-octadien-3-ol), Menthol (5-methyl-2- (propan-2-yl)-cyclohexan-1-ol) and Vitamin A ((2E,4E,6E,8E)-3,7-dimethyl- 9-(2,6,6- trimethylcyclohex-1-enyl)nona-2,4,6,8-tetraen-1-ol).

[0226] Citral occurs as (2Z)- and (2E)-isomers: the (2Z))-lsomer, neral, as depicted in formula (Vb-1) and the (2E)-lsomer, geranial, as depicted in formula (Va-1).

[0227] The term citral obtainable (or obtained) according to the method of the present invention may be any mixture of the two isomers, preferably a mixture having a mass ratio of neral : geranial of between 40 . 60 to 60 : 40, in particular between 45 . 55 to 55 : 45, between 48 . 52 to 52 : 48, between 49 . 51 to 51 : 49, or (approximately) 50 : 50.

[0228] As known in the art, neral and geranial may be optionally separated from one another. For instance, neral and geranial may be separated by distillation. This allows adjusting the mass ratio of neral : geranial to a desired degree.

[0229] Citral obtainable (or obtained) according to the invention may comprise geranial of the formula (Va-1) and / or of neral of the formula (Vb-1).

[0230] A further aspect of the present invention relates to geranial of the formula (Va-1), neral of the formula (Vb-1) or a mixture thereof obtainable (or obtained) from a method of the present invention.

[0231] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing (iso)prenal and / or (iso)prenol and the further steps directed to citral above mutatis mutandis apply to the preparation and product characteristics of geranial of the formula (Va- 1), neral of the formula (Vb-1) or a mixture thereof. It will be understood that the geranial of the formula (Va-1), neral of the formula (Vb-1) and a mixture thereof may have certain characteristics. A further aspect of the present invention relates to a method of preparing citral involving the following steps: i) providing 3-methyl-3-butene-1-ol (isoprenol) and / or 3-methyl-2-butene-1-ol (prenol) obtainable by the present invention according to any of claims 23 or 24, optionally isomerizing isoprenol to prenol; ii) producing 3-methyl-2-butenal (prenal) and / or 3-methyl-3-butenal (isoprenal) from the prenol and / or isoprenol by oxidative dehydration by means of an oxygen-containing gas on a silver support catalyst; iii) producing additional prenal from a mixture containing isoprenal by isomerization; iv) producing unsaturated acetal 3-methyl-2-butenal-diprenylacetal from the prenol and the prenal using an acidic catalyst; and v) obtaining citral from the 3-methyl-2-butenal-diprenylacetal by cleavage and subsequently rearranging.

[0232] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing the unsaturated esters and / or of preparing unsaturated alcohols mutatis mutandis may apply to the method of preparing citral and precursors (e.g., isoprenol, prenol, isoprenal, prenal, and acetal) as well as downstream processes (e.g., to menthol, geraniol, isopulegol, linalool, vitamin A etc.).

[0233] In a further aspect, the invention provides a method for the preparation of prenol, comprising the step of: d) isomerizing isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

[0234] In a further aspect, the invention provides a method for the preparation of prenal and / or isoprenal, comprising the steps of: d) optionally, isomerizing isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and e) providing prenal by at least one of e-i) and e-ii): e-i) subjecting isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and e-ii) oxidizing prenol obtained in step d) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase. In a still further aspect, the invention provides a method for the preparation of 3,7-dimethyl- octa-2,6-dienal (citral) comprising the steps of: d) isomerizing isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; e) providing prenal by at least one of e-i) and e-ii): e-i) subjecting isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and e-ii) oxidizing prenol obtained in step d) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; f) condensing prenol obtained in step d) with prenal obtained in step e) to obtain diprenyl acetal of prenal; and g) subjecting diprenyl acetal of prenal obtained in step f) to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

[0235] Aspect - Preparation of Prenol

[0236] This aspect of the invention relates to the preparation of 3-methyl-2-buten-1-ol (prenol), comprising b) isomerizing isoprenol, obtained according to the method of the present invention and / or obtained according to the method of any of claims 1 to 24, to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

[0237] As used herein and hereinafter, the term “reactant stream” refers to a stream comprising a reactant or reactants consumed in the course of a chemical reaction. In this sense, the reactant stream may further comprise solvent(s), catalyst(s), additive(s) and / or any other substance involved in the chemical reaction.

[0238] Generally, the isomerization of isoprenol to 3-methyl-2-buten-1-ol (prenol) may be carried out over a supported noble metal, preferably in the presence of hydrogen.

[0239] A preferred catalyst is a fixed bed catalyst containing palladium and selenium or tellurium or a mixture of selenium and tellurium supported on silicon dioxide.

[0240] The catalyst contains 0.1 to 2.0% by weight of palladium and 0.01 to 0.2% by weight of selenium, tellurium or a mixture of selenium and tellurium, based on the total weight of the catalyst. The BET surface area is, for example, in the range of 100 to 150 m2 / g, in particular in the range of 110 to 130 m2 / g. The BET surface area is determined by N2 adsorption according to DIN 66131.

[0241] The pore volume in the pore diameter range from 3 nm to 300 pm is preferably 0.8 to 0.9 cm3 / g, in particular 0.8 to 0.85 cm3 / g. Thereby, 80 to 95%, preferably 85 to 93% of this pore volume is in the pore diameter range of 10 to 100 nm. The pore volume is determined by Hg porosimetry.

[0242] Preferably, the catalyst contains 0.2 to 0.8% by weight, in particular 0.4 to 0.6% by weight of palladium. Preferably, the catalyst contains 0.02 to 0.08, in particular 0.04 to 0.06 wt% selenium, tellurium or a mixture of selenium and tellurium, preferably selenium. In addition to the active components mentioned, other metals may be present on the catalyst in small amounts. Preferably, only palladium, selenium and / or tellurium, in particular only palladium and selenium, are present on the silica support.

[0243] The described isomerization of isoprenol to prenol on a fixed-bed catalyst is also described in EP-A-841090, to which express reference is made.

[0244] The isomerization is carried out at a temperature in the range of 50 to 150 °C, preferably in the range of 60 to 130 °C, more preferably in the range of 70 to 120 °C to produce a reaction mixture of prenol and isoprenol. The isoprenol can be recycled. Further details are provided in W02008 / 037693.

[0245] Generally, a regeneration cycle is performed periodically, to remove accumulated coke from the catalyst. The regeneration cycle can be initiated when the pressure drop increased above a threshold value, or at arbitrary time intervals, for example once a week. A regeneration cycle consists of sending diluted air or air for a defined period of time, for example 6 to 24 h, over the reactor while increasing the salt bath temperature, for example 400 to 450 °C, to allow coke combustion.

[0246] The unreacted isoprenol from the isoprenol isomerization method may be used, i.e. recycled for the isoprenol isomerization.

[0247] Aspect - Providing Prenal and / or Isoprenal

[0248] In addition, the invention relates to the preparation of prenal and / or isoprenal, comprising d) optionally, isomerizing isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and e) providing prenal by at least one of e-i) and e-ii): e-i) subjecting isoprenol, obtained according to the method described above and / or obtained according to the method of any of claims 1 to 24, to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and e-ii) oxidizing prenol obtained in step d) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0249] The isoprenol obtained as described above is converted to prenal, involving isomerization and an oxidative dehydrogenation in any order. Thus, it is possible to first isomerize isoprenol to prenol, and subsequently oxidize prenol to prenal; or, to first oxidatively dehydrogenate isoprenol to isoprenal, and optionally isomerize at least part of the isoprenal to prenal.

[0250] Oxidizing Prenol to Prenal

[0251] The prenol obtained as described above may be oxidized so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0252] Suitable oxidants include hydrogen peroxide and oxygen, in particular oxygen.

[0253] The oxidation is preferably carried out in the presence of a liquid phase and with oxygen as the oxidant. The liquid phase preferably comprises at least 25 wt.-% of water, more preferably at least 50 wt.-% of water or at least 70 wt.-% of water, based on the total weight of the liquid phase, determined at a temperature of 20 °C and a pressure of 1 bar. It has been found that these conditions allow for a simple and efficient method for preparing prenal from prenol.

[0254] The oxidation is typically carried out in the presence of at least one oxidation catalyst selected from the group consisting of platinum, palladium and gold. Preferably, the at least one oxidation catalyst comprises platinum. In a preferred embodiment, the at least one oxidation catalyst is a supported catalyst.

[0255] The oxidation is suitably carried out at a temperature of 20 to 100 °C, preferably, 25 to 80 °C, in particular 30 to 70 °C, in particular 35 to 50 °C. In another embodiment the oxidation is carried out at a temperature of 20 to 70 °C. The oxidation is suitably carried out under a partial pressure of oxygen between 0.2 and 8 bar.

[0256] Further details of the oxidation reaction may be found in WO2023 / 222895 A1 , which herewith is incorporated by reference in its entirety.

[0257] Oxidative Deyhdrogenation of Isoprenol

[0258] Oxidative dehydrogenation of isoprenol typically comprises bringing a reactant stream, in particular a gaseous reactant stream, comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in particular at least one silver-containing heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen. The at least one heterogeneous catalyst may consist of an inert support having a smooth surface having an active layer of silver. Alternatively, massive (full-metal) silver bodies may be used. In an embodiment, the non-reacted isoprenol from the isomerization of isoprenol to prenol, which corresponds to the step b), is used as feed to the dehydrogenation step.

[0259] Hence, in an embodiment, the method includes separating an unreacted isoprenol stream from a prenol containing product stream obtained in step b) and directing the unreacted isoprenol stream at least partially to step c-i).

[0260] In an embodiment, oxidative dehydrogenation is carried out by passing the isoprenol through a plurality of reaction tubes of a shell-and-tube heat exchange reactor comprising

[0261] - a shell-side heat exchange passage for circulating a heat transfer medium and a reaction passage comprising the plurality of reaction tubes;

[0262] - an inlet for introducing the reactant stream to the reaction passage; and

[0263] - an outlet from the reaction passage for recovering an effluent stream from the reaction tubes; wherein the reaction tubes comprise a reactant pre-heating zone adjacent to the inlet, and a reaction zone downstream of the reactant pre-heating zone, the reaction zone having a catalytically active wire matrix insert having silver at least on a part of its surface.

[0264] The term “reactant pre-heating zone” denotes a section of the reaction tube, i.e. a section inside the reaction tube, where essentially no catalytic oxidative dehydrogenation reaction occurs and where the gaseous stream through the reaction tubes is heat-exchanged via the tube wall with the circulating heat transfer medium. The pre-heating zone upstream of the reaction zone involves net heat flow into the reaction tube and ensures that the reactant stream is sufficiently heated up to a temperature close to or at the reaction temperature when it reaches the reaction zone.

[0265] Upon contact with the catalytic surface, the oxidative dehydrogenation reaction immediately starts. Otherwise, in the event when a “cold” reactant stream reaches the catalytic surface such that the reaction onset temperature of the reaction is not reached, coke formation may occur. Less coke formation advantageously leads to a prolonged reactor operation without the necessity of burning off the coke from the catalytic surface.

[0266] Preferably, the reactant pre-heating zone is adapted to allow for laminar flow of the reactant inside the reactant pre-heating zone. This means, the reactant pre-heating zone is devoid of any obstacles to the reactant flow that triggers a laminar-to-turbulent flow transition. Hence, the reactant pre-heating zone preferably has an essentially free cross section, i.e. the preheating zone is empty.

[0267] In the case of an “essentially free cross section”, the reactant pre-heating zone may be empty. Alternatively, the reactant pre-heating zone may accommodate fixtures made of a material having zero or limited catalytic activity, which fixtures have a negligible cross-section in a plane perpendicular to the longitudinal axis of the reaction tube. Said fixtures may be attached to the catalytically active wire matrix which is present in the reaction zone and allow to easily place said wire-matrix insert into or remove the same from the reaction zone. For example, the negligible mounting may be a stainless steel wire or rod.

[0268] This setup allows for heating up only the portion of the entire reactant stream that travels near the hot reaction tube wall. Consequently, the portion of the reactant stream flowing in the center of the reaction tube is not heated to the reaction temperature and blind reactions of the unstable starting materials are thus reduced or even avoided. A “blind reaction” is an unselective oxidative reaction that occurs in the absence of the catalyst. Once the reactant stream reaches the reaction zone, the oxidative dehydrogenation reaction is initiated. Due to the exothermic nature of this reaction, energy is released and the remainder of the reactant stream is rapidly heated to the reaction onset temperature, and the reaction proceeds. This fast heat up of the predominant part of the reaction mixture reduces unwanted side-reactions and thus leads to an increased selectivity.

[0269] Alternatively, the reactant pre-heating zone may have a wire matrix insert having zero or limited catalytic activity. The wire matrix insert may reduce or eliminate temperature gradients without creating any obstruction to flow that would promote turbulent flow characteristics. A wire matrix insert is considered as having zero catalytic activity (or in other words, as being “inert”) if it does not catalyze the gas-phase partial oxidation reaction in question to a significant degree, and the chemical composition of a stream passing the wire matrix insert does not change significantly. Similarly, a matrix insert is considered as having limited catalytic activity if its catalytic activity is less than the activity of a reaction zone. In an embodiment, the wire matrix insert having zero or limited catalytic activity is made of an inert material, preferably stainless steel.

[0270] Herein, the term “reaction zone” denotes a region of the reaction tube where the catalytic gasphase partial oxidation reaction occurs. The reaction zone comprises a catalytically active wire matrix insert having at least on a part of its surface a catalytically active precious metal. Due to the more open structure of the wire matrix contained in the reaction zone as compared to a packing of individual elements, a larger proportion of the reaction heat is discharged to the reaction tube wall by radiation and does not have to be dissipated by the reactant stream. Due the unique flow characteristic of the reactant stream through the reaction tube with the wire matrix insert in place, heat transfer via the tube wall is improved. Formation of prominent hotspots can be avoided. This in turn, avoids deposition of organic constituents of the reactant stream on the surface of the active catalyst material with concomitant pressure drop. Overall, less regular maintenance in the form of regeneration and / or replacement of the catalyst is required. The number of annual operating hours can be increased and the existing production capacities can be fully utilized, reducing operation cost and increasing profit.

[0271] In contrast to individually present catalyst bodies, the wire matrix inserts can be formed contiguously, or in one piece. Hence, placing the wire matrix inserts in the catalyst containment region of the reaction tubes, and removal therefrom is much facilitated. The “reaction zone” may be comprised of a single contiguous reaction zone. Alternatively, the reaction zone may comprise an alternating series of regions having catalytically active wire matrix inserts and regions having an essentially free cross section or having wire matrix inserts having zero or limited catalytic activity.

[0272] A “wire matrix insert” is understood to be a self-supporting skeletal-like structure made of coiled, bent or crimped metal wire which is adapted to be inserted into a reaction tube of a shell-and-tube reactor. The wire matrix insert has a more voluminous structure than a longitudinal wire.

[0273] A fixture such as a stainless steel wire or rod may be attached to the wire matrix insert which allows for easily placing the wire-matrix insert into or removing the same from the reaction zone.

[0274] In an embodiment, the catalytically active wire matrix inserts comprise an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are longitudinally arranged and helically shifted, that is, neighboring wire loops have an angular offset. The loops may be formed by helically bending the wire over the length of the wire matrix insert. In view of the ease of manufacture, the elongated core preferably comprises at least two longitudinal core wire members, which are twisted around each other to form core wire windings, and the wire loops are accommodated in the core wire windings.

[0275] The wire loops may be formed from one wire, or more than one intertwined wire, preferably 4 intertwined wires.

[0276] The wire matrix insert comprised in the reaction zone has silver at least on a part of its surface a catalytically active precious metal. The wire constituting the wire loops may be a massive silver wire, or a wire coated with silver. The core wire may be made of brass alloys, or highgrade steels. The coating layer of silver superimposed on the surface of the core has a thickness of, e.g., 10 pm. In general, however, a massive silver wire has better service life and is preferred. If the wire loops are formed from more than one intertwined wire, at least one of the intertwined wires is made of a massive silver wire, or a wire coated with silver while the other intertwined wires can be made of an inert material.

[0277] A silver wire which is of the same composition throughout its cross section and comprises at least 92.5 wt.-% Ag can suitably be used. The silver wire is helically bent to form wire loops, and combined with at least two longitudinal core wire members, which are twisted around each other to form core wire windings, and the wire loops are accommodated in the core wire windings. The longitudinal core wire members can also be silver wire or inert metal wire.

[0278] In a preferred embodiment, the catalytically active wire matrix inserts comprise an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are longitudinally arranged and helically shifted, and the wire loops comprise a massive silver wire. Further details of oxidative dehydrogenation carried out by passing the isoprenol through a plurality of reaction tubes of a shell-and-tube heat exchange reactor as described above may be found in WO2023 / 241952 A1 , which herewith is incorporated by reference in its entirety.

[0279] When isoprenol is subjected to oxidative dehydrogenation, it may be favorable to maintain in the reactant stream a weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol of less than 0.04, preferably less than 0.03, in particular less than 0.02, or less than 0.01. In another embodiment, the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol is maintained at less than 0.002, or less than 0.001 and optionally at least 100 ppm.

[0280] The weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol in the reactant stream may be maintained at a certain level or less. Reducing the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol in the reactant stream beyond a certain point, however, reaches a point of rapidly diminishing return. Aldehydes, preferably prenal and / or formaldehyde removal involves additional equipment and operating costs. An economic balance must be taken between the improvement due to reducing the ratio and the cost of maintaining such a ratio. Hence, the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol is preferably not lower than 0.0005. In an alternative embodiment the weight ratio is not lower than 0.005.

[0281] Reactor clogging and pressure drop increase are significantly affected by the presence of aldehydes, preferably prenal and / or formaldehyde in the reactant stream. Catalyst-fouling reactions of condensation and polymerization are believed to be the principal reactions involved in carbon or coke formation on the catalyst. It is thought that this carbon formation involves thermal condensation of aldehydes, preferably prenal and / or formaldehyde or of aldehydes, preferably prenal and / or formaldehyde with the olefinic hydrocarbons isoprenol and (iso)prenal. In the presence of the catalyst, the primary condensation products tend to undergo dehydrogenation and polymerization type reactions and to settle on the catalyst and undergo further dehydrogenation and decomposition until carbonaceous deposits are formed.

[0282] The method of the invention according to the aspect may satisfy the following condition 1), and preferably the following condition 2), or the method meets at least one of the following conditions 1) and 2):

[0283] 1) Step e-i) is characterized by maintaining in the reactant stream a weight ratio of aldehydes to isoprenol of less than 0.04.

[0284] 2) Step d) is characterized by maintaining in the reactant stream a concentration of aldehydes of less than 0.5% by weight, preferably less than 0.4% by weight, in particular less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reactant stream, and, optionally, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, based on the total weight of the reactant stream. Reducing the weight ratio of aldehydes, preferably prenal to isoprenol in the reactant stream can be accomplished in several different ways. In an embodiment, aldehydes, preferably prenal, are removed from the unreacted isoprenol stream prior to combining the unreacted isoprenol stream with the crude isoprenol stream.

[0285] In an embodiment, the unreacted isoprenol stream is combined with the crude isoprenol stream, preferably prenal is removed from the combined stream.

[0286] Aldehydes, preferably prenal and / or formaldehyde, may be removed from isoprenol streams by a conventional separating method such as distillation, selective adsorption and or selective reaction, in particular by the purification method involving the pressure-swing distillation.

[0287] Pre-Treating (Iso)Prenol by Nitrogen Removal Prior to Oxidative Deyhdrogenation or Oxidation

[0288] Prior to contacting with the at least one oxidative dehydrogenation catalyst or with the at least one oxidation catalyst, respectively, the (iso)prenol may advantageously be treated to remove organically bound nitrogen from the (iso)prenol by contacting the (iso)prenol with a weakly acidic solid adsorbent. In other words, the (iso)prenol may be depleted of organically bound nitrogen by this method.

[0289] The term “organically bound nitrogen” is intended to denote any compound containing at least one nitrogen atom directly bound to one or more carbon atoms. For example, such compounds containing at least one nitrogen atom may be selected from amines, such as ethylamine, trimethylamine, aniline, pyridine or piperidine. An amine particularly significant in practice is hexamethylenetetramine (urotropin). (Iso)prenol may comprise about 5 to 30 ppm of organically bound nitrogen.

[0290] The weakly acidic solid adsorbents have been found to be capable of adsorbing organically bound nitrogen in the presence of abundant (iso)prenol while not interfering with the reactive carbon-carbon double bond.

[0291] The weakly acidic adsorbent may include an adsorbent material having sufficient acidity to adsorb the organically bound nitrogen from the (iso)prenol. In an embodiment, the solid adsorbent is a crosslinked resin having phosphonic functional groups. Preferably, the resin polymer is a vinyl aromatic copolymer, preferably crosslinked polystyrene and more preferably a polystyrene divinylbenzene copolymer. Other polymers having a phosphonic functional group may also be used. Preferably, the crosslinked resin having phosphonic functional groups is of the macroporous type. A preferred solid adsorbent is Purolite S956.

[0292] The resin is typically used in bead form and loaded into a column. The (iso)prenol is passed through the column, contacting the resin beads. During contact, the organically bound nitrogen in the (iso)prenol reacts with the functional group and an exchange occurs where a proton is transferred to the nitrogen and an ionic bond is formed to the anionic site of the resin. Contact is maintained until a threshold level is reached i.e. the breakthrough concentration. At this breakthrough point, the method reaches an equilibrium where additional organically bound nitrogen cannot be removed effectively. The flow is halted and the column is backwashed with water, preferably deionized or softened water. By flowing in reverse, the resin is fluidized and solids captured by the beads are loosened and removed.

[0293] In another embodiment, the solid adsorbent is a silica-alumina hydrate. Numerous silica- alumina catalyst compositions and processes for their preparation are described in the patent literature, see, e.g., US4,499,197.

[0294] Preferably, the alumina content of the silica-alumina hydrate is from about 10 to about 90 wt.-% of AI2O3. The preferred range of alumina content is from about 30 to about 70 wt.-% of AI2O3. The introduction of silicon dioxide into aluminum oxide leads to the introduction of acidic centers. The number of acidic centers can be controlled by the amount of introduced silicon dioxide. The number of acidic centers increases with the amount of introduced silicon dioxide up to a maximum number of acidic centers, and decreases again with a further increasing amount of silicon dioxide after having reached the maximum number of acidic centers.

[0295] Examples of commercially available silica-alumina hydrates are Siral® available from Sasol Germany Gmbh, Hamburg, Germany. Siral® is based on orthorhombic aluminum oxide hydroxide (boehmite; AIOOH) and doped with SiO2. Various Siral® grades having different ratios of AI2O3 to SiO2 are available: Siral 1 (Al2O3 / SiO2 = 99 / 1), Siral 5 (Al2O3 / SiO2 = 95 / 5), Siral 10 (AI2O3 / SiO2= 90 / 10), Siral 20 (AI2O3 / SiO2= 80 / 20), Siral 28M (AI2O3 / SiO2= 72 / 28), Siral 30 (Al2O3 / SiO2 = 70 / 30), Siral 40 (AhO3 / SiO2 = 60 / 40). Siral 40 is especially preferred.

[0296] In an embodiment, the (iso)prenol is passed over a bed of the weakly acidic solid adsorbent. Suitably, said step of “passing over a bed” denotes that a layer (“bed”) of the weakly acidic solid adsorbent is provided in a customary reaction vessel known to the skilled person which may preferably be equipped with a stirring device, e.g. in a stirred-tank reactor. The (iso)prenol is then introduced into the reaction vessel and guided through the same in a manner that it gets into contact with the weakly acidic solid adsorbent.

[0297] Alternatively, the weakly acidic solid adsorbent may be provided in a reaction tube, e.g. of a tubular reactor and the (iso)prenol then continuously flows through said reaction tube(s) while getting into contact with the weakly acidic solid adsorbent.

[0298] In an alternative embodiment, the (iso)prenol comprises, after contacting the alcohol stream with a weakly acidic solid adsorbent, less than 2 ppm of organically bound nitrogen. Herein, “ppm” denotes wt.-ppm of compounds incorporating organically bound nitrogen, relative to the total weight of the (iso)prenol.

[0299] For example, the content of organically bound nitrogen in the (iso)prenol may be determined by Kjeldahl analysis. Alternatively, an oxidative combustion method with a chemiluminescence detector according to DIN 51444 may be used. Aspect - Preparation of Citral

[0300] In addition, the invention relates to the preparation of 3,7-dimethyl-octa-2,6-dienal (citral), comprising the steps of providing isoprenol as described above, d) isomerizing the obtained isoprenol to obtain prenol as described above, and e) providing prenal by at least one of steps e-i) and e-ii) as described above, and further f) condensing prenol obtained in step d) with prenal obtained in step e) to obtain diprenyl acetal of prenal; and g) subjecting diprenyl acetal of prenal obtained in step f) to cleaving conditions to obtain citral.

[0301] In particular, 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by a method comprising the steps of:

[0302] - condensing the prenal with prenol in the presence of at least one catalyst in a reaction column while withdrawing an acetal fraction comprising the diprenyl acetal of prenal from the reaction column;

[0303] - subjecting the acetal fraction in a cleaving column to cleaving conditions in the presence of at least one catalyst while withdrawing from the cleaving column a cleaving fraction containing at least one of prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl- 1 ,5-hexadiene, and optionally containing citral; and

[0304] - reacting the cleaving fraction in a plug-flow type reactor to obtain citral.

[0305] The overall reaction sequence is illustrated by the reaction scheme below.

[0306] -pwnol prenol prenal diprenyl acetal - formyl-1 ,5-hexadiene butadienyl) ether

[0307] The unsaturated acetal 3-methyl-2-butenal-diprenyl acetal (herein referred to as “diprenyl acetal of prenal” or “diprenyl acetal”) is formed from prenol and prenal using at least one catalyst. For this purpose, prenal may be reacted together with prenol in the presence of catalytic amounts of at least one acid and with separation of the water formed during the reaction in a reaction column.

[0308] It has been found that when the conversion rate of diprenyl acetal of prenal is driven to full conversion, the concentration of by-products increases sharply. Accordingly, it is preferred that the conversion rate of diprenyl acetal of prenal is maintained at above 90% and below 100%. Preferably, the conversion rate of diprenyl acetal of prenal in step b) is maintained equal to or below 99.5%, preferably equal to or below 99%, such as equal to or below 98%, or equal to or below 97.5%, or equal to or below 97%. Preferably, the conversion rate of diprenyl acetal of prenal is maintained above 91 %, such as above 92%, or above or 93%, or above 94%, or above 95%. In suitable embodiments, the conversion rate of diprenyl acetal of prenal in is above 94% and equal to or below 99%, such as above 95% and equal to or below 98%. Lower conversion rates will render the method economically unprofitable or will otherwise necessitate recovery and recycling of unreacted diprenyl acetal. Complete conversion is however undesirable as it results in a drop of yield of citral building blocks and increasing by-products- formation. The conversion rate is governed by various parameters including cleaving temperature, nature and concentration of the catalyst(s) and residence time in the cleaving column.

[0309] The resulting 3-methyl-2-butenal diprenyl acetal (diprenyl acetal) is cleaved in the presence of at least one catalyst in a cleaving column with elimination of 3-methyl-2-buten-1-ol (prenol) to give prenyl (3-methylbutadienyl) ether. Claisen rearrangement of the obtained prenyl (3- methylbutadienyl) ether yields 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene which subsequently undergoes Cope rearrangement yielding 3,7-dimethyl-2,6-octadienal (citral).

[0310] Cleaving is carried out in the presence of at least one catalyst, preferably an acid catalyst. The catalyst can be a single catalytic species or a combination of two or more different catalytic species. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid. In an embodiment, the catalyst comprises phosphoric acid. In a preferred embodiment, the concentration of the phosphoric acid in the bottoms of the cleaving column is maintained above 100 ppm and below 1500 ppm, preferably above 200 ppm and below 1000 ppm. Higher concentrations of (acid) catalyst may result in reduced yields of citral building blocks.

[0311] Condensation of prenol with prenal is carried out in the presence of at least one catalyst, preferably an acid. The catalyst can be a single catalytic species or a combination of two or more different catalytic species. In an embodiment, the catalyst in is nitric acid. Preferably, the concentration of the nitric acid is below 500 ppm, more preferably in the range of from 100 to 300 ppm, relative to the total amount of the starting materials prenol and prenal. Lower amounts of (acid) catalyst may result in a low conversion in the reaction column. Higher amounts of (acid) catalyst may disadvantageously result in increased formation of by-products and in decreased selectivities.

[0312] Preferably, the acetal fraction is continuously subjected to cleaving conditions in a cleaving column. “Cleaving conditions” denotes reaction conditions selected such that the diprenyl acetal contained in the acetal fraction is cleaved to prenyl (3-methylbutadienyl) ether which may subsequently rearrange to 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene and citral. The acetal fraction comprises diprenyl acetal as a main constituent. The acetal fraction does not necessarily need to consist of pure diprenyl acetal, but may also comprise prenol, prenal and citral building blocks.

[0313] Cleaving is carried out in the presence of at least one catalyst, preferably at least one acid catalyst. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid.

[0314] Suitably, the continuous cleaving in the cleaving column may be carried out in the lower part or the sump of the distillation column acting as cleaving column. Preferably, the acetal fraction and / or the catalyst(s) are introduced into the lower part of the distillation column, into the sump of the distillation column or into the evaporator of the distillation column.

[0315] If desired, a high-boiling inert compound can be introduced into the sump of the cleaving column in order to ensure a minimum filling level of the sump and the evaporator. Suitable high-boiling inert compounds are selected from liquid compounds which are inert under the reaction conditions and have a higher boiling point than citral and diprenyl acetal. For example, the high-boiling inert compounds may be selected from hydrocarbons such as tetradecane, pentadecane, hexadecane, octadecane, eicosane; or ethers such as diethylene glycol dibutyl ether; white oils; kerosene oils; or mixtures thereof.

[0316] Suitably, the distillation conditions are selected such that the diprenyl acetal is predominantly retained in the lower part or the sump of the distillation column. During the cleaving reaction, a cleaving fraction is continuously withdrawn from the cleaving column, the cleaving fraction containing at least one of prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5- hexadiene, and optionally containing citral. For the ease of reference, prenyl (3-methyl- butadienyl) ether, 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene and citral are collectively referred to as “citral building blocks”. This is because the former are intermediates on the reaction route to citral and can be converted into citral in the subsequent passage through the plug-flow type reactor.

[0317] Additionally, the prenol formed during the cleaving reaction may be continuously removed from the reaction mixture, generally at the top of the cleaving column.

[0318] The cleaving fraction together with the formed prenol may be withdrawn at the top of the distillation column.

[0319] Alternatively and preferably, it is also possible to withdraw the cleaving fraction in liquid or vaporous form at a side draw of the distillation column.

[0320] The cleaving fraction may be reacted in a plug-flow type reactor to obtain citral. To this end, the cleaving fraction is guided through the plug-flow type reactor at a suitable temperature for carrying out the rearrangement reaction(s) yielding citral. By employing a combination of a highly back-mixed cleaving column and a plug-flow reactor, it is possible to increase the selectivity and the yield of the cleaving reaction. All of the catalyst(s) required for the cleaving reaction is / are preferably introduced into the cleaving column and preferably, no catalyst is introduced into the plug-flow reactor.

[0321] In an embodiment, prenol eliminated in the cleaving reaction is recycled to the condensation reaction. This allows for improved yields to be achieved in the method of the invention.

[0322] In particular, the inventive method may comprise recycling prenol obtained in step e) to step d); wherein the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene of the prenol recycled from step e) into step d) is controlled such that the concentration of 2,4,4-trimethyl-3-formyl- 1 ,5-hexadiene in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal; and wherein the concentration of citral of the prenol recycled from step e) into step d) is controlled such that the concentration of citral in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal.

[0323] Further Conversion to Menthol or Linalool

[0324] The thus obtained citral is a useful intermediate for, e.g., menthol or linalool.

[0325] Menthol may be prepared from citral via a method comprising the steps of

[0326] - catalytic hydrogenation of citral produced by using the above described methods to obtain citronellal;

[0327] - cyclization of citronellal to obtain isopulegol in the presence of at least one acidic catalyst; and

[0328] - catalytic hydrogenation of isopulegol to obtain menthol.

[0329] The overall reaction sequence is illustrated by the reaction scheme below.

[0330] The hydrogenation of citral to obtain citronellal may be achieved by hydrogenation in the presence of a rhodium-phosphine catalyst.

[0331] Menthol (p-menthal-3-ol) is a naturally occurring active ingredient that is widely used in pharmaceuticals, cosmetics and the food industry. Menthol has a cooling effect when it comes into contact with mucous membranes, especially the oral mucosa. In natural sources, for example peppermint oil, menthol occurs in the form of four diastereomeric enantiomer pairs. The following formula depicts the main component, (-)-menthol or L-menthol, which has desired taste and other sensory properties.

[0332] IIIPAC name: 1 R, 2S, 5R 2-isopropyl-5-methylcyclohexanol

[0333] Isopulegol (5-methyl-2-(1-methylethenyl)-cyclohexanol) has three asymmetric carbon atoms and therefore four stereoisomers, each occurring as a pair of enantiomers. (1 R,3R,4S)- (-)lsopulegol is also known as L-isopulegol.

[0334] A further aspect of the invention is directed to a process for the preparation of isopulegol, preferably optically active isopulegol, preferably L-isopulegol, comprising the steps of o) optionally separating the citral obtainable (or obtained) according to the invention, preferably according to claim 25, into geranial of the formula (Va-1) and neral of the formula (Vb-1), i) preparation of optically active citronellal by asymmetric hydrogenation of citral obtainable (or obtained) according to the invention, preferably according to claim 25, geranial of the formula (Va-1) or of neral of the formula (Vb-1) by the process according to the invention, ii) cyclization of the optically active citronellal prepared in this way to give optically active isopulegol in the presence of a suitable acid, preferably a Lewis acid.

[0335] A further aspect of the present invention relates to isopulegol, which may be optionally optically active isopulegol, preferably L-isopulegol, obtainable (or obtained) from a method of the present invention.

[0336] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing (iso)prenal and / or prenol, citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of isopulegol. It will be understood that the obtainable (or obtained) isopulegol has certain characteristics.

[0337] A further aspect of the invention is directed to a process for preparation of menthol comprising the steps of catalytic hydrogenation of citral obtainable (or obtained) according to the invention, preferably according to claim 25, to obtain citronellal; cyclization of citronellal prepared in this way to obtain isopulegol in the presence of at least one acidic catalyst; and catalytic hydrogenation of isopulegol prepared in this way to obtain menthol.

[0338] A further aspect of the present invention is directed to a process for the preparation of optically active menthol using citral obtained by the process according to the invention. A further aspect of the invention is directed to a process for the preparation of optically active menthol, preferably L-menthol, comprising the steps of o) optionally separating the citral obtainable (or obtained) according to the invention, preferably according to claim 25, into geranial of the formula (Va-1) and neral of the formula (Vb-1), i) preparation of optically active citronellal by asymmetric hydrogenation of citral obtainable (or obtained) according to the invention, preferably according to claim 25, geranial of the formula (Va-1) or of neral of the formula (Vb-1) by the process according to the invention, ii) cyclization of the optically active citronellal prepared in this way to give optically active isopulegol in the presence of a suitable acid, preferably a Lewis acid, and iii) hydrogenation of the optically active isopulegol prepared in this way to give optically active menthol.

[0339] A further aspect of the present invention relates to menthol, which may be optionally optically active menthol, preferably L-menthol, obtainable (or obtained) from a method of the present invention.

[0340] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing (iso)prenal and / or prenol, citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of menthol. It will be understood that the obtainable (or obtained) menthol has certain characteristics.

[0341] The cyclization of citronellal to isopulegol may be achieved by cyclization in the presence of at least one Lewis-acidic aluminum-containing catalyst, such as a bis(diarylphenoxy)aluminum compound, which may be used in the presence of an auxiliary, such as a carboxylic anhydride. The isopulegol may be recovered from the catalyst-containing reaction product by distillative separation to give an isopulegol-enriched top product and an isopulegol-depleted bottom product. From the bottom product, the at least one catalyst may be regenerated. The isopulegol obtainable in this way by the cyclization of citronellal can be further purified by suitable separating and / or purification methods, in particular by crystallization, and be at least largely freed from undesired impurities or by-products.

[0342] The hydrogenation of isopulegol may be achieved by hydrogenation in the presence of at least one heterogeneous nickel-containing catalyst, preferably at least one heterogeneous nickeland copper-containing catalyst.

[0343] Further details regarding the reaction sequence from citral to menthol may be found in US2013 / 46118A1 , which is incorporated by reference herein.

[0344] In one aspect, the invention thus relates to an improved method for the preparation of menthol by producing citral using the above methods and then producing menthol from the citral. Menthol may be prepared as described herein or by other methods known in the art. Linalool may be prepared from citral via a process comprising catalytic hydrogenation of citral to obtain nerol and / or geraniol, and isomerization thereof.

[0345] A further aspect of the present invention relates to linalool obtainable (or obtained) from a method of the present invention.

[0346] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing (iso)prenal and / or prenol, citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of linalool. It will be understood that the obtainable (or obtained) linalool has certain characteristics.

[0347] The hydrogenation of citral to obtain nerol and / or geraniol may be achieved by hydrogenation in the presence of at least one supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably a ruthenium catalyst supported on carbon black, ruthenium / iron catalyst supported on carbon, comprising 0.1 to 10% by weight of ruthenium and 0.1 to 5% by weight of iron. Reference is made to EP1318128A2 and WO2017 / 060243.

[0348] The isomerization of nerol and / or geraniol to obtain linalool may be achieved by isomerization in the presence of a tungsten catalyst, especially a dioxotungsten (VI) complex, very especially a dioxotungsten(VI) complex of the general formula (VI), wherein Li and

[0349] L2 are independently of each other a ligand selected from the group consisting of the aminoalcohols, the aminophenols and mixtures thereof; and m and n are each 1 or 2. Further details regarding the isomerization of geraniol may be found in W003 / 048091 and WO03 / 047749.

[0350] In one aspect, the invention thus relates to an improved process for the preparation of linalool by producing citral using the above processes and then producing linalool from the citral. Linalool may be prepared as described herein or by other methods known in the art.

[0351] Further Conversion to Vitamin A

[0352] The obtained citral is also a useful intermediate for the synthesis of vitamin A.

[0353] A further aspect of the invention is directed to a process for the preparation of vitamin A acetate comprising the steps of

[0354] - converting Citral (VII) obtainable (or obtained) according to the invention, preferably according to claim 25, into pseudoionone (VIII),

[0355] - reacting pseudoionone (VIII) to obtain p-ionone (IX),

[0356] - transforming p-ionone (IX) into p-vinylionol of formula (X),

[0357] - phosphorylation of p-vinylionol of formula (X) to yield the Cis-salt of formula (XI), and - reacting the Cis-salt of formula (XI) with the Cs-acetate of formula (XII) to yield vitamin A acetate of formula (XIII).

[0358] A further aspect of the present invention relates to vitamin A obtainable (or obtained) from a method of the present invention.

[0359] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing (iso)prenal and / or prenol, citral and further steps and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of vitamin A. It will be understood that the obtainable (or obtained) vitamin A has certain characteristics.

[0360] Vitamin A acetate may be prepared from citral via the reaction sequence illustrated by the reaction scheme below.

[0361] Citral (VII) can be converted into pseudoionone (VIII) in reaction step A. Said pseudoionone can be reacted in synthetic step B to obtain p-ionone (IX), which is further transformed into p- vinylionol of formula (X). Phosphorylation of p-vinylionol of formula (X) can yield the Cis-salt of formula (XI), which upon reacting it with the Cs-acetate of formula (XII) can yield vitamin A acetate of formula (XIII).

[0362] Accordingly, Vitamin A acetate may be prepared from citral via a process comprising the steps of - converting Citral (VII) obtained by using the above described processes into pseudoionone (VIII),

[0363] - reacting pseudoionone (VIII) to obtain 0-ionone (IX),

[0364] - transforming 0-ionone (IX) into p-vinylionol of formula (X),

[0365] - phosphorylation of p-vinylionol of formula (X) to yield the Cis-salt of formula (XI), and

[0366] - reacting the Cis-salt of formula (XI) with the Cs-acetate of formula (XII) to yield vitamin A acetate of formula (XIII).

[0367] Reaction step A can be realized in the presence of a base selected form metal hydroxides, in particular alkali metal hydroxides and earth alkali metal hydroxides. Said base acts as a catalyst and can be added in one or several portions as e.g. disclosed in EP0062291A1 and W02004 / 041764A1.

[0368] Cyclisation of pseudoionone (VIII) into p-ionone (IX) in step B is realized in the presence of an acid, preferably in the presence of a mineral acid. A method of realizing step B is disclosed in EP0133 668A2 and in US3,840,601. The vinylionol (X) can be obtained by reacting the compound of formula (IX) with a Grignard reagent. The Cis-salt of formula (XI) can be obtained from vinylionol (X) in the presence of a phosphine. A suitable method of obtaining compound (XI) is disclosed in W02005 / 058811A1.

[0369] Vitamin A acetate (XIII) can finally be obtained by subjecting the Cis-salt of formula (XI) to Wittig conditions in the presence of the acetate of formula (XII). Details of such a Wittig reaction are disclosed in W02005058811A1.

[0370] The present invention also embraces the products obtainable (or obtained) from a method according to the present invention.

[0371] It will be understood that the definitions and preferred embodiments as laid out in the context of preparing the unsaturated esters and / or of preparing unsaturated alcohols and / or citral and / or downstream products thereof mutatis mutandis may apply to the obtainable (or obtained products.

[0372] A further aspect of the present invention relates to an ester of formula (I) obtainable (or obtained) from a method according to the present invention, preferably according to any of claims 1 to 22.

[0373] A further aspect of the present invention relates to an alcohol of formula (IV) obtainable (or obtained) from a method according to the present invention, preferably according to any of claims 23 or 24.

[0374] A further aspect of the present invention relates to an aldehyde or ketone of formula (IV’) obtainable (or obtained) from a method according to the present invention, preferably according to any of claims 26 or 27. A further aspect of the present invention relates to citral obtainable (or obtained) from a method according to the present invention, preferably according to claim 25.

[0375] The examples below serve to illustrate the invention without limiting it in any way.

[0376] Examples

[0377] Example 1

[0378] A crimp vial equipped with stirring bar is charged with sodium decatungstate (W O32Na4, 0.01 mmol, 2.5 mol%), Co(dmgH)2Cl2 (0.005 mmol, 1.25 mol%) and dimethylglyoxime (0.02 mmol) and is sealed with a PTFE rubber septum. The reaction vial is degassed on a Schlenk line via syringe needle by repeated evacuation and flushing with argon. The ester shown in table 1 (0.4 mmol) and acetonitrile (3 mL) are added via syringe needle. The vial is placed in a metal shell which is drained by cooling liquid and tempered by a thermostat. The reaction mixture is irradiated with a 365 nm LED set-up from the bottom side of the vial for an appropriate time.

[0379] Table 1

[0380] 1)Reaction conditions: NADT (2.5 mol%), Co(dmgH2)2Cl2 (1.25 mol%), dmgH2 (5 mol%), MeCN, Ar, 3h 20°C 365 nm LED (3.7 W)

[0381] 2)Reaction conditions: NADT (2.5 mol%), Co(dmgH2)2Cl2 (1.25 mol%), dmgH2 (5 mol%), MeCN, Ar, 5h 20°C 365 nm LED (3.7 W)

[0382] 3)Additional products:

[0383] Examples 2 to 7

[0384] Example 1 is repeated, except that the educts shown in table 2 are used and the following reaction conditions are employed: NADT (2.5 mol%), Co(dmgH2)2Cl2 (1.25 mol%), dmgH2 (5 mol%), MeCN, Ar, 5h 20°C 365 nm LED (3.7 W).

[0385] Table 2

[0386] Example Educt Produces)1’

[0387] ) Confirmed via GC / GCMS using reference material. ) Approximated yield(s) based on GC area.

Claims

Claims1 . A method for the production of unsaturated esters of formula30R\ A 1(I), comprising the step a) photochemical conversion of esters of formulain neat form, or in a solvent in the presence of a catalyst system with light, wherein R1is a hydrogen atom, a Ci-C4alkyl group, or a monocyclic or polycyclic aromatic hydrocarbon group, which may optionally be substituted, especially by one, or more groups R4;R2is an aliphatic, or an alicyclic group;R3denotes the aliphatic, or the alicyclic group R2, comprising one additional double bond; andR4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CChMe, SO3H, NO2, CO2H, a tert-butyl group and a phenyl group.

2. The method according to claim 1 , wherein:R1is defined as in claim 1 ;R2is a linear or branched C2-C2salkyl group, a linear or branched C2-C2salkenyl group, a Cs-Ciomonocycloalkyl group, which may optionally be substituted by one or more C1- C4alkyl groups, or a CyCispolycycloalkyl group, which may optionally be substituted by one or more Ci-C4alkyl groups; andR3is defined as in claim 1 ; and if present, R4is defined as in claim 1.

3. The method according to any of claims 1 or 2, whereinR1is a hydrogen atom, a Ci-C4alkyl group, or a monocyclic or polycyclic aromatic hydrocarbon group, which may optionally be substituted, especially by one, or more groups R4, especially a hydrogen atom, a Ci-C4alkyl group, a phenyl group, which may optionally be substituted by one, or more groups R4;R2is an alkyl group, or a cycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups, especially a C2-C2salkyl group, or a Cs-C cycloalkyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups;R3denotes a single alkenyl group, or two, or more different alkenyl groups, or a single cycloalkenyl group, which may optionally be substituted by one, or more Ci-C4alkyl groups, or two, or more different cycloalkenyl groups, which may optionally be substituted by one, or more Ci-C4alkyl groups, especially a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; or a single Cs-C cycloalkenyl group,which may optionally be substituted by one, or more Ci-C4alkyl groups, or two, or more different Cs-C cycloalkenyl groups, which may optionally be substituted by one, or more Ci-C4alkyl groups; andR4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CChMe, SO3H, NO2, CO2H, a tert-butyl group and a phenyl group, especially F, Cl, Br, I, CN, CO2Me, SO3H and NO2.

4. The method according to any of claims 1 to 3, wherein:R1is defined as in any of claims 1 to 3, preferably is a linear or branched Cs-C alkyl group, a linear or branched Cs-C alkenyl group, in particular -CH3;R2is a linear or branched Cs-C alkyl group, a linear or branched Cs-C alkenyl group, a Cs-Ciomonocycloalkyl group, which may optionally be substituted by one or more C1- C4alkyl groups, or a CyCispolycycloalkyl group, which may optionally be substituted by one or more Ci-C4alkyl groups; andR3is defined as any of claims 1 to 3; and if present, R4is defined as any of claims 1 to 3.

5. The method according to any of claims 1 to 4, wherein:R1is defined as any of claims 1 to 4, preferably is a linear or branched Cs-C alkyl group, a linear or branched Cs-C alkenyl group, in particular -CH3;R2is selected from the group consisting of ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n- heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl or n-dodecyl, isopropyl, sec-butyl, 2- methylpropyl (isobutyl), 1 -methylbutyl, 2-methylbutyl, 3-methylbutyl, 1 ,1 -dimethylpropyl, 1 -ethylpropyl, 1 -methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1 ,1- dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,2-dimethylbutyl, 1 ,3-di methyl butyl, 2,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1 -methyl hexyl, 2-methylhexyl, 3- methylhexyl, 4-methylhexyl, 5-methylhexyl, 1 ,1 -dimethylpentyl, 2,2-dimethylpentyl, 3,3- dimethylpentyl, 4,4-dimethylpentyl, 1 ,2-dimethylpentyl, 1 ,3-dimethylpentyl, 1 ,4- dimethylpentyl, 2,3-dimethylpentyl, 2,4-dimethylpentyl, 3,4-dimethylpentyl, 1- ethylpentyl, 2-ethyl pentyl, 3-ethylpentyl, 1 -methylheptyl, 2-methylheptyl, 3- methylheptyl, 4-methylheptyl, 5-methylheptyl, 6-methylheptyl, 1 ,1 -di methyl hexyl, 2,2- dimethylhexyl, 3, 3-di methyl hexyl, 4,4-dimethylhexyl, 5, 5-di methyl hexyl, 1 ,2- dimethylhexyl, 1 ,3-di methyl hexyl, 1 ,4-dimethylhexyl, 1 , 5-di methyl hexyl, 2,3- dimethylhexyl, 2,4-dimethylhexyl, 2,5-dimethylhexyl, 3,4-dimethylhexyl, 3,5- dimethylhexyl, 1-ethylhexyl, 2-ethylhexyl, 3-ethylhexyl, 4-ethylhexyl, 3,5,5- tri methyl hexyl, citronellyl, and geranyl; and R3is defined as any of claims 1 to 4; and if present, R4is defined as any of claims 1 to 4.

6. The method according to any of claims 1 to 5, wherein:(a) an unsaturated ester of formula (I) having a chemical structure 1c1 or 1c2 or a mixture thereof as obtained from the step (a) of photochemical conversion of an ester of formula (II) having a chemical structure 2c:1c1 1c22c wherein R1is selected from the group consisting of H, methyl, ethyl, n-propyl and phenyl; b) an unsaturated ester of formula (I) having the chemical structure as obtained from an ester of formula (II) having the chemical structure: c) an unsaturated ester of formula (I) having the chemical structureor or a mixture thereof as obtained from an ester of formula (II) having the chemical structure: d) an unsaturated ester of formula (I) having the chemical structureor or a mixture thereof as obtained from an ester of formula (II) having the chemical structure:e) an unsaturated ester of formula (I) having the chemical structureor a mixture of two or more thereof as obtained from an ester of formula (II) having the chemical structure: f) an unsaturated ester of formula (I) having the chemical structure as obtained from an ester of formula (II) having the chemical structure:g) an unsaturated ester of formula (I) having the chemical structureas obtained from an ester of formula (II) having the chemical structure:

7. The method according to any of claims 1 to 6, whereinR1is a hydrogen atom, a Ci-C4alkyl group, a phenyl group, which may optionally be substituted by one, or more groups R4;R2is a C2-C2salkyl group, or a Cs-C cycloalkyl group, which may optionally be substituted by one, or more groups Ci-C4alkyl groups; andR3denotes a single C2-C2salkenyl group, or two, or more different C2-C2salkenyl groups; or a single Cs-C cycloalkenyl group, or two, or more different C3- C cycloalkenyl groups, which may optionally be substituted by one, or more groups Ci-C4alkyl groups; andR4may be the same, or different in each occurrence and is selected from F, Cl, Br, I, CN, CO2Me, SO3H and NO2.

8. The method according to any of claims 1 to 7, wherein the ester of formula (II) is obtained from an alcohol obtained from a bio-based source, in particular bio-based fusel oil, and optionally an acid from a bio-based source, in particular acetic acid.

9. The method according to any of claims 1 to 8, wherein the solvent is selected from acetone, ethylacetate, dimethylacetamide, dichloromethane, 1,2-dichloroethane, methyl-tert-butylether, acetonitrile, dimethylsulfoxide (DMSO), tert.-alcohols, water, N- methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF) and mixtures thereof, in particular the solvent is an aprotic solvent, which is selected from acetone, ethylacetate, dimethylacetamide, dichloromethane, 1,2-dichloroethane, methyl-tert-butylether, acetonitrile, dimethylsulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF) and mixtures thereof.

10. The method according to any of claims 1 to 9, wherein the solvent is present in step a) in an amount of 99 to 0 % by weight, especially 95 to 20 % by weight based on the amount of solvent and esters of formula (II).

11. The method according to any of claims 1 to 10, wherein the esters of formula (II) are photochemically converted in neat form, i.e. no solvent is added in step a).

12. The method according to any of claims 1 to 11 , wherein the catalyst system comprises at least one photoredox catalyst and optionally at least one hydrogen evolution catalyst.

13. The method according to claim 12, wherein the photoredox catalyst comprises at least one compound of formula [W O32]X4, wherein X is Li+, Na+, K+, Rb+, Cs+, N(R7)4+or P(R7’)4+, wherein R7and R7’ may be the same, or different in each occurrence and are selected from a hydrogen atom, a Ci-Csalkyl group and a phenyl group and R7’ may be the same, or different in each occurrence and are selected from a Ci-Csalkyl group and a phenyl group.

14. The method according to any of claims 1 to 13, wherein the catalyst system comprises at least one hydrogen evolution catalyst, which is an organometallic complex on basis of palladium, nickel, cobalt, or ruthenium; especially an organometallic complex offormula , or Co(dmgH)2R5R5(V) (a complex having cobalt as central atom in oxidation state II or III, which is planarly coordinated by 2 dimethylglyoxime (dmgH) ligands (complex of formulawherein R5and R5’ are independently of each other Cl, Br, or a group of formula, wherein R6is a hydrogen atom, -CN, -OMe, -tBu, or -CChMe.

15. The method according to claim 14, wherein the hydrogen development catalyst is an organometallic complex of formula Co(dmgH)2R5R5(V), wherein R5and R5’ are Cl; or R5and R5’ are Br; orR5and R5’ are a group of formula16. The method according to any of claims 1 to 15, wherein in step a) the photochemical conversion is done with light, preferably monochromatic light, preferably in thewavelength range of from 320 to 580 nm, especially in the wavelength range of from 365 to 470 nm.

17. The method according to claim 16, wherein the photochemical conversion with light is accomplished by use of at least one LED (light emitting diode), or laser.

18. The method according to any of claims 1 to 17, wherein step a) is done in a temperature range of -20 to 100 ° C.

19. The method according to any of claims 1 to 18, wherein step a) is done in a pressure range from 50 mbar to 10 bar.

20. The method according to any of claims 1 to 19, wherein step a) is carried out in a pumping circuit of a reaction vessel with back-mixing, in a continuously stirred reaction vessel, or in a continuous flow reactor.

21. The method according to any of claims 1 to 20, comprising as step 0)esterification of alcohols of formula (III) to obtain esters of formula (II), wherein R2is defined in any one of claims 1 to 722. The method according to any of claims 1 to 21 , comprising the additional steps b1) separation of the unsaturated esters of formula (I), the esters of formula (II) and the solvent from the catalyst system, b2) separation of the solvent from the unsaturated esters of formula (I) and the esters of formula (II), b3) separation of the unsaturated esters of formula (I) from the esters of formula (II), and b4) recycling of the catalyst system, the solvent and the esters of formula (II) to step a).

23. The method according to any of claims 1 to 22, comprising the additional step c) removal of the ester group of the unsaturated esters of formula (I) to obtain unsaturatedalcohols of formula (IV), wherein R3is defined in any one of claims 1 to 7.

24. The method according to claim 23, wherein removal of the ester group comprises cleavage of the ester group, preferably under basic conditions, and optionally isolating the alcohol of formula (IV).

25. A method of preparing citral involving the following steps: i) providing 3-methyl-3-butene-1-ol (isoprenol) and / or 3-methyl-2-butene-1-ol (prenol) obtainable by the present invention according to any of claims 23 or 24, optionally isomerizing isoprenol to prenol; ii) producing 3-methyl-2-butenal (prenal) and / or 3-methyl-3-butenal (isoprenal) from the prenol and / or isoprenol by oxidative dehydration by means of an oxygencontaining gas on a silver support catalyst; iii) producing additional prenal from a mixture containing isoprenal by isomerization; iv) producing unsaturated acetal 3-methyl-2-butenal-diprenylacetal from the prenol and the prenal using an acidic catalyst; and v) obtaining citral from the 3-methyl-2-butenal-diprenylacetal by cleavage and subsequently rearranging.

26. The method according to any of claims 1 to 22, comprising the additional step c’) removal of the ester group of the unsaturated esters of formula (I) to obtain aldehydes or ketones of formula R3’=O (IV’), wherein R3’ is defined as R2in any one of claims 1 to 7.

27. The method according to claim 26, wherein removal of the ester group comprises cleavage of the ester group, preferably under basic conditions, and optionally isolating the aldehyde and / or ketone of formula (IV’).

28. An ester of formula (I) obtainable from a method according to any of claims 1 to 22.

29. An alcohol of formula (IV) obtainable from a method according to any of claims 23 or 24.

30. An aldehyde or ketone of formula (IV’) obtainable from a method according to any of claims 26 or 27.31 . Citral obtainable from a method according to claim 25.

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