Photoisomerization of geranial and neral

Abstract

The present invention relates to a method, comprising the isomerization of neral of formula (II) to geranial of formula (I), or the isomerization of geranial to neral, characterized in that (a) the isomerization is accomplished by irradiation with light. The method according to the present invention makes it possible to convert an undesired isomer, which is an unavoidable by-product generated during the production of neral and / or geranial, into a desired isomer highly efficiently, thus greatly improving the economic performance of the method for producing the desired isomer (in high yield by avoiding cost intensive distillation and with avoiding possible side products).

Description

[0001] Photoisomerization of Geranial and Neral

[0002] Description

[0003] The present invention relates to a method, comprising the isomerization of neral of formula r the isomerization of geranial to neral, characterized in that (a) the isomerization is accomplished by irradiation with light. The method according to the present invention makes it possible to convert an undesired isomer, which is an unavoidable by-product generated during the production of neral and / or geranial, into a desired isomer highly efficiently, thus greatly improving the economic performance of the method for producing the desired isomer (in high yield by expending as few energy as possible and with avoiding possible side products).

[0004] Prior Art

[0005] CN112142583A relates to a method for preparing neral of formula from geranial of formula characterized in that the geranial is isomerized into neral in the presence of carbon monoxide and a catalyst obtained by reacting triethylaluminum with bis-diarylphenol ligand of formula which R1, R2, R3and R4are independently of one another selected from Ce-Cisaryl, R5is selected from hydrogen, Ci-Cealkyl and Ci-Ceperfluoroalkyl.

[0006] CN107879914A relates to a method for preparing neral by efficiently rectifying citral which comprises the following steps: the citral containing two isomers neral and geranial is rectified and separated by a rectifying tower, and a neral product is obtained at the tower top of the rectifying tower, wherein iodine and phosphorus catalytic active centers are loaded on a stripping section filler of the rectifying tower, so that a material which flows through the tower bottom and is rich in geranial is catalytically converted into a material with an increased neral proportion. CN112125782A relates to a method for preparing high-purity nerol and geranial by hydrogenating citral which is characterized by comprising the steps of selectively carrying out catalytic hydrogenation reaction on nerol in citral to obtain nerol by adopting a novel transition metal composite catalyst, carrying out no catalytic reaction on geranial, and carrying out rectification separation on reaction products to obtain the high-purity nerol and geranial; wherein, the novel transition metal composite catalyst comprises a transition metal compound and a chiral spiro bisoxazoline ligand with a spiro indane skeleton.

[0007] RU2579122 relates to a method of obtaining the geranial from the mixture of isomers of geranial and neral (citral), based on conducting of the transformations of citral in the presence of acid catalysts, preferably montmorillonite clay, wherein the isolation of geranial from the reaction mixture can be achieved by column chromatography or distillation at reduced pressure.

[0008] Wout A. M. Wolken et al., J. Agric. Food Chem., 48 (2000) 5401 -5405 reports that under alkaline conditions, amino acids or proteins catalyze the deacetylation of citral, a major aroma component, resulting in methylheptenone and acetaldehyde formation. 3- Hydroxycitronellal is an intermediate in this reaction. Amino acids also catalyze the cis-trans isomerization of the pure isomers of citral, geranial, and neral. The glycine-catalyzed isomerization of geranial is a relatively fast process (Figure 1A). After 80 min, an equilibrium was reached with 60% of geranial and 40% neral. Starting with neral, the same equilibrium was reached (Figure 1 B).

[0009] O.S. Bokareva, Computational and Theoretical Chemistry 1149 (2019) 8-16 relates to a quantum-chemical study of structures and conformational dynamics of trans- and ciscrotonaldehydes in excited electronic states.

[0010] Jens Holz et al., Adv. Synth. Catal. 2017, 359, 4379 - 4387 describes the highly chemoselective hydrogenation of neral affording citronellal. The reaction has been conducted with homogeneous rhodium complexes. A high activity and chemoselectivity in favor of the desired citronellal is achieved at 0.1 MPa and room temperature. Under the same conditions, geranial is also reduced to citronellal.

[0011] CN116474824A relates to a catalyst for isomerising neral to geranial, characterized in that: the catalyst is one or more of citric acid, tartaric acid, amino acid and malic acid and a method for increasing the geranial content in citral, which comprises adding the catalyst to citral.

[0012] R. C. Cookson et al., Tetrahedron 19 (1963) pages 1995-2007 concerns the cyclization of citral to 2-isopropenyl-5-methylcyclopentane-carboxaldehyde by UV irradiation using a medium-pressure mercury arc lamp. It is stated on page 1 , footnote * that the sample used was a 1 :1 mixture of cis and trans isomers and the interconversion between the geometrical isomers (of citral) is probably faster than cyclisation. W. L. Dilling, Chemical Reviews (1966) pages 373-393 relates to the intramolecular photochemical cycloaddition reactions of nonconjugated olefins. Cis- and trans citral on irradiation in either cyclohexane or ethanol results in the cycloaddition product XVIII and a hydrogen migration product XIX (page 376, column 2, paragraph 1).

[0013] Yuko Iwanami et al., Journal of Agricultural and Food Chemistry 45 (1997) pages 463-466 relates to the changes of lemon flavour components in aqueous solution during UV irradiation. Citral under light irradiation decreased rapidly with Z-E isomerization and new peaks, such as I, II and III appeared (page 464, column 2, paragraph 6 and page 465, column 1 , paragraph 2 and Fig. 1). In addition, Citral in ethanol was irradiated by UV light under nitrogen, wherein products 3 to 12 shown in Fig. 2 are obtained.

[0014] An object is to provide a method for efficiently obtaining neral, or geranial.

[0015] It is a further object of the present invention to convert an undesired isomer, which is an unavoidable by-product generated during the production of neral and geranial, into a desired isomer highly efficiently, thus greatly improving the economic performance of the method for producing the desired isomer (in high yield by avoiding cost intensive distillation and with avoiding possible side products).

[0016] Surprisingly, it was found that the above objects could be solved with the method according to the invention.

[0017] Accordingly, the present invention relates to a method, comprising the isomerization of neral of formula citral B (cis form)) to geranial of formula citral A (trans form)), or the isomerization of geranial to neral, characterized in that (a) the isomerization is accomplished by irradiation with light.

[0018] In other words, the present invention relates to a method for isomerization of neral of formula (II; citral B (cis form)) to geranial of formula (I; citral A (trans form)), or the isomerization of geranial to neral, characterized in that (a) the isomerization is accomplished by irradiation with light.

[0019] In other words, the present invention relates to producing a desired isomer (real or geranial), comprising the isomerization of neral of formula (II; citral B (cis form)) to geranial of formula (I; citral A (trans form)), or the isomerization of geranial to neral, characterized in that (a) the isomerization is accomplished by irradiation with light. In a preferred embodiment, the method is a method for producing a desired isomer, in particular neral or geranial, optionally comprised in a mixture comprising neral and geranial.

[0020] In other words, the method of the present invention may be a method for the preparation of enriched or pure neral and / or geranial.

[0021] It will be understood that the produced that produced neral may either be pure neral or may be enriched neral, i.e., neral comprised in a mixture that further comprises geranial and optionally one or more further components that has a higher neral content as the mixture before isomerization (of step a)). It will be understood that the produced that produced geranial may either be pure geranial or may be enriched geranial, i.e., geranial comprised in a mixture that further comprises neral and optionally one or more further components that has a higher geranial content as the mixture before isomerization (of step a)).

[0022] In a preferred embodiment, irradiation with light is irradiation with monochromatic light.

[0023] In a preferred embodiment in step a) the geranial is irradiated with monochromatic light, or in step a) the neral is irradiated with monochromatic light.

[0024] It will be understood that the irradiation of geranial does not exclude the concomitant irradiation of neral and vice versa, in particular when a mixture consisting of or comprising both, neral and geranial, is irradiated. Irradiation of neral may be irradiation of pure neral or of a mixture consisting of or comprising both, neral and geranial, in particular enriched neral (i.e., a mixture comprising a higher content of neral). One or more irradiations may lead to an altered, preferably more desirable, ratio of neral : geranial.

[0025] In a preferred embodiment, the method of the present invention is for increasing the content of neral in a composition, in particular a mixture comprising geranial and neral, comprising the step a) isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light.

[0026] In a preferred embodiment, the method of the present invention is for increasing the content of geranial in a composition, in particular a mixture comprising geranial and neral, comprising the step a) isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light.

[0027] In a particularly preferred embodiment of the present invention (where a sensitizer is present in step a)) monochromatic light, is all radiation at least 90 % of its power and at most 100 % thereof being emitted in the range from 350 nm to 490 nm, i.e. at least 90 % and at most 100 % of the radiant flux of the monochromatic light is emitted in the range from 350 nm to 490 nm. Its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum.

[0028] In another particularly preferred embodiment of the present invention (where no sensitizer is present in step a)) monochromatic light, is all radiation at least 90 % of its power and at most 100 % thereof being emitted in the range from 300 nm to 420 nm, i.e. at least 90 % and at most 100 % of the radiant flux of the monochromatic light is emitted in the range from 300 nm to 420 nm. Its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum.

[0029] Said defined halfwidth gives a highly structured lighting profile, which results in an improved yield of neral / geranial.

[0030] The method according to the present invention makes it possible to convert an undesired isomer, which is an unavoidable by-product generated during the production of neral and / or geranial, into a desired isomer highly efficiently, thus greatly improving the economic performance of the method for producing the desired isomer (in high yield by avoiding cost intensive distillation and with avoiding possible side products).

[0031] In addition, the method of the present invention is commercially attractive as it enables the efficient, demand-oriented production of either neral, or geranial and, hence, offers more flexibility in the production of aroma chemicals.

[0032] The present invention enables the efficient production of neral, or geranial.

[0033] Accordingly, in a first embodiment the present invention relates to

[0034] N.1 A method for the preparation of enriched or pure neral, comprising the steps a) the isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light; b) the separation, especially the (continuous) separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure neral, and enriched or pure geranial; and c) optionally recycling of the enriched or pure geranial obtained in step b) to step a).

[0035] N.2 The method according to embodiment N.1, additionally comprising as step 0) the provision of enriched or pure geranial by distil lative separation of mixtures comprising geranial and neral.

[0036] N.3 The method according to embodiments N.1 , or N.2, wherein in step a) a solution of the geranial in a solvent is irradiated with light.

[0037] N.4 The method according to embodiment N.3, wherein the solvent is selected from water, dichloromethane, trichloromethane, tetrachloromethane, CS2, Ci- C4alcohols, chlorobenzene, fluorobenzene, trifluoromethylbenzene, acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetra hydrofuran (THF), ethylacetate, acetone and mixtures thereof.

[0038] N.5 The method according to any of embodiments N.1 to N.4, wherein in step a) the geranial is irradiated with light in the presence of a sensitizer.

[0039] N.6 The method according to embodiment G.5, wherein the sensitizer is selected from [lr(dF(CF3)ppy)2(bpy)]PF6, (lr[dF(CF3)ppy]2(dtbbpy))PF6, (lr[dF(CF3)ppy]2(dtbpy))PF6, (lr[dF(Me)ppy]2(dtbbpy))PF6, [lr(dtbbpy)(ppy)2]PF6, lr(ppy)3, Ru(bpy)3CI3, [Ru(bpy)3](PFe)2, benzophenone, thioxanthen-9-one, Michler's Ketone, 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 and mixtures thereof.

[0040] N.7 The method according to embodiments N.5, or N.6, wherein in step a) the geranial is irradiated with monochromatic light in the wavelength range of 350 to 490 nm.

[0041] N.8 The method according to embodiments N.1 , or N.2, wherein in step a) the geranial in neat form is irradiated with light, i.e. no solvent and sensitizer are present.

[0042] N.9 The method according to embodiment N.8, wherein in step a) the geranial is irradiated with monochromatic light in the wavelength range of 300 to 420 nm.

[0043] N.10 The method according to any of the preceding embodiments, wherein step a) is done in a temperature range of -20 to 100 ° C.

[0044] N.11 The method according to any of the preceding embodiments, wherein step a) is done in a pressure range from 1 mbar to 20 bar.

[0045] N.12 The method according to any of the preceding embodiments, wherein step a) is carried out in a continuously stirred reaction vessel, in a pumping circuit, or in a continuous flow reactor.

[0046] N.13 The method according to any of the preceding embodiments, wherein the irradiation with light is accomplished by use of at least one LED (light emitting diode), or laser. N.14 The method according to any of the preceding embodiments, which is a method for producing menthol and comprises the additional steps d.1) to g.1): d.1) catalytic hydrogenation of neral obtained by the method of the present invention (e.g., according to any of claims 1-3, 6-16, 18-23), preferably of the neral obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)), to give citronellal, e.1) cyclization of citronellal to give isopulegol in the presence of an acidic catalyst, f.1) optionally purification of isopulegol, preferably by crystallization and g.1) catalytic hydrogenation of isopulegol to give menthol.

[0047] N.15 The method according to embodiment N.14, comprising the additional steps d.2) to g.2): d.2) asymmetric catalytic hydrogenation of neral obtained by the method of the present invention (e.g., according to any of claims 1-3, 6-16, 18-23), preferably of the neral obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)), to give optically active citronellal, e.2) cyclization of optically active citronellal to give optically active isopulegol in the presence of an acidic catalyst, f.2) optionally purification of optically active isopulegol, preferably by crystallization and g.2) catalytic hydrogenation of optically active isopulegol to give optically active menthol.

[0048] In contrast to the teaching of the prior art the photochemical isomerisation of geranial advantageously leads to a mixture of neral and geranial, in which neral is present in an amount of more than 50% the amount of geranial.

[0049] In a second embodiment the present invention relates to

[0050] G.1 a method for the preparation of enriched or pure geranial, comprising the steps a) the isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light; b) the separation, especially the (continuous) distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure geranial, and enriched or pure neral; and c) optionally recycling of the enriched or pure neral obtained in step b) to step a).

[0051] G.2 The method according to embodiment G.1, additionally comprising as step 0) the provision of enriched or pure neral by distillative separation of mixtures comprising geranial and neral.

[0052] G.3 The method according to embodiments G.1 , or G.2, wherein in step a) a solution of the neral in a solvent is irradiated with light.

[0053] G.4 The method according to embodiment G.3, wherein the solvent is selected from water, dichloromethane, trichloromethane, tetrachloromethane, CS2, Ci- C4alcohols, chlorobenzene, fluorobenzene, trifluoromethylbenzene, acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetra hydrofuran (THF), ethylacetate, acetone and mixtures thereof.

[0054] G.5 The method according to any of embodiments G.1 to G.4, wherein in step a) the neral is irradiated with light in the presence of a sensitizer.

[0055] G.6 The method according to embodiment G.5, wherein the sensitizer is selected from [lr(dF(CF3)ppy)2(bpy)]PF6, (lr[dF(CF3)ppy]2(dtbbpy))PF6, (lr[dF(CF3)ppy]2(dtbpy))PF6, (lr[dF(Me)ppy]2(dtbbpy))PF6, [lr(dtbbpy)(ppy)2]PF6, lr(ppy)3, Ru(bpy)3CI3, [Ru(bpy)3](PFe)2, benzophenone, thioxanthen-9-one, Michler's Ketone, 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 and mixtures thereof.

[0056] G.7 The method according to embodiments G.5, or G.6, wherein in step a) the neral is irradiated with monochromatic light in the wavelength range of 350 to 490 nm.

[0057] G.8 The method according to embodiment G.7, or G.8, wherein in step a) the neral in neat form is irradiated with light, i.e. no solvent and sensitizer are present.

[0058] G.9 The method according to embodiment G.8, wherein in step a) the neral is irradiated with monochromatic light in the wavelength range of 300 to 420 nm.

[0059] G.10 The method according to any of the preceding embodiments, wherein step a) is done in a temperature range of -20 to 100 ° C.

[0060] G.11 The method according to any of the preceding embodiments, wherein step a) is done in a pressure range from 1 mbar to 20 bar.

[0061] G.12 The method according to any of the preceding embodiments, wherein step a) is carried out in a continuously stirred reaction vessel, in a pumping circuit, or in a continuous flow reactor.

[0062] G.13 The method according to any of the preceding embodiments, wherein the irradiation with light is accomplished by use of at least one LED (light emitting diode), or laser. G.14 The method according to any of the preceding embodiments, which is a method for producing linalool and comprises the additional steps d’) and e’): d’) catalytic hydrogenation of geranial and / or neral obtained by the method of the present invention, preferably of geranial obtained by the method of the present invention,, in particular of geranial obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)), especially catalytic hydrogenation of the geranial obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)) in the presence of a supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably a ruthenium catalyst supported on carbon black; to obtain geraniol; and e’) isomerization of geraniol in the presence of a catalyst, especially a tungsten catalyst, very especially a dioxotungsten (VI) complex to obtain linalool.

[0063] In a first preferred embodiment the present invention is directed to a method for the preparation of enriched or pure neral, comprising the step a) the isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light.

[0064] The enriched or pure geranial is preferably provided by separation of mixtures comprising geranial and neral. The enriched or pure geranial is preferably provided by distillative separation of mixtures comprising geranial and neral. Optionally, separation may be partly or complete separation, in other words, may include obtaining pure geranial, and / or pure neral, and / or a mixture comprising or consisting of both, neral and geranial with a desired neral : geranial mass ratio that may differ from the neral : geranial mass ratio of the feed of separation.

[0065] Distillative means for partly or completely separating neral and geranial from one another, in other words enriching either neral in a fraction and / or geranial in a fraction, may be conducted by any means suitable for this purpose. Examples for suitable distillation steps are taught in W02009 / 068444.

[0066] Accordingly, the present invention is directed to a method for the preparation of enriched or pure neral, comprising the steps

[0067] 0) the distillative separation of mixtures comprising geranial and neral to give enriched or pure geranial and neral; a) the isomerization of the enriched or pure geranial obtained in step 0) to a mixture of neral and geranial by irradiation with light.

[0068] The mixture comprising geranial and neral obtained in step a) is preferably distillatively separated to give, the product, enriched or pure neral and enriched or pure geranial, which is recycled to step a). The distillative separation is preferably a continuous distillative separation.

[0069] Accordingly, the present invention is directed to a method for the preparation of enriched or pure neral, comprising, preferably continuously, the steps a) the isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light; b) the continuous distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure neral, and enriched or pure geranial; and c) optionally recycling of the enriched or pure geranial obtained in step b) to step a).

[0070] As used herein, the term “product, enriched or pure neral, and enriched or pure geranial” may be understood as at least two separated streams: a first stream that is enriched or pure neral and a second stream that is enriched or pure geranial. In this context, the first stream that is enriched or pure neral may preferably be considered as the desired product and the second product as side product that may optionally be recycled (e.g., in step c)).

[0071] As used herein, the term “product, enriched or pure geranial, and enriched or pure neral” may be understood as at least two separated streams: a first stream that is enriched or pure neral and a second stream that is enriched or pure geranial. In this context, the first stream that is enriched or pure geranial may preferably be considered as the desired product and the second product as side product that may optionally be recycled (e.g., in step c)).

[0072] In a particularly preferred embodiment the present invention is directed to a method for the preparation of enriched or pure neral, comprising, preferably continuously, the steps 0) the distillative separation of mixtures comprising geranial and neral to give enriched or pure geranial and neral; a) the isomerization of the enriched or pure geranial obtained in step 0) to a mixture of neral and geranial by irradiation with light; b) the continuous distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure neral, and enriched or pure geranial; and c) optionally recycling of the enriched or pure geranial obtained in step b) to step a).

[0073] In a second preferred embodiment the present invention is directed to a method for the preparation of enriched or pure geranial, comprising the step a) the isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light.

[0074] The enriched or pure geranial is preferably provided by distillative separation of mixtures comprising geranial and neral.

[0075] Accordingly, the present invention is directed to a method for the preparation of enriched or pure geranial, comprising the steps

[0076] 0) the distillative separation of mixtures comprising geranial and neral to give enriched or pure geranial and neral; a) the isomerization of the enriched or pure neral obtained in step 0) to a mixture of neral and geranial by irradiation with light. The mixture comprising geranial and neral obtained in step a) is preferably distillatively separated to give the product, enriched or pure geranial, and enriched or pure neral, which is recycled to step a). The distillative separation is preferably a continuous distillative separation.

[0077] Accordingly, the present invention is directed to a method for the preparation of enriched or pure geranial, comprising, preferably continuously, the steps a) the isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light; b) the distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure geranial, and enriched or pure neral; and c) optionally recycling of the enriched or pure neral obtained in step b) to step a).

[0078] In a particularly preferred embodiment the present invention is directed to a method for the preparation of enriched or pure geranial, comprising, preferably continuously, the steps 0) the distillative separation of mixtures comprising geranial and neral to give enriched or pure geranial and neral; a) the isomerization of the enriched or pure neral obtained in step 0) to a mixture of neral and geranial by irradiation with light; b) the distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure geranial, and enriched or pure neral; and c) optionally recycling of the enriched or pure neral obtained in step b) to step a).

[0079] Step 0)

[0080] The method of the present invention comprises preferably additionally as step 0) the distillative separation of mixtures comprising geranial and neral to give enriched or pure geranial or neral.

[0081] Suitable feed materials in step 0) are substance mixtures which comprise neral and geranial, preferably those which consist predominantly of the double-bond isomers neral and geranial. Among these, preference is given to those substance mixtures which comprise at least 90% by weight to 100% by weight, particularly preferably at least 95 to 98% by weight (in each case based on the total amount of the respective substance mixture) of geranial and neral or consist thereof in the specified fractions and in addition can comprise to a low extent, i.e. in a fraction of up to 10% by weight, preferably up to 5% by weight (in each case based on the total amount of the respective substance mixture) also further components such as, for example, isomers, by-products or impurities. One preferred feed material is synthetically produced citral, especially that which has been obtained by thermal cleavage of 3-methyl-2- buten-1-al diprenylacetal with elimination of prenol to give cis / trans-prenyl (3- methylbutadienyl) ether, Claisen rearrangement thereof to give 2,4,4-trimethyl-3-formyl-1,5- hexadiene and subsequent Cope rearrangement thereof, as described, for example, in EP992477 and European patent application no. 23177047.0. This comprises typically about 45 to about 55% by weight of neral as well as about 55 to about 45% by weight and about 1 to 5% by weight of further compounds and / or impurities. The method according to the invention comprises, as additional inserted step, the aforementioned production method of citral starting from 3-methyl-2-buten-1-al diprenylacetal.

[0082] A substance mixture which is used in step 0) consists preferably of 30 to 70% by weight, preferably of 40 to 60% by weight, of neral, of 70 to 30% by weight, preferably of 60 to 40% by weight, of geranial and of 0 to 5% by weight of further components, where the percentages add up to 100% by weight.

[0083] In a preferred embodiment, the method according to the invention comprises, as step 0), the distillative separation of the geranial- and neral-containing mixtures to give enriched or pure geranial or neral. Reference is made to W02009 / 068444.

[0084] Moreover, the distillative separation of geranial- and neral-containing mixtures can be carried out advantageously by means of a dividing wall column or an interconnection of thermally coupled columns. In this way, neral in particular is accessible in pure or enriched form through distillative separation of substance mixtures comprising geranial and neral.

[0085] In a preferred embodiment a continuous method for producing neral in pure or enriched form by distillative removal of neral from substance mixtures comprising neral and geranial is inserted, the distillative removal being carried out in a dividing wall column or in an interconnection of two distillation columns in the form of a thermal coupling having 80 to 200 theoretical plates and one or more side take-off points at an absolute operating pressure of from 5 to 200 mbar.

[0086] The distillative removal is usually carried out by separating the neral and geranial comprising substance mixture used into, in each case, one or more low-boiling, medium-boiling and high-boiling fraction or fractions, and removing neral in pure or enriched form as mediumboiling fraction at the side take-off point of the dividing wall column used or the interconnection of two distillation columns in the form of a thermal coupling in liquid or gaseous form.

[0087] Accordingly, the distillative separating is preferably a continuous method for isolating neral in pure or enriched form by distillative removal of neral from substance mixtures comprising neral and geranial, the distillative removal being carried out in a dividing wall column or in an interconnection of two distillation columns in the form of a thermal coupling having 80 to 200 theoretical plates and one or more side take-off points at an absolute operating pressure, i.e. at an absolute pressure in the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling of from 5 to 200 mbar.

[0088] The dividing wall column has 80 to 200, preferably 100 to 180, theoretical plates and one or more, preferably 1 to 3, particularly preferably 1 or 2, side take-off points. The method for producing pure or enriched neral to be carried out preferably within the context of the method according to the invention is carried out at an absolute operating pressure in the dividing wall column or in the interconnection of two distillation columns in the form of a thermal coupling of from 5 to 200 mbar, preferably from 5 to 100 mbar, particularly preferably from 5 to 70 mbar and very particularly preferably from 10 to 50 mbar and especially preferably from 10 to 40 mbar. Preferably, the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling is operated here such that the absolute top pressure is 10 to 50 mbar, preferably 10 to 40 mbar. Likewise preferably, the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling is operated here such that the absolute bottom pressure is 5 to 200 mbar, preferably 10 to 100 and particularly preferably 20 to 50 mbar.

[0089] The reflux ratio can be varied within wide limits and is usually about 5 : 1 to about 2000 : 1, preferably about 20 : 1 to 1000 : 1. Also advantageous is a dephlegmator procedure, i.e. only the return stream is condensed in the top condenser of the column and fed back to the column. In such an energetically favorable case of partial condensation, the top product to be discharged is produced exclusively in the aftercooler, which can be operated at a lower temperature.

[0090] The term “neral in enriched form” is to be understood as meaning neral-containing substance mixtures which have a higher content of neral than the neral / geranial comprising substance mixture used in step 0). For instance, such enrichment may be increasing the content of neral by at least 10 % by weight, by at least 25 % by weight, by at least 50 % by weight, by at least 75 % by weight, or by at least 85 % by weight, in comparison to the neral / geranial comprising substance mixture used in step 0). Preferably, the term neral in enriched form (or equivalent terms such as pure or enriched neral or similar terms) is to be understood as meaning neral which has a purity, i.e. a neral content, of from 80 to 95% by weight, preferably from 85 to 95% by weight and very particularly preferably from 90 to 95% by weight based on the total amount of the respective substance mixtures. The method according to the invention also permits the production of neral (cis-citral) in pure form. The term “neral in pure form” is to be understood as meaning neral with a content greater than or equal to 95, 96 or 97% by weight, preferably greater than or equal to 98% by weight and particularly preferably 98 to 99.5% by weight based on the total amount of the respective substance mixtures. Particularly preferably, the term “neral in pure form” is to be understood as meaning neral which has a geranial content of up to 1% by weight, preferably of from 0.05 to 0.5% by weight and particularly preferably from 0.1 to 0.3% by weight. Likewise preferably, the neral in pure form accessible according to the invention has a content of isocitrals, such as, for example, isocitrals of the formulae (IV), (V) and (VI)

[0091] (IV) (V) (VI) of up to 2% by weight, preferably of from 0.1 to 1 % by weight, where all of the data within the context of the present invention refer to the total amount of the respective substance mixtures.

[0092] The term “geranial in enriched form” (or equivalent terms such as pure or enriched geranial or similar terms) is to be understood as meaning geranial-containing substance mixtures which have a higher content of geranial than the neral / geranial comprising substance mixture used in step 0). For instance, such enrichment may be increasing the content of geranial by at least 10 % by weight, by at least 25 % by weight, by at least 50 % by weight, by at least 75 % by weight, or by at least 85 % by weight, in comparison to the neral / geranial comprising substance mixture used in step 0). Preferably, the term geranial in enriched form is to be understood as meaning geranial which has a purity, i.e. a geranial content, of from 80 to 95% by weight, preferably from 85 to 95% by weight and very particularly preferably from 90 to 95% by weight based on the total amount of the respective substance mixtures. The method according to the invention also permits the production of geranial (trans-citral) in pure form. The term “geranial in pure form” is to be understood as meaning geranial with a content greater than or equal to 95, 96 or 97% by weight, preferably greater than or equal to 98% by weight and particularly preferably 98 to 99.5% by weight based on the total amount of the respective substance mixtures. Particularly preferably, the term “neral in pure form” is to be understood as meaning neral which has a geranial content of up to 1% by weight, preferably of from 0.05 to 0.5% by weight and particularly preferably from 0.1 to 0.3% by weight. Likewise preferably, the geranial in pure form accessible according to the invention has a content of isocitrals, such as, for example, isocitrals of the formulae (IV), (V) and (VI) of up to 2% by weight, preferably of from 0.1 to 1% by weight, where all of the data within the context of the present invention refer to the total amount of the respective substance mixtures.

[0093] The method according to any of claims 1 to 17, wherein the method is for producing a desired isomer mixture, in particular wherein an educt mixture comprising neral and geranial having a first mass ratio of neral : geranial is subjected to irradiation with light in step a) and wherein, after irradiation, a product mixture is obtained that has a second mass ratio of neral : geranial that differs from the first mass ratio of neral : geranial.

[0094] In a preferred embodiment, an educt mixture (comprising neral and geranial e.g., a neral / geranial comprising substance mixture used in step 0)) having a first mass ratio of neral : geranial is subjected to irradiation with light in step a) and wherein, after irradiation, a product mixture is obtained that has a second mass ratio of neral : geranial that differs from the first mass ratio of neral : geranial. In a preferred embodiment, the first mass ratio of neral : geranial and the second first mass ratio of neral : geranial differ from one another by at least 5%, by at least 10%, by at least 20%, by at least 30%, by at least 50%, or by at least 75%.

[0095] The feed, i.e. the substance mixture to be used, can be fed in liquid or gaseous form into the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling, preferably into the dividing wall column, and be separated there into a top and bottom fraction and also one or more, preferably into two or more, side take-offs as described above. In one side take-off, the neral product of value is produced in the desired purity. In one particular embodiment, a postcondenser is connected downstream of the top condenser of the column and is cooled with cooling liquid (for example sols), and a low-neral low-boiling fraction is also produced therein.

[0096] For the continuous distillative fractionation of multisubstance mixtures, according to the prior art, various process variants can be used. In the simplest case, the feed mixture is fractionated into two fractions, a low-boiling top fraction and a high-boiling bottom fraction. When separating feed mixtures into more than two fractions, it is necessary to use a plurality of distillation columns according to this process variant. In order to limit the apparatus complexity, columns with liquid or vaporous side take-offs are used if possible in the separation of multisubstance mixtures.

[0097] Dividing wall columns are described, for example, in US 2,471 ,134; US 4,230,533; EP 0 122 367; EP 0 126288; EP 0 133 510; Chem. Eng. Technol. 10 (1987) 92 - 98; Chem.-lng.- Tech. 61 (1989) No.1 , 16 - 25; Gas Separation and Purification 4 (1990) 109 - 114; Process Engineering 2 (1993) 33 - 34; Trans IChemE 72 (1994) Part A 639 - 644 and Chemical Engineering 7 (1997) 72 - 76.

[0098] Step a)

[0099] The method of the present invention comprises a) the isomerization of neral, especially of enriched or pure neral to a mixture of neral and geranial by irradiation with light, or a) the isomerization of geranial, especially of enriched or pure geranial to a mixture of neral and geranial by irradiation with light.

[0100] In an embodiment of the present invention a solution of geranial, or neral in a solvent is irradiated with light. The solvent is preferably selected from water, dichloromethane, trichloromethane, tetrachloromethane, CS2, Ci- C4alcohols, chlorobenzene, fluorobenzene, trifluoromethylbenzene, acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), ethylacetate, acetone and mixtures thereof.

[0101] In an alternative preferred embodiment of the present invention no solvent is present in step a), i.e. geranial, and / or neral in neat form are irradiated with light, in particular monochromatic light. In a preferred embodiment, in step a) the geranial in neat form is irradiated with light, in particular monochromatic light, or the method according to any of claims 1 , 4 and 5, wherein in step a) the neral in neat form is irradiated with light, in particular monochromatic light, or the method according to any of claims 1 to 5, wherein in step a) a mixture comprising or consisting of geranial and neral in neat form is irradiated with light, in particular monochromatic light.

[0102] Geranial and / or neral may be irradiated with light in the presence of a sensitizer.

[0103] A “(photo)sensitizer” in terms of the present invention is an organic molecule (generally a dye), or transition metal complex which, when subjected to irradiation (generally to electromagnetic radiation in the UV, in the visible or in the near IR region) is subject to “Dexter excitation transfer” (electron exchange excitation transfer): Excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap of the wavefunctions of the energy donor (sensitizer) and the energy acceptor (neral / geranial). It is the dominant mechanism in triplet-triplet energy transfer.

[0104] The sensitizer is preferably selected from [lr(dF(CF3)ppy)2(bpy)]PFe, (lr[dF(CF3)ppy]2(dtbbpy))PF6, (lr[dF(CF3)ppy]2(dtbpy))PF6, (lr[dF(Me)ppy]2(dtbbpy))PF6, [lr(dtbbpy)(ppy)2]PFe, lr(ppy)3, Ru(bpy)3CI3, [Ru(bpy)3](PFe)2, benzophenone, thioxanthen-9- one, Michler's Ketone, 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 and mixtures thereof.

[0105] In said embodiment the irradiation is done with light in the wavelength range of 300 to 800 nm, in particular in the absorption range of the sensitizer, usually in the wavelength range of 350 to 490 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.

[0106] '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).

[0107] In a preferred embodiment, irradiation comprises or consists of irradiation with light of a wavelength in the range of between 300 and 1000 nm, of between 300 and 490 nm, of between 300 and 420 nm, of between 350 and 490 nm, of between 350 and 370 nm, of between 400 and 410 nm. In a preferred embodiment, irradiation comprises or consists of irradiation with light of a wavelength of (approximately) 365 nm, or of (approximately) 405 nm.

[0108] In a preferred embodiment, in step a) the geranial is irradiated with monochromatic light.

[0109] In a preferred embodiment, in step a) the neral is irradiated with monochromatic light.

[0110] It will be understood that such monochromatic light may also be light within the abovereferenced preferred wavelength ranges.

[0111] Monochromatic light consists of a (small) bandwidth of wavelengths. It will be understood that monochromatic light may be understood in the broadest sense as generally understood in the art. For instance, monochromatic light may exhibit a halfwidth of 0 to 50 nm, of + / - 0.1 to + / - 40 nm, of + / - 0.5 to + / - 30 nm, or of + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum.

[0112] A person skilled in the art will generally understand that monochromatic light preferably exhibits light with a full width at half maximum (FWHM) (which may optionally be considered as halfwidth) of not more than 150 nm, preferably not more than 100 nm, more preferably not more than 75 nm, even more preferably not more than 60 nm, in particular not more than 50 nm, or nor more than 30 nm or nor more than 25 nm, or not more than 10 nm. For example, an FWHM may be between 1 and 100 nm, between 5 and 60 nm, or between 10 and 60 nm, or between 10 and 30 nm, or between 20 and 30 nm, or between 20 and 60 nm, or between 20 and 60 nm, or between 5 and 20 nm, or between 1 and 10 nm, or between 2 and 20 nm.

[0113] In a preferred embodiment, when applying monochromatic light and / or irradiation light consisting of a certain wavelength range, at least 80%, more preferably at least 90%, in particular at least 99% of the applied light intensity (e.g., given in candela) may be the designated light.

[0114] A filter isolates monochromatic light from a broadband light source; lasers or LEDs generate monochromatic light directly.

[0115] In said embodiment the irradiation is preferably done with monochromatic light in the wavelength range of 300 to 800 nm, in particular in the absorption range of the sensitizer, usually in the wavelength range of 350 to 490 nm.

[0116] E.1. The method according to any of the preceding embodiments, where in step (a) the reaction mixture, which consists of neral, or geranial and sensitizer and optionally solvent, is irradiated with light in the wavelength range of from 300 to 800 nm.

[0117] E.2. The method according to embodiment E.1 , where in step (a) the reaction mixture consists of neral, or geranial and sensitizer and solvent. E.3. The method according to embodiment E.1 , or E2, where in step (a) the reaction mixture consists of neral, or geranial and sensitizer.

[0118] E.4. The method according to any of embodiments E.1 to E.3, where in step (a) the reaction mixture is irradiated with light in the wavelength range of from 350 to 490 nm.

[0119] E.5. The method according to any of the preceding embodiments, where in step (a) the reaction mixture is irradiated with monochromatic light.

[0120] E.6. The method according to embodiment E.13, where irradiation 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 300 to 800 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 300 to 800 nm.

[0121] E.7. The method according to embodiment E.6, where irradiation 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 350 to 490 nm.

[0122] E.8. The method according to any of embodiments E.6, or E.7, where irradiation in step (a) is carried out using an electroluminescent lighting device emitting monochromatic light, where the electroluminescent lighting device consists of at least one LED.

[0123] To perform the isomerization efficiently, the amount of photosensitizer used in the reaction is at a concentration that ensures absorbance of light in the photoreactor in the range of 0.1 to 3.0, preferably in the range of 0.5 to 2.5, and most preferably in the range of 1.0 to 2.0.

[0124] “Absorbance” is defined as the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls).

[0125] 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 350 nm to 490 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 photosensitizer 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 350 nm to 490 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 350 nm to 490 nm and its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 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 neral / geranial.

[0126] In an alternative more preferred embodiment of the present invention no sensitizer is present in step a).

[0127] In said embodiment the irradiation is preferably done with monochromatic light in the wavelength range of 300 to 800 nm, in particular in the wavelength range of 300 to 420 nm.

[0128] F.1 The method according to any of the preceding embodiments, where in step (a) the reaction mixture, consisting of neral, or geranial and optionally solvent, is irradiated with light in the wavelength range of from 300 to 800 nm.

[0129] F.2 The method according to embodiment F.1 , where in step (a) the reaction mixture consists of neral, or geranial and solvent.

[0130] F.3 The method according to embodiment F.1 , or F.2, where in step (a) the reaction mixture consists of neral, or geranial.

[0131] F.4 The method according to any of embodiments F.1 to F.3, where in step (a) the reaction mixture is irradiated with light in the wavelength range of from 300 to 420 nm.

[0132] F.5 The method according to any of the preceding embodiments, where in step (a) the reaction mixture is irradiated with monochromatic light.

[0133] F.6 The method according to embodiment F.5, where irradiation 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 300 to 800 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 300 to 800 nm.

[0134] F.7 The method according to embodiment F.6, where irradiation 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 300 to 420 nm.

[0135] F.8 The method according to embodiments F.6, or F.7, where irradiation in step (a) is carried out using an electroluminescent lighting device emitting monochromatic light, where the electroluminescent lighting device consists of at least one LED.

[0136] In a preferred embodiment, in step a) the geranial is irradiated with light, in particular monochromatic light, in the wavelength range of 300 to 420 nm; or in step a) the neral is irradiated with light, in particular monochromatic light, in the wavelength range of 300 to 420 nm; in step a) a mixture comprising or consisting of geranial and neral in neat form is irradiated with light, in particular monochromatic light, wherein preferably at least 90 % and at most 100 % of the radiant flux of the light, in particular monochromatic light, is emitted in the range from 300 nm to 420 nm and its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum, and / or wherein preferably the geranial in neat form, the neral in neat form or a mixture comprising or consisting of neral and geranial in neat form is irradiated with light, in particular monochromatic light, as noted above.

[0137] 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 300 nm to 420 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 organic solvent. However, the great majority of embodiments of monochromatic light only contains small amounts of light portions beyond 300 nm to 420 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 300 nm to 420 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 300 nm to 420 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 300 nm to 420 nm and its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 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 neral / geranial.

[0138] The irradiation with light is preferably accomplished by the use of a LED (light emitting diode), or laser.

[0139] 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.

[0140] The light emitted by the LED unit is chosen such that at least a part of the light can be used in the isomerisation. 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 300 and 800 nm, more preferably between 300 and 420 nm, or 350 and 490 nm. 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 neral / geranial. 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.

[0141] In a preferred embodiment, in step a) the geranial is irradiated with light, in particular monochromatic light, in the wavelength range of 350 to 490 nm; or in step a) the neral is irradiated with light, in particular monochromatic light, in the wavelength range of 350 to 490 nm, wherein preferably at least 90 % and at most 100 % of the radiant flux of the light, in particular monochromatic light, is emitted in the range from 350 nm to 490 nm and its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum, and / or wherein preferably the geranial and / or neral is irradiated in the presence of a sensitizer.

[0142] In a further elaborated embodiment of the inventive process at least 90 % of the power of said monochromatic light and at most 100 % of said power being emitted in the range from 300 nm to 420 nm, or 350 to 490 nm. Working at this somewhat smaller wavelength range still provides good conversion rates into and high yields of neral / geranial, 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.

[0143] The irradiation may be carried out in a continuously stirred reaction vessel, in a pumping circuit, or in a continuous flow reactor. The lighting device and photochemical reactor described in WO2021 / 233951 and / or W02023 / 011951A1 may be used in conducting step a) of the process of the present invention.

[0144] Step a) is preferably done in a temperature range of -20 to 100 ° C.

[0145] Step a) is preferably done in a pressure range from 1 mbar to 20 bar.

[0146] Step b)

[0147] In a preferred embodiment the method according to the invention comprises, as step b) the separation of the mixture comprising geranial and neral obtained in step a) to give enriched or pure geranial and enriched or pure neral, especially by way of distillation, or chromatographic separation processes.

[0148] In a more preferred embodiment, the method according to the invention comprises, as step b) the (continuous) distillative separation of the mixture comprising geranial and neral obtained in step a) to give enriched or pure geranial and enriched or pure neral. Reference is made to W02009 / 068444 and the above detailed illustration of step a). With respect to step b) the same preferences apply as for step 0). In case the irradiation with light in step a) is done in the presence of solvent and / or sensitizer step b) involves also the distillative removal of solvent and the separation of the sensitizer, which remains as residue in the distillation, and optionally its reuse in step a).

[0149] The distillative separating is a continuous method for isolating geranial in pure or enriched form by distillative removal of geranial from substance mixtures comprising neral and geranial, the distillative removal being carried out in a dividing wall column or in an interconnection of two distillation columns in the form of a thermal coupling having 80 to 200 theoretical plates and one or more side take-off points at an absolute operating pressure, i.e. at an absolute pressure in the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling of from 5 to 200 mbar.

[0150] The distillative separating is preferably a continuous method for isolating neral in pure or enriched form by distillative removal of neral from substance mixtures comprising neral and geranial, the distillative removal being carried out in a dividing wall column or in an interconnection of two distillation columns in the form of a thermal coupling having 80 to 200 theoretical plates and one or more side take-off points at an absolute operating pressure, i.e. at an absolute pressure in the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling of from 5 to 200 mbar.

[0151] The dividing wall column has 80 to 200, preferably 100 to 180, theoretical plates and one or more, preferably 1 to 3, particularly preferably 1 or 2, side take-off points.

[0152] Step b) is carried out at an absolute operating pressure in the dividing wall column or in the interconnection of two distillation columns in the form of a thermal coupling of from 5 to 200 mbar, preferably from 5 to 100 mbar, particularly preferably from 5 to 70 mbar and very particularly preferably from 10 to 50 mbar and especially preferably from 10 to 40 mbar. Preferably, the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling is operated here such that the absolute top pressure is 10 to 50 mbar, preferably 10 to 40 mbar. Likewise preferably, the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling is operated here such that the absolute bottom pressure is 5 to 200 mbar, preferably 10 to 100 and particularly preferably 20 to 50 mbar.

[0153] The reflux ratio can be varied within wide limits and is usually about 5 : 1 to about 2000 : 1, preferably about 20 : 1 to 1000 : 1. Also advantageous is a dephlegmator procedure, i.e. only the return stream is condensed in the top condenser of the column and fed back to the column. In such an energetically favorable case of partial condensation, the top product to be discharged is produced exclusively in the aftercooler, which can be operated at a lower temperature.

[0154] The feed, i.e. the substance mixture to be used, can be fed in liquid or gaseous form into the dividing wall column or the interconnection of two distillation columns in the form of a thermal coupling, preferably into the dividing wall column, and be separated there into a top and bottom fraction and also one or more, preferably into two or more, side take-offs as described above. In one side take-off, the neral product of value (or the geranial product of value) is produced in the desired purity.

[0155] In a particularly preferred embodiment the present invention is directed to a method for the preparation of enriched or pure neral.

[0156] In another preferred embodiment the present invention is directed to a method for the preparation of enriched or pure geranial.

[0157] Step 0) includes the distillative separation of mixtures comprising geranial and neral to give enriched or pure geranial and neral.

[0158] A substance mixture which is used in step 0) consists preferably of 30 to 70% by weight, preferably of 40 to 60% by weight, of neral, of 70 to 30% by weight, preferably of 60 to 40% by weight, of geranial and of 0 to 5% by weight of further components, where the percentages add up to 100% by weight.

[0159] In step a) the enriched or pure geranial obtained in step 0) is isomerized to a mixture of neral and geranial by irradiation with light. Preferably the enriched or pure geranial in neat form is irradiated with light, i.e. no solvent and photosensitizer are present. The enriched or pure geranial is isomerized to the (photo)equilibrium between neral and geranial by irradiation with light. Surprisingly, the isomerization of neral, or geranial by irradiation with light does not lead to the thermodynamic equilibrium between neral and geranial (ca. 40 % neral / ca. 60 % geranial according to Fig. 1 of Wolken et al., J. Agric. Food Chem. 2000, 48, 5401-5405). That is, in the mixture of neral and geranial obtained by irradiation with light the amount of neral is generally higher than the amount of geranial. Said fact is advantageous for the production of neral.

[0160] Step a) is preferably done in a temperature range of -20 to 100 ° C.

[0161] Step a) is preferably done in a pressure range from 1 mbar to 20 bar.

[0162] Step b) involves the distillative separation of the mixture comprising geranial and neral obtained in step a) to give enriched or pure geranial and neral. The product neral is collected.

[0163] The (undesired) enriched or pure geranial obtained in step b) is recycled according to step c) in step a), i.e. isomerized to the (photo)equilibrium between neral and geranial by irradiation with light. The recycling of the (undesired) enriched or pure geranial obtained in step b) to step a) allows for improved yields of enriched or pure neral to be achieved in the process of the invention.

[0164] It is economical from the perspective of equipment to use a continuous reaction apparatus to efficiently obtain the neral continuously by distillation while performing photoisomerization of geranial in a take-off section of the distillation column.

[0165] As indicated above, the mass ratio of neral : geranial of an educt such as, e.g., a mixture comprising neral and geranial, may be altered by means of conducting a method of the present invention. Thus, an obtainable (or obtained) product may bear special characteristics.

[0166] A further aspect of the present invention thus relates to a mixture comprising neral and geranial obtainable (or obtained) from a method of the present invention.

[0167] It will be understood that the definitions and preferred embodiments as laid out in the context of the method of isomerizing neral to geranial and / or geranial to neral of the present invention above mutatis mutandis apply to the mixture obtainable (or obtained) from such method.

[0168] In a preferred embodiment, the mixture has a mass ratio of neral : geranial of >1.25 or <0.75, in particular of >1.5. In a preferred embodiment, the mixture has a mass ratio of neral : geranial of >1.75 or >2.0 or >3.0. This may be achieved by isomerization of step a) optionally in combination with enriching neral. In an alternative preferred embodiment, the mixture has a mass ratio of neral : geranial of <0.7 or >0.6 or >0.5. This may be achieved by isomerization of step a) optionally in combination with enriching geranial.

[0169] In a further alternative preferred embodiment, the mixture has a mass ratio of neral : geranial between 1 : 1.25 to 1.25 : 1.

[0170] Further Conversion to Menthol or Linalool

[0171] The neral, or geranial obtained according to the method of the present invention are useful intermediates in the production of, for example, menthol, or linalool.

[0172] The neral, or geranial, especially neral obtained according to the method of the present invention may be used for producing menthol, especially optically active menthol.

[0173] Accordingly, the method according to the present invention may comprise

[0174] - the additional steps d.1) to g.1): d.1) catalytic hydrogenation of neral obtained by the method of the present invention (e.g., according to any of claims 1-3, 6-16, 18-23), preferably of neral obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)), to give citronellal, e.1) cyclization of citronellal to give isopulegol in the presence of an acidic catalyst, f.1) optionally purification of isopulegol, preferably by crystallization and g.1) catalytic hydrogenation of isopulegol to give menthol. Reference is made to W02009068444 and D. Dylong et al, Flavour Fragr. J., 37 (2022) 195-209.

[0175] The overall reaction sequence is illustrated by the reaction scheme below.

[0176] Step d.1): Catalytic hydrogenation of neral and / or geranial to give citronellal

[0177] According to stage d.1) of the method for producing menthol, a catalytic hydrogenation of neral is carried out to give citronellal, preferably a catalytic hydrogenation of neral produced as described above. Reference is made to WO2009768444, Jakel C, Paciello R. The asymmetric hydrogenation of enones - access to a new L-menthol synthesis; in: Blaser H-U, Federsel H-J, eds. Asymmetric Catalysis on Industrial Scale: Challenges, Approaches, and Solutions. 2nded. Wiley-VCH; 2010:187-205; and Stolle A, Gallert T, Schmdger C, Ondruschka B. Hydrogenation of citral: a wide-spread model reaction for selective reduction of a,p-unsaturated aldehydes. RSC Adv. 2013;3(7):2112-2153.

[0178] An improved method, and one which can be used preferably within the context of the present invention, for producing optically active carbonyl compounds by asymmetric hydrogenation of a,p-unsaturated carbonyl compounds in the presence of optically active transition metal catalysts that are soluble in the reaction mixture and which have at least one carbon monoxide ligand is known from W02006 / 040096, to which reference is hereby made in its entirety and which, including all of the preferred embodiments, should be considered part of the present disclosure.

[0179] According to WO2016 / 097242 A 1 the catalytic activity of the optically active transition metal catalysts used for the homogeneous-catalytic asymmetric hydrogenation of geranial / neral, which comprise rhodium as catalytically active transition metal, can be significantly increased by adding a phosphine compound, such as, for example,

[0180] - (2-(diphenylphosphoryl)-1-methylpropyl))diphenylphosphane, including its (R,R)-enantiomer (=(R,R)-chiraphos monoxide) and its (S,S)-enantiomer (=(S,S)-chiraphos monoxide), and its racemate (compounds (1-1)),

[0181] - cyclopentyldiphenylphosphine (compound (I-2),

[0182] - 2-butyldiphenylphosphine (compound (I-3),

[0183] - cyclohexyldiphenylphosphine (compound (I-4),

[0184] - isopropyldiphenylphosphine (compound (I-5),

[0185] - [5-(diphenylphosphanylmethyl)-2,2-dimethyl-1,3-dioxolan-4-yl]methyldiphenyl-phosphane monoxide, including its (4S,5S) and (4R,5R) enantiomers and its racemate (compounds (I-6),

[0186] - [5-(1-diphenylphosphanylethyl)-2,2-dimethyl-1,3-dioxolan-4-yl]ethyldiphenyl-phosphane monoxide, including its (4S,5S) and (4R,5R) enantiomers and its racemate (compounds (I-7),

[0187] - [2-diphenylphosphanylcyclohexyl]diphenylphosphane monoxide, including its (1S,2S) and (1 R,2R) enantiomers and its racemate (compounds (I-8),

[0188] - [4-diphenylphosphanyltetrahydrofuran-3-yl]diphenylphosphane monoxide, including its (3S,4S) and (3R,4R) enantiomers and its racemate (compounds (I-9),

[0189] - [2-diphenylphosphanyl-3-bicyclo[2.2.1]hept-5-enyl]diphenylphosphane monoxide, including its (1S,2R,3R,4R), (1R,2S,3R,4R), (1S,2R,3S,4S), (1 R,2S,3S,4S) isomers, and enantiomer and diastereomer mixtures thereof (compounds (1-10),

[0190] - [1-benzyl-4-diphenylphosphanylpyrrolidin-3-yl]diphenylphosphane monoxide, including its (3S,4S) and (3R,4R) enantiomers and its racemate (compounds (1-11),

[0191] - [3-diphenylphosphanyl-1-methylbutyl]diphenylphosphane monoxide, including its (1S,3S) and (1R,3R) enantiomers and its racemate (compounds (1-12), and mixtures thereof, without adversely affecting their stability and selectivity in a significant manner.

[0192] The racemic or optically active citronellal accessible in this way is usually produced in high yield and in particular high chemical and optical purity. Depending on the requirements placed on the chemical purity of the resulting citronellal to be further reacted in the course of further step d.1 ), preferably D-citronellal, it can be further purified by separation and / or purification methods known per se to the person skilled in the art. A prepurification of the citronellal-containing product mixture obtained by the presented hydrogenation by means of a falling film evaporator and subsequent fine distillation of the citronellal has proven to be advantageous.

[0193] A further aspect of the invention is directed to a process for the preparation of citronellal, preferably optically active citronellal, comprising the steps of d.0) optionally separating the citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of >1 , preferably >1.5, into geranial and neral; and d.1) preparation of optically active citronellal by asymmetric hydrogenation of the citral of the step d.0) or of citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of >1, preferably >1.5.

[0194] A further aspect of the present invention relates to citronellol, which may be optionally optically active cintronellol, obtainable (or obtained) from a method of the present invention.

[0195] It will be understood that the definitions and preferred embodiments as laid out in the context of the methods of the present invention 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.

[0196] Isopulegol (5-methyl-2-(1-methylethenyl)-cyclohexanol) may have three asymmetric carbon atoms and therefore four stereoisomers, each occurring as a pair of enantiomers.

[0197] (1 R,3R,4S)-(-)lsopulegol is also known as L-isopulegol.

[0198] 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 (d.0) optionally separating the citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of >1, preferably >1.5, into geranial and neral; d.1) preparation of optically active citronellal by asymmetric hydrogenation of the citral of the step d.0) or of citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of >1, preferably >1.5; and e.1) cyclization of the citronellal of step d.1) to give isopulegol in the presence of a suitable acid, preferably a Lewis acid.

[0199] 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.

[0200] It will be understood that the definitions and preferred embodiments as laid out in the context of the methods of the present invention 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.

[0201] A further aspect of the invention is directed to a process for preparation of menthol, preferably L-menthol, comprising the steps of d.0) optionally separating the citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of >1, preferably >1.5, into geranial and neral; d.1) preparation of optically active citronellal by asymmetric hydrogenation of the citral of the step d.0) or of citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of >1, preferably >1.5; e.1) cyclization of the citronellal of step d.1) to give isopulegol in the presence of a suitable catalyst; f.1.) optionally purification of isopulegol such as, e.g., by crystallization; and g.1) catalytic hydrogenation of isopulegol prepared in any of steps e.1) or f.1) to obtain menthol.

[0202] A further aspect of the present invention is directed to a process for the preparation of optically active menthol using neral and / or geranial, in particular pure neral or a mixture of neral and geranial that comprises a neral : geranial mass ratio of >1, preferably >1.5, obtained by the process according to the invention.

[0203] 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 optionally separating the citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, into geranial and neral; preparation of optically active citronellal by asymmetric hydrogenation of the citral; 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 hydrogenation of the optically active isopulegol prepared in this way to give optically active menthol.

[0204] A further aspect of the present invention relates to menthol, which may be optionally optically active menthol, in particular L-menthol, obtainable (or obtained) from a method of the present invention.

[0205] It will be understood that the definitions and preferred embodiments as laid out in the context of the methods of the present invention 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.

[0206] Preferred embodiments of the preparation of menthol are provided herein.

[0207] Step e.1): Cyclization of citronellal to give isopulegol

[0208] According to step e.1) of the method according to the invention, a cyclization of citronellal which has been obtained by the above-described step d.1) by catalytic hydrogenation of neral, to give isopulegol is carried out in the presence of an acidic catalyst. Reference is made to W02006092433A1 and US7550633. The cyclization of citronel lai to isopulegol under acidic conditions has been known for a long time. An overview of the available acidic or Lewis-acidic reagents or catalysts can be found, for example, under E. J. Lenardao, G. V. Botteselle, F. de Azambuja, G. Perin, R. G. Jacob Tetrahedron 2007, 63, 6671-6712.

[0209] A broad diversity of systems is known as customary catalysts and reagents, such as for example: silica gel or aluminum oxide or mixtures thereof, as disclosed e.g. in W02004 / 089299, zeolites, as described e.g. for the case of boron-containing zeolites in W02004 / 101480. Further customary acidic or Lewis-acidic catalysts are, for example, zinc bromide, as described e.g. in Synthesis 1978, 147-148 and in EP1053974A1 or else tungsten-containing acids as described in BR2005002489A.

[0210] Moreover, EP1225163A describes the cyclization of citronellal to isopulegol in the presence of tris(2,6-diphenylphenol)aluminum catalysts. Tris(2,6-diphenylphenol)aluminum is known in the literature and as catalyst for selective 1 ,4-functionalizations of a,p-unsaturated carbonyl compounds and for specific Claisen rearrangements, for example in Angew. Chem. Int. Ed. 2004, 43, 994. The specified catalyst system is also suitable for use in the course of step e.1) of the method according to the invention.

[0211] W02007 / 039342 and W02007 / 039366 likewise disclose aluminum-containing homogeneous catalysts, specifically those which have one or more siloxide ligands on the aluminum. The disclosed aluminum-siloxide compounds are suitable as catalysts for intramolecular Prins reactions, including the cyclization of citronellal to isopulegol.

[0212] Preference is given to carrying out the cyclization of citronellal to isopulegol according to step e.1) in the presence of an aluminum-containing catalyst, specifically in the presence of a Lewis-acidic aluminum-containing catalyst.

[0213] A method for the cyclization of citronellal to isopulegol that is particularly preferred within the context of step e.1) of the method according to the invention is described in W02006 / 092433, to which reference is hereby made in its entirety and the disclosure of which, including all preferences and embodiments, should be considered part of the present disclosure.

[0214] 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. Step f.1): Purification of isopulegol by crystallization

[0215] According to step f.1) of the method according to the invention, a purification of isopulegol obtainable as described above according to step e.1) of the method according to the invention by crystallization is carried out.

[0216] The crystallization of isopulegol is known to the person skilled in the art and disclosed, for example, in US5,663,460. The patent describes the purification of (-)-n-isopulegol by crystallization from petroleum ether or advantageously from acetone at temperatures of from -20°C to -60°C. Here, an increase in the optical purity can also be achieved.

[0217] In addition, US3,218,361 discloses a method for the crystallization of isopulegol from substance mixtures comprising isopulegol and diastereomers of isopulegol. The crystallization is carried out here at temperatures below 0°C, preferably below -30°C and for example at -65°C and can be carried out either from the solution or from the melt.

[0218] W02007 / 023109, to which reference is hereby made in its entirety and the disclosure of which including all preferences and embodiments is to be considered part of the present disclosure, discloses a method for producing enriched isopulegol, specifically enriched L- isopulegol by crystallization from a melt comprising L-isopulegol.

[0219] Such a method for the purification of isopulegol, specifically of optically active L-isopulegol by melt crystallization constitutes a preferred method for the purification of isopulegol by crystallization according to step f.1).

[0220] The isopulegol obtained by crystallization as described above according to step e.1) of the method according to the invention can also be further purified by further separating methods, preferably by distillation. In this connection, the use of dividing wall columns or interconnections of columns in the form of a thermal coupling have proven to be advantageous from the point of view of processing and costs.

[0221] Step g.1): Catalytic hydrogenation of isopulegol to give menthol

[0222] According to step g.1) of the method according to the invention, a catalytic hydrogenation of isopulegol obtained according to step f.1) to menthol is carried out. Reference is made to W02009068444A2 and W02009013192A2.

[0223] Within the context of one preferred embodiment, the catalytic hydrogenation of racemic or optically active isopulegol according to step g.1) of the method according to the invention is carried out in the presence of a heterogeneous nickel-containing catalyst. When using enantiomer-enriched or enantiomerically pure isopulegol, preferably L-isopulegol, the catalytic hydrogenation according to step g.1) is preferably carried out in the presence of a heterogeneous nickel- and copper-containing catalyst.

[0224] DE577036 discloses a method for producing synthetic menthol by hydrogenation of thymol.

[0225] Nickel, nickel / copper and cobalt catalysts are described as suitable catalysts. Specific nickel catalysts have also been used for the catalytic hydrogenation of piperitol to give menthol, as described in GB1 ,503,723.

[0226] EP1532091 discloses a method for producing racemic menthol by catalytic hydrogenation of isopulegol which has been used in the form of a diastereomer mixture of 70.1% isopulegol, 18.1% neo-isopulegol, 6.8% iso-isopulegol and 2.6% neoiso-isopulegol. The catalyst used was Raney nickel doped with iron and chromium. This gave menthol in the form of a mixture of the possible diastereomers which consisted to 61.4% of menthol and to 35.6% of the further diastereomers of menthol.

[0227] A further route to menthol is that of processes for the diastereoselective cyclization of citronellal to isopulegol, as described, for example, in the aforementioned EP1225163 or W02006 / 092433. The isopulegol obtained in this way can then be hydrogenated to menthol in a further step.

[0228] R.H. Pickard et al. described, in J. Chem. Soc. 1920, 1248-1263, the production of L-menthol by catalytic hydrogenation of L-isopulegol in the presence of colloidal palladium.

[0229] B. Dudley Sully et al. describe, in P.& E.O.R. 1068, 235-366, the production of L-menthol by hydrogenation of L-isopulegol in the presence of Raney nickel at a temperature of 120°C.

[0230] EP1053974 discloses a method for the catalytic hydrogenation of isopulegol to menthol in the presence of a catalyst of 5% palladium on carbon at a hydrogen pressure of 5 bar.

[0231] EP0394842 relates to catalysts for the hydrogenation of aliphatic unsaturated compounds, which comprises nickel and copper and is characterized by a content of from 20 to 75% by weight of nickel oxide, 10 to 75% by weight of zirconium dioxide and 5 to 50% by weight of copper oxide, in each case based on the oxidic, unreduced catalyst. Examples of substrate specified are: butyne-2-diol-1 ,4, butene-2-diol-1 ,4 and 2-ethylhexen-2-al.

[0232] According to one particularly preferred embodiment within the context of step g.1) of the method according to the invention, a method for producing racemic or optically active menthol ( ) is carried out by catalytic hydrogenation of racemic or optically active isopulegol the presence of hydrogen and a catalyst comprising

[0233] 30 to 70% by weight of oxygen-containing compounds of nickel, calculated as NiO, 15 to 45% by weight of oxygen-containing compounds of zirconium, calculated as ZrO2,

[0234] 5 to 30% by weight of oxygen-containing compounds of copper, calculated as CuO and

[0235] 0.1 to 10% by weight of oxygen-containing compounds of molybdenum, calculated as MoOa, where the data in % by weight are based on the dry unreduced catalyst.

[0236] A catalyst that is particularly preferred for use in the course of step g.1) of the method according to the invention consists to 49 to 53% by weight of NiO, to 15 to 19% by weight of CuO, to 28 to 32% by weight of ZrO2and to 1 to 2% by weight of MoOa, and optionally to 0 to 3% by weight of further components, such as, for example, graphite, the fractions by weight of the individual components selected in each case adding up to 100% by weight. Catalysts of this type are known and can be produced, for example, as described in EP0696572, to which reference is made in this regard in its entirety. The catalysts in the course of step g.1) can be produced, for example, as described on pages 65 to 67 of W02009 / 068444. h.1) Fine distillation of menthol

[0237] To guarantee the highest quality standards, especially with regard to the sensory, specifically olfactory properties of the resulting menthol the method according to the invention therefore comprises, in the context of a preferred embodiment as further optional step h.1 ), the distil lative purification of racemic and / or optically active menthol preferably by means of a dividing wall column.

[0238] In a particularly preferred embodiment, the present invention also relates to a method for producing optically active menthol, comprising the steps d.2) asymmetric catalytic hydrogenation of neral and / or geranial to give optically active citronellal, e.2) cyclization of optically active citronellal obtained according to step d.2) to give optically active isopulegol in the presence of an acidic catalyst, f.2) purification of optically active isopulegol obtained according to step e.2), preferably by crystallization and g.2) catalytic hydrogenation of optically active isopulegol obtained according to any of steps e.2) or f.2) to give optically active menthol.

[0239] According to step d.2), an asymmetric hydrogenation of neral is carried out. Preferably, an asymmetric catalytic hydrogenation, as described above under step d), of pure or enriched neral is carried out. In this way, optically active citronellal is accessible, if desired, depending on the configuration of the asymmetric catalytic hydrogenation, in the form of one of the two enantiomers, preferably in the form of D-citronellal.

[0240] The optically active citronellal obtainable according to step d.2) can then be cyclized according to step e.2) to give optically active isopulegol in the presence of an acidic catalyst. Suitable acidic catalysts which may be mentioned are the acidic or Lewis-acidic catalysts described above in step d.1), such as the diarylphenoxyaluminum compounds.

[0241] The optically active isopulegol obtainable in this way is, according to step f.2) within the context of this preferred embodiment, purified by crystallization. Preference is given to carrying out a crystallization from the melt as described under step f.1). Within the context of this preferred embodiment of the method according to the invention, purified L-isopulegol is obtained.

[0242] Within the context of this embodiment of the method according to the invention, according to step g.2) the optically active isopulegol obtainable in this way is then catalytically hydrogenated to give optically active menthol. The catalytic hydrogenation of isopulegol to menthol is known to the person skilled in the art and can be carried out using a wide variety of customary heterogeneous hydrogenation catalysts. It has proven to be advantageous to carry out the catalytic hydrogenation in the presence of the nickel-, copper-, zirconium- and molybdenum-containing catalysts described above under step g.1).

[0243] A particular advantage of the method according to the invention that should be emphasized is that it opens up the route to optically active, preferably practically enantiomerically and diastereomerically pure L-menthol. A higher content of neral in the asymmetric hydrogenation leads to a higher enantiomer excess of the optically active citronellal formed.

[0244] In addition, the neral, or geranial, especially geranial obtained according to the method of the present invention may be used for producing linalool.

[0245] A further embodiment of the present invention is directed to a method for producing linalool and comprises the additional steps d’) and e’): d’) catalytic hydrogenation of geranial and / or neral obtained by the method of the present invention, preferably of geranial obtained by the method of the present invention, in particular of geranial obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)), especially catalytic hydrogenation of geranial obtained in any of steps 0), a) and / or b) (e.g., 0) and / or b), or a) and / or b)) in the presence of a supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably a ruthenium catalyst supported on carbon black; to obtain geraniol; and e’) isomerization of geraniol in the presence of a catalyst, especially a tungsten catalyst, very especially a dioxotungsten (VI) complex to obtain linalool.

[0246] Linalool may be prepared from geranial via a process comprising catalytic hydrogenation of geranial to obtain geraniol and isomerization thereof to linalool.

[0247] The hydrogenation of geranial to obtain geraniol may be achieved by hydrogenation in the presence of a 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.

[0248] The crude mixture of geraniol can be separated by rectification (see DE10223974) or can be used without purification in the isomerization of geraniol to linalool.

[0249] The isomerization of 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 (III), wherein Li and 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.

[0250] Additional Catalysts, which can be used in the isomerization, are, for example, described in CN105218312B and CN111087343B.

[0251] One aspect of the invention hence is for an improved process for the preparation of linalool. The preparation of linalool may be done as described herein or by other methods known in the art.

[0252] A further aspect of the present invention relates to linalool obtainable (or obtained) from a method of the present invention.

[0253] It will be understood that the definitions and preferred embodiments as laid out in the context of methods 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. A further aspect of the invention is directed to a process for the preparation of vitamin A or vitamin A acetate comprising the steps of

[0254] - converting citral having a neral : geranial mass ratio as obtainable (or obtained) according to the invention, preferably having a neral : geranial mass ratio of <1 , preferably <0.75, into pseudoionone,

[0255] - reacting pseudoionone to obtain p-ionone,

[0256] - transforming p-ionone (IX) into p-vinylionol,

[0257] - phosphorylation of p-vinylionol to yield the C15-salt, and

[0258] - reacting the C15-salt with the C5-acetate to yield vitamin A acetate.

[0259] A further aspect of the present invention relates to vitamin A obtainable (or obtained) from a method of the present invention.

[0260] It will be understood that the definitions and preferred embodiments as laid out in the context of methods and obtainable (or obtained) products above mutatis mutandis apply to the preparation and product characteristics of vitamin A and vitamin A acetate. It will be understood that the obtainable (or obtained) vitamin A and vitamin A acetate has certain characteristics.

[0261] The examples below serve to illustrate the invention without limiting it in any way.

[0262] Examples

[0263] Example 1

[0264] General procedure

[0265] The sensitizer (as indicated) is weighed into the borosilicate glass ampoule, which is then sealed. The ampoule is evacuated via a cannula and filled with argon. This procedure is repeated several times. The solution of geranial / neral in acetonitrile (as indicated (AON)) is added via a syringe. The prepared ampoule is irradiated for the specified time, wavelength, and radiometric power at 20 °C. The reaction mixture is then analyzed by gas chromatography with flame ionization detector. The values given are GC area percentages (without solvent).

[0266] GC method:

[0267] Agilent 6890N Gas Chromatograph having SSL-Injector and FID

[0268] Column: CP-Sil 5 CB; 50m x 0.25mm x 0.12pm

[0269] Carrier gas: Hydrogen

[0270] Carrier gas flow: 12.0 Psi; const, pressure

[0271] Injector temperature: 250°C

[0272] Split: 1:100

[0273] Injection volume: 1.0 pL

[0274] Detector: FID

[0275] Detector temperature: 300 °C

[0276] Oven Program:

[0277] Run time: 20.0 min

[0278] Examples 1 to 5 - Photochemical Isomerisation of Geranial

[0279] 1)Radient flux (365 nm): 0.21 W; Radient flux (405 nm): 0.77 W.2)[lr(dtbbpy)(ppy)2]PF6.

[0280] 3)Michler's Ketone (4,4'-bis-(dimethylamino)-benzophenon).

[0281] 4)2CzPN (4,5-bis(carbazol-9-yl)-1 ,2-dicyanobenzene).

[0282] Examples 1 to 5 show that the geranial content of the mixture can be changed from 96 a% to ca. 50 a% by photochemical isomerization. An excess of neral is obtained by extending the irradiation time. Reference is made to Example 13.

[0283] Examples 6 to 9 - Photochemical Isomerisation of Neral

[0284] 1)Radient flux (365 nm): 0.21 W; Radient flux (405 nm): 0.77 W.2> [lr(dtbbpy)(ppy)2]PF6.

[0285] 4)2CzPN (4,5-bis(carbazol-9-yl)-1 ,2-dicyanobenzene).

[0286] 5)Diacetyl (2,3-butandione)

[0287] 6)a% = (peak) area%. Examples 6 to 9 show that the neral content of the mixture can be changed from 99 a% to ca. 60 a% by photochemical isomerization.

[0288] Examples 10 to 12 - Photochemical Isomerisation of 100 mg Geranial or Neral in 2.5 ml

[0289] ACN without Photocatalyst

[0290] 1)Radient flux (365 nm): 0.21 W; Radient flux (405 nm): 0.77 W.

[0291] ^ Composition: 99.0 % Neral and 0.24 % Geranial and 0.76 % side products (determined by GC).

[0292] ^ Composition: 3.5 % Neral and 96.2 % Geranial and 0.3 % side products (determined by GC).

[0293] It is apparent from Examples 10 to 12 that, given sufficient reaction time, an excess of Neral is obtained regardless of the starting material and wavelength.

[0294] Example 13

[0295] A 150 ml reaction flask made of borosilicate is several times evacuated and filled with argon. It is charged with almost pure geranial (26.8 g, 169.3 mmol geranial and 6.1 mmol neral), and irradiated for 5h with 365 nm LEDs (radient flux 10 W) at 20 °C. From time-to-time samples are taken, and the reaction mixtures are measured by1H-NMR spectroscopy. The molar ratios of geranial and neral are provided in the following table.

[0296] 1H-NMR (500 MHz), Bruker, 298 K. The spectra were measured unlocked, without addition of deuterated solvents and tetramethyl silane. The quantification was performed by normalizing the aldehyde signals of neral and geranial to the sum of their signals in the first spectrum, recorded at 0 h. Since chemical shift referencing according to IIIPAC was not possible, the aldehyde signal of geranial was identified as the most lowfield shifted, high intensity doublet (J = 8 Hz). The aldehyde signal of Neral was identified as the low intensity doublet (J = 8 Hz)

[0297] Example 14

[0298] Example 14 is repeated, except that the reaction mixture is analyzed by quantitative GC. Examples 13 and 14 demonstrate that in a mixture of neral and geranial obtained by irradiation with light the amount of neral is higher than the amount of geranial. Said fact is advantageous for the production of neral.

Claims

Claims1. A method, comprising the isomerization of neral of formulageranial of formular the isomerization of geranial to neral, characterized in that (a) the isomerization is accomplished by irradiation with light.

2. The method according to claim 1, which is a method for the preparation of enriched or pure neral, comprising the steps a) the isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light; b) the separation, especially the distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure neral, and enriched or pure geranial; and c) optionally recycling of the enriched or pure geranial obtained in step b) to step a).

3. The method according to claim 2, additionally comprising as step 0) the provision of enriched or pure geranial by distillative separation of mixtures comprising geranial and neral.

4. The method according to claim 1 , which is a method for the preparation of enriched or pure geranial, comprising the steps a) the isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light; b) the separation, especially the distillative separation of the mixture comprising geranial and neral obtained in step a) to give the product, enriched or pure geranial, and enriched or pure neral; and c) optionally recycling of the enriched or pure neral obtained in step b) to step a).

5. The method according to claim 4, additionally comprising as step 0) the provision of enriched or pure neral by distillative separation of mixtures comprising geranial and neral.

6. The method according to any of claims 1 to 3, wherein in step a) a solution of the geranial in a solvent is irradiated with light, or the method according to any of claims 1 , 4, or 5, wherein in step a) a solution of the neral in a solvent is irradiated with light, wherein the solvent is preferably selected from water, dichloromethane, trichloromethane, tetrachloromethane, CS2, Ci-C4alcohols, chlorobenzene, fluorobenzene, trifluoromethylbenzene, acetonitrile, dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), ethylacetate, acetone and mixtures thereof.

7. The method according to any of claims 1 to 3, or 6, wherein in step a) the geranial is irradiated with light, in particular monochromatic light, in the presence of a sensitizer, or the method according to any of claims 1 , 4, 5, or 6, wherein in step a) the neral is irradiated with light, in particular monochromatic light, in the presence of a sensitizer.

8. The method according to claim 7, wherein the sensitizer is selected from [lr(dF(CF3)ppy)2(bpy)]PF6, (lr[dF(CF3)ppy]2(dtbbpy))PF6, (lr[dF(CF3)ppy]2(dtbpy))PF6, (lr[dF(Me)ppy]2(dtbbpy))PF6, [lr(dtbbpy)(ppy)2]PF6, lr(ppy)3, Ru(bpy)3CI3, [Ru(bpy)3](PFe)2, benzophenone, thioxanthen-9-one, Michler's Ketone, 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 and mixtures thereof.

9. The method according to any of claims 1 to 8, wherein in step a) the geranial is irradiated with light, in particular monochromatic light, in the wavelength range of 350 to 490 nm; or in step a) the neral is irradiated with light, in particular monochromatic light, in the wavelength range of 350 to 490 nm, wherein preferably at least 90 % and at most 100 % of the radiant flux of the light, in particular monochromatic light, is emitted in the range from 350 nm to 490 nm and its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum, and / or wherein preferably the geranial and / or neral is irradiated in the presence of a sensitizer in accordance with any of claims 7 or 8.

10. The method according to any of claims 1 to 3, wherein in step a) the geranial in neat form is irradiated with light, in particular monochromatic light, or the method according to any of claims 1 , 4 and 5, wherein in step a) the neral in neat form is irradiated withlight, in particular monochromatic light, or the method according to any of claims 1 to 5, wherein in step a) a mixture comprising or consisting of geranial and neral in neat form is irradiated with light, in particular monochromatic light.

11. The method according to any of claims 1-8 or 10, wherein in step a) the geranial is irradiated with light, in particular monochromatic light, in the wavelength range of 300 to 420 nm; or in step a) the neral is irradiated with light, in particular monochromatic light, in the wavelength range of 300 to 420 nm; in step a) a mixture comprising or consisting of geranial and neral in neat form is irradiated with light, in particular monochromatic light, wherein preferably at least 90 % and at most 100 % of the radiant flux of the light, in particular monochromatic light, is emitted in the range from 300 nm to 420 nm and its monomodal emission spectrum preferably exhibiting a halfwidth of not more than + / - 100 nm, preferably not more than + / - 60 nm, such as + / - 10 to + / - 30 nm, in relation to the wavelength of the emission maximum, and / or wherein preferably the geranial in neat form, the neral in neat form or a mixture comprising or consisting of neral and geranial in neat form is irradiated with light, in particular monochromatic light, in accordance with claims 10.

12. The method according to any of claims 1 to 11 , wherein step a) is done in a temperature range of -20 to 100 ° C.

13. The method according to any of claims 1 to 12, wherein step a) is done in a pressure range from 1 mbar to 20 bar.

14. The method according to any of claims 1 to 13, wherein step a) is carried out in a continuously stirred reaction vessel, in a pumping circuit, or in a continuous flow reactor.

15. The method according to any of claims 1 to 14, wherein the irradiation with light is accomplished by use of at least one LED (light emitting diode), or laser.

16. The method according to any of claims 1-3 or 6-15, wherein in step a) the geranial is irradiated with monochromatic light.

17. The method according to any of claims 1 or 4-15, wherein in step a) the neral is irradiated with monochromatic light.

18. The method according to any of claims 1 to 17, wherein the method is a method for producing a desired isomer, in particular neral or geranial, optionally comprised in a mixture comprising neral and geranial.

19. The method according to any of claims 1 to 18, wherein the method is a method for isomerization geranial to neral or for isomerization of neral to geranial.

20. The method according to any of claims 1 to 19, wherein the method is a method for the preparation of enriched or pure neral.

21. The method according to any of claims 1 , 6-16, or 18-20, which is a method for increasing the content of neral in a composition, in particular a mixture comprising geranial and neral, comprising the step a) isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light.

22. The method according to any of claims 1 , 6-16, or 18-21 , which is a method for the preparation of enriched or pure neral, comprising the step a) isomerization of the enriched or pure geranial to a mixture of neral and geranial by irradiation with light.

23. The method according to any of claims 21 or 22, additionally comprising as step 0) the provision of enriched or pure geranial by distil lative separation of mixtures comprising geranial and neral.

24. The method according to any of claims 1 , 6-15, or 17-20, which is a method for increasing the content of geranial in a composition, in particular a mixture comprising geranial and neral, comprising the step a) isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light.

25. The method according to any of claims 1 , 6-15, 17-20, or 24, which is a method for the preparation of enriched or pure geranial, comprising the step a) isomerization of the enriched or pure neral to a mixture of neral and geranial by irradiation with light;26. The method according to any of claims 24 or 25, additionally comprising as step 0) the provision of enriched or pure neral by distillative separation of mixtures comprising geranial and neral.

27. The method according to any of claims 1-3, 6-16, 18-23, which is a method for producing menthol and comprises the additional steps d.1) to g.1): d.1) catalytic hydrogenation of neral obtained by the method of any one of claims 1-3, 6-16, 18-23, preferably the neral obtained in any of steps 0), a) and / or b), to give citronellal, e.1) cyclization of citronellal to give isopulegol in the presence of an acidic catalyst, f.1) optionally purification of isopulegol, preferably by crystallization, and g.1) catalytic hydrogenation of isopulegol to give menthol.

28. The method according to claim 27, comprising the additional steps d.2) to g.2):d.2) asymmetric catalytic hydrogenation neral obtained by the method of any one of claims 1-3, 6-16, 18-23, preferably of the neral obtained in any of steps 0), a) and / or b), to give optically active citronellal, e.2) cyclization of optically active citronellal to give optically active isopulegol in the presence of an acidic catalyst, f.2) optionally purification of optically active isopulegol, preferably by crystallization, and g.2) catalytic hydrogenation of optically active isopulegol to give optically active menthol.

29. The method according to any of claims 1 to 26, preferably any of claims 1 , 4-15, 17-19, or 24-26, which is a method for producing linalool and comprises the additional steps d’) and e’): d’) catalytic hydrogenation of geranial and / or neral obtained by the method of any one of claims 1 to 26, in particular geranial of any of claims 1 , 4-15, 17-19, or 24-26, preferably the geranial obtained in any of steps 0), a) and / or b), especially catalytic hydrogenation of the geranial obtained in any of steps 0), a) and / or b) in the presence of a supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably a ruthenium catalyst supported on carbon black; to obtain geraniol; and e’) isomerization of geraniol in the presence of a catalyst, especially a tungsten catalyst, very especially a dioxotungsten (VI) complex to obtain linalool.

30. A mixture comprising neral and geranial obtainable from a method of any of claims 1 to 26.

Images