Selective hydrogenation of conjugated dienes

Abstract

The present disclosure relates to a process for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene using a suitable heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive. The heterogeneous palladium-containing catalyst and the additive make up a catalyst system that is also the subject of the present disclosure.

Description

[0001] SELECTIVE HYDROGENATION OF CONJUGATED DIENES

[0002] Field of the Disclosure

[0003] The present disclosure relates to the field of organic synthesis. More particularly, it provides a process for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene using a suitable heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive.

[0004] Background of the Disclosure

[0005] The selective hydrogenation of the y-6 C=C bond of dienes conjugated to electron withdrawing groups in a compound, such as conjugated dienals (a,y-dienals) or conjugated dienones (a,y-dienones), is a transformation that is useful for the synthesis of the corresponding monoene. Monoenes that can be accessed by such a transformation are important in many industrial areas, such as the field of perfumery. Indeed, hydrogenation can take place at three different sites (i.e., at any of the two C=C bonds or the C=O group in the case of dienals and dienones). Moreover, for selective hydrogenation of the y-6 C=C bond of dienes conjugated to electron withdrawing groups to be industrially useful, the process should be achieved with an acceptable conversion and with a reasonable turn-over (complex load and reaction time). Methods of selective hydrogenation of conjugated dienals or conjugated dienones to the corresponding monoenes are known, but the reversed selectivity is obtained in which the alkene closest to the carbonyl (i.e., the a-p C=C bond) is reduced.

[0006] For example, WO2012 / 150053 (Firmenich SA) describes a process for the reduction by hydrogenation of a C6-C20 conjugated dienals into the corresponding deconjugated enal, wherein the process is carried out in the presence of a catalytic system comprising a base and a rhodium complex having certain ligands. WO2022 / 223363 (Firmenich SA) describes a base-free catalytic system comprising a specific rhodium complex for the reduction of a conjugated dienal or dienone into the corresponding deconjugated enal or deconjugated enone.

[0007] WO2023137133 (BASF SE) describes a method for selective hydrogenation of (2 ,3) / (4,5) unsaturated dienones using a rhodium or ruthenium complex without the need for nitrogen containing additives such as pyridine, pyrazine, quinoline, and quinoxaline. The conversion of pseudoionone to geranylacetone is exemplified.

[0008] G. Buchi and H. Wiiest (Helv. Chim. Acta 54, 1971 , 1767-1776) describe the hydrogenation of p-damascenone to form p-damascone (also known as Dorinone beta), purporting to achieve the desirable reduction of the y-6 C=C bond. Disadvantageously, however, the process requires overall conditions that are not industrially viable.

[0009] Therefore, there is an ongoing need for improved processes to selectively hydrogenate the y-6 C=C bond of dienes conjugated to electron withdrawing groups in a compound, such as conjugated dienals or conjugated dienones.

[0010] Summary of the Disclosure

[0011] The following aspects of the present disclosure seek to address one or more of the problems described hereinabove.

[0012] In a first aspect, the present disclosure relates to a process for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene, the process comprising reacting the compound comprising a conjugated diene, which has the structure of formula (I),

[0013] RI-CR2=CR3-CR4=CR5-R6 (I), wherein

[0014] Ri and R2are each, independently, H, (Ci-Ci2)alkyl, or aryl, Rs, R4, and R5 are each, independently, H or (C1-C12)al kyl, or, alternatively, Ri or R2 and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4 are attached form a carbocyclic or heterocyclic ring,

[0015] Re is -CN, -NO2, aryl, or -L-Ra, wherein L is -(C=O)-, -(C=O)O-, or -(C=O)NR’-, wherein R’ is H or (Ci-Ci2)alkyl, and Rais H, (Ci-Ci2)alkyl, (C2-Ci2)alkenyl, (C2- Ci2)alkynyl, or aryl; with a heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive, thereby forming the corresponding compound comprising a monoene, which has the structure of formula (II),

[0016] RI-CHR2-CHR3-CR4=CR5-R6 (II), wherein R1 to Re are as defined above.

[0017] In a second aspect, the present disclosure relates to the use of a heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive for selectively hydrogenating a compound comprising a conjugated diene, which has the structure of formula (I),

[0018] RI-CR2=CR3-CR4=CR5-R6 (I), wherein

[0019] R1 and R2 are each, independently, H, (Ci-Ci2)alkyl, or aryl,

[0020] R3, R4, and R5 are each, independently, H or (C1-C13)al kyl, or, alternatively, R1 or R2 and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3and R4are attached form a carbocyclic or heterocyclic ring,

[0021] Re is -CN, -NO2, aryl, or -L-Ra, wherein L is -(C=O)-, -(C=O)O-, or -(C=O)NR’-, wherein R’ is H or (Ci-Ci2)alkyl, and Rais H, (Ci-Ci2)alkyl, (C2-Ci2)alkenyl, (C2- Ci2)alkynyl, or aryl; to form the corresponding compound comprising a monoene, which has the structure of formula (II), RI-CHR2-CHR3-CR4=CR5-R6 (II), wherein Ri to Re are as defined above.

[0022] In a third aspect, the present disclosure relates to a catalyst system for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene, the catalyst system comprising a heterogeneous palladium-containing catalyst and an additive.

[0023] Brief Description of the Figures

[0024] FIG. 1 shows the hydrogenation of an exemplary substrate in which the L and Ragroups are varied with corresponding beta isomer product selectivity (GC %) as a function of conversion (GC %).

[0025] FIG. 2 shows the various dithioether additives used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0026] FIG. 3 shows the various dithioether additives and amounts of the additives used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0027] FIG. 4 shows the various sterically-hindered dithioether additives used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0028] FIG. 5 shows the various sterically-hindered dithioether additives and amounts of the said additives used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GC %). FIG. 6 shows the various sulfur-containing additives used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GO %).

[0029] FIG. 7 shows the various temperatures used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0030] FIG. 8 shows the various hydrogen pressures used to hydrogenate safranone in an exemplary process according to some aspects of the present disclosure with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0031] FIG. 9 shows the various sulfur-containing additives and amounts of said additives used to hydrogenate ethyl damascenate in an exemplary process according to some aspects of the present disclosure with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %).

[0032] FIG. 10 shows the various nitrogen-containing additives used to hydrogenate ethyl damascenate in an exemplary process according to some aspects of the present disclosure with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %).

[0033] FIG. 11 shows the various nitrogen-containing additives used to hydrogenate ethyl damascenate in an exemplary process according to some aspects of the present disclosure with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %).

[0034] FIG. 12 shows the various nitrogen-containing additives used to hydrogenate ethyl damascenate in an exemplary process according to some aspects of the present disclosure with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %).

[0035] FIG. 13 shows the various solvents used to hydrogenate ethyl damascenate in an exemplary process according to some aspects of the present disclosure with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %).

[0036] Detailed Description

[0037] As used herein, the terms “a”, “an”, or “the” means “one or more” or “at least one” unless otherwise stated.

[0038] While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of’ or “consist of’ the various components, substances and steps. As used herein the term “consisting essentially of’ shall be construed to mean including the listed components, substances or steps and such additional components, substances or steps which do not materially affect the basic and novel properties of the composition or method. In some embodiments, a composition in accordance with embodiments of the present disclosure that “consists essentially of’ the recited components or substances does not include any additional components or substances that alter the basic and novel properties of the composition.

[0039] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this specification pertains.

[0040] It should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations. As used herein, and unless otherwise indicated, the term “about” or “approximately” means an acceptable error for a particular value as determined by one of ordinary skill in the art, which depends in part on how the value is measured or determined. In certain embodiments, the term “about” or “approximately” means within 1 , 2, 3, or 4 standard deviations. In certain embodiments, the term “about” or “approximately” means within 50%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range.

[0041] Throughout the present disclosure, various chemical names and structures may be recited. Unless otherwise stated, any stereoisomers, such as enantiomers, diastereomers, anomers, epimers, and the like; and geometric isomers, such as cis / trans or E / Z isomers, of the recited chemical name or structure are contemplated. As would be understood by those of ordinary skill in the art, stereoisomers may possess one stereocenter, giving rise to enantiomers, or more than one stereocenter, giving rise to diastereomers, each stereocenter having one of two different stereochemistries (i.e. , R or S). Enantiomers may be characterized by their ability to rotate oncoming plane-polarized light to the right, designated as dextrorotatory, “(+)” or“D”, or to the left, designated as levorotatory, or“L”. Enantiomers may exist as racemic mixtures or scalemic mixtures. Geometric isomers refer to isomers in which the spatial relationship of atoms around a double bond are different, typically designated E or Z according to conventional understanding in the chemical art. Geometric isomers may also exist as mixtures of E and Z isomers. All of the aforementioned isomeric variations of the chemical names or structures recited herein are included.

[0042] As used herein, the terminology "(Cx-Cy)", “Cx-Cy”, or“Cx-y” in reference to an organic group, wherein x and y are each integers, means that the group may contain from x carbon atoms to y carbon atoms per group.

[0043] As used herein, the term "alkyl" means a monovalent straight or branched saturated hydrocarbon radical, more typically, a monovalent straight or branched saturated (C1-C22) hydrocarbon radical, such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, hexyl, octyl, hexadecyl, octadecyl, eicosyl, behenyl, tricontyl, and tetracontyl. As used herein, the term "alkenyl" means an unsaturated straight or branched hydrocarbon radical, more typically an unsaturated straight, branched, (C2-C22) hydrocarbon radical, that contains one or more carbon-carbon double bonds, including, for example, ethenyl (vinyl), n-propenyl, and iso-propenyl, and allyl.

[0044] As used herein, the term "aryl" means a monovalent unsaturated hydrocarbon radical containing one or more six-membered carbon rings in which the unsaturation may be represented by three conjugated double bonds. Aryl radicals include monocyclic aryl and polycyclic aryl. “Polycyclic aryl” refers to a monovalent unsaturated hydrocarbon radical containing more than one six-membered carbon ring in which the unsaturation may be represented by three conjugated double bonds wherein adjacent rings may be linked to each other by one or more bonds or divalent bridging groups or may be fused together. Aryl radicals may be substituted at one or more carbons of the ring or rings. Examples of aryl radicals include, but are not limited to, phenyl, methylphenyl, isopropylphenyl, tert-butylphenyl, methoxyphenyl, dimethylphenyl, trimethylphenyl, chlorophenyl, trichloromethylphenyl, triisobutyl phenyl, anthracenyl, naphthyl, phenanthrenyl, fluorenyl, and pyrenyl.

[0045] As used herein, the term “carbocycle” or “carbocyclic ring” refers to a saturated or partially unsaturated cyclic hydrocarbon ring structure. Examples of carbocyclic rings include, but are not limited to, cyclooctadiene, cyclohexadiene, cyclohexene, cyclohexane, cyclooctatriene, and the like.

[0046] As used herein, the term “heterocycle” or “heterocyclic ring” refers to a saturated or partially unsaturated cyclic ring structure that includes one or more non-carbon atoms in the ring. Non-carbon atoms include, but are not limited to, O, N, S, and the like. Examples of heterocyclic rings include, but are not limited to, morpholine, piperadine, piperazine, pyrroline, pyrazole, and pyrrolidine.

[0047] Any substituent or radical described herein may optionally be substituted at one or more carbon atoms with one or more, same or different, substituents described herein. For instance, an alkyl group may be further substituted with an aryl group or another alkyl group. Any substituent or radical described herein may also optionally be substituted at one or more carbon atoms with one or more substituents selected from the group consisting of halogen, such as, for example, F, Cl, Br, and I; nitro (NO2), cyano (CN), amino (NH2), and hydroxy (OH).

[0048] Throughout the present disclosure, various publications may be incorporated by reference. Should the meaning of any language in such publications incorporated by reference conflict with the meaning of the language of the present disclosure, the meaning of the language of the present disclosure shall take precedence, unless otherwise indicated.

[0049] In the first aspect, the present disclosure relates to a process for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene, the process comprising reacting the compound comprising a conjugated diene, which has the structure of formula (I),

[0050] RI-CR2=CR3-CR4=CR5-R6 (I), wherein

[0051] R1 and R2 are each, independently, H, (Ci-Ci2)alkyl, or aryl,

[0052] R3, R4, and R5 are each, independently, H or (C1-C12)al kyl, or, alternatively, R1 or R2 and Rs together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4 are attached form a carbocyclic or heterocyclic ring,

[0053] Re is -CN, -NO2, aryl, or -L-Ra, wherein L is -(C=O)-, -(C=O)O-, or -(C=O)NR’-, wherein R’ is H or (Ci-Ci2)alkyl, and Rais H, (Ci-Ci2)alkyl, (C2-Ci2)alkenyl, (C2- Ci2)alkynyl, or aryl; with a heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive, thereby forming the corresponding compound comprising a monoene, which has the structure of formula (II),

[0054] RI-CHR2-CHR3-CR4=CR5-R6 (II), wherein R1 to Re are as defined above. The compound according to formula (I) acts as the substrate to be selectively hydrogenated. According to the present disclosure, the process selectively hydrogenates the y-6 C=C bond of the diene, which is conjugated to Re within the compound. As would be understood by those of ordinary skill in the art, the y-6 C=C bond refers to the C=C bond farthest from Re. The process results in the formation of the corresponding monoene, which is represented by the structure of formula (II).

[0055] In an embodiment, Ri, R4, and R5 are not H, and R2 and R3 are H.

[0056] In another embodiment, R1 or R2 and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4 are attached form a carbocyclic or heterocyclic ring.

[0057] In an embodiment, the compound comprising a conjugated diene has the structure of formula (la), wherein Rb and Rcare each, independently, (Ci-Ci2)alkyl, and R1 to Re are as defined above, and the corresponding compound comprising a monoene has the structure of formula (Ila) wherein Rb, Rc, and R1 to Re are as defined above. In an embodiment, R2 and R3 is H, and R4 is not H.

[0058] In an embodiment, Re is -L-Ra, wherein L is -(C=O)- or -(C=O)O-, and Rais H, (C1- Ci2)alkyl or (C2-Ci2)alkenyl.

[0059] In another embodiment, the compound comprising a conjugated diene is selected from the group consisting of:

[0060] In an embodiment, the corresponding compound comprising a monoene is selected from the group consisting of:

[0061] It would be understood by those of ordinary skill in the art that the desired reaction product, i.e. , the compound of formula (II), may be formed along with other side reaction products, and that some residual starting material, i.e., the compound of formula (I), may remain at the end of the reaction, resulting in a mixture. Therefore, mixtures comprising the compound of formula (II) in combination with side reaction products, residual starting material, or both, are contemplated in the present disclosure.

[0062] According to the present disclosure, the heterogeneous palladium-containing catalyst facilitates the hydrogenation reaction. Suitable heterogeneous palladium- containing catalysts are those comprising palladium deposited on calcium carbonate or barium sulfate, typically combined with various forms of lead or sulfur. In an embodiment, the heterogeneous palladium-containing catalyst is a Lindlar catalyst. In another embodiment, the heterogeneous palladium-containing catalyst comprises palladium deposited on calcium carbonate and combined with lead. In some embodiments, the heterogeneous palladium-containing catalyst comprises (i) 85 to 99.85%, typically 89 to 96%, by weight of CaCOs, based on the total weight of the catalyst, (ii) 0.1 to 10%, typically 3 to 7%, by weight of Pd, based on the total weight of the catalyst, (iii) 0.05 to 10%, typically 1 to 5%, by weight of Pb, based on the total weight of the catalyst. Such heterogeneous palladium-containing catalysts are commercially available and include, but are not limited to, A310050-5, A305050-5, A304060-5, A305060-5, and A306060-5 (available from Johnson Matthey), Noblyst® P8059 (available from Evonik), and the like.

[0063] The amount of the heterogeneous palladium-containing catalyst is not particularly limited. However, good results may be obtained using less than stoichiometric amounts. In an embodiment, the heterogeneous palladium-containing catalyst is present in a catalytic amount. In another embodiment, the heterogeneous palladium-containing catalyst is present such that palladium is present in an amount of from 0.005 to 0.4 mol.%, typically 0.01 to 0.4 mol %, more typically from 0.01 to 0.2 mol %, relative to the amount of the compound comprising a conjugated diene. In an embodiment, the heterogeneous palladium-containing catalyst is present such that palladium is present in an amount of from 0.005 to 0.3 mol.%, typically 0.006 to 0.2 mol %, more typically from 0.007 to 0.1 mol %, relative to the amount of the compound comprising a conjugated diene. In some embodiments, the heterogeneous palladium-containing catalyst is present such that palladium is present in an amount of from 0.008 to 0.4 mol.%, typically 0.009 to 0.2 mol %, more typically from 0.009 to 0.1 mol %, relative to the amount of the compound comprising a conjugated diene.

[0064] The pressure of hydrogen gas used in the process described herein is not particularly limited. However, in some embodiments, the hydrogen gas is present at a pressure of from 1 to 100 bars, typically 1 to 50 bars, more typically 1 to 30 bars. The process of the present disclosure includes the use of an additive. The additive may be a nitrogen-containing compound, a phosphorus-containing compound, a sulfur-containing compound, or any combination thereof. In an embodiment, the additive is a nitrogen-containing compound, a sulfur-containing compound, or a combination thereof, that does not comprise phosphorus. In an embodiment, the additive is a phosphorus-containing compound.

[0065] In an embodiment, the additive is a nitrogen-containing compound, typically selected from the group consisting of diamines, typically ethylenediamine, trans-1 ,2- diaminocyclohexane, 1 ,3-diaminopropane, 1 ,4-diaminobutane, 1 ,6-diaminohexane, N,N-dimethylethylenediamine, N,N’-dimethylethylenediamine and N,N,N’,N’- tetramethylethylenediamine; polyamines, typically diethylenetriamine, pentaethylenehexamine, and tris(2-aminoethyl)amine; and monoamines, typically butylamine, decylamine, dipropylamine and triethylamine; nitrogen-containing heterocyclic compounds, typically quinoline, 2,2’-bipyridine, 1 ,10-phenanthroline, 2,9-dimethyl-1 ,10-phenanthroline, and 4,7-diphenyl-1 ,10-phenanthroline, pyridine, 2- aminopyridine, 2-picolylamine, and any combination thereof.

[0066] In an embodiment, the additive is a phosphorus-containing compound. Exemplary phosphorus-containing compounds include, but are not limited to, phosphines, such as monophosphines of the formula PR3, wherein each occurrence of R is a C1-C12 group, such as linear, branched or cyclic alkyl, alkoxy or aryloxy group which may be optionally substituted; substituted or unsubstituted phenyl, diphenyl, 2-furanyl, naphthyl, or di-naphthyl group. Exemplary monophosphines include, but are not limited to, trimethylphosphine, tributylphosphine, triphenylphosphine, trioctylphosphine, triisobutylphosphine, triethylphosphine, 2-dicyclohexylphosphino- 2’,4’,6’-triisopropylbiphenyl, diphenylphosphine, tricyclohexylphosphine, 2- dicyclohexylphosphino-2’,6’-dimethoxybiphenyl, tri-tert-butylphosphine, tri(o- tolyl)phosphine, 2-(dicyclohexylphosphino)-3,6-dimethoxy-2’,4’,6’-triisopropyl-1 ,T- biphenyl, 2-d i-tert-butyl phosph ino-2’ ,4’ ,6’-triisopropyl biphenyl , di( 1 -adamantyl)-n- butylphosphine, (2-biphenyl)di-tert-butylphosphine, 2-dicyclohexylphosphino-2’-(N,N- dimethylamino)biphenyl, 2-(di-tert-butylphosphino)-2’,4’,6’- triisopropyl-3,6- dimethoxy-1 , 1 ’-biphenyl, (2-biphenyl)dicyclohexylphosphine, (4-(N,N- dimethylamino)phenyl)di-tert-butyl phosphine, and the like. In an embodiment, the additive is triphenylphosphine.

[0067] In an embodiment, the additive is a sulfur-containing compound. Exemplary sulfur- containing compounds include, but are not limited to, compounds selected from the group consisting of 3,6-dithia-1 ,8-octanediol; dithioethers, typically 1 ,8- bis(ethylthio)octane, 1 ,5-bis(ethylthio)pentane, 1 ,4- bis(ethylthio)butane, 1 ,3- bis(ethylthio)propane, 1 ,2-bis(ethylthio)ethane, and 1 ,2-bis(t-butylthio)ethane; monothiols, typically dodecanethiol; dithiols, typically 2,2'- (ethylenedioxy)diethanethiol and 1 ,8-octanedithiol, and any combination thereof.

[0068] The amount of the additive used in the process described herein is not particularly limited. However, in an embodiment, the additive is present in an amount of from 0.1 to 15 mol %, typically 2.5 to 10 mol%, relative to the amount of palladium present. In another embodiment, the additive is present in an amount of from 3 to 7 mol %, typically 4 to 6 mol%, relative to the amount of palladium present. In another embodiment, the additive is present in an amount of from 1000 to 4000 mol.%, relative to the amount of palladium present. In yet another embodiment, the additive is present in an amount of from 1000 to 5000 mol.%, relative to the amount of palladium present.

[0069] The selective hydrogenation may be conducted with or without solvent. In an embodiment, the process is conducted using solvent, typically selected from the group consisting of methanol, ethanol, butanol, acetone, ethyl acetate, DMF, toluene, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, methyl tert-butyl ether, or a combination thereof. In another embodiment, the process is conducted in the absence of any solvent, such as those described herein.

[0070] The process may be conducted over a broad range of temperatures. A person of ordinary skill in the art would be able to select the suitable temperature as a function of the melting and boiling point as well as of the specific properties of any solvents used as well as the desired time of reaction or conversion. However, in an embodiment, the process is conducted at a temperature of from 10 to 150 °C, typically 20 to 120 °C, more typically 60 to 100 °C. In another embodiment, the process is conducted at a temperature of from 40 to 100 °C.

[0071] The hydrogenation process may be characterized by selectivity for the desired monoene product, which is the compound of formula (II). As used herein, selectivity is defined as the (amount of compound of formula (II) x 100) I (initial amount of compound of formula (I) - amount of residual amount of compound of formula (I)). The amount of compound of formula II, initial amount of compound of formula (I), and amount of residual amount of compound of formula (I) may be quantified using any technique known to those of ordinary skill in the art, such as gas chromatography. In an embodiment, the selectivity is at least 75%, at least 85%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99%.

[0072] Upon reaction completion, the desired products may be isolated using methods known to those of ordinary skill in the art. For example, the reaction vessel may be cooled down, depressurized, and then purged with nitrogen. The reaction mixture may then be filtered to remove the spent catalyst. After light compounds are evaporated under vacuum, typically when solvent is used, the crude product is then flash distilled to determine the quantity of residues formed during the reaction and yield can then calculated based the amount of distilled product.

[0073] In the second aspect, the present disclosure relates to the use of a heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive for selectively hydrogenating a compound comprising a conjugated diene, which has the structure of formula (I),

[0074] RI-CR2=CR3-CR4=CR5-R6 (I), wherein

[0075] Ri and R2 are each, independently, H, (Ci-Ci2)alkyl, or aryl, R3, R4, and R5 are each, independently, H or (C1-C12)al kyl, or, alternatively, R1 or R2 and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4 are attached form a carbocyclic or heterocyclic ring, Re is -CN, -NO2, aryl, or -L-Ra, wherein L is -(C=O)-, -(C=O)O-, or -(C=O)NR’-, wherein R’ is H or (Ci-Ci2)alkyl, and Rais H, (Ci-Ci2)alkyl, (C2-Ci2)alkenyl, (C2- Ci2)alkynyl, or aryl; to form the corresponding compound comprising a monoene, which has the structure of formula (II),

[0076] RI-CHR2-CHR3-CR4=CR5-R6 (II), wherein R1 to Re are as defined above.

[0077] In a third aspect, the present disclosure relates to a catalyst system for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene, the catalyst system comprising a heterogeneous palladium-containing catalyst and an additive.

[0078] The compound comprising a conjugated diene and the corresponding compound comprising a monoene are those compounds described herein and the features are applied here, mutatis mutandis. The catalyst system comprises the heterogeneous palladium-containing catalyst and the additive as described herein and the features are applied here, mutatis mutandis.

[0079] The processes and catalysts according to the present disclosure are further illustrated by the following non-limiting examples.

[0080] Examples

[0081] Unless otherwise indicated, the following general procedure was used for the selective hydrogenation reaction. A compound according to formula (I), solvent, when used, heterogeneous palladium-containing catalyst, and additive were loaded together into a 100 mL or 1 L autoclave equipped with a mechanical stirring device including a hollow shaft for efficient hydrogen gas injection into the liquid phase, pressure and temperature internal sensors, a dip tube for reaction mixture sampling during the reaction and some heating / cooling system for internal temperature regulation. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 25°C for 20 to 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bars) before being pressurized to the required hydrogen pressure via a hydrogen tank equipped with a way-out pressure regulator and an internal pressure sensor to follow and determine hydrogen consumption. The reaction mixture was then heated to required temperature under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. Hydrogenation reaction progress was monitored by both hydrogen gas consumption and GC analysis of reaction mixture samples obtained using autoclave dip tube. Upon reaction completion, autoclave was then cooled down to 20°C. It was then depressurized and purged with nitrogen (3 times, 5 bars) and reaction mixture was then transferred to a round-bottomed flask upon spent catalyst filtration. After light compounds evaporation under vacuum, when solvent was used, crude product was then flash distilled to determine the quantity of residues formed during the reaction and yield was then calculated based on GC selectivity of distilled product.

[0082] Example 1

[0083] A series of compounds complying with the formula in which L and Rawere varied, were subjected to the hydrogenation process of the present disclosure to form the corresponding beta isomer. For each compound, the compound, the catalyst (Lindlar catalyst A-310050-5 obtained from Johnson- Matthey, 0.0772-0.0912 mol% Pd, 1 wt%), and additive (3,6-dithia-1 ,8-octanediol; 5 mol% I Pd) were loaded together into a 100 mL or 1 L autoclave equipped as described hereinabove. The reaction was conducted neat. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (1 bar to 50 bars). The reaction mixture was then heated to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. Hydrogenation reaction progress was monitored by both hydrogen gas consumption and GC analysis of reaction mixture samples obtained using autoclave dip tube. The reaction was worked-up as described hereinabove. FIG. 1 shows the L and Ragroups with corresponding beta isomer product selectivity (GC %) as a function of conversion (GC %).

[0084] Example 2

[0085] Safranone (1-(2,6,6-trimethylcyclohexa-1 ,3-dien-1-yl)ethan-1-one) was subjected to the hydrogenation process of the present disclosure to form the corresponding product ( “BMK”, 1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethan-1-one). A series of dithioether additives were used. The compound, the catalyst (Lindlar catalyst A- 310050-5 obtained from Johnson-Matthey, 0.0772 mol% Pd, 1 wt%), and additive (5 mol% I Pd) were loaded together into a 100 mL or 1 L autoclave equipped as described hereinabove. The reaction was conducted neat. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (30 bars). The reaction mixture was then heated to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. FIG. 2 shows the various dithioether additives with corresponding BMK product selectivity (GC %) as a function of conversion (GC %). The BMK product selectivity resulting from the use of 3,6-dithia-1 ,8-octanediol as the additive is also shown. Example 3

[0086] The process of Example 2 was conducted, except that the amount of the additive was used at 5 mol% / Pd or 15 mol% I Pd. FIG. 3 shows the various dithioether additives and amounts of the additives used with corresponding BMK product selectivity (GC %) as a function of conversion (GO %).

[0087] Example 4

[0088] The process of Example 2 was conducted, except that a series of sterically-hindered dithioethers were used. FIG. 4 shows the various sterically-hindered dithioether additives used with corresponding BMK product selectivity (GC %) as a function of conversion (GC %). The BMK product selectivity resulting from the use of 3, 6-dithia- 1 ,8-octanediol as the additive is also shown.

[0089] Example 5

[0090] The process of Example 4 was conducted, except that the amount of the additive was used at 5 mol% / Pd or 15 mol% I Pd. FIG. 5 shows the various dithioether additives and amounts of the additives used with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0091] Example 6

[0092] The process of Example 2 was conducted, except that a series of sulfur-containing additives were used. FIG. 6 shows the various sulfur-containing additives used with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0093] Example 7

[0094] The process of Example 2 was conducted, except that the amount of the catalyst was modified to 0.0193 mol% (0.25 wt%), the additive was replaced with 1 ,10- phenanthroline (10 mol. equiv / Pd), and the temperature was varied from 40°C to 80°C. FIG. 7 shows the various temperatures used with corresponding BMK product selectivity (GC %) as a function of conversion (GO %).

[0095] Example 8

[0096] The process of Example 7 was conducted, except that the temperature was fixed at 80°C and the hydrogen pressure was 10, 30, or 50 bars. FIG. 8 shows the various hydrogen pressures used with corresponding BMK product selectivity (GC %) as a function of conversion (GC %).

[0097] Example 9

[0098] Ethyl damascenate (ethyl 2, 6, 6-trimethylcyclohexa-1 ,3-diene-1 -carboxylate) was subjected to the hydrogenation process of the present disclosure to form the corresponding product, beta ethyl cyclogeraniate (ethyl 2,6,6-trimethylcyclohex-1- ene-1 -carboxylate). A series of sulfur-containing additives were used. Ethyl damascenate, the catalyst (Lindlar catalyst A-310050-5 obtained from Johnson- Matthey, 0.0912 mol% Pd, 1 wt%), and additive (5 mol% / Pd or 15 mol% I Pd) were loaded together into a 100 mL or 1 L autoclave equipped as described hereinabove. The reaction was conducted neat. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (50 bars). The reaction mixture was then heated to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. FIG. 9 shows the various sulfur-containing additives and amounts of additive with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %).

[0099] Example 10 The process of Example 9 was conducted, except that the additive was replaced with a first series of nitrogen-containing additives (10 mol. equiv I Pd), and the reaction temperature was set to 40°C. FIG. 10 shows the various nitrogencontaining additives used with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GO %). The beta ethyl cyclogeraniate product selectivity resulting from the use of 3,6-dithia-1 ,8-octanediol as the additive is shown as a reference.

[0100] Example 11

[0101] The process of Example 10 was conducted, except that the additive was replaced with a second series of nitrogen-containing additives (10 mol. equiv I Pd). FIG. 11 shows the various nitrogen-containing additives used with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %). The beta ethyl cyclogeraniate product selectivity resulting from the use of 3,6-dithia-1 ,8- octanediol as the additive is shown as a reference.

[0102] Example 12

[0103] The process of Example 10 was conducted, except that the additive was replaced with a third series of nitrogen-containing additives (10 mol. equiv I Pd). FIG. 12 shows the various nitrogen-containing additives used with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %). The beta ethyl cyclogeraniate product selectivity resulting from the use of 3,6-dithia-1 ,8- octanediol as the additive is shown as a reference.

[0104] Example 13

[0105] The process of Example 10 was conducted, except that the additive was replaced with 1 , 10-phenanthroline (10 mol. equiv I Pd) and various solvents were used. FIG. 13 shows the various solvents used with corresponding beta ethyl cyclogeraniate product selectivity (GC %) as a function of conversion (GC %). The beta ethyl cyclogeraniate product selectivity resulting from the neat process is also shown. Example 14

[0106] Beta damascenone ((E)-1-(2,6,6-trimethylcyclohexa-1 ,3-dien-1-yl)but-2-en-1-one) was subjected to the hydrogenation process of the present disclosure to form the corresponding product, dorinone beta ((E)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)but-2- en-1 -one). Beta damascenone, the catalyst (Lindlar catalyst A-310050-5 obtained from Johnson-Matthey, 0.089-0.357 mol% Pd, 1-4 wt%), additive (3,6-dithia-1 ,8- octanediol, 0 or 5 mol% I Pd), and solvent (when used) were loaded together into a 100 mL or 1 L autoclave equipped as described hereinabove. The reaction was conducted neat or in AcOEt solvent. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (1-15 bars). The reaction mixture was then heated to 20°C to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. Table 1 below summarizes the reaction conditions and resulting selectivity for dorinone beta.

[0107] Table 1. conducted in same manner as Ex. C3 but with a plain shaft as opposed to hollow shaft

[0108] As shown in Table 1 , the process according to the present disclosure in which catalyst in combination with the additive was used in the absence of solvent resulted in dorinone beta selectivities greater than 90% (Ex. A-C). For comparison, when neither additive nor solvent was used, selectivities were very low (Comparative Ex. C1 and C2). For further comparison, when solvent, but no additive, was used according to a known process (G. Buchi and H. Wiiest (Helv. Chim. Acta 54, 1971 , 1767-1776)), selectivities were also low (Comparative Ex. C3 and C4).

[0109] Example 15

[0110] Ethyl damascenate (ethyl 2, 6, 6-trimethylcyclohexa-1 ,3-diene-1 -carboxylate) was subjected to the hydrogenation process of the present disclosure to form the corresponding product, beta ethyl cyclogeraniate (ethyl 2,6,6-trimethylcyclohex-1-ene- 1 -carboxylate). Ethyl damascenate, the catalyst (Lindlar catalyst A-310050-5 obtained from Johnson-Matthey, 0.0092-0.092 mol% Pd, 0.1-1 wt%) and additive (3,6-dithia- 1 ,8-octanediol, 0 or 5 mol.% / Pd or 1 ,10-phenanthroline, 0 or 10 mol. equiv. / Pd) were loaded together without solvent into a 100 mL or 1 L autoclave equipped as described hereinabove. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (1-50 bars). The reaction mixture was then heated to 40°C to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. Table 2 below summarizes the reaction conditions and resulting selectivity for beta ethyl cyclogeraniate.

[0111] Table 2.

[0112] As shown in Table 2, the process according to the present disclosure in which catalyst in combination with 3 ,6-dithia- 1 ,8-octanediol or 1 , 10-phenanthroline additive was used resulted in beta ethyl cyclogeraniate selectivities greater than 95% (Ex. D-H). For comparison, when no additive was used, selectivities were significantly lower (comparative Ex. C5-C10).

[0113] Example 16

[0114] Methyl damascenate (methyl 2, 6, 6-trimethylcyclohexa-1 ,3-diene-1 -carboxylate) was subjected to the hydrogenation process of the present disclosure to form the corresponding product, beta methyl cyclogeraniate (methyl 2,6,6-trimethylcyclohex-1- ene-1 -carboxylate). Methyl damascenate, the catalyst (Lindlar catalyst A-3100050-5 obtained from Johnson-Matthey, 0.0085-0.085 mol% Pd, 0.1-1 wt%) and additive (3,6- dithia-1 ,8-octanediol, 0 or 5 mol.% I Pd or 1 ,10-phenanthroline, 0 or 10 mol. equiv. I Pd) were loaded together without solvent into a 100 mL or 1 L autoclave equipped as described hereinabove. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (1-50 bars). The reaction mixture was then heated to 40°C to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. Table 3 below summarizes the reaction conditions and resulting selectivity for beta methyl cyclogeraniate.

[0115] Table 3.

[0116] As shown in Table 3, the process according to the present disclosure in which catalyst in combination with 3,6-dithia-1 ,8-octanediol or 1 ,10-phenantroline additive was used resulted in beta methyl cyclogeraniate selectivities greater than 93% (Ex. I-M). For comparison, when no additive was used, selectivities were significantly lower (comparative Ex. C11-C13).

[0117] Example 17

[0118] Safranone (1-(2,6,6-trimethylcyclohexa-1 ,3-dien-1-yl)ethan-1-one)) was subjected to the hydrogenation process of the present disclosure to form the corresponding product, “BMK” (1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethan-1-one)). Safranone, the catalyst (Lindlar catalyst A-3100050-5 obtained from Johnson-Matthey, 0.0077-0.077 mol% Pd, 0.1-1 wt%) and additive (3,6-dithia-1 ,8-octanediol, 0 or 5 mol. % / Pd or 1 ,10- phenantroline, 0 or 10 mol. equiv. I Pd) were loaded together without solvent into a 100 mL or 1 L autoclave equipped as described hereinabove. The autoclave was sealed and then purged with nitrogen without stirring (3 times, 5 bars) and then under stirring (3 times, 5 bars). The autoclave was then pressurized with nitrogen (1 bar) and left stirring at 20°C to 30°C for 30 minutes for a catalyst activity moderation phase. Upon catalyst activity moderation phase completion, autoclave was then purged with hydrogen under stirring (3 times, 1 bar) before being pressurized to the required hydrogen pressure (1-50 bars). The reaction mixture was then heated to 40°C to 80°C under required hydrogen pressure, the autoclave being continuously maintained to this hydrogen pressure value during the whole reaction. Table 4 below summarizes the reaction conditions and resulting selectivity for “BMK”.

[0119] Table 4.

[0120] As shown in Table 4, the process according to the present disclosure in which catalyst in combination with 3,6-dithia-1 ,8-octanediol or 1 ,10-phenanthroline additive was used resulted in “BMK” selectivities greater than 94% (Ex. N-R). For comparison, when no additive was used, selectivities were noticeably lower (comparative Ex. C14-C17).

[0121] The disclosed subject matter has been described with reference to specific details of particular embodiments thereof. It is not intended that such details be regarded as limitations upon the scope of the disclosed subject matter except insofar as and to the extent that they are included in the accompanying claims.

[0122] Therefore, the exemplary embodiments described herein are well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the exemplary embodiments described herein may be modified and practiced in different but equivalent manners apparent to those of ordinary skill in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the exemplary embodiments described herein. The exemplary embodiments described herein illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and / or any optional element disclosed herein.

Claims

WHAT IS CLAIMED IS:1 . A process for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene, the process comprising reacting the compound comprising a conjugated diene, which has the structure of formula (I),RI-CR2=CR3-CR4=CR5-R6 (I), whereinRi and R2are each, independently, H, (Ci-Ci2)alkyl, or aryl,R3, R4, and R5 are each, independently, H or (C1-C12)al kyl, or, alternatively, R1 or R2and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4 are attached form a carbocyclic or heterocyclic ring,Re is -CN, -NO2, aryl, or -L-Ra, wherein L is -(C=O)-, -(C=O)O-, or -(C=O)NR’-, wherein R’ is H or (Ci-Ci2)alkyl, and Rais H, (Ci-Ci2)alkyl, (C2-Ci2)alkenyl, (C2- Ci2)alkynyl, or aryl; with a heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive, thereby forming the corresponding compound comprising a monoene, which has the structure of formula (II),RI-CHR2-CHR3-CR4=CR5-R6 (II), wherein R1 to Re are as defined above.

2. The process according to claim 1 , wherein R1 or R2and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4 are attached form a carbocyclic or heterocyclic ring.

3. The process according to claim 2, wherein the compound comprising a conjugated diene has the structure of formula (la),(la), wherein Rb and Rcare each, independently, (Ci-Ci2)alkyl, andRi to Re are as defined above, and the corresponding compound comprising a monoene has the structure of formula (Ila)wherein Rb, Rc, and Ri to Re are as defined above.

4. The process according to claim 3, wherein Re is -L-Ra, wherein L is -(C=O)- or -(C=O)O-, and Rais H, (Ci-Ci2)alkyl or (C2-Ci2)alkenyl.

5. The process according to claim 4, wherein the compound comprising a conjugated diene is selected from the group consisting of:

6. The process according to any one of claims 1 to 5, wherein the heterogeneous palladium-containing catalyst is a Lindlar catalyst.

7. The process according to any one of claims 1 to 6, wherein palladium is present in a catalytic amount, typically in an amount of from 0.005 to 0.4 mol.%.

8. The process according to any one of claims 1 to 7, wherein the hydrogen gas is present at a pressure of from 1 to 100 bars, typically 1 to 50 bars, more typically 1 to 30 bars.

9. The process according to any one of claims 1 to 8, wherein the additive is a nitrogen-containing compound, typically selected from the group consisting of diamines, typically ethylenediamine, trans-1 ,2-diaminocyclohexane, 1 ,3- diaminopropane, 1 ,4-diaminobutane, 1 ,6-diaminohexane, N,N- dimethylethylenediamine, N,N’-dimethylethylenediamine and N,N,N’,N’- tetramethylethylenediamine; polyamines, typically diethylenetriamine, pentaethylenehexamine, and tris(2-aminoethyl)amine; and monoamines, typically butylamine, decylamine, dipropylamine and triethylamine; nitrogen-containing heterocyclic compounds, typically quinoline, 2,2’-bipyridine, 1 ,10-phenanthroline, 2,9-dimethyl-1 ,10-phenanthroline, and 4,7-diphenyl-1 ,10-phenanthroline, pyridine, 2- aminopyridine, 2-picolylamine, and any combination thereof.

10. The process according to any one of claims 1 to 8, wherein the additive is a phosphorus-containing compound.11 . The process according to any one of claims 1 to 8, wherein the additive is a sulfur-containing compound, typically selected from the group consisting of 3,6- dithia-1 ,8-octanediol; dithioethers, typically 1 ,8-bis(ethylthio)octane, 1 ,5- bis(ethylthio)pentane, 1 ,4- bis(ethylthio)butane, 1 ,3-bis(ethylthio)propane, 1 ,2- bis(ethylthio)ethane, and 1 ,2-bis(t-butylthio)ethane; monothiols, typically dodecanethiol; dithiols, typically 2, 2'-(ethylenedioxy)diethanethiol and 1 ,8- octanedithiol, and any combination thereof.

12. The process according to any one of claims 1 to 11 , wherein the additive is present in an amount of from 0.1 to 15 mol %, typically 2.5 to 10 mol%, relative to the amount of palladium present or wherein the additive is present in an amount of from 1000 to 5000 mol.%, relative to the amount of palladium present.

13. The process according to any one of claims 1 to 12, wherein the process is conducted in the absence of any solvent.

14. Use of a heterogeneous palladium-containing catalyst in the presence of hydrogen gas and an additive for selectively hydrogenating a compound comprising a conjugated diene, which has the structure of formula (I),RI-CR2=CR3-CR4=CR5-R6 (I), whereinRi and R2are each, independently, H, (Ci-Ci2)alkyl, or aryl,Rs, R4, and R5 are each, independently, H or (C1-C12)al kyl, or, alternatively, R1 or R2and R5 together with the carbon atoms to which they are attached and the carbon atoms to which R3 and R4are attached form a carbocyclic or heterocyclic ring,Re is -CN, -NO2, aryl, or -L-Ra, wherein L is -(C=O)-, -(C=O)O-, or -(C=O)NR’-, wherein R’ is H or (Ci-Ci2)alkyl, and Rais H, (Ci-Ci2)alkyl, (C2-Ci2)alkenyl, (C2- Ci2)alkynyl, or aryl; to form the corresponding compound comprising a monoene, which has the structure of formula (II),RI-CHR2-CHR3-CR4=CR5-R6 (II), wherein R1 to Re are as defined above.

15. A catalyst system for selectively hydrogenating a compound comprising a conjugated diene to form the corresponding compound comprising a monoene, the catalyst system comprising a heterogeneous palladium-containing catalyst and anadditive, wherein the additive is a nitrogen-containing compound, a sulfur-containing compound, or a combination thereof.

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