WO2025191084A1 - Rhodium-carbonyl compounds for use as selective hydrogenation catalysts - Google Patents

Abstract

The present invention relates to rhodium compounds of the formula (I) [(L1)2Rh]+k[A]k- (I), where L1 is a bidentate phosphor-organic ligand, in particular a chiral bisphosphine ligands, preferably a ligand of the formula (II) as described herein, such as (2R,3R)-bis(diphenylphosphino)butane or (2S,3S)-bis(diphenylphosphino)butane, and A is a rhodium carbonyl anion optionally containing n mono- and bidentate ligands L2 and n is 0, 1 or 2, in particular 0, k is 1, 2 or 3; wherein [A]k- in formula (I) is preferably an anion of the formula [Rh(CO)4-n(L2)n]-, where L2 if present, is a monodentate organic ligand other than CO or, if n = 2, two ligands L2 may together form a bidentate organic ligand. The present invention further relates to the use of the rhodium compounds of the formula (I) as catalysts in a selective catalytic hydrogenation of an α,β-unsaturated olefinic double bond of an α,β-unsaturated carbonyl compound.

Description

Rhodium-carbonyl compounds for use as selective hydrogenation catalysts

Description

The present invention relates to rhodium-carbonyl compounds, in particular to ionic rhodium complexes, which contain at least one bidentate phosphor-organic ligand. The invention further relates to the preparation of these complexes and their use as catalysts for the selective catalytic hydrogenation of the a,p-unsaturated olefinic double bonds of a,p-unsaturated carbonyl compounds, in particular for the enantioselective hydrogenation of the a,p-unsaturated double bonds in prochiral a,p-unsaturated carbonyl compounds.

BACKGROUND OF THE INVENTION

Olefins can be hydrogenated by a variety of homogeneous catalysts using hydrogen. Rhodium-, iridium- or ruthenium-based transition metal complexes are usually used as noble metal catalysts. For a review, see H.-U. Blaser et al. in Applied Homogeneous Catalysis with Organometallic compounds, editors B. Cornils, W. A. Herrmann, M. Beller, R. Paciello, Wiley-VCH, New York, Vol. 3, 2018, 621-690.

Catalysts containing rhodium and chiral phosphine ligands are used for the enantioselective hydrogenation of a,p-unsaturated olefinic double bonds of a,p-unsaturated carbonyl compounds, e. g. for the enantioselective hydrogenation of citral. The active catalyst is usually prepared from suitable rhodium precursors, such as Rh(CO)2acac, and the desired chiral phosphine ligand, e. g. (2S,3S)-(- )-bis(diphenyl- phosphino)butane or (2 R,3R)-(+)-bis(diphenylphosphino)butane, which are referred to as (-)-chiraphos and (+)-chiraphos, respectively (see WO 2006/040096 and WO 2008/132057).

Frequently, the rhodium precursor, e. g. Rh(CO)2acac, and the chiral phosphine ligand, e. g. chiraphos, are fed separately into the reactor and converted to the active catalyst under H2/CO atmosphere. In case of Rh(CO)2acac and chiraphos, the formation of the active monohydridodicarbonyl complex (chiraphos)Rh(CO)2H from Rh(CO)2acac and chiraphos, has been reported, e. g. by Jakel et al. Adv. Synth. Catal 2008, 350, 2708-2714 and Paciello et al. in Asymmetric Catalysis on Industrial Scale: Challenges, Approaches and Solutions, editor H.-U. Blaser et al, Wiley-VCH, New York, 2010, 187-205).

Chapuis et al. describe in Helv. Chim. Acta, 2001, volume 84, pages 230 -242, 30 footnote 4, the asymmetric hydrogenation of geranial or neral to optically active citronellal in the presence of a catalyst based on Rh4(CO)i2 and (R,R)-chiraphos.

T.-P. Dang et al. describe in J. Mol. Cat., 1982, vol. 16, pages 51 - 59, a process for the homogeneous catalytic hydrogenation of a,p-unsaturated aldehydes using rhodium carbonyl compound, such as RhHCO(PPh3)3 or Rh6(CO)i6, and a chiral diphosphine as catalysts. However, only low conversion rates are reported. US 4,473,505 discloses Ethy l-linkages bound to heteroatoms which are suitable as ligands for use in catalytic processes for the conversion of hydrocarbons and carbon monoxide. Typically, the compounds bear a cationic tetra substituted phosphonium atom and a second phosphine group, the latter being capable of coordinating to transition metals.

The known methods have the disadvantage that a mixture of the rhodium precursor, e.g. Rh(CO)2acac, and the diphosphine ligand L, e.g. Chiraphos, can lead to a precipitation of Rh(L)2⁺ salts and thus to a loss of activity. For example, Paciello et al. have reported in Asymmetric Catalysis on Industrial Scale (loc. cit.) that very stable Rh(L)2acac can form from Rh(CO)2acac and bisphosphine ligands, which shows little catalytic activity (see Ch. Jakel, R. Paciello "The Asymmetric Hydrogenation of Enones - Access to a New L-Menthol Synthesis in H.-U. Blaser et al. (ed.) Asymmetric catalysis on Industrial Scale, 2^nd ed., Wiley VCH). Rh(L)2acac is formed instantaneously with CO release upon mixing the diphosphine ligand with Rh(CO)2acac. Indeed, Rh(L)2acac is often (for many different types of bisphosphines) poorly soluble and may precipitate when Rh(CO)2acac and bisphosphine ligands are mixed (see Journal of Molecular Catalysis A: Chemical 270, 2007, 241-249 when 1 ,2-bis(diphenylphosphino)ethane is used as a ligand). Many salts of type Rh(L)2⁺X- (X- are e. g. acac or BF4-; L refers to a bisphosphine ligand) were prepared by precipitation as described in the literature (see Canad. J. Chem. 1979, 180-187) and were described as ineffective hydrogenation catalysts. As a consequence when feeding into a reactor Rh(CO)2acac and bisphosphine ligands have to be kept strictly separated to avoid the described precipitation.

There is therefore a need for Rh-complexes with bidentate phosphor-organic ligands, in particular with chiral bisphosphine ligands, especially with chiraphos or structurally similar chiral bis(diphenylphosphine) ligands which overcome the problems described above. These Rh-complexes should ideally contain rhodium and the chiral diphosphine ligand in a molar ratio of 1 :1 and ideally not contain any interfering coligands such, such as COD or DBCOT (dibenzocyclooctatetraene), or counterions such as PFf, BARF (tetrakis{3,5-bis(trifluoromethyl)phenyl]borate or BF . The Rh-complexes should also be capable to be preformed to the catalytically active species in a H2/CO atmosphere. In particular, the Rh-complexes should ideally be producible from technically readily available precursors such as [Rh4(CO)i2], RhCla or Rh(CO)2acac. The Rh-complexes should be suitable as catalysts for the selective catalytic hydrogenation of the a,p-unsaturated olefinic double bonds of a,p-unsaturated carbonyl compounds, in particular for the enantioselective hydrogenation of the a,p-unsaturated double bonds in prochiral a,p-unsaturated carbonyl compounds. In particular, the Rh-complexes should be suitable as catalysts for the selective catalytic hydrogenation of cis-citral and trans-citral to the desired citronellal with high enantioselectivity and without isomerization.

SUMMARY OF THE INVENTION

It was surprisingly found that the above objectives are achieved by rhodium compounds of the formula (I) as described herein.

The present invention therefore relates to rhodium compounds of the formula (I) [(Li)₂Rh]*_k[A]k (l), where

L¹ is a bidentate phosphor-organic ligand, in particular a chiral bisphosphine ligands, preferably a ligand of the formula (II) as described herein, such as (2R,3R)-bis(diphenylphosphino)butane or (2S,3S)-bis(diphenylphosphino)butane, and

A is a rhodium carbonyl anion optionally containing n mono- and bidentate ligands L² and n is 0, 1 or 2, in particular 0, k is 1 , 2 or 3; wherein [A]^k- in formula (I) is preferably an anion of the formula [Rh (CO)4-_n( ²)_n]^_, where

L² if present, is a monodentate organic ligand other than CO, in particular a monodentate phosphine ligand, or, if n = 2, two ligands L² may together form a bidentate organic ligand, such as a bisphosphine ligand.

The present invention further relates to the use of the rhodium compounds of the formula (I) as catalysts in a selective catalytic hydrogenation of an a,p-unsaturated olefinic double bond of an a,p-unsaturated carbonyl compound.

The present invention also relates to a process for a catalytic hydrogenation of an a,p-unsaturated olefinic double bond of an a,p-unsaturated carbonyl compound, which comprises subjecting the o,|3-unsaturated carbonyl compound to a homogeneous catalytic hydrogenation in the presence of a compound of the formula (I).

Specifically, the present invention relates to the use or process described above wherein the o,p- unsaturated double bond in a prochiral o,|3-unsaturated carbonyl compound, such as neral or geranial, is subjected to an enantioselective hydrogenation to give the corresponding chiral product, such as citronellal.

The invention is associated with several benefits. The rhodium compounds of the formula (I), are stable, solid compounds which can be well handled and which are suitable as catalysts for the selective catalytic hydrogenation of the o,|3-unsaturated olefinic double bonds of o,|3-unsaturated carbonyl compounds, in particular for the enantioselective hydrogenation of the o,|3-unsaturated double bonds in prochiral a, |3- unsaturated carbonyl compounds. In particular, the Rh-complexes are suitable as catalysts for the selective catalytic hydrogenation of cis-citral and trans-citral to the desired citronellal with high enantioselectivity and without isomerization. The rhodium compounds of the formula (I) can be preformed to the catalytically active species in a H2/CO atmosphere. The use of the catalyst of the formula (I) provides the "correct” stoichiometry of ligand L to rhodium and avoids the necessity for separate feeding of the rhodium precursor and the bidentate ligand and the problems associated therewith. The rhodium compounds of the formula (I) contain rhodium and the chiral diphosphine ligand in a molar ratio of 1 : 1 and do not contain any interfering co-ligands or counterions. DETAILED DESCRIPTION OF THE INVENTION

The rhodium compound of formula (I) according to the invention consists of 1 , 2 or 3, in particular one cation [(L¹)2Rh]⁺ and one rhodium carbonyl anion [A]^k- which may be one to three times negatively charged. The anion [A]^k- is in particular the singly negatively charged [Rh(CO)4-_n(L²)_n]', i. e. k = 1. Accordingly, both the anion and the cation of the compound contain rhodium, frequently having different valences. Since L¹ is a bidentate phosphor-organic ligand, rhodium and ligand L¹ in the compounds of formula (I) are typically already present in a molar ratio of 1 :1 , which is preferred in rhodium catalysts for homogeneous hydrogenations. Typically, in the cation of the formula [(L¹)2Rh]⁺ both phosphorous atoms of each L¹ are coordinated to the Rh atom in the cation of the formula [(L¹)2Rh]⁺.

According to the invention the rhodium compounds of formula (I) are suitable as catalysts for the selective hydrogenation of an a,p-unsaturated olefinic double bond of an a,p-unsaturated carbonyl compound. They are in particular useful in the preparation of optically active carbonyl compounds such as aldehydes, ketones, esters, lactones or lactams by asymmetric, i.e. the enantioselective, hydrogenation of the corresponding carbonyl compounds which have an ethylenic double bond in a,p position relative to the carbonyl group. According to the invention, the ethylenic double bond in the a,p position relative to the carbonyl group is hydrogenated in the presence of a rhodium compound (I) to give a carbon-carbon single bond, wherein the tetrahedral carbon atom newly provided in the p-position carries four different substituents and is obtained in non-racemic form. Accordingly, in the context of the present invention, the term asymmetric hydrogenation is to be understood as meaning a hydrogenation during which the two enantiomeric forms of the hydrogenation product are not obtained in equal amounts.

In the definitions of the variables given in the formulae above and below, collective terms are used which are generally representative of the respective substituents. The meaning C_n- to C_m- indicates the respective possible number of carbon atoms in the particular substituents or substituent moiety.

In the context of the present invention, the expression "alkyl" comprises linear or branched alkyl groups having 1 to 4, 6, 12 or 25 carbon atoms. These include, for example, Ci- to Ce-alkyl, such as methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, 2-pentyl, 2-methylbutyl, 3-methylbutyl,

1.2-dimethylpropyl, 1 ,1 -dimethylpropyl, 2,2-dimethylpropyl, 1 -ethylpropyl, n-hexyl, 2-hexyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1 ,2-dimethylbutyl, 1 ,3-dimethylbutyl, 2,3-dimethylbutyl, 1 , 1 -dimethylbutyl,

2.2-dimethylbutyl, 3,3-dimethylbutyl, 1 ,1 ,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1 -ethylbutyl, 2-ethylbutyl, 1-ethyl-2-methylpropyl and the like. Preferably "alkyl" is linear or branched Ci- to Ce-alkyl groups.

In the context of the present invention, the term "branched” includes single-branched groups, i. e. groups having a single branching site, and multi-branched groups, i. e. groups having more than 1 , e. g. 2, 3, 4, 5 or 6 branching sites.

In the context of the present invention, the expression "cycloalkyl" comprises cyclic, saturated hydrocarbon groups having 3 to 6, 12 or 25 carbon ring members, e.g. Ca-Cs-cycloalkyl such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl, or C7-Ci2-bicycloalkyl. In the context of the present invention, the expression "alkoxy" is an alkyl group having 1 to 6 carbon atoms bonded via an oxygen, e.g. Ci- to Ce-alkoxy, such as methoxy, ethoxy, n-propoxy, 1 -methylethoxy, butoxy, 1 -methylpropoxy, 2-methy I propoxy, 1 ,1 -dimethylethoxy, pentoxy, 1 -methylbutoxy, 2-methylbutoxy, 3-methylbutoxy, 1 , 1 -dimethylpropoxy, 1 ,2-dimethylpropoxy, 2,2-dimethylpropoxy, 1 -ethyl propoxy, hexoxy, 1 -methylpentoxy, 2-methylpentoxy, 3-methy I pentoxy, 4-methylpentoxy, 1, 1 -di methyl butoxy, 1 ,2-dimethylbutoxy, 1 ,3-dimethylbutoxy, 2,2-dimethylbutoxy, 2,3-dimethylbutoxy, 3,3-dimethylbutoxy, 1 -ethylbutoxy, 2-ethylbutoxy, 1,1,2-trimethylpropoxy, 1 ,2,2-trimethylpropoxy, 1-ethyl-1 -methylpropoxy or 1 -ethyl-2-methylpropoxy. Preferably, "alkoxy" is Ci- to C4-alkoxy.

In the context of the present invention, the term "5, 6 or 7 membered carbocyclic group” refers to a saturated or unsaturated monocyclic, non-aromatic or aromatic hydrocarbon group with 5, 6 or 7 carbon atoms. These include, for example, cyclopentyl, cyclopentenyl, cyclohexyl, cyclohexenyl and benzyl.

In the context of the present invention, the term "6, 7 or 8 membered carbobicyclic group” refers to saturated or unsaturated bicyclic hydrocarbon group with 6, 7 or 8 carbon atoms. These include, for example, bicyclo[2.2.1]heptanyl, bicyclo[2.2.1]heptenyl, bicyclo[2.1.1]hexanyl and bicyclo[2.2.2]octyl.

In the context of the present invention, the term "5, 6 or 7 membered heterocyclic group” refers to a saturated or unsaturated, non-aromatic or aromatic ring or ring system containing 3 to 6 carbon atoms and at least one heteroatom selected from 0, S and N. Suitable non-aromatic heterocyclic groups include e.g. tetrahydrofuranyl, dihydrofuranyl, 1 ,4-dioxanyl, morpholinyl, 1 ,4-dithianyl, piperazinyl, piperidinyl, 1 ,3- dioxolanyl, imidazolidinyl, imidazolinyl, pyrrolinyl, pyrrolidinyl, tetrahydropyranyl, dihydropyranyl, oxathiolanyl, dithiolanyl, 1 ,3-dioxanyl, 1 ,3-dithianyl, oxathianyl, thiazolidinyl and thiomorpholinyl. Suitable aromatic heterocyclic groups include e.g. 2- or 3-thienyl, 2- or 3-furanyl, pyrrolyl, imidazolyl, pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, triazolyl, tetrazolyl, 2-, 3-, or 4-pyridyl, pyridazinyl, pyrazinyl and pyrimidinyl.

In the context of the present invention, the expression "C1-C4 alkanolate" refers to the the conjugate base of the corresponding C1-C4 alkanol. These include, for example, methanolate, ethanolate, propanolate, butanolate, isopropanolate, fert-butanolate and isobutanolate.

In the context of the present invention, the expression "C1-C4 alkanoate" refers to the the conjugate base of the corresponding C1-C4 alkanoic acid. These include, for example, formate, acetate, propanoate, butanoate, isopropanoate, tert-butanoate and isobutanoate.

In the context of the present invention, the expression "alkenyl" comprises linear or branched hydrocarbon radicals having 2 to 4, 6, 12 or 25 carbon atoms which comprise at least one double bond, for example 1, 2, 3 or 4 double bonds. These include, for example, C2-Ce-alkenyl such as ethenyl, 1 -propenyl, 2-propenyl,

1 -methylethenyl, 1-butenyl, 2-butenyl, 3-butenyl, 1-methyl-1 -propenyl, 2-methyl-1 -propenyl, 1-methyl-2- propenyl, 2-methyl-2-propenyl, 1 -pentenyl, 2-pentenyl, 3-pentenyl, 4-pentenyl, 1-methyl-1-butenyl,

2-methyl-1-butenyl, 3-methyl-1-butenyl, 1-methyl-2-butenyl, 2-methy l-2-buteny 1, 3-methyl-2-butenyl, 1-methyl-3-butenyl, 2-methyl-3-butenyl, 3-methyl-3-butenyl, 1 ,1-dimethyl-2-propenyl, 1 ,2-dimethyl- 1 -propenyl, 1 ,2-dimethyl-2-propenyl, 1-ethyl-1 propenyl, 1-ethyl-2-propenyl, 1 -hexenyl, 2-hexenyl,

3-hexenyl, 4-hexenyl, 5-hexenyl, 1-methyl-1 -pentenyl, 2-methyl-1 -pentenyl, 3-methyl-1 -pentenyl, 4-methyl- 1 -pentenyl, 1 -methyl-2-pentenyl, 2-methyl-2-pentenyl, 3-methyl-2-pentenyl, 4-methyl-2-pentenyl, 1-methyl-

3-pentenyl, 2-methyl-3pentenyl, 3-methyl-3-pentenyl, 4-methyl-3-pentenyl, 1-methyl-4-pentenyl, 2-methyl-

4-pentenyl, 3-methyl-4-pentenyl, 4-methyl-4-pentenyl, 1 ,1-dimethyl-2-butenyl, 1 ,1-dimethyl-3-butenyl, 1,2- dimethyl-1-butenyl, 1 ,2-dimethyl-2-butenyl, 1 ,2-dimethyl-3-butenyl, 1 ,3-dimethyl-1-butenyl, 1 ,3-dimethyl-2- butenyl, 1 ,3-dimethyl-3-butenyl, 2,2-dimethyl-3-butenyl, 2,3-dimethyl-1-butenyl, 2,3-dimethyl-2-butenyl,

2.3-dimethyl-3-butenyl, 3,3-dimethyl-1-butenyl, 3,3-dimethyl-2-butenyl, 1-ethyl-1-butenyl, 1-ethyl-2-butenyl, 1-ethyl-3-butenyl, 2-ethyl-1-butenyl, 2-ethyl-2-butenyl, 2-ethyl-3-butenyl, 1 ,1 ,2-trimethyl-2-propenyl, 1- ethyl-1-methyl-2-propenyl, 1-ethyl-2-methyl-1 -propenyl and 1-ethyl-2-methyl-2-propenyl. Preferably, "alkenyl" is linear C2- to Ci2-alkenyl groups or branched C3- to Ci2-alkenyl groups having in each case 1 to 3 double bonds, particularly preferably linear C2- to Ce-alkenyl groups or branched C3- to Ce-alkenyl groups having in each case one double bond.

In the context of the present invention, the expression "alkylene" refers to divalent hydrocarbon radicals having 2 to 25 carbon atoms. The divalent hydrocarbon radicals can be linear or branched. These include, for example, C2-Ci6-alkylene groups, such as 1 ,4-butylene, 1,5-pentylene, 2-methyl-1 ,4-butylene, 1,6- hexylene, 2-methyl-1,5-pentylene, 3-methyl-1,5-pentylene, 1 ,7-heptylene, 2-methyl-1 ,6-hexylene, 3- methyl-1,6-hexylene, 2-ethyl-1,5-pentylene, 3-ethyl-1,5-pentylene, 2,3-dimethyl-1 ,5-pentylene, 2,4- dimethyl-1 ,5-pentylene, 1 ,8-octylene, 2-methyl-1,7-heptylene, 3-methyl-1 ,7-heptylene, 4-methyl-1 ,7- heptylene, 2-ethyl-1 ,6-hexylene, 3-ethyl-1,6-hexylene, 2,3-dimethyl-1 ,6-hexylene, 2,4-dimethyl-1 ,6- hexylene, 1,9-nonylene, 2-methyl-1,8-octylene, 3-methyl-1 ,8-octylene, 4-methyl-1 ,8-octylene, 2-ethyl-1,7- heptylene, 3-ethyl-1 ,7-heptylene, 1 ,10-decylene, 2-methyl-1 ,9-nonylene, 3-methyl-1,9-nonylene, 4-methyl- 1 ,9-nonylene, 5-methyl-1 ,9-nonylene, 1,11 -undecylene, 2-methyl-1 ,10-decylene, 3-methyl-1,10-decylene,

5-methyl-1 ,10-decylene, 1 ,12-dodecylene, 1 ,13-tridecylene, 1 ,14-tetradecylene, 1,15-pentadecylene, 1,16- hexadecylene and the like. Preferably, "alkylene" is linear C2- to Ci2-alkylene groups or branched C3- to Ci2-alkylene groups, in particular linear C2- to Ce-alkylene groups or branched C3- to Ce-alkylene groups.

In the mono- or multi-branched or substituted alkylene groups, the carbon atom at the branching point or the carbon atoms at the respective branching points or the carbon atoms carrying a substituent can have, independently of one another, a R or S configuration or both configurations in equal or different proportions.

In the context of the present invention, the expression "alkenylene" refers to divalent hydrocarbon radicals having 2 to 25 carbon atoms, which can be linear or branched, where the main chain has one or more double bonds, for example 1, 2 or 3 double bonds. These include, for example, C2- to Cis-alkenylene groups, such as ethylene, propylene, 1-, 2-butylene, 1-, 2-pentylene, 1-, 2-, 3-hexylene,

1.3-hexadienylene, 1 ,4-hexadienylene, 1-, 2-, 3-heptylene, 1 ,3-heptadienylene, 1 ,4-heptydienylene, 2,4- heptadienylene, 1-, 2-, 3-octenylene, 1 ,3-octadienylene, 1 ,4-octadienylene, 2,4-octadienylene, 1-, 2-, 3-nonenylene, 1-, 2-, 3-, 4-, 5-decenylene, 1-, 2-, 3-, 4-, 5-undecenylene, 2-, 3-, 4-, 5-, 6-dodecenylene,

2.4-dodecadienylene, 2,5-dodecadienylene, 2,6-dodecadienylene, 3-, 4-, 5-, 6-tridecenylene, 2.5-tridecadienylene, 4,7-tridecadienylene, 5,8-tridecadienylene, 4-, 5-, 6-, 7-tetradecenylene, 2,5- tetradecadienylene, 4,7-tetradecadienylene, 5,8-tetradecadienylene, 4-, 5-, 6-, 7-pentadecenylene,

2.5-pentadecadienylene, 4,7-pentadecadienylene, 5,8-pentadecadienylene, 1 , 4, 7-pentadecatrieny lene, 4,7, 11-pentadecatrienylene, 4,6,8-pentadecatrienylene, 4-, 5-, 6-, 7-, 8-hexadecenylene, 2,5- hexadecadienylene, 4,7-hexadecadienylene, 5,8-hexadecadienylene, 2,5,8-hexadecatrienylene, 4,8, 11- hexadecatrienylene, 5,7,9-hexadecatrienylene, 5-, 6-, 7-, 8-heptadecenylene, 2,5-heptadecadienylene,

4.7-heptadecadienylene, 5,8-heptadecadienylene, 5-, 6-, 7-, 8-, 9-octadecenylene, 2,5-octadecadienylene,

4.7-octadecadienylene, 5,8-octadecadienylene and the like. Preferably, "alkenylene" is linear C3- to C12- alkenylene groups or branched C4- to Ci2-alkenylene groups having in each case one or two double bonds, in particular linear C3- to Cs-alkenylene groups with one double bond.

The double bonds in the alkenylene groups can be present independently of one another in the E and also in the Z configuration or as a mixture of both configurations.

In the context of the present invention, the expression "halogen" comprises fluorine, chlorine, bromine and iodine, preferably fluorine, chlorine or bromine.

In the context of the present invention, the expression "aryl" comprises a mono- to trinuclear aromatic ring system comprising 6 to 14 carbon ring members. These include, for example, Ce- to Cw-aryl, such as phenyl or naphthyl.

In the context of the present invention, the expression "hetaryl" comprises mono- to trinuclear aromatic ring system comprising 6 to 14 carbon ring members, where one or more, for example 1 , 2, 3, 4, 5 or 6, carbon atoms are substituted by a nitrogen, oxygen and/or sulfur atom. These include, for example, C3- to Cg-hetaryl groups, such as 2-furyl, 3-furyl, 2-thienyl, 3-thienyl, 2-pyrrolyl, 3-pyrrolyl, 3-isoxazolyl, 4- isoxazolyl, 5-isoxazolyl, 3-isothiazolyl, 4-isothiazolyl, 5-isothiazolyl, 3-pyrazolyl, 4-pyrazolyl, 5-pyrazolyl, 2- oxazolyl, 4-oxazolyl, 5-oxazolyl, 2-thiazolyl, 4-thiazolyl, 5-thiazolyl, 2-imidazolyl, 4-imidazolyl, 1 ,2,4- oxadiazol-3-yl, 1 ,2,4-oxadiazol-5-yl, 1 ,2,4-thiadiazol-3-yl, 1 ,2,4-thiadiazol-5-yl, 1 ,2,4-triazol-3-yl, 1 ,3,4- oxadiazol-2-yl, 1 ,3,4-thiadiazol-2-yl, 1 ,3,4-triazol-2-yl, 2-pyridinyl, 3-pyridinyl, 4-pyridinyl, 3-pyridazinyl, 4- pyridazinyl, 2-pyrimidinyl, 4-pyrimidinyl, 5-pyrimidinyl, 2-pyrazinyl, 1 ,3,5-triazin-2-yl, 1 ,2,4-triazin-3-yl, 2- indolyl, 3-indolyl, 4-indolyl, 5-indolyl, 6-indolyl, 7-indolyl and the like. Preferably, "hetaryl" is C5- to Ce- hetaryl.

In the context of the present invention, the expression "aralkyl" comprises a mono- to dinuclear aromatic ring system, comprising 6 to 10 carbon ring members, bonded via an linear or branched Ci- to Ce-alkyl group. These include, for example, C7- to Ci2-aralkyl, such as phenylmethyl, 1 -phenylethyl, 2-phenylethyl, 1 -phenylpropyl, 2-phenylpropyl, 3-phenylpropyl and the like.

In the context of the present invention, the expression "alkylaryl" comprises mono- to dinuclear aromatic ring systems comprising 6 to 10 carbon ring members which are substituted with one or more, for example 1 , 2 or 3, linear or branched Ci- to Ce-alkyl radicals. These include e.g. C7- to Ci2-alkylaryl, such as 1 -methylphenyl, 2-methylphenyl, 3-methyl phenyl, 1 -ethylphenyl, 2-ethylphenyl, 3-ethylphenyl, 1 -propylphenyl, 2-propylphenyl, 3-propy I phenyl, 1 -isopropylphenyl, 2-isopropylphenyl, 3-isopropylphenyl, 1 -butylphenyl, 2-butylphenyl, 3-butylphenyl, 1 -isobutylphenyl, 2-isobutylphenyl, 3-iso-butylphenyl, 1-sec- butylphenyl, 2-sec-butylphenyl, 3-sec-butylphenyl, 1-tert-butylphenyl, 2-tert-butyl phenyl, 3-tert-butylphenyl,

1-(1 -pentenyl) phenyl, 2-(1-pentenyl)phenyl, 3-(1-pentenyl)phenyl, 1-(2-pentenyl)phenyl, 2-(2- pentenyl)phenyl, 3-(2-pentenyl)phenyl, 1-(3-pentenyl)phenyl, 2-(3-pentenyl)phenyl, 3-(3-pentenyl)phenyl, 1 -(1 -(2-methy Ibuty l))pheny 1 , 2-(1 -(2-methy I bu ty I)) phenyl , 3-(1 -(2-methy I bu ty I)) phenyl , 1 -(2-(2- methylbutyl))phenyl, 2-(2-(2-methylbutyl))phenyl, 3-(2-(2-methylbutyl))phenyl, 1-(3-(2-methylbutyl))phenyl,

2-(3-(2-methylbutyl))phenyl, 3-(3-(2-methylbutyl))phenyl, 1-(4-(2-methylbutyl))phenyl, 2-(4-(2- methylbutyl))phenyl, 3-(4-(2-methylbutyl))phenyl, 1-(1-(2,2-dimethylpropyl))phenyl, 2-(1-(2,2- dimethylpropyl))phenyl, 3-(1-(2,2-dimethylpropyl))phenyl, 1-(1-hexenyl)phenyl, 2-(1-hexenyl)phenyl, 3-(1- hexenyl)phenyl, 1-(2-hexenyl)phenyl, 2-(2-hexenyl)phenyl, 3-(2-hexenyl)phenyl, 1-(3-hexenyl)phenyl, 2-(3- hexenyl)phenyl, 3-(3-hexenyl)phenyl, 1 -(1-(2-methylpentenyl))phenyl, 2-(1-(2-methylpentenyl))phenyl, 3- (1-(2-methylpentenyl))phenyl, 1-(2-(2-methylpentenyl))phenyl, 2-(2-(2-methylpentenyl))phenyl, 3-(2-(2- methylpentenyl))phenyl, 1-(3-(2-methylpentenyl))phenyl, 2-(3-(2-methylpentenyl))phenyl, 3-(3-(2- methylpentenyl)) phenyl, 1-(4-(2-methylpentenyl))phenyl, 2-(4-(2-methylpentenyl))phenyl, 3-(4-(2-methyl- penteny l))phenyl , 1 -(5-(2-methy I penteny l))pheny 1, 2-(5-(2-methy I pentenyl))pheny 1 , 3-(5-(2-methyl- penteny l))phenyl , 1 -(1 -(2,2-di methy Ibuteny l))pheny I, 2-(1 - (2,2-dimethy Ibuteny l))pheny I, 3-(1 -(2,2- dimethylbutenyl))phenyl, 1-(3-(2,2-dimethylbutenyl))phenyl, 2-(3-(2,2-dimethylbutenyl))phenyl, 3-(3-(2,2- dimethylbutenyl))phenyl, 1-(4-(2,2-dimethylbutenyl))phenyl, 2-(4-(2,2-dimethylbutenyl))phenyl, 3-(4-(2,2- dimethylbutenyl)) phenyl and the like.

The remarks made below concerning preferred embodiments of the variables (substituents) are valid on their own as well as preferably in combination with each other concerning the rhodium compounds of formula (I) and the bidentate ligands of formula (II), as well as concerning the uses according to the invention.

The rhodium compound of the present invention contains two bidentate phosphor-organic ligands L¹ per molecule. The term bidentate phosphor-organic ligand refers to a compound which has two phosphor atoms that are capable of coordinating the rhodium atom in the cation [(L¹)2Rh]⁺. Preferably, the phosphor atoms have 3 P-C bonds. In particular, at the C-atom of the at least one of the P-C bonds is part of an aromatic ring, e. g. a benzene ring. For the purpose of the invention, the phosphor-organic ligands L¹ is preferably chiral.

Preferably, the bidentate phosphor-organic ligand L¹ of the rhodium compound of formula (I) of the invention is a ligand of the formula (II), (II) where

Ar are identical or different and selected from the group consisting of Ce-C -aryl, in particular phenyl, which is unsubstituted or carries one or more substituents which are selected from Ci-Ce-alkyl, Ca-Ce-cycloalkyl, phenyl, Ci-Ce-alkoxy, phenoxy and amino, and where Ar is especially unsubstituted phenyl;

R^a is selected from the group consisting of Ci-Ce-alkyl and Ca-Ce-cycloalkyl, preferably is Ci-Ca-alkyl, and in particular is methyl;

R^b is selected from the group consisting of hydrogen and Ci-Ce-alkyl, preferably is hydrogen or C1-C3- alkyl, more preferably is hydrogen or methyl, and in particular is methyl; or

R^a, R^b together with the carbon atoms, to which they are bound form a 5, 6 or 7 membered carbocyclic group, a 6, 7 or 8 membered carbobicyclic group or a 5, 6 or 7 membered heterocyclic group, where the carbocyclic group, the carbobicyclic group and the heterocyclic group are unsubstituted or substituted by 1, 2, 3 or 4 substituents R^cx^c, which are selected from Ci-Ce-alkyl, phenyl and benzyl; where R^a and R^b, together with the carbon atoms to which they are bonded, are preferably

1.2-cyclopentylene, 1,2-cyclohexylene, bicyclo[2.2.1]heptylene, in particular 2,3- bicyclo[2.2.1 ]heptylene, bicyclo[2.2.1 ]heptenylene, in particular 2,3-bicyclo[2.2.1 ]heptenylene, bicyclo[2.1.1]hexylene, in particular 2,3-bicyclo[2.1.1]hexylene, bicyclo[2.2.2]octylene, in particular

2.3-bicyclo[2.2.2]octylene, tetrahydrofuranylene, e. g. 3,4-tetrahydrofuranylene, pyrrolidinylene, e. g. 3,4-pyrrolidinylene, tetrahydropyranylene and morpholinylene, in particular 1,2-cyclohexylene,

2.3-bicyclo[2.2.1]heptenylene and N-benzyl-pyrrolidin-3,4-diyl.

The two carbon atoms in formula (II) to which the radicals R^a and R^b are bonded can both have a (R) or (S) configuration. Preferably, the compounds of the formula (II) are chiral, I. e. the carbon atoms bearing the radicals R^a and R^b have either both (R) configuration or (S) configuration. Thus, the compound of formula (II) can be present as the pure (R,R)- or the pure (S,S)-enantiomer or a mixture thereof, such as a racemic mixture.

Here and in the following, a pure stereoisomer is understood as meaning chiral substances which, with regard to the desired stereoisomer, have an enantiomeric excess (ee) of at least 80% ee, in particular at least 90% ee and specifically at least 95% ee.

Within the above preferred group of embodiments of the invention particular preference is given to bidentate phosphor-organic ligands L¹ which are selected from the group consisting of compounds of the formulae (Ila) to (lid) and the enantiomers thereof:

In this context even more preference is given to the ligand L¹ of the formula (I l-a) and its enantiomer, i.e. to the compounds (2R,3R)-(+)-bis(diphenylphosphino)butane (also called herein (R,R)-chiraphos) and (2S, 3S)-(-)-bis(diphenylphosphino)butane (also called herein (S,S)-chiraphos).

The bidentate phosphor-organic ligands L¹ of the rhodium compound (I) according to the invention, in particular the ligands of formula (II), are preferably present in enantiopure form and in particular have an enantiomeric excess of at least 90% ee, especially at least 95% ee.

In formula (I) of the rhodium compound according to the present invention the variable k is of the value 1, 2 or 3. The value of k represents the anionic charge of the anion [A]^k\ Consequently, the molar ratio of the cations [(L¹)2Rh]⁺ to the anion A is 1, 2 or 3. In other words rhodium compound (I) therefore consists of one, two or three cations [(L¹)2Rh]⁺ and one rhodium carbonyl anion [A]^k-. In a preferred group of embodiments the variable k is 1 and thus the rhodium compound (I) corresponds to the formula [(L¹)₂RhHA]-.

The anion [A]- is a rhodium carbonyl anion, which means that the anion consists of at least one rhodium atom, e. g. 1 to 7 rhodium atoms, and at least 1 CO ligand, e. g. 1 to 16 CO ligands, and optionally containing n mono- and bidentate ligands L².

Suitable monodentate ligands L² preferably have a phosphorous atom which is capable of coordinating the Rh atom of the anion [A]- , for example a compound of the formula (A) R^d

R^c— P (A) where the substituents R^c, R^d, and R^e, independently of each other, are selected from Ci-Ce-alkyl, Ci-Ce- alkoxy, Ce-Cio-aryl and Ce-Cw-aryloxy, in particular from methyl, ethyl, methoxy, ethoxy, phenyl and phenoxy. Particularly preferred monodentate ligands L² in this context are triphenylphosphine, methyldiphenylphosphine, dimethyl(phenyl)phosphine, trimethylphosphite and triphenylphosphite.

Suitable monodentate ligands L² preferably have a phosphorous atom which is capable of coordinating the Rh atom of the anion [A]-. Bidentate ligands L² are preferably selected from the group consisting of 1 ,2- bis(diphenylphosphino)ethane, 1 ,2-bis(diphenylphosphino)ethane monoxide, the ligands of formula (II), as defined above in connection with the ligands L¹, and the monooxides of the ligands of formula (II), wherein preference is given to those ligands (II) mentioned herein as preferred, as well as to their monoxides.

Preferably, the anion [A]- of the rhodium compound [(L¹)2Rh]⁺[A]- is of the formula [Rh(CO)4-_n(L²)_n]', where L² is a monodentate organic ligand other than CO or, if n = 2, two ligands L² may together form a bidentate organic ligand; and n is 0, 1 or 2.

Accordingly, the rhodium compound of this preferred group of embodiments is of the formula [(L¹)₂Rh]⁺[Rh(CO)4-n(L²)_n]-, which is hereinafter also referred to as formula (I. a).

In the anions [Rh(CO)4-_n(L²)_n]' of the preferred rhodium compound of formula (I. a), the one or two ligands L², if present, are mono- or bidentate ligands. Monodentate ligands L² are preferably selected from the phosphines of the formula (A) above.

Examples of suitable anions [A]- are [Rh(CO)4]’, [Rh(CO)3L]-, where L is triphenylphosphite or triphenylphosphine, [Rh(CO)₂L₂]-, where L is triphenylphosphite triphenylphosphine or the two L are together dppe (= 1 ,2-bis(diphenyllphosphino)ethane). Further examples of suitable anions [A]- are [Rh₆(CO)i₅]²-, [Rh₆(CO)io]² ,[Rh₆(CO)ii]²-, [Rh₆(CO)₉]²-, [Rh₇(CO)i₆]³-, [Rh₄(CO)₈]²- and [Rh₃(CO)₅]-.

In a particular preferred group of embodiments the number n of the ligands L² in the rhodium carbonyl anion [A]^k- of the compound of the formula (I) is 0. Thus, according to this group of embodiments the rhodium compound (I) is preferably of the formula (I. a), where n = 0, i.e. of the formula [(L¹)₂Rh]⁺[Rh(CO)4]‘ , which is hereinafter also referred to as formula (l.a-1).

A very preferred embodiment of the present invention relates to the compound of formula (l.a-1), wherein L¹ is a compound of the formula (Ila), in particular (R,R)-chiraphos or (S,S)-chiraphos.

The compound of formula (l.a-1), wherein L¹ is a compound of the formula (Ila), in particular in particular (R,R)-chiraphos or (S,S)-chiraphos, can be isolated as a crystalline solid by crystallization from tetrahydrofurane. The crystallization is furthered by addition of aliphatic hydrocarbon, such as pentane or hexane. Thereby brown-green platelets of the respective compound are obtained.

These crystals obtained are a solvate of [Rh(chiraphos)2][Rh(CO)4] containing about one molar equivalent of THF as revealed by single crystal X-ray diffraction. The crystalline compound has the following crystallographic properties, determined at 100 K and using CuKo of the wavelength 1.54178 A.

Z refers to the number of molecules [Rh(chiraphos)2][Rh(CO)4] per unit cell.

The molecular structure of [Rh((R,R)-chiraphos)2][Rh(CO)4] as calculated from the single crystal X-ray diffraction data is presented in figures 1 a and 1 b.

The present invention further relates to method for producing a rhodium compound of the formula (I) as defined herein, which comprises the reaction of a compound of the formula (III) with a compound of the formula (IV)

[(L¹)₂Rh]X (III)

M_k[A]k- (IV) where L¹, A and k are as defined herein and in particular have the meanings mentioned herein as preferred;

M is an alkal imetal ion, such as Li⁺, Na⁺ or K⁺, or a quaternary or a ternary ammonium ion, such as tetramethylammonium ion, tetraethylammonium ion, tetrapropylammonium ion, tetrabutylammonium ion, trimethylammonium ion or triethylammonium ion, in particular is Na⁺ or K⁺ and especially is K⁺; and

X is an anion selected from the group of acetyl acetonate, BF4, (804)0.5. C1-C4 alkanolate, such as methanolate or ethanolate, halogenide, such as chloride or bromide, and C1-C4 alkanoate, such as acetate, and in particular is acetyl acetonate (herein also called acac).

The use of the coefficient 0.5 in the moiety "(804)05” above, accounts for the fact that the anion (SO4)²' is doubly negatively charged, while the counterion [(L¹)2Rh]⁺ only has a single positive charge. As apparent from the foregoing, in order to produce one of the preferred rhodium compound of the formulae (I. a) or (l.a-1) as defined herein above, i.e. a rhodium compound of formulae [(L¹)2Rh]⁺[Rh(CO)4- n(L²)_n]' or [(L¹)2Rh]⁺[Rh(CO)4]‘, by the above method, a compound (IV) that is of the formulae (IV.a) or (IV.a-1) has to be used:

M⁺[Rh(CO)₄-n(L²)n]- (IV.a),

M⁺[Rh(CO)₄]- (IV.a-1), where M, L², and n are as defined herein and in particular have the meanings mentioned herein as preferred.

Particularly preferred compounds of formula (III) are selected from the group of [Rh((R,R)-chiraphos)2]X and [Rh((S,S)-chiraphos)2]X, wherein the anion X in each case is preferably acetyl acetonate (acac). Particularly preferred compounds of formula (IV) are selected from those of formula (IV.a-1), wherein M is preferably Na⁺ or K⁺, especially K⁺. Hence, the compound K[Rh(CO)4] is a specifically preferred compound of formula (IV). Accordingly, very particular preference is given to the rhodium compound of the formula (I) which is selected from [Rh((R,R)-chiraphos)2][Rh(CO)4] and [Rh((S,S)-chiraphos)2][Rh(CO)4], and in particular is [Rh((R,R)-chiraphos)2][Rh(CO)4].

The method for producing a compound of formulae (I), (I. a) or (l.a-1) is carried out by bringing a compound (III) into contact with a compound (IV). In a preferred embodiment a solution of the compound (III) is contacted with a solution of the compound (IV). To this end, the compound (III) is typically provided as dissolved in a polar aprotic solvent, such as a sulfoxide, e.g. dimethyl sulfoxide (DMSO), dimethylformamide (DMF), N-methyl-2-pyrrolidone, acetonitrile, gamma-butyrolactone or carbonate solvents, e.g. dimethyl carbonate, and in particular DMSO. The compound (IV) is usually provided as dissolved in an aqueous solvent, such as especially a mixture of water and a water-miscible organic solvent preferably selected from Ci-Ca-alkanols, e.g. methanol, ethanol, 1 -propanol or 2-propanol, and in particular a 30:70 to 70:30 (v/v), especially 45:55 to 55:45 (v/v) mixture of water and 2-propanol.

According to the above preferred embodiment, the reaction of the compounds (III) and (IV) is preferably initiated by adding the solution of compound (III) to the solution of the compound (IV) with agitation. The reaction is carried out at a temperature of at least 5 °C. Preferably, the temperature is in the range of 10 to 80 °C, more preferably 15 to 60 °C, even more preferably 18 to 50 °C, such as 18 to 40 °C, most preferably 19 to 30 °C or 20 to 25 °C.

For preparing a compound of formulae (I. a) or (l.a-1), or a compound (I), i.e. a compound [(L¹)2Rh]k[A], wherein k =1 , the molar ratio of the total amount of the compound of formula (III) to the total amount of the compound of formulae (IV), (IV.a) or (IV.a-1) is preferably in the range of 0.7:1 to 1.3: 1 , more preferably 0.8: 1 to 1.2: 1 , most preferably 0.9: 1 to 1.1 : 1. Accordingly, if the variable k in formula (I) is 2 or 3, the compounds (III) and (IV) are accordingly used in proportionally higher ratios. During the reaction of a compound of formula (III) with a compound of formula (IV), (IV.a) or (IV.a-1) the crude product typically precipitates as a solid from the reaction mixture. Upon completion of the reaction the precipitate is usually filtered off, preferably under inert conditions, and subjected to work-up measures well established in the art, such as washing the filtrate especially with water, dissolving the filtrate in a suitable solvents, such as tetrahydrofuran, drying the obtained solution with a suitable drying agent, such as magnesium sulfate, and crystallization from a suitable solvent or solvent mixture, such as aliphatic or aromatic hydrocarbons or mixtures containing an aliphatic or aromatic hydrocarbon and a polar aprotic solvent, e. g. an aliphatic or alicyclic ether, e. g. a mixture of tetrahydrofuran and aliphatic hydrocarbon, e. g. n-pentane and the like. In this way, for example, the compound [Rh((R,R)-chiraphos)2][Rh(CO)4] which is a compound of formula (l.a-1), was obtained in crystalline form as a solvate containing 1 molar equivalent of tetrahydrofuran, which can be characterized by single crystal X-ray diffraction.

The starting materials of formula (III) and formulae (IV), (IV.a) or (IV.a-1) used in the process according to the invention for the preparation of compounds (I) are readily accessible from commercially available or easily producible rhodium compounds.

For example, [Rh((R,R)-chiraphos)2]⁺acac-, which is a compound of formula (III), can be produced by first preparing solutions of each 1 molar equivalent of dicarbonyl(acetylacetonato)rhodium(l), i.e. Rh(CO)2acac, and about 2 molar equivalents of ('R,R)-chiraphos, preferably in the same suitable solvent, such as THF. The two solutions are then mixed under inert and substantially anhydrous conditions and allowed to react preferably at a temperature in the ange of 10 to 30°C. A precipitate is formed which upon completion of the reaction is filtered off, washed with a suitable solvent, such as pentane, and dried. The thus obtained product [Rh((R,R)-chiraphos)2]⁺acac- can without further purification directly be used as the compound of formula (III) in the method of the invention for preparing a compound of formula (I).

Rhodium compounds comprising the anion [Rh(CO)4]^_ are known in the art and are described, e.g. in J. L. Vidal et al., Journal of Organometallic Chemistry 1983, 241 (3), 395-416 and in J. L. Vidal et al., Inorganic Chemistry 1981 , 20(1), 249-254. In addition, L. Garlaschelli et al., Inorganic Chemistry 1990, 28, 21 1-215, describe an efficient synthesis of K[Rh (CO)4] starting from RhCh trihydrate. Using essentially the procedure of Garlaschelli et al. K[Rh(CO)4], which is a compound of formula (IV.a-1), can be produced using either a rhodium(l) compound, such as Rh(CO)2acac, or a rhodium(lll) compound, such as RhCh hydrate, as starting material. Initially, the respective starting material is dissolved in a polar aprotic solvent, such as DMSO or DMF, especially DMSO, under inert and substantially anhydrous conditions. After adding a molar excess of powdered potassium hydroxide, a low pressure of carbon monoxide is passed through the agitated reaction mixture at ambient temperature. Upon completion the of the reaction the obtain solution of K[Rh (CO)4] can be directly introduced in the method of the invention to produce a compound of formula (I).

The present invention additionally relates to the use of a compound of the formula (I), (I. a) or (l.a-1) as defined herein above as a catalyst in a selective catalytic hydrogenation of an o,|3-unsaturated olefinic double bond of an o,|3-unsaturated carbonyl compound. Accordingly, the invention also relates to a process for a catalytic hydrogenation of an o,|3-unsaturated olefinic double bond of an o,|3-unsaturated carbonyl compound, which comprises subjecting the o,|3- unsaturated carbonyl compound to a homogeneous catalytic hydrogenation in the presence of a compound of the formula (I), (I. a) or (l.a-1), as defined above.

Suitable starting compounds for the use or process according to the invention are generally all types of prochiral carbonyl compounds which have a double bond at the 2 position. Preferably, the prochiral a,|3- unsaturated carbonyl compound is a prochiral, a,|3-unsaturated ketone or in particular a prochiral, a,|3-unsaturated aldehyde.

Accordingly, the use or process according to the invention is preferably suitable for the preparation of optically active aldehydes or ketones by asymmetric hydrogenation of prochiral a,|3-unsaturated aldehydes or ketones. The use or process according to the invention is particularly preferably suitable for the preparation of optically active aldehydes by asymmetric hydrogenation of prochiral a,|3-unsaturated aldehydes.

In a preferred embodiment of the use or process according to the invention, the prochiral a,|3-unsaturated carbonyl compound is selected from compounds of the formula (V) in which

R⁵, R⁶are different from one another and each is a linear, branched or cyclic hydrocarbon radical having 1 to 25 carbon atoms which is saturated or has one or more, e.g. 1 , 2, 3, 4 or 5, preferably nonconjugated ethylenic double bonds, and which is unsubstituted or carries one or more, e.g. 1 , 2, 3 or 4, identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to C10- ary I and C3- to Cg-hetaryl;

R⁷ is hydrogen or a linear, branched or cyclic hydrocarbon radical having 1 to 25 carbon atoms which is saturated or has one or more, e.g. 1 , 2, 3, 4 or 5, preferably nonconjugated ethylenic double bonds, and which is unsubstituted or carries one or more, e.g. 1 , 2, 3 or 4, identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to Cw-aryl and C3- to Cg-hetaryl; or

R⁷ together with one of the radicals R⁵ or R⁶, can also be a 3- to 25-membered alkylene group, in which 1 , 2, 3 or 4 nonadjacent CH2 groups can be replaced by 0 or N-R^9c, where the alkylene group is saturated or has one or more, e.g. 1 , 2, 3, 4 or 5, preferably nonconjugated ethylenic double bonds, and where the alkylene group is unsubstituted or carries one or more, e.g. 1 , 2, 3 or 4, identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ci- to C4-alkyl, Ce- to Cw-aryl and C3- to Cg-hetaryl, where two substituents also together can be a 2- to 10-membered alkylene group, where the 2- to 10-membered alkylene group is saturated or has one or more, e.g. 1, 2, 3 or 4, nonconjugated ethylenic double bonds, and where the 2- to 10-membered alkylene group is unsubstituted or carries one or more, e.g. 1 , 2, 3 or 4, identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to Cw-aryl and C3- to Cg- hetaryl; where

R⁸ is hydrogen, Ci- to Ce-alkyl, Ce- to Cw-aryl, C7- to Cw-aralkyl or C7- to Cw-alkylaryl;

R^9a, R^9b are in each case independently of one another hydrogen, Ci- to Ce-alkyl, Ce- to Cw-aryl, C7- to Cw-aralkyl or C7- to Cw-alkylaryl, or

R^9a and R^9b together can also be an alkylene chain having 2 to 5 carbon atoms, which can be interrupted by N or 0; and

R^9c is hydrogen, Ci- to Ce-alkyl, Ce- to Cw-aryl, C7- to Cw-aralkyl or C7- to Cw-alkylaryl.

The linear, branched or cyclic hydrocarbon radicals having 1 to 25 carbon atoms specified in the definition of radicals R⁵, R⁶ and R⁷ are usually linear Ci- to C25-alkyl groups, linear C2- to C25-alkenyl groups, linear C4- to C25-alkadienyl groups, branched C3- to C25-alkyl groups, branched C3- to C25-alkenyl groups, branched C5- to C25-alkadieny I groups, and also C3- to C25-cycloalky I groups or C3- to C24-cycloalky I groups which are substituted by one or more, e.g. by 1 , 2, 3 or 4, Ci- to C4-alkyl groups, as defined above. The cyclic hydrocarbon radicals both also include cyclic hydrocarbon radicals which have a phenyl ring which optionally carries one or more, e.g. 1 , 2, 3, 4, 5 or 6, Ci-C4-alkyl groups, where the phenyl ring is bonded directly to the ethylenically unsaturated double bond or the carbonyl group in formula (V) or is bonded via a Ci-Ce-alkylene group.

An alkenyl group is understood as meaning a linear or branched aliphatic hydrocarbon radical which is monounsaturated. An alkdienyl group is understood as meaning a linear or branched aliphatic hydrocarbon radical which is diunsaturated. The 3- to 25-membered alkylene groups specified in the definition of the radical R⁷ that are saturated are generally linear or branched C3- to C25-alkylene groups, as defined above.

The 3- to 25-membered alkylene groups which have one or more, e.g. 1 , 2, 3 or 4, nonconjugated ethylenic double bonds specified in the definition of the radical R⁷ are generally linear or branched C3- to C25-alkenylene groups, as defined above.

Preferably, one of the radicals R⁵, R⁶ is methyl or ethyl, in particular methyl, and the other radical is an linear, branched or cyclic hydrocarbon radical having 3 to 25 carbon atoms which is saturated or has one or more. e.g. 1 , 2, 3, 4 or 5, preferably nonconjugated ethylenic double bonds, and which is unsubstituted or carries one or more, e.g. 1, 2, 3 or 4, identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to Cw-aryl and C3- to Cg-hetaryl.

In particular, one of the radicals R⁵, R⁶ is methyl or ethyl, in particular methyl, and the other radical is an linear, branched or cyclic hydrocarbon radical having 3 to 25 carbon atoms which is saturated or has one or more, e.g. 1, 2 or 3, preferably nonconjugated ethylenic double bonds.

R⁷ is in particular hydrogen.

In a very preferred embodiment of the use or process according to the invention, the prochiral a,|3- unsaturated carbonyl compound is selected from compounds of the general formula (Va) and (Vb) in which

R⁵, R⁶ is in each case an linear or branched hydrocarbon radical having 2 to 25, in particular having 3 to 20, carbon atoms which is saturated or has 1, 2, 3, 4 or 5 nonconjugated ethylenic double bonds.

Accordingly, the use or process according to the invention can be applied, by way of asymmetric hydrogenation of prochiral a,|3-unsaturated aldehydes or ketones of the general formulae (V), (Va) or (Vb), to prepare the corresponding a,|3-saturated aldehydes or ketones of the formula (VI) in optically active form, where the carbon atom which carries the radicals R⁵ and R⁶ constitutes the asymmetry center generated by the hydrogenation.

In formula (VI), R⁵, R⁶ and R⁷ have the meanings specified for formula (V), in particular those specified for the formulae (Va) and (Vb).

The asymmetric, i.e. enantioselective hydrogenation according to the invention of the a,|3-unsaturated aldehydes of the formulae (Va) or (Vb) renders accessible the corresponding a,|3-saturated aldehydes. The compounds of the formulae (Va) and (Vb) constitute E/Z double-bond isomers relative to one another. In principle, the correspondingly optically active aldehydes are accessible starting from both double-bond isomers of the formulae (Va) and (Vb). Depending on the choice of the enantiomeric form of the catalyst of formulae (I), (I. a) or (l.a-1), i.e. depending especially on whether the (+) or (-) enantiomer of the chiral ligand L¹ is chosen, preferably one of the enantiomers of the optically active aldehyde is obtained by means of the process of the invention from the E or Z double-bond isomer of formulae (Va) or (Vb) used. The same is true for the aforementioned substrate and product classes. In principle, it is also possible to react mixtures of the two double-bond isomers in a manner according to the invention. This gives mixtures of the two enantiomers of the desired target compound.

Examples of optically active aldehydes or ketones which can be prepared using the process according to the invention include the following compounds:

The use or process according to the invention is particularly preferably suitable for the preparation of optically active citronellal of the formula (VII) in which * denotes the asymmetry center; by asymmetric hydrogenation of geranial of the formula (Va-1) or of neral of the formula (Vb-1 )

Mixtures of geranial and neral can also be reacted in the manner according to the invention, wherein, as described above, mixtures of D- or L-citronel lai are obtained which are optically active if the two enantiomers are not present therein in equal parts.

Of particular preference in the context of the process according to the invention is the preparation according to the invention of D-ci tronell al by asymmetric hydrogenation of neral or geranial.

The preparation process according to the invention is carried out in the presence of the rhodium compound of formulae (I), (I. a) or (l.a-1) as homogeneous hydrogenation catalyst which preferably possesses optically activity and contains ligands L¹ that are optically active.

The catalytic rhodium compounds (I), (I. a) or (l.a-1) are used according to the invention in amounts typically used in asymmetric hydrogenation of a,p-unsaturated aldehydes which in a batch procedure is usually in the range of about 0.001 to about 1 mol%, preferably in the range of about 0.002 to about 0.5 mol%, in particular in the range of about 0.005 to about 0.2 mol%, based on the rhodium atoms present in relation to the amount of substrate to be hydrogenated. In continuous procedures the catalytic rhodium compounds (I), (I. a) or (l.a-1) are typically used in amounts of 100 to 10,000 ppm, in particular in the range of 200 to 5,000 ppm, based on the reaction mixture.

In a preferred embodiment of the process according to the invention, the rhodium catalyst of formulae (I), (I. a) or (l.a-1) is pretreated before the hydrogenation with a gas mixture comprising carbon monoxide and hydrogen, and/or the asymmetric hydrogenation is carried out in the presence of carbon monoxide additionally introduced to the reaction mixture.

This means that the rhodium catalyst (I), (I. a) or (l.a-1), which is soluble in the reaction mixture, i.e. homogeneous, is either pretreated before the asymmetric hydrogenation with a gas mixture which comprises carbon monoxide and hydrogen (i.e. a so-called preformation is carried out) or the asymmetric hydrogenation is carried out in the presence of carbon monoxide additionally introduced into the reaction mixture or a preformation is carried out and then the asymmetric hydrogenation is carried out in the presence of carbon monoxide additionally introduced to the reaction mixture.

Preferably, in this preferred embodiment, the catalyst (I), (I. a) or (l.a-1) is pretreated with a gas mixture comprising carbon monoxide and hydrogen and the asymmetric hydrogenation is carried out in the presence of carbon monoxide additionally introduced to the reaction mixture.

In this preferred embodiment, the specified pretreatment is carried out with a gas mixture comprising 20 to 90% by volume carbon monoxide, 10 to 80% by volume hydrogen and 0 to 5% by volume further gases, where the specified volume fractions add up to 100% by volume, at a pressure of 5 to 100 bar.

Additionally, excess carbon monoxide is separated off from the resulting catalyst prior to use in the asymmetric hydrogenation.

The term excess carbon monoxide is to be understood here as meaning the carbon monoxide which is present in the resulting reaction mixture in gaseous or dissolved form and is not bonded to the final rhodium catalyst or its precursor catalyst (I), (I. a) or (l.a-1). Accordingly, the excess carbon monoxide not bonded to the catalyst is removed at least largely, i.e. to an extent that any residual amounts of dissolved carbon monoxide do not make themselves noticeably disruptive in the subsequent hydrogenation. This is usually ensured if about 90%, preferably about 95% or more of the carbon monoxide used for the preformation are separated off. Preferably, the excess carbon monoxide is removed completely from the catalyst obtained by preformation.

The separation off of the excess carbon monoxide from the resulting catalyst or from the reaction mixture comprising the catalyst can take place in different ways. Preferably, the rhodium catalyst or the mixture comprising the catalyst obtained by preformation is decompressed to a pressure of up to about 5 bar (absolute), preferably, specifically when carrying out the preformation in the pressure range from 5 to 10 bar, to a pressure of less than 5 bar (absolute), preferably to a pressure in the range from about 1 bar to about 5 bar, preferably 1 to less than 5 bar, particularly preferably to a pressure in the range from 1 to 3 bar, very particularly preferably to a pressure in the range from about 1 to about 2 bar, particularly preferably to atmospheric pressure, meaning that gaseous, nonbound carbon monoxide escapes from the product of the preformation.

The aforementioned decompression of the preformed catalyst can take place for example using a high- pressure separator, as is known per se to the person skilled in the art. Separators of this type in which the liquid is in the continuous phase are described for example in: Perry's Chemical Engineers' Handbook, 1997, 7th edition, McGraw-Hill, p. 14.95 and 14.96; the prevention of a possible drop entrainment is described on pages 14.87 to 14.90. The decompression of the preformed catalyst can take place in one stage or two stages until the desired pressure in the range from 1 bar to about 5 bar is reached, during which the temperature usually drops to 10 to 40°C.

Alternatively, the separation off of excess carbon monoxide can be achieved by so-called stripping of the catalyst or of the mixture comprising the catalyst with a gas, advantageously with a gas that is inert under the reaction conditions. Here, the term stripping is understood by the person skilled in the art as meaning the introduction of a gas into the catalyst or the reaction mixture comprising the catalyst, as described for example in W. R. A. Vauck, H. A. Muller, Grundoperationen chemischer Verfahrenstechnik [Basic operations of chemical processing technology], Deutscher Verlag fiir Grundstoffchemie Leipzig, Stuttgart, 10th edition, 1984, page 800. Suitable inert gases which may be mentioned here by way of example are: hydrogen, helium, neon, argon, xenon, nitrogen and/or CO2, preferably hydrogen, nitrogen, argon.

Preferably, the asymmetric hydrogenation is then carried out with hydrogen which has a carbon monoxide content in the range from 50 to 3000 ppm, in particular in the range from 100 to 2000 ppm, specifically in the range from 200 to 1000 ppm and very specifically in the range from 400 to 800 ppm.

If a preforming of the catalyst is carried out, usually the rhodium catalyst (I), (I. a) or (l.a-1) and if desired the substrate to be hydrogenated asymmetrically are dissolved in a suitable solvent or solution medium that is inert under the reaction conditions, such as, for example, ether, tetrahydrofuran, methyltetrahydrofuran, toluene, xylene, chlorobenzene, octadecanol, biphenyl ether, texanol, Marlotherm, Oxo Oil 9N (hydroformylation products from isomeric octenes, BASF Aktiengesel Ischaft), ci tronellal and the like. The substrate to be reacted, the product or any high-boiling by-products produced during the reaction can also serve as solution medium. A gas mixture which comprises hydrogen and carbon monoxide is injected into the resulting solution, advantageously in a suitable pressurized reactor or autoclave, at a pressure of usually about 5 to about 350 bar, preferably from about 20 to about 200 bar and particularly preferably from about 50 to about 100 bar. Preferably, for the preformation a gas mixture is used which comprises about

30 to 99% by volume hydrogen,

1 to 70% by volume carbon monoxide and 0 to 5% by volume further gases, where the data in % by volume must add up to 100% by volume.

For the preformation, particular preference is given to using a gas mixture which comprises about

40 to 80% by volume hydrogen,

20 to 60% by volume carbon monoxide and 0 to 5% by volume further gases, where the data in % by volume must add up to 100% by volume.

A gas mixture particularly preferred for the preformation is so-called synthesis gas, which usually consists to about 35 to 55% by volume of carbon monoxide alongside hydrogen and traces of further gases.

The preformation of the catalyst is usually carried out at temperatures of from about 25°C to about 100°C, preferably at about 40°C to about 80°C. If the preformation is carried out in the presence of the substrate to be hydrogenated asymmetrically, the temperature is advantageously selected such that it does not result, to a troublesome extent, in an isomerization of the double bond to be hydrogenated. The preformation is usually terminated after about 1 h to about 24 h, often after about 1 to about 12 h.

Following the optional preformation to be carried out, the asymmetric hydrogenation of the selected substrate is carried out in accordance with the invention. Following a preceding preformation, the hydrogenation of selected substrate can generally be successfully carried out with or without the introduction of additional carbon monoxide. If no preformation is carried out, the asymmetric hydrogenation according to the invention can be carried out either in the presence of carbon monoxide introduced into the reaction system or without the introduction of carbon monoxide. Advantageously, a preformation is carried out as described and additional carbon monoxide is added to the reaction mixture during the asymmetric hydrogenation.

If carbon monoxide is introduced into the reaction system, the introduction can be carried out in various ways: thus, for example, the carbon monoxide can be admixed with the hydrogen used for the asymmetric hydrogenation, or else be metered in directly in gaseous form into the reaction solution. A further option consists for example in adding compounds to the reaction mixture which readily release carbon monoxide, such as for example formates or oxalyl compounds.

The carbon monoxide is preferably admixed with the hydrogen used for the asymmetric hydrogenation.

The asymmetric hydrogenation according to the invention is advantageously carried out at a pressure of about 5 to about 200 bar, in particular about 10 to about 100 bar, specifically at about 60 to about 100 bar and a temperature of generally about 0°C to about 100°C, preferably about 0°C to about 30°C, in particular at about 10°C to about 30°C.

The selection of the solvent to be used for carrying out the asymmetric hydrogenation according to the invention is less important. In any case, advantages according to the invention also arise in different solvents. Suitable solvents are, for example, those mentioned for carrying out the preformation according to the invention. With particular advantage, the asymmetric hydrogenation is carried out in the same solvent as the preformation optionally carried out beforehand.

Suitable reaction vessels for carrying out the asymmetric hydrogenation according to the invention are in principle all those which permit reactions under the specified conditions, in particular pressure and temperature, and are suitable for hydrogenation reactions, such as, for example, autoclaves, tubular reactors, bubble columns, etc.

If the hydrogenation of the process according to the invention is carried out using high-boiling, generally viscous solvents, as are described for example above in connection with the use in the course of the pretreatment of the catalyst (for example the specified solvents octadecanol, biphenyl ether, texanol, Marlotherm®, Oxo Oil 9N) or if the hydrogenation is carried out without the additional use of solvents, but with accumulation of the high-boiling components arising as by-products to a low degree (such as, for example, dimers or trimers which are formed by reactions of the starting materials and/or the products and subsequent secondary reactions), it may be advantageous to provide good gas introduction and good thorough mixing of gas phase and condensed phase. This is achieved for example by carrying out the hydrogenation step of the process according to the invention in a gas circulation reactor. Gas circulation reactors are known per se to the person skilled in the art and are described for example in P. Trambouze, J.-P. Euzen, Chemical Reactors, Ed. Technip, 2004, p. 280-283 and P. Zehner, R. Benfer, Chem. Eng. Sci. 1996, 51, 1735-1744, and also e.g. in EP 1 140 349. In principle, it is also possible to carry out the reaction in the product as solvent, for example in citronellal when the starting material is neral or geranial.

When using a gas circulation reactor as specified above, it has proven to be particularly advantageous to introduce the gas or gas mixture (hydrogen comprising carbon monoxide) to be used in parallel to the starting materials to be introduced into the reactor and/or the circulating reaction mixture or the catalyst by means of a single nozzle or a two-material nozzle into the gas circulation reactor. Here, the two-material nozzle is notable for the fact that liquid and gas to be introduced into the reactor arrive through two separate concentric tubes under pressure to the mouth of the nozzle, where they are combined with one another.

The use or process according to the invention can be successfully carried out with and without the addition of tertiary amines. Preferably, the use or process according to the invention is carried out in the absence, i.e. without the addition of additional tertiary amines or in the presence of only catalytic amounts of additional tertiary amines. The amount of amine used here can be between 0.5 and 500 mol equivalents, based on the amount of rhodium present in the catalyst used, and preferably is 1 to 100 mol equivalents based on the amount of the rhodium. The choice of tertiary amine is not critical. As well as short-chain alkylamines, such as, for example, triethylamine, it is also possible to use long-chain alkylamines, such as for example tridodecylamine. In the context of a preferred embodiment, the hydrogenation process according to the invention is carried out in the presence of a tertiary amine, preferably tridodecylamine, in an amount of about 2 to 30 mol equivalents, preferably about 5 to 20 mol equivalents and particularly preferably 5 to 15 mol equivalents, based on the amount of rhodium present in the catalyst used.

The reaction is advantageously terminated when the target compound is present in the desired yield and the desired optical activity, i.e. with the desired enantiomer excess (ee) in the reaction mixture, as can be established by the person skilled in the art by means of routine experiments for example by means of chromatographic methods. Usually, the hydrogenation is terminated after about 1 to about 150 h, often after about 2 to about 24 h.

By means of the use or process according to the invention it is possible to provide optically active carbonyl compounds, in particular optically active aldehydes, in high yields and enantiomer excesses. Usually, the desired asymmetrically hydrogenated compounds are obtained in an enantiomer excess of at least 80% ee, often with an enantiomer excess with about 85 to about 99% ee. In this connection, it is to be noted that the maximum achievable enantiomer excess may depend on the purity of the substrate used, in particular with regard to the isomer purity of the double bond to be hydrogenated. Consequently, suitable starting substances are in particular those which have an isomer ratio of at least about 90:10, preferably at least about 95:5 with regard to the E/Z double-bond isomers.

By means of the preforming and/or by means of the carbon monoxide additionally introduced into the reaction system, the homogeneous catalyst of formulae (I), (I. a) or (l.a-1) used can be stabilized, as a result of which, on the one hand, the service life of the catalysts is considerably increased and, on the other hand, the reusability of the catalyst is facilitated.

Thus, for example, the resulting reaction product can be removed from the reaction mixture by processes known per se to the person skilled in the art, such as e.g. by distillation, and the catalyst that is left behind can be used in the course of further reactions, optionally after repeated preformation.

The process according to the invention can accordingly be operated either discontinuously or semicontinuously as well as continuously and is suitable in particular for reactions on an industrial scale.

In a particularly preferred embodiment of the use or process according to the invention, neral or geranial, which comprises up to about 5 mol%, preferably up to about 2 mol%, of the respective double-bond isomers, is converted to optically active citronellal. According to this preferred embodiment it is also preferred to use a rhodium compound of formula (I), especially of formulae (I. a) or (l.a-1), which comprises optical active bidentate ligand L¹ having an enantiomeric excess of at least 90% ee, in particular at least 95% ee. In this context it is further preferred to use a ligand L¹ that is selected from the compounds of the formulae (Ila) to (lid) and the enantiomers thereof, and in particular is the compound of formula (Ila) or its enantiomer, i.e. (R,R)-chiraphos or (S,S)-chiraphos.

In a particularly preferred embodiment of the use or process according to the invention, neral which comprises up to about 5 mol%, preferably up to about 2 mol% of geranial is hydrogenated in the presence of a rhodium compound of formula (I), especially of formulae (I. a) or (l.a-1), to yield D-citronellal, where the compound (I), (I. a) or (l.a-1) includes (R,R)-chiraphos as ligand L¹ having an enantiomeric excess of at least 90% ee, in particular at least 95% ee. In another rparticularly preferred embodiment of the use or process according to the invention, neral which comprises up to about 5 mol%, preferably up to about 2 mol% of geranial is hydrogenated in the presence of a rhodium compound of formula (I), especially of formulae (I. a) or (l.a-1), to yield L-citronellal, where the compound (I), (I. a) or (l.a-1) includes (S,S - chiraphos as ligand L¹ having an enantiomeric excess of at least 90% ee, in particular at least 95% ee. In these particularly preferred embodiments it is also preferred that the rhodium catalyst (I), (I. a) or (l.a-1) is preformed under the conditions mentioned above and then the asymmetric hydrogenation is carried out in the presence of hydrogen which comprises in particular 50 to 3000 ppm of carbon monoxide.

A further aspect of the present invention relates to a process for the preparation of optically active menthol using optically active citronellal prepared by the process according to the invention. The preparation of optically active menthol proceeding from optically active citronellal is known. A key step here is the cyclization of optically active citronellal to optically active isopulegol, as described for example in EP 1 225 163 A2. The process for the preparation of optically active menthol comprises the following steps: i) preparation of optically active citronell al by asymmetric hydrogenation of geranial of the formula (Va-1) or of neral of the formula (Vb-1 ) by the process according to the invention,

II) cyclization of the optically active citronell al prepared in this way to give optically active isopulegol in the presence of a Lewis acid, and ill) hydrogenation of the optically active isopulegol prepared in this way to give optically active menthol.

As shown below in diagrammatic form for the preparation of L-menthol of the formula (IX), optically active citronellal prepared according to the invention can be cyclized in the presence of a suitable acid, in particular a Lewis acid to give L-isopulegol of the formula (XII) and then hydrogenated to L-menthol.

A further aspect of the present invention accordingly relates to the use of optically active citronellal prepared by the process according to the invention for preparing optically active menthol. In particular, the invention relates to the use of the D-citronellal prepared by the process according to the invention for preparing optically active L-menthol.

The examples below serve to illustrate the invention without adversely affecting it in any way:

Figures 1a and 1b: ORTEP plots of the molecular structure of [Rh(chiraphos)2][Rh(CO)4]*THF caclulated from the single cry Istal X-ray data.

EXAMPLES

Abbreviations: acac: acetylacetonate

(R,R)-chiraphos: (2R,3R)-bis(diphenylphosphino)butane

THF: tetrahydrofuran

DMSO: dimethyl sulfoxide v/v: volume to volume %ee: percent enantiomeric excess Example 1.1 : Preparation of [Rh((R,R)-chiraphos)2][Rh(CO)4]

Step a): Preparation of a solution of KfRhfCO) in DMSO (Solution 1 , concentration: 13,8 mq/mL) startinq from RhCI₃

In a 250 mL three-neck flask rhodium trichloride trihydrate (40.03% Rh, 560 mg) was dissolved in 40 mL DMSO (dry) under argon to give a red-brown solution. The apparatus was then briefly purged with nitrogen and a weak flow of carbon monoxide is passed through the apparatus and potassium hydroxide (1 .5 g) was added. A weak flow of carbon monoxide was then passed through the apparatus for 7 hours while stirring, causing the suspension to change color from red-brown to orange. The gas injection was stopped overnight. The next day, finely triturated potassium hydroxide (750 mg) was added and carbon monoxide was passed through the apparatus for another 5 hours with stirring. The mixture initially turned brown, and later almost colorless towards the end of the reaction (weak orange coloration).

In a Schlenk tube [Rh((R,R)-chiraphos)2]acac (2.0 g) was dissolved under Ar in water (HPLC grade, 35 mL) and 2-propanol (35 mL) to give a red solution. To this solution was added solution 1 (35 mL), whereupon a solid precipitated immediately. After stirring for 30 min at room temperature, the yellow solid was separated using an inert gas frit. The solid was then washed with water (3 x 5 mL) and pre-dried under vacuum. Afterwards the solid was dissolved in THF (20 mL) and dried over MgSC (discoloration to green-brown). The solution was filtered and concentrated to dryness in vacuo. A brown-green solid (1.11 g, yield: 53 %) was obtained that crystallized from THF/pentane into platelets.

These crystals obtained are a solvate of [Rh((R,R)-chiraphos)2][Rh(CO)4] containing one molar equivalent of THF as revealed by single crystal X-ray diffraction summarized below.

¹H NMR (500 MHz, d₈-THF): 5 [ppm] = 7.5-7.1 (m, 40H, Ar-H), 3.7 (s, 4H, 2 CH₂), 1.9 (m, 4H, 2 CH), 0.7 (bs, 12H, CH₃);

¹³C NMR (125 MHz, d₈-THF): 5 [ppm] = 206.9 (d, J¹ _c,Rh = 74 Hz), 137.3 (s, Ar-C), 132. 6 (d, Ar-C), 132.0 (d, Ar-C), 131.0 (d, Ar-C), 129.4 (d, Ar-C), 129.0 (d, Ar-C), 38.8 (CH), 13.9 (CH₃);

³¹P-NMR (200 MHz, d₈-THF): 5 [ppm] = 60.0 (s, J² _P,_Rh = 128 Hz);

IR (Nujol): Deo = 1891 cm’¹ (vs);

Elemental analysis from solution, ealed.: Rh : P = 1 : 2.0, found: Rh : P = 1 : 1.93; structure data as summarized in Table below were obtained by single crystal X-ray diffraction analysis employing a Bruker X8 PROSPECTOR diffractometer with an APEX-II CCD detector using monochromated Cu-Ko radiation (A = 1.54178 A). The details of data collection and refinement are summarized in table 2.

The structure was solved and refined using the Bruker SHELXTL Software Package. The final anisotropic full-matrix least-squares refinement on F² with 680 variables converged at R1 = 2.73%, for the observed data and wR2 = 6.81 % for all data. Nonhydrogen atoms were assigned anisotropic thermal parameters. Hydrogen atoms at carbon atoms were placed in calculated positions.

Table 1 : crystal data for [Rh((R,R)-chiraphos)2][Rh(CO)4] x THF Absolute structure parameter 0.076(6) Largest diff. peak and hole 0.941 and -1.104 eA³ R.M.S. deviation from mean 0.080 eA³

Example 1.2: Preparation of [Rh((R,R)-chiraphos)2][Rh(CO)4]

Step a): Preparation of a solution of KfRhfCO)^ in DMSO (Solution 1 , concentration: 17 mg/mL) starting from Rh(CO)2acac

In a 250 mL three-neck flask rhodium Rh(CO)2acac (dicarbonyl(acetylacetonato)rhodium(l), 690 mg) was dissolved in 40 mL DMSO (dry) under argon to give a red-brown solution. The apparatus was then briefly purged with nitrogen and a weak flow of carbon monoxide is passed through the apparatus and potassium hydroxide (1 .5 g) was added. A weak flow of carbon monoxide was then passed through the apparatus for 7 hours while stirring, causing the suspension to change color from red-brown to orange. The gas injection was stopped overnight. The next day, finely triturated potassium hydroxide (750 mg) was added and carbon monoxide was passed through the apparatus for another 5 hours with stirring. The mixture initially turned brown, but towards the end of the reaction it became almost orange.

In a Schlenk tube [Rh((R,R)-chiraphos)2]acac (2.0 g) was dissolved under Ar in a mixture of water (HPLC grade, 35 mL) and 2-propanol (35 mL) to give a red solution. To this solution was added solution 1 (35 mL), whereupon a solid precipitated immediately. After stirring for 30 min at room temperature, the yellow solid was separated using an inert gas frit. The solid was then washed with water (3 x 5 mL) and pre-dried under vacuo. Afterwards the solid was dissolved in THF (20 mL) and dried over MgSC>4 (discoloration to green-brown). The solution was filtered and concentrated to dryness in vacuo. A brown-green solid (1.64 g) was obtained that crystallized from THF/pentane into platelets.

³¹P-NMR (200 MHz, d₈-THF): 5 [ppm] = 60.0 (s, J² _R,_Rh = 128 Hz);

IR (Nujol): Deo = 1891 cm-¹ (vs); elemental analysis from solution, ealed.: Rh : P = 1 : 2.0, found: Rh : P = 1 : 2.06.

Example 2: Preparation of Rh[(R,R)-chiraphos]2acac

In the glovebox, (R,R)-chiraphos (1 g, 9.4 mmol) and Rh(CO)2acac (1.2 g, 4.7 mmol) were each separately dissolved in absolute THF (2 x 7 mL) and then mixed while stirring, with gas evolution being observed. The resulting orange-colored solution was stirred for two hours. A yellow solid precipitated, which was isolated by filtration. The solid was washed with pentane and then dried in vacuo, yielding a yellow solid (3.65 g, yield: 74%).

¹H NMR (500 MHz, CDCI₃): 5 [ppm] = 7.5-7.1 (m, 40H, Ar-H), 1.9 (m, 4H, CH), 0.7 (bs, 12H, CH₃);

¹³C NMR (125 MHz, CDCI3): 5 [ppm] = 136.1 (s, Ar-C), 131. 2 (d, Ar-C), 131.1 (d, Ar-C), 130.3 (d, Ar-C), 128.6 (d, Ar-C), 128.1 (d, Ar-C), 37.6 (CH), 13.6 (CH3);

³¹P NMR (200 MHz, CDCI3): 5 [ppm] = 60.1 (s, J² _P,_Rh =130 Hz). Examples 3.1, 3.2 and 3.C: Asymmetric hydrogenation of neral in the presence of carbon monoxide

In each case, the amount of rhodium catalyst indicated in Table 2 was dissolved in THF (20 mL) and stirred in a 100 mL steel autoclave (V2A steel, manufacturer Premex, magnetically coupled gassing stirrer, 1000 rpm) at 80 bar synthesis gas (H2/CO = 1 :1, v/v) at 70°C for 16 hours. Each solution was then cooled to 25°C and nitrogen was passed through the solution for two hours (8 l/h). After purging with nitrogen, neral (20 g, ratio of the double bond isomers neral/geranial = 98.3:0.4) was added to the autoclave via an airlock. The reaction pressure was adjusted to 80 bar by pressurizing hydrogen gas containing 1000 ppm carbon monoxide. Conversion rates, yields and enantionmeric excesses were determinated by gas chromatography. The conversion rates of neral were determined after 4 hours of reaction time and the yields of D-citronel lai and its optical purities were also determined after 20 hours. These results are summarized in Table 2

Table 2: Summary of the Examples 3.1, 3.2 according to the invention and comparative Example 3.3

1) molar amount of catalyst based on the molar amount of substrate

2) Example 3.2 was carried out using 25 ml of a mixture of citronellal and THF (4:1 v/v) instead of 20 ml THF as solvent

Claims

Claims

1. A rhodium compound of the formula (I)

[(L¹)₂Rh]⁺k[A]^k-(l), where

L¹ is a bidentate phosphor-organic ligand and

A is a rhodium carbonyl anion optionally containing n mono- and bidentate ligands L² and n is 0, 1 or 2 k is 1 , 2 or 3.

2. The compound of claim 1 , where [A]^k- is an anion of the formula [Rh(CO)₄-n(L²)n]-, where

L² is a monodentate organic ligand other than CO or, if n = 2 two ligands L² may together form a bidentate organic ligand; n Is O, 1 or 2.

3. The compound of any one of the preceding claims, where the bidentate phosphor-organic ligand L¹ is of the formula (II), where

R^a is selected from the group consisting of Ci-Ce-alkyl and Cs-Ce-cycloalkyl;

R^b is selected from the group consisting of hydrogen and Ci-Ce-alkyl; or

R^a, R^b together with carbon atoms, to which they are bound form a 5, 6 or 7 membered carbocyclic group, a 6, 7 or 8 membered carbobicyclic group or a 5, 6 or 7 membered heterocyclic group, where the carbocyclic group, the carbobicyclic group and the heterocyclic group are unsubstituted or substituted by 1 , 2, 3 or 4 substituents R^cv^c, which are selected from Ci-Ce- alkyl, phenyl and benzyl.

Ar are identical or different and selected from the group consisting of Ce-C -ary I which is unsubstituted or carries one or more substituents which are selected from Ci-Ce-alkyl, Ca-Ce-cycloalkyl, phenyl, Ci-Ce-alkoxy, phenoxy and amino.

4. The compound of any one of the preceding claims, where the ligand L¹ is present in enantiopure form or has an enantiomeric excess of at least 90% ee.

5. The compound of any one of the preceding claims, where the ligand L¹ is selected from the group consisting of the compounds of the formula (Ila) - (lid) and the enantiomers thereof:

(ll-d)

6. The compound of any one of the preceding claims, where n is 1 or 2 and where the ligand L² is a monodentate phosphor-organic ligand or, if n = 2 two ligands L² may together form a bidentate phosphor-organic ligand.

7. The compound of claim 6, where the ligand L² is selected from triphenylphosphine, methyldiphenylphosphine, dimethyl(phenyl)phosphine, trimethylphosphite, 1 ,2- bis(diphenylphosphino)ethane, 1 ,2-bis(diphenylphosphino)ethane monooxide or a ligand of the formula (II) as defined in claim 2, or a monoxide of a ligand of the formula (II).

8. The compound of any one of claims 1 to 5, where n = 0.

9. The compound of the formula (I) as defined in claim 1, wherein [A]^k- is an anion of the formula [Rh(CO)4]', wherein k is 1 and the ligand L¹ is as defined in claim 5 and wherein L¹ is in particular 2R,3R)-(+)-bis(diphenylphosphino)butane or (2S, 3S)-(-)-bis(diphenylphosphino)butane.

10. A method for producing a compound of the formula (I) as defined in any one of the preceding claims, which comprises the reaction of a compound of the formula (III) with a compound of the formula (IV)

[(L¹)₂Rh]X (III)

M_k[A]^k- (IV) where L¹, A and k are as defined in any one of claims 1 to 9;

M is an alkalimetal ion or a quaternary or a ternary ammonium ion; and

X is an anion selected from the group of acetyl acetonate, BF4, (804)0.5,

C1-C4 alkanolate, halogenide and C1-C4 alkanoate.

11 . The use of a compound of the formula (I) as defined in any one claims 1 to 9 as a catalyst in a selective catalytic hydrogenation of an o,|3-unsaturated olefinic double bond of an o,|3-unsaturated carbonyl compound.

12. A process for a catalytic hydrogenation of an o,|3-unsaturated olefinic double bond of an o,p- unsaturated carbonyl compound, which comprises subjecting the o,|3-unsaturated carbonyl compound to a homogeneous catalytic hydrogenation in the presence of a compound of the formula (I) as defined in any one of the claims 1 to 8.

13. The use of claim 11 or the process of claim 12, where the selective hydrogenation of the o,|3- unsaturated double bond in a prochiral o,|3-unsaturated carbonyl compound is an enantioselective hydrogenation.

14. The use or process of any one of claims 11 or 13, where the o,|3-unsaturated carbonyl compound is a compound of the formula (V) in which

R⁵, R⁶ are different from one another and each is a linear, branched or cyclic hydrocarbon radical having 1 to 25 carbon atoms which is saturated or has one or more, nonconjugated ethylenic double bonds, and which is unsubstituted or carries one or more identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to Cw-aryl and Cato Cg-hetaryl;

R⁷ is hydrogen or a linear, branched or cyclic hydrocarbon radical having 1 to 25 carbon atoms which is saturated or has one or more nonconjugated ethylenic double bonds, and which is unsubstituted or carries one or more identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to Cw-aryl and C3- to Cg-hetaryl; or

R⁷ together with one of the radicals R⁵ or R⁶, can also be a 3- to 25-membered alkylene group, in which 1 , 2, 3 or 4 nonadjacent CH2 groups can be replaced by 0 or N-R^9c, where the alkylene group is saturated or has one or more nonconjugated ethylenic double bonds, and where the alkylene group is unsubstituted or carries one or more identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ci- to C4-alkyl, Ce- to Cw-aryl and C3- to Cg-hetaryl, where two substituents also together can be a 2- to 10-membered alkylene group, where the 2- to 10-membered alkylene group is saturated or has one or more nonconjugated ethylenic double bonds, and where the 2- to 10-membered alkylene group is unsubstituted or carries one or more identical or different substituents which are selected from OR⁸, NR^9aR^9b, halogen, Ce- to Cw-aryl and C3- to Cg-hetaryl; where

R⁸ is hydrogen, Ci- to Ce-alkyl, Ce- to Cw-aryl, C7- to Cw-aralkyl or C7- to Cw-alkylaryl;

R^9a, R^9b are in each case independently of one another hydrogen, Ci- to Ce-alkyl, Ce- to Cw- aryl, C7- to Cw-aralkyl or C7- to Cw-alkylaryl, or

R^9a and R^9b together can also be an alkylene chain having 2 to 5 carbon atoms, which can be interrupted by N or 0; and

R^9c is hydrogen, Ci- to Ce-alkyl, Ce- to Cw-aryl, C7- to Cw-aralkyl or C7- to Cw-alkylaryl.

15. The use or process of claim 14, where R⁷ is hydrogen.

16. The use or process of claim 15, where the compound of the formula (V) is represented by one of the following formulae (Va) and (Vb) or a mixture thereof: wherein

R⁵, R⁶ is in each case a linear or branched hydrocarbon radical having 2 to 25 carbon atoms which is saturated or has 1 , 2, 3, 4 or 5 nonconjugated ethylenic double bonds.

17. The use or process of claim 16, where the compound of the formula (V) is represented by one of the following formulae (Va-1 ) and (Vb-1 ) or a mixture thereof:

18. The use or process of any one of claims 11 to 17, which has at least one of the elements a) to c): a) pretreatment of the compound of the formula (I) before the hydrogenation with a gas mixture comprising carbon monoxide and hydrogen; b) the hydrogenation of the a,p-unsaturated carbonyl compound is effected with hydrogen which has a carbon monoxide content in the range from 50 to 3000 ppm, in particular in the range from 100 to 2000 ppm, specifically in the range from 200 to 1000 ppm and very specifically in the range from 400 to 800 ppm; c) the hydrogenation of the a,p-unsaturated carbonyl compound is effected at a hydrogen pressure of from 5 to 200 bar, in particular at a hydrogen pressure of from 10 to 100 bar.