Preparation of Optically Active Citronellal

1. Field of the Invention

The present invention relates to a process for the preparation of optically active citronellal, which is also referred to as chiral citronellal, by the asymmetric hydrogenation of neral [E-3,7-dimethyloctadien-2,6-al] or geranial [Z-3,7-dimethyloctadien-2,6-al] which are achiral isomeric constituents of citral.

2. Description of the Prior Art

Chiral citronellal is a valuable intermediate in organic synthesis; in particular, d-citronellal is used for the preparation of (-) (1S)-menthol, hereafter simply 1-menthol, by a process which employs the cyclization of d-citronellal into (-) (1S)-isopulegol under the influence of a proton catalyst, or by a thermal route, followed by the hydrogenation of the isopulegol into 1-menthol [cf., J. C. Leffingwell and R. E. Shackelford, Cosmetics and Perfumery, 89, 70-78 (1974)].

The d-citronellal used for the synthesis of 1-menthol is obtained from various essential oils largely comprised of d-citronellal and, in particular, from citronella oil. The use of chiral citronellal of natural origin is not entirely satisfactory insofar as, because of the fluctuation in the prices of natural products, it periodically happens that the price of d-citronellal is higher than that of natural menthol. It is therefore of importance to industry to have available a source of chiral citronellal which has a relatively stable price and leads to a synthetic 1-menthol having a lower cost price than that of natural menthol.

The synthetic achiral citronellal obtained via the hydrogenation of citral [a mixture of neral and geranial] could constitute a valuable source of chiral citronellal and especially of d-citronellal; however, no industrial process exists for resolving racemic citronellal into its enantiomers, with the result that industry does not have any process available which makes it possible to obtain chiral citronellals by a synthetic route.

Accordingly, a primary object of the present invention is to provide a process for the preparation of optically active citronellal, by hydrogenating neral or geranial in the presence of a catalyst comprising a complex which is soluble in the reaction medium and which is formed from a rhodium derivative and a chiral phosphine.

The term chiral phosphine denotes a phosphine or diphosphine in which at least one of the organic radicals bonded to the phosphorous atom contains at least one chiral carbon atom, and/or in which at least one of the phosphorus atoms is chiral.

The soluble complex of the rhodium derivative and the chiral phosphine can be prepared for immediate use or formed "in situ" under the reaction conditions by merely employing the constituents of the complex. The latter procedure, which has the advantage of simplicity, is generally preferred.

Rhodium derivatives containing radicals of a diverse nature are used as rhodium derivatives which are suitable for carrying out the process according to the invention. They can be rhodium salts of mineral or organic acids or rhodium complexes in which the ligands can be replaced by the chiral phosphine. It is possible to use, for example, rhodium halides such as hydrated rhodium trichloride, and complexes of rhodium with olefins, these complexes having the general formula:

in which X represents a halogen atom, for example, chlorine or bromine, x is an integer from 1 to 4 and L is an olefin or an aliphatic or cycloaliphatic diolefin, such as ethylene, propylene, butene, isobutene, butadiene, hexa-1,5-diene, hepta-1,4-diene, octa-1,5-diene, isoprene, cyclohexa-1,3-diene or cycloocta-1,5-diene; examples of such complexes which may be mentioned are .mu.,.mu.'-dichloro-bis-(cyclohexa-1,3-dienerhodium), .mu.,.mu.'-dichloro-bis-(cycloocta-1,5-dienerhodium) and .mu.,.mu.'-dichloro-bis-(diethylenerhodium); it is also possible to use rhodium carbonyl complexes and their derivatives such as those corresponding to the general formula:

in which L.sub.1 represents a mono- or poly-dentate ligand and especially an achiral phosphine of the general formula:

in which R represents an achiral alkyl, cycloalkyl or aryl radical having from 1 to 10 carbon atoms, such as the methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, cyclohexyl, phenyl or toluyl radicals. L.sub.1 is preferably triphenylphosphine. The various rhodium carbonyls, and especially tetrarhodium dodecacarbonyl and hexarhodium hexadecacarbonyl, are preferably used from among the above-mentioned rhodium derivatives. Rhodium complexes in the cationic form can also be used.

Both monophosphines and diphosphines can be used as the chiral phosphine which is suitable for carrying out the invention. Diphenylmenthylphosphine, phenyldimenthylphosphine and trimenthylphosphine are mentioned as examples of chiral monophosphines. However, it is preferred to use chiral diphosphines of the general formula: ##STR1## in which: R.sub.1 and R.sub.2, which are identical or different, represent hydrocarbon radicals having from 1 to 15 carbon atoms and A represents a valence bond or a divalent organic radical which is optionally substituted by one or more inert functional groups, at least one of the radicals R.sub.1, R.sub.2 and A being chiral.

More specifically, R.sub.1 and R.sub.2, which are preferably identical, represent alkyl radicals having from 1 to 10 carbon atoms [methyl, ethyl, isobutyl, sec.-butyl, sec.-pentyl or 2-ethylhexyl radicals], cycloalkyl radicals having from 4 to 8 cyclic carbon atoms [cyclobutyl, 1-methylcyclobutyl, cyclohexyl, 1-methylcyclohexyl or 2-methylcyclohexyl radicals], or aryl or alkylaryl radicals [phenyl, naphthyl or toluyl radicals]. A represents:

a linear or branched alkylene radical having from 1 to 10 carbon atoms, a cycloalkylene radical which has from 3 to 7 cyclic carbon atoms and is optionally substituted by 1 to 3 alkyl groups having from 1 to 4 carbon atoms, an arylene radical or a divalent polycyclic radical, these radicals being optionally substituted by 1 or more inert functional groups and, in particular, by 1 to 3 alkoxy groups having from 1 to 4 carbon atoms,

a divalent heterocyclic group [pyridylene or 1,3-dioxacyclopent-4,5-ylene] having 1 or 2 hetero-atoms such as oxygen and/or nitrogen,

a linkage consisting of 1 or more divalent alkylene and/or cycloalkylene and/or heterocyclic and/or polycyclic radicals such as those defined previously, or

a linkage consisting of alkylene groups such as those defined previously and tertiary amino groups which can optionally be bonded directly to the phosphorus atoms via the nitrogen atom.

Methylene, ethylene, propylene and 2-ethylpropylene radicals may be mentioned as examples of alkylene radicals; A can also represent a cyclobutylene, cyclohex-1,4-ylene or 2-methylcyclohex-1,4-ylene radical, or ortho- or p-phenylene radical or a 2,3-dimethoxybut-1,4-ylene radical.

As examples of divalent chiral radicals A formed by a linkage consisting of alkylene and cycloalkylene or heterocyclic, polycyclic or amino radicals, there are mentioned those of the formulae ##STR2##

Among the chiral diphosphines which can be used in the process according to the invention, the following are mentioned strictly as illustrative: 1,2-bis-(diphenylphosphinomethyl)-cyclobutane (DPCB), 1,2-bis-(dimethylphosphinomethyl)-cyclobutane, 1,2-bis-(di-n-butyl-phosphinomethyl)-cyclobutane, 1,2-bis-(dioctylphophinomethyl)-cyclobutane, 1,2-bis(ditolylphosphinomethyl)-cyclobutane, 1,2-bis-(dinaphthylphosphinomethyl)-cyclobutane, 1,2-bis-(ethylhexylphosphinomethyl)-cyclobutane, 1,2-bis-(diphenylphosphinomethyl)-cyclopentane, 1,2-bis-(diphenylphosphinomethyl)-cyclohexane, 4,5-bis-(dimethylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane, 4,5-bis-(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane (DIOP), 4,5-bis-(ditolylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane, 1,2-bis-(dimethylphosphinomethyl)-acenaphthene, 1,2-bis-(dibutylphosphinomethyl)-acenaphthene, 1,2-bis-(diphenylphosphinomethyl)-acenaphthene (DPA), 1,2-bis-(ditolylphosphinomethyl)-acenaphthene, 1,4-bis-(diphenylphosphino)-2,3-dimethoxybutane (DDB), tetramenthyl diphosphine and bis-(N,N'-diphenylphosphino)-bis-[N,N'-(1-phenylethyl)]-1,4-diazabutane.

From among those phosphines previously mentioned, the 1,2-bis-(diarylphosphinomethyl)-cyclobutanes described in French Pat. No. 73/18,319, hereby expressly incorporated by reference, are preferably used.

Methylcyclohexyl-ortho-methoxyphenylphosphine, methylcyclohexylphenylphosphine and benzylphenylmethylphosphine are mentioned as examples of phosphines having chiral phosphorus atoms.

The complexes derived from Rh.sub.4 (CO).sub.12 or Rh.sub.6 (CO).sub.16 and from 1,2-bis-(diarylphosphinomethyl)-cyclobutanes are very particularly suitable for the asymmetric hydrogenation of neral and geranial to give enantiomers of citronellal, because they simultaneously provide a high hydrogenation rate, a good selectivity with respect to citronellal and a good optical purity.

The amount of rhodium derivative employed in the process of the invention, and expressed in gram atoms of metal per mol of diene aldehyde to be hydrogenated, can vary over wide limits. Whether the preformed complex is used or whether the derivative which is capable of generating this complex under the reaction conditions is used, the amount can be selected such that the number of gram atoms of rhodium per mol of aldehyde is between 1.times.10.sup.-4 and 1.times.10.sup.-1.

When the rhodium derivative/chiral phosphine complex is prepared "in situ", the amount of phosphine employed in the process depends on the nature of the phosphine and on that of the rhodium derivative. This amount, expressed as the number of gram atoms of phosphorus per gram atom of rhodium, is such that this ratio can vary between 0.5 and 10; the ratio of P/Rh is preferably between 1 and 6. However, ratios of P/Rh which are greater than 10 could be employed without departing from the scope of the present invention, but no particular advantage would be gained thereby.

The temperature at which the hydrogenation is carried out is not critical and can also vary over wide limits. It is generally between 0.degree. and 150.degree. C. and preferably between 10.degree. and 100.degree. C. The same applies to the hydrogen pressure which can vary between 0.1 and 100 bars, and preferably between 0.5 and 50 bars.

Although the diene aldehyde which is subjected to the asymmetric hydrogenation should preferably be as pure as possible, that is to say, virtually free from its isomer, it is possible to employ neral containing up to 15% of geranial, and vice versa.

Similarly, it is preferable to use a chiral phosphine which does not contain its enantiomer, although it is possible to carry out the reaction with a chiral phosphine containing less than 15% of its enantiomer.

The asymmetric hydrogenation of neral or geranial is preferably carried out in a solvent which is inert towards the aldehyde and the catalyst. Hydrocarbons [hexane, heptane, cyclohexane, benzene and toluene], alcohols [methanol and ethanol] and nitriles [acetonitrile and benzonitrile] are mentioned as examples of solvents.

In order to further illustrate the present invention and the advantages thereof, the following specific examples are given, it being understood that the same are intended only as illustrative and in nowise limitative. In these examples, the term optical purity OP will denote the ratio of the rotatory power [.alpha..sub.1 ].sub.D of the product obtained by the process to the rotatory power [.alpha.].sub.D of the pure product measured under the same conditions, this ratio being multiplied by 100; that is to say

The term optical yield denotes the value of the optical purity of the product which would be obtained by using an optically pure phosphine.

18.2 mg of Rh.sub.6 (CO).sub.16 (1.02.times.10.sup.-4 g atoms of Rh) and 67.5 mg of (+)-(DPCB), that is to say, 0.15 millimol, were introduced into a 50 cm.sup.3 glass round-bottomed flask equipped with a dip-tube gas inlet, a thermometer, a magnetic stirrer and a glass stopper which makes it possible to add reagents or withdraw samples of the reaction mixture by means of a syringe, the apparatus was then purged with nitrogen and 20 cm.sup.3 of toluene were injected. The contents of the flask were stirred for 1 hour under a nitrogen atmosphere and 1.79 g [that is to say 11.77 millimols] of geranial containing 5% of neral were then added. The apparatus was purged with hydrogen and the contents of the flask were then kept under a hydrogen pressure of 1 bar for 4 hours at 25.degree. C. The reaction was stopped and the reaction mixture was analyzed by gas phase chromatography; the degree of conversion of the geranial was 100% and the yield of citronellal was 99%. The solvent was evaporated off and the residue was then distilled under reduced pressure. 1.28 g of 1-citronellal having a rotatory power [.alpha.].sub.D.sup.25 of -8.76.degree. (measured on a solution in hexane containing 6 g per 100 cm.sup.3) and [.alpha.].sub.D.sup.25 of -9.1.degree. (measured in the absence of solvent) are thus collected. The rotatory power [.alpha.].sub.D.sup.25 of pure 1-citronellal, measured on a solution in cyclohexane containing 6 g/100 cm.sup.3, was -15.6.degree.. On the basis of this value, the OP of the product obtained was 56%. The rotary power [.alpha.].sub.D.sup.25 of pure 1-citronellal (determination without solvent) was -16.degree. [cf. Donell et al., Australian J. Chem., 19, 525 (1966)].

The procedure of Example 1 was followed, using the following amounts:

After distillation, 9.6 g of citronnellal were collected, for which [.alpha.].sub.D.sup.25 =+10.15.degree. (solution in hexane containing 6 g/100 cm.sup.3), this being an optical purity of 65%.

The procedure of Example 1 was followed, the (+)-DPCB being replaced by (+)-DIOP. The ratio of geranial/Rh was 120 and ratios of P/Rh of 4 and 6 were used successively.

Under these conditions, the following results were obtained:

The procedure of Example 1 was followed, (+)-DPCB being replaced by (-)-DPCB. The ratio of the number of mols of geranial (G) to the number of gram atoms of rhodium (G/Rh) was equal to 123 and the ratio of P/Rh was equal to 4. The (-)-DPCB had an optical purity of 95.5%.

The reaction time was 18 hours, the degree of conversion of the geranial was 99% and the yield of citronellal is 99%. The optical purity of the d-citronellal obtained was 49% ([.alpha.].sub.D.sup.25 =+7.7.degree., solution in hexane containing 6 g/100 cm.sup.3). If the purity of the phosphine was taken into account, the optical yield increased to 52%.

The procedure of Example 5 was followed, the geranial being replaced by neral and the other conditions being otherwise identical. The following results were obtained:

The reaction was carried out in accordance with the method of operation of Example 1 and under the same pressure and temperature conditions, Rh.sub.6 (CO).sub.16 being replaced by Rh.sub.4 (CO).sub.12. The ratio of P/Rh is 3 and the ratio of G/Rh was 120. The results were as follows:

The procedure of Example 7 was followed, the geranial being replaced by neral. (The ratio of N/Rh was 140.) The following results were obtained:

The procedure of Example 1 was followed, bringing the ratio of N/Rh to 750 instead of 115 and the ratio of P/Rh to 2. The following results were obtained:

The reaction was carried out in accordance with the method of operation and the temperature and pressure conditions of Example 1, varying the nature of the aldehyde, the chiral phosphine and the rhodium derivative. The other conditions and the results obtained are included in the following table:

A 35 cm.sup.3 glass ampoule containing 10 cm.sup.3 of toluene, 18.3 mg of (-)-DPCB, 91.9 mg of RhH(CO) (P.phi..sub.3).sub.3 and 1.91 g of geranial were introduced into a 125 cm.sup.3 stainless steel autoclave equipped with a system for agitation by shaking. The autoclave was closed and hydrogen was introduced up to a pressure of 25 bars. These conditions were maintained for 17 hours, the autoclave was then degassed and the contents of the ampoule were treated and analyzed as in Example 1.

The degree of conversion of the geranial was 69%, the yield of citronellal relative to the geranial converted was 99% and the optical purity was 60% (rotatory power [.alpha.].sub.D.sup.25 =+6.6.degree. measured on the pure product).

While the invention has been described in terms of various preferred embodiments, the skilled artisan will appreciate that various modifications, substitutions, omissions, and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following claims.