Asymmetric Hydrogenation of Aromatic-Substituted Olefins Using Organorhenium Catalyst

This invention relates to a process for the catalytic reduction of aromatic-substituted olefins. More specifically, this invention relates to a process for asymmetrically, catalytically reducing aromatic-substituted olefins using a mixture of a rhenium halide and an optically active organophosphorous compound.

Enantioselective catalysis using chiral metal complexes provides one of the most general and flexible methods for achieving asymmetric organic reactions. Metallic elements possess a variety of catalytic activities, and permutations of 15 organic ligands or other auxiliary groups directing the steric course of the reaction are practically unlimited. Efficient ligands must be endowed with, for example, suitable functionality, appropriate chirality, a structure capable of differentiating space either electronically or sterically and skeletal rigidity or flexibility.

Among the asymmetric organic reactions catalyzed by chiral transition metal complexes, asymmetric hydrogenation has been one of the best studied, due in large part to the fact that it is the basis for the first commercialized catalytic asymmetric process. See, for example, ApSimon, et al., Tetrahedron, 1986, 42, 5157.

Some of the more interesting of the asymmetric hydrogenation catalysts are those derived from BINAP [2,2'-bis(diphenylphosphino)-1,1'-binaphthyl]. See, for example, U.S. Pat. Nos.: 4,691,037; 4,739,084; 4,739,085; 4,764,629; 4,994,607; and 4,766,227. Unlike the more classical models of chiral (asymmetric) molecules, chirality in the case of the BINAP compounds arises from the restricted rotation about the single bond joining the naphthalene rings. Isomers arising from this type of asymmetry are termed atropisomers.

BINAP-based Ru(II) and Rh(I) complexes induce high enantioselectivity in catalytic reactions. See Noyori and Takaya, Acc. Chem. Res., 1990, 23, 345.

The BINAP ruthenium complexes are dramatically different than the rhodium ones. They have been used to catalyze a variety of asymmetric hydrogenations, including the hydrogenation of enamides and alkyl and aryl-substituted acrylic acids. See Noyori, et al., Modern Synthetic Methods, 1989, 5, 115, incorporated herein by reference. Unlike the rhodium catalyzed reductions, ruthenium(II) carboxylate complexes possessing the BINAP ligand are efficient catalysts for the enantioselective hydrogenation of .alpha.,.beta.-unsaturated carboxylic acids. According to Ohta, et al, J. Org. Chem, 52, 3174 (1982), the carboxyl moiety of the substrate, and not other oxygen containing groups, is responsible for the stereoselective reaction. Asymmetric reductions of noncarboxyl-containing substrates by ruthenium complexes are inefficient.

While the chiral phosphine complexes of rhodium and ruthenium are well known to catalyze asymmetric reactions, rhenium has not been established as a metal capable of forming complexes that promote such enantioselective transformations.

The present invention involves a novel method for the use of rhenium compounds, when admixed in an appropriate solvent with ligands having optical activity, can be used as in situ catalyst to effect the asymmetric reduction of certain unsaturated organic compounds.

In the present specification, alkyl means straight or branched chain alkyl having 1 to 20 carbon atoms and includes, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secondary butyl, tertiary butyl, pentyl, isopentyl, neopentyl, hexyl, heptyl, octyl, 2-ethylhexyl, 1,1,3,3-tetramethylbutyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, and eicosyl.

Cycloalkyl means cyclic alkyl having 3 to 7 carbon atoms and includes cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and cycloheptyl.

Substituted aryl means phenyl or naphthyl substituted by at least one substituent selected from the group consisting of halogen (chlorine, bromine, fluorine or iodine), amino, nitro, hydroxy, alkyl, alkoxy, which means straight or branched chain alkoxy having 1 to 10 carbon atoms, and includes, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, secondary butoxy, tertiary butoxy, pentyloxy, isopentyloxy, hexyloxy, heptyloxy, octyloxy, nonyloxy and decyloxy, haloalkyl which means straight or alkyl having 1 to 8 carbon atoms which is substituted by at least one halogen, and includes, for example, chloromethyl, bromomethyl, fluoromethyl, iodomethyl, 2-chloroethyl, 2-bromoethyl, 2-fluoroethyl, 3-chloropropyl, 3-bromopropyl, 3-fluoropropyl, 4-chlorobutyl, 4-fluorobutyl, dichloromethyl, dibromomethyl, difluoromethyl, diiodomethyl, 2,2-dichloroethyl, 2,2-dibromoethyl, 2,2-difluoroethyl, 3,3-dichloropropyl, 3,3-difluoropropyl, 4,4-dichlorobutyl, 4,4-difluorobutyl, trichloromethyl, trifluoromethyl, 2,2,2-trifluoroethyl, 2,3,3-trifluoropropyl, 1,1,2,2-tetrafluoroethyl and 2,2,3,3-tetrafluoropropyl.

Haloalkyl means straight or branched chain alkyl having 1 to 10 carbon atoms which is substituted by at least one halogen as mentioned above.

Phenylalkyl means that the alkyl moiety is straight or branched chain alkyl having 1 to 8 carbon atoms and includes, for example, benzyl, 2-phenylethyl, 1-phenylethyl, 3-phenylpropyl, 4-phenylbutyl, 5-phenylpentyl, 6-phenylhexyl and 8-phenyloctyl.

Substituted phenylalkyl means above-mentioned phenylalkyl which is substituted by at least one substituent selected from the group consisting of halogen, amino, nitro, hydroxy, alkyl, alkoxy and haloalkyl on the phenyl nucleus.

Chiral phosphine compound means an optically active alkyl or aryl substituted trivalent phosphorus compound. Examples of such compounds are:

1,2-ethanediyl-bis(o-methoxyphenyl)phenylphosphine (DIPAMP);

N,N'-bis(.alpha.-methylbenzyl)-N,N'-bis(diphenylphosphine) ethylenediamine (PNNP);

2,3-bis(diphenylphosphino)butane (CHIRAPHOS);

1,2-bis(diphenylphosphino)propane (PROPHOS);

2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenyl-phosphino)butane (DIOP);

2,4-t-butyl 4-(diphenylphosphino)-2-(diphenylphosphinomethyl)-1-pyrrolidine-carboxylat e (BPPM);

2,4-bis(diphenylphosphino)pentane (SKEWPHOS);

2,5-bis(diphenylphosphino)hexane (BDPH);

1,2-bis(diphenylphosphino)-1-phenylethane (PHENPHOS);

1,2-bis(diphenylphosphino)-1-cyclohexylethane (CYCPHOS);

.alpha.-[1,2-bis(diphenylphosphino)ferrocenyl]-ethyldimethylamine (BPPFA); and

trans-4,5-bis[(5H-dibenzophospholyl)methyl]-2,2-dimethyl-1,3-dioxolane (DIPHOL).

A detailed description of suitable phosphines for the present invention is disclosed in Asymmetric Synthesis, Vol. 5, Ed. by James D. Morrison, Academic Press, Orlando (1985), incorporated herein by reference.

The enantioselective preparations of the present invention are carried out optionally using amine salts of .alpha.-aryl olefinic carboxylic acids. When amine salts are used, they are derived from a wide variety of primary, secondary or tertiary hydrocarbyl amines. They include the aromatic amines, aliphatic amines, mixed aromatic/aliphatic amines, as well as heterocyclic and cycloaliphatic amines. Such hydrocarbyl amino compounds are illustrated by methylbenzylamine, ethyldiisopropylamine, N-methylpiperidine, triethylamine, pyrrole, etc. They react readily with the carboxylic acid function of the .alpha.-aryl olefinic carboxylic acid to produce amine salts, usually by preparing a solution of equimolar amounts of the two reactants. The resulting amine salts, generated in situ or preformed, are used in the subsequent step of the process of this invention.

The carboxylic acids have the formula ##STR2## where R and R.sub.1 are the same or different and are hydrogen, alkyl, cycloalkyl, or haloalkyl and Ar is aryl or substituted aryl. Preferably R and R.sub.1 are the same or different and are hydrogen or alkyl. The amine salts are also useful in the process of the present invention. Most preferred in the above carboxylic acid is where R and R.sub.1 are the same and are hydrogen or methyl. They are reduced (hydrogenated) asymmetrically by a catalytic process employing a mixture of (i) a rhenium compound and (ii) an optically active ligand in an appropriate solvent where the ligand is ##STR3## where R' is hydrogen (BINAP), alkyl, haloalkyl, aryl or substituted aryl. It is neither necessary nor economically desirable to isolate the chiral metal catalyst that may be formed in this mixture prior to hydrogenation of the substrate.

Rhenium-containing compounds of use in this invention include the halides such as rhenium(III) chloride, rhenium(IV) chloride, rhenium(V) chloride, rhenium (VI) chloride, I5 rhenium(III) bromide, rhenium(IV) bromide, rhenium(V) bromide, rhenium(III) (2,4-pentanedionate), rhenium(IV) (2,4-pentanedionate) or rhenium(V) (2,4-pentanedionate).

The preferred rhenium compounds of use in the process of the present invention are the rhenium halides. The most preferred is rhenium(V) chloride.

The chiral organophosphorous compound admixed with the rhenium compound is preferably one where all R' are the same and are hydrogen or C.sub.1 to C.sub.6 linear or branched alkyl. Most preferably all R' are the same and are hydrogen, methyl, ethyl, propyl or isopropyl.

The asymmetric catalytic hydrogenations utilizing the catalyst mixture of (i) and (ii) above is mixed with a solution of an .alpha.-aryl olefinic carboxylic acid or its amine salt, typically in a molar ratio of (i):(ii) of 10:1 to 1:10, preferably 8:1 to 1:8, most preferably 1:1.

The molar ratio of (i) to the olefinic acid or its amine salt is between about 1 to 20 to about 1 to 20,000, preferably about 1 to 100 to about 1 to 10,000, most preferably about 1 to 5,000 to about 1 to 10,000.

The combination of the catalyst mixture, the olefinic carboxylic acid (or amine salt of such acid) and suitable organic solvent, provide a system suitable for hydrogenation at elevated hydrogen pressure, i.e., pressures above about 75 psig.

To achieve enantioselective hydrogenation of a free .alpha.-olefinic carboxylic acid, a mixture of (i) and (ii) in the hydrogenation solvent must be given time (typically 1 to 5 hours) to become activated, either with or without hydrogen pressure at room temperature or at elevated temperature, before the substrate is introduced.

The chemical reactions described above are generally disclosed in terms of their broadest application to the preparation of the compounds of this invention. Occasionally, the reactions may not be applicable as described to each compound included within the disclosed scope. The compounds for which this occurs will be readily recognized by those skilled in the art. In all such cases, either the reactions can be successfully performed by conventional modifications known to those skilled in the art, e.g., by appropriate protection of interfering groups, by changing to alternative conventional reagents, by routine modification of reaction conditions, and the like, or other reactions disclosed herein or otherwise conventional, will be applicable to the preparation of the corresponding compounds of this invention. In all preparative methods, all starting materials are known or readily prepared from known starting materials.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

All solvents used in the hydrogenation were reagent grade and were sparged with nitrogen for at least 2 hours to remove oxygen. The S-BINAP and rhenium(V) chloride used were commercially available materials. Conversions were determined by GC (area %). Optical purities were determined by HPLC using a chiral column. To a 100-ml pressure reactor in a nitrogen-filled glove box was added 22 mg (0.061 mmol) of ReCl.sub.5 [rhenium(V)-chloride], 43 mg (0.069 mmol) of S-BINAP, 590 mg (1.67 mmol) of tri-n-octylamine, 15 ml of tetrahydrofuran (inhibited), 15 ml of methanol and 312 mg (1.53 mmol) of 2-(4-isobutylphenyl)acrylic acid (UA). The reactor was purged With hydrogen and stirred (300 rpm) at 22.degree. C. for 44 hours with 1000 psi hydrogen. GC showed that 5% ibuprofen was formed. The temperature was raised to 60.degree. C. and the reactor was stirred at 60.degree. C. for 48 hours with 1000 psi hydrogen. The conversion of UA to ibuprofen was 55% and the optical purity was 38% (S).