Abstract:
A process of preparing chiral alcohols by the asymmetric catalytic hydrogenolysis of epoxides using rhodium or ruthenium catalysts containing chiral phosphine ligands.

Description:
FIELD OF THE INVENTION 
     This invention relates to a process of preparing chiral alcohols by the asymmetric catalytic hydrogenolysis of epoxides using rhodium or ruthenium catalysts containing chiral phosphine ligands. 
     SUMMARY OF RELATED ART 
     Chiral alcohols are a class of chemicals used for the synthesis of natural products and drugs. Traditionally, these compounds are made either by fermentation or by means of cost-intensive resolution of racemic mixtures. 
     Non-traditional methods of preparing chiral alcohols are disclosed in B. R. James, Adv. Organomet. Chem. Ser., 1979, 17, 319 and R. Noyori et. al., J. Am. Chem. Soc., 1987, 109, 5856, which describe the synthesis of chiral alcohols from the asymmetric hydrogenation of prochiral ketones. K. B. Sharpless et. al., Pure and Appl. Chem., Vol. 55, No. 4, 589, 1983 disclose the hydrogenolysis of chiral epoxides with LiAlH 4  or NaBH 4  to make certain types of chiral alcohols. 
     None of the above references disclose the process of preparing chiral alcohols by the present process. 
     SUMMARY OF THE INVENTION 
     The present invention is a process to prepare chiral alcohols by the asymmetric hydrogenolysis of epoxides using rhodium or ruthenium catalysts containing chiral phosphine ligands. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is illustrated by the following scheme: ##STR1## wherein R 1  is selected from hydrogen, alkyl, aryl and carboxyl-containing groups. R 2  is a carboxyl-containing group. Examples of R 1  include methyl, ethyl, propyl, phenyl, benzyl, carboxyl, carboxymethyl, carboxyethyl, carboxybutyl and their salts. Examples of R 2  include carboxyl, carboxyethyl, carboxymethyl, carboxybutyl and their salts. 
     The epoxide suitable for the present invention is any epoxide with at least one chiral center having a carboxyl-containing group. A preferred epoxide is of meso form which, upon asymmetric hydrogenolysis, produces one enantiomer of the chiral compound as a dominant species. Examples of suitable epoxides include cis-epoxysuccinic acid disodium salt, sodium 3-methyl-3-phenylglycidate, sodium 3-sec-butyl-3-methylglycidate, sodium 2,3-epoxybutyrate, 2,3-epoxy-3-(2-methoxyphenyl)propionic acid sodium salt, 2,3-epoxy-2,3-dimethylsuccinic acid disodium salt, 2,3-epoxy-1,4-butanedicarboxylic acid disodium salt, lithium 3-phenylglycidate and sodium glycidate, and other acid salts. When an essentially pure enantiomer of a non-symmetrical, chiral epoxide is used, the regiospecific hydrogenolysis can provide an essentially pure chiral alcohol product, e.g., ##STR2## where * indicates a chiral center and R is hydrogen, alkyl, aryl or carbonyl-containing groups. When a racemic mixture of non-symmetrical epoxide is used, a racemic hydroxyl product is obtained at total conversion due to the retention of the chiral center, e.g., ##STR3## where R is defined above. 
     However, at partial conversion, one diastereomer of the starting material reacts faster than the other and a kinetic resolution can take place such that the product contains a dominant enantiomer, even though the starting material contains a 50/50 mixture of the two diastereomers. 
     The catalyst suitable for the present invention is any rhodium or ruthenium asymmetric hydrogenation catalyst containing a chiral phosphine ligand. Other noble metal asymmetric hydrogenation catalysts give little or no yield of the desired chiral hydroxyl compound. Suitable rhodium catalysts include catalysts of the structure Rh(diene)(chiral phosphine)A, where diene includes norbornadiene and cyclooctadiene or two molecules of alkenes such as ethylene. A is a counter ion, such as for example BF 4 , PF 6 , ClO 4 , halides, pseudohalides and carboxylates. Chiral phosphines are phosphine ligands containing one or more chiral centers. Examples of suitable rhodium catalysts include [rhodium(1,5-cyclooctadiene)(R,R-1,2-ethanediylbis-(o-methoxyphenyl)phenylphosphine]tetrafluoroborate ([Rh(COD)(R,R-DIPAMP)]Bf 4 ), [rhodium (2,5-norbornadiene)(R-1,2-bis(diphenylphosphino)cyclohexylethane]hexafluorophospate ([Rh(NBD)(R-Cycphos)]PF 6 ), [rhodium(2,5-norbornadiene)(2R,3R-bis(diphenylphosphine)butane]perchlorate ([Rh(NBD)(R,R-Chiraphos)]ClO 4 ), [rhodium(1,5-cyclooctadiene)(2R,3R-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane]tetrafluoroborate ([Rh(COD)(R,R-Diop)BF 4 ), [rhodium(2,5-norbornadiene)(R-1,2-bis(diphenylphosphino)propane)]perchlorate ([Rh(R-Prophos]ClO 4 ), [rhodium(2,5-norbornadiene)(R-1,2-bis(diphenylphosphino)phenylethane)]perchlorate ([Rh(NBD)(R-Phenphos)]ClO 4 ), [rhodium(norbornadiene)((+)-2,3-O-Isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane)]tetrafluoroborate, ([Rh(NBD)((+)-DIOP]BF 4 ), [rhodium(norbornadiene)((R)-α-[ (s)-2,1&#39;-Bis(diphenylphosphino)ferrocenyl]ethyldimethylamine)]tetrafluoroborate, ([Rh(NBD)(BPPFA)]BF 4 ), [rhodium(norbornadiene)(s,s)-2,4-bis(diphenylphosphino)pentane)]tetrafluoroborate, ([Rh(NBD)(skewphos)]BF 4 ), [rhodium(norbornadiene)((-)-N-t-Butoxycarbonyl-4-diphenylphosphino-2-diphenylphosphosphinomethylpyrrolidine)]tetrafluoroborate, ([Rh(NBD)(BPPM)]BF 4 ), chloro(norbornadiene)(N,N&#39;-bis[(R)-α-methylbenzyl]-N,N&#39;-bis(diphenylphosphino)ethylenediamine)rhodium, (Rh(NBD)(PNNP)Cl), [rhodium(norbornadiene)(trans-4,5-bis(5H-dibenzophospholylmethyl)-2,2-dimethyl-1,3-dioxolane)]tetrafluoroborate, ([Rh(NBD)(DIPHOL)]BF 4 ), chloro(norbornadiene)((R)-(+)-2,2&#39;-bis(diphenylphosphino)-1,1&#39;-binaphthyl)rhodium, (Rh(NBD)(R-BINAP)Cl), chloro(norbornadiene)((R)-2,3-bis(diphenylphosphino)propane)rhodium, (Rh(NBD)(R-PROPHOS)Cl), chloro((s,s)-2,3-bis(diphenylphosphino)butane)rhodium dimer, ([Rh(CHIRAPHOS)Cl] 2 ), [rhodium(cyclooctadiene)((R,R)-N-benzyl-3,4-bis(diphenylphosphino)pyrrolidine)]tetrafluoroborate, ([Rh(COD)(N-Bz-pyrphos)]BF 4 ), [rhodium(norbornadiene)((R)-1,2-bis(diphenylphosphino)-cyclohexylethane)]tetrafluoroborate, ([Rh(NBD)(cycphos)]BF 4 ), chloro(norbornadiene)(N,N&#39;-bis[(s)-α-(1-naphthyl)ethyl]-N,N&#39;-bis(diphenylphosphino)ethylenediamine)rhodium, (Rh(NBD)((s)-naphthylethyl-PNNP)Cl), and dichlorotetrakis(dimethylsulfoxide)ruthenium+(s,s)-2,3-bis(diphenylphosphino)butane, (Ru(DMSO) 4  Cl 2  +CHIRAPHOS). For the preparation of [Rh(R,R-Dipamp)(COD)]BF 4 , see Vineyard, B. D., Knowles, W. S., Sabacky, M. J. Bachman, G. L. and Weinkauff, D. J., J. Amer. Chem. Soc., 1977, 99, 5046. For the preparation of [Rh(Diop)(COD)]BF 4 , see Kagan, H. B. and Dang, T. P., J. Amer. Chem. Soc., 1972, 94, 6429. For the preparation of [Rh(R-Cycphos) NBD]PF 6 , see Riley, D. P. and Shumate, R. E., J. Org. Chem., 1980, 45, 5187. A detailed description of suitable catalysts for the present invention is disclosed in &#34;Asymmetric Synthesis&#34;, Vol. 5, Ed. by James D. Morrison, Academic Press, Orlando (1985). The above references are hereby incorporated by reference. 
     The catalyst concentration is not critical and can vary widely depending on reaction conditions. The catalyst concentration can be in the range of 1×10 -6  to 5×10 -1  M, a preferred concentration being in the range of about 1×10 -4  to 5×10 -2  M. Higher catalyst concentrations increase the reaction rate. Excess catalyst has little or no adverse effect on the selectivity to the desired hydroxyl compounds. 
     The reaction temperature of the present invention is a temperature sufficient to initiate and sustain the reaction. The temperature can vary from sub-ambient temperature, (ambient being about 23° C.), to beyond the boiling point of the solvent system. A preferred reaction temperature is in the range of 0° C. to 100° C. Higher reaction temperatures increase the reaction rate. In the presence of water, higher reaction temperatures, e.g., 60° C. or higher, cause the hydrolysis of the epoxide, resulting in lower selectivity to the hydroxyl compound, while lower reaction temperatures, e.g., less than 20° C., decrease the reaction rate. 
     The reaction takes place in a solvent system suitable for solubilizing the catalyst and substrate, such as ethers, alcohols, nitriles, ketones and mixtures thereof. Examples of suitable solvents include water, tetrahydrofuran, methanol, acetonitrile and acetone. The amount of solvent present can vary widely. It is desirable to have sufficient solvent for the reaction system to be one phase. The solvent to substrate ratio can vary in the range of 1000:1 to 1:1, a preferred range being from 100:1 to 2:1. 
     The reaction pressure is atmospheric pressure or higher. A preferred range is from atmospheric (103 kPa), to about 3,000 psig (21,000 kPa), and a preferred range is from 20 psig (140 kPa) to 1000 psig (7000 kPa). When the solvent is an alcohol, the reaction can take place without an external H 2  source, since the solvent is a hydrogen source. It is preferred, however, to have an external source of hydrogen, e.g. H 2  gas, which increases the rate of reaction. The higher the H 2  pressure, the higher the reaction rate. The H 2  can be introduced to the reaction mass by any convenient means, such as pressurized sources, e.g., gas cylinders or pumps. 
     The reaction time is dependent upon catalyst concentration, substrate concentration, hydrogen pressure and reaction temperature. The reaction time ranges from about 5 minutes to about 150 h. Under reaction conditions where the reaction rate is relatively slow, it is advantageous to have a longer reaction time to favor higher conversions of the reactants. A preferred reaction time is from about 1 h to about 75 h. 
    
    
     The following examples are for illustrative purposes only and are not intended to limit the claimed invention in any manner. 
     EXAMPLES 
     Examples 1 through 17 were performed according to the following procedure: A 100 ml Fisher-Porter reaction bottle was charged with the substrate, cis-epoxysuccinic acid disodium salt, catalyst, and solvent system under a N 2  atmosphere. The bottle was pressurized with hydrogen gas as indicated, stirred and allowed to react. The crude product was obtained by evaporating the solvent. The % conversion was determined by NMR, and the enantiomer excess (e.e.) was determined by chiral GLC after derivatization. A method for GLC analysis of chiral alcohol compounds is disclosed by W. A. Konig et. al., J. Chromatogr., 1982, 238, 427. 
     
                                           TABLE 1__________________________________________________________________________Asymmetric Hydrogenolysis of cis-EpoxysuccinicAcid Disodium Salt                                          Reaction   Catalyst          Substrate                     Solvents  Temp.                                    P.sub.H.sbsb.2                                          Time %     %Ex.   (mg)              (g)  (g)       (° C.)                                    (psig, kPa)                                          (h)  Conversion                                                     e.e.__________________________________________________________________________1  [Rh(NBD)((+)-DIOP]BF.sub.4                0.5  H.sub.2 O/THF                               60   80    72   100   25.8 (D)   (50)                   (10.5/14)2  [Rh(NBD)(BPPFA)]BF.sub.4                0.5  H.sub.2 O/THF                               60   80    14   100   10.2 (D)   (50)                   (6/13)3  [Rh(NBD)((+)-DIOP)]BF.sub.4                0.5  H.sub.2 O/THF/CH.sub.3 OH                               23   200   14   100   30.2 (D)   (50)                   (7.6/14/4)4  [Rh(NBD)(skewphos)]BF.sub.4                0.36 H.sub.2 O/THF/CH.sub.3 OH                               23   600   72   100   28.5 (D)   (20)                   (7.6/6/1.65  [Rh(NBD)(skewphos)]BF.sub.4                0.84 H.sub.2 O/THF/CH.sub.3 OH                               23   20    72   100   28 (D)   (50)                   (15.8/14/3.7)6  Rh(NBD)(PNNP)Cl   0.6  D.sub.2 O/THF/CH.sub.3 OD                               60   80    14   100   33.5 (D)   (100)                  (5/10/4)7  Rh(NBD)(PNPP)Cl   0.67 H.sub.2 O/THF/CH.sub.3 OH                               25   80    14   100   37 (D)   (50)                   (6.5/4.5/4)8  [Rh(COD)(DIPAMP)]BF.sub.4                0.66 H.sub.2 O/THF/CH.sub.3 OH                               25   80    14   10    7 (L)   (50)                   (6.6/4.5/4)9  [Rh(NBD)(DIPHOL)]BF.sub.4                0.6  H.sub.2 O/THF/CH.sub.3 OH                               25   80    14   20    33 (L)   (40)                   (6.7/9.7/3.7)10 [Rh(NBD)(BPPM)]BF.sub.4                0.6  H.sub.2 O/THF/CH.sub.3 OH                               25   80    14   100   23.2 60   (40)                   (6.7/9.7/3.7)11 Rh(NBD)(R-BINAP)Cl                0.8  H.sub.2 O/CH.sub.3 OH                               25   80    14   100   19.5 (L)   (120)                  (9/16)12 Rh(NBD)(R-PROPHOS)Cl                0.8  H.sub.2 O/CH.sub.3 OH                               25   80    14   100   17.6 (L)   (80)                   (9/16)13 [Rh(CHIRAPHOS)Cl].sub.2                0.6  H.sub.2 O/THF/CH.sub.3 OH                               25   80    14   30    6 (D)   (30)                   (7.9/5.4/4.8)14 [Rh(NBD)(cycphos)]BF.sub.4                2.3  H.sub.2 O/CH.sub.3 OH                               60   60    14   100   23 (L)   (20)                   (6/13)15 [Rh(NBD)(+)-DIOP]BF.sub.4                0.37 H.sub.2 O/CH.sub.3 OH                               25   None  124  ˜5                                                     20 (D)   (50)16 RuCl.sub.2 (DMSO).sub.4 + CHIRAPHOS                0.34 H.sub.2 O/CH.sub.3 OH                               60   80    14   100   23 (D)   (50)                   (7/11)17 Rh(NBD)((S)-Naphthyl-                0.2  H.sub.2 O/CH.sub.3 OH                               23   80    72   95    59 (L)   ethyl-PNNP)Cl          (5/16)__________________________________________________________________________