Ferrocenyl Diphosphines Containing Stereogenic Phosphorus Atoms. Synthesis and Application in the Rhodium-Catalyzed Asymmetric Hydrogenation

Enantiomerically pure (R)-2-methoxyphenyl(phenyl)-O-methylphosphinite borane (1a) and (R)-1-naphthyl(phenyl)-O-methylphosphinite borane (1b) react with 1,1‘-dilithioferrocene to give (S,S)-1,1‘-bis(2-methoxyphenyl(phenyl)phosphino)ferrocene borane (2a) and (S,S)-1,1‘-bis(1-naphthyl(phenyl)phosphino)ferrocene borane (2b). Deboranation with morpholine yields the diphosphines (S,S)-1,1‘-bis(2-methoxyphenyl(phenyl)phosphino)ferrocene (3a) and (S,S)-1,1‘-bis(1-naphthyl(phenyl)phosphino)ferrocene (3b) in high yields and high diastereo- and enantiomeric purity, as determined on the corresponding phosphine oxides 4a,b. X-ray investigations support the (S,S) configuration of 2a and disclose the coordination properties of 3a in the platinum derivative [PtCl2(3a)] (5a). The rhodium(I) derivatives [Rh(COD)(3a,b)]BF4 (6a,b) have been prepared and tested in the enantioselective catalytic hydrogenation of various olefins and ketones. Excellent enantioselectivity is achieved in the asymmetric hydrogenation of methyl (Z)-α-N-methyl-acetamidocinnamate to (R)-N-methyl-N-acetylphenylalanine methyl ester with 6a (97% ee). Methyl (Z)-α-acetamidocinnamate is hydrogenated to (R)-N-acetylphenylalanine methyl ester with up to 96% ee with 6b as the catalyst.