The mechanism of asymmetric hydrogenation. Chiral bis(diphenylphosphino)-α-phenylalkane complexes in catalytic and structural studies

(R)-1,2-Bis(diphenylphosphino)phenylethane has been synthesised in good overall yield from (S)-mandelic acid [hydroxy(phenyl)acetic acid]; its cationic rhodium(I) solvated complex has been shown to be effective in catalytic asymmetric hydrogenation with optical yields of up to 88% having been observed. The n.m.r. spectra of complexes of this phosphine are discussed. Racemic (R*,R*)-1,3-bis(diphenylphosphino)-1,3-diphenylpropane and (R*,R*)-1,4-bis(diphenylphosphino)-1,4-diphenylbutane have been prepared. These form a range of rhodium complexes with dehydroamino-acids whose structures may be assigned on the basis of characteristic n.m.r. P–Rh and P–P coupling constants.