Optically Active Alkyldiphenylphosphines as Chiral Ligands in Catalytic Asymmetric Hydrogenation. Importance of Substrate-Ligand Cooperation for Improving the Optical Yield

(-)-(R)-1-methylpropyldiphenylphosphine and (+)-(R)-1,2-dimethylpropyldiphenylphosphine have been synthesized by alkylation of the diphenylphosphide anion with the methansulphonates of the corresponding optically active alcohols. The enantiomeric excess of the phosphines, evaluated by 31P_NMR measurements on samples of chloro-{N,N-dimethyl-(S)-1-phenylethylamine-2-C,N](phosphine)palladium, was found to be substantially identical to that of the starting alcohols. The phosphines have been employed in the preparation of the cycloocta-1,5-diene-bis(phosphine)rhodium tetrafluoroborate complexes, these being used as catalysts in the asymmetric hydrogenation of (E)-alpha-methylcinnamic acid, (E)-2,3-dimethylacrylic acid, and a-ethylstyrene. The optical yields, 1-13%, depend on the structure of both the phosphines and the substrates. In the case of simple olefinic substrates the optical yields obtained either with the above phosphines or with other phosphines reported in the literature are less than 10%, thus they are practically independent of the structure of the phosphine used. In contrast the optical yields obtained with substrates bearing a carboxylic group in addition to the double bond are higher, and are remarkably affected by the structure of the phosphine.