Asymmetric hydrogenation catalyzed by (S,S)-R-BisP*-Rh and (R,R)-R-miniPHOS complexes: Scope, limitations, and mechanism

A new class of chiral C-2-symmetric bis(trialkyl)phosphine ligands has been prepared and used in Rh(I)-catalyzed asymmetric hydrogenation reactions. T he Ligands, 1,2-bis(alkylmethylphosphino)ethanes 1 a-g (abbreviated as BisP *, alkyl = t-butyl, 1-adamantyl, 1-methylcyclohexyl, 1,1-diethylpropyl, cyc lopentyl, cyclohexyl, isopropyl) and 1,2-bis(alkylmethylphosphino)methanes 2 a-d (abbreviated as MiniPHOS, alkyl = t-butyl, cyclohexyl, isopropyl, phe nyl) are prepared by a simple synthetic approach based on the air-stable ph osphine-boranes. These new ligands give the corresponding Rh(I) complexes, which are effective catalytic precursors for the asymmetric hydrogenation o f a representative series of dehydroamino acids and itaconic acid derivativ es. Enantioselectivities observed in these hydrogenations are universally h igh and in many cases exceed 99%. X-Ray characterization of four precatalys ts, study of the pressure effects, deuteration experiments, and characteriz ation of the wide series of intermediates in the catalytic cycle are used f or the discussion of the possible correlation between the structure of the catalysts and the outcome of the catalytic asymmetric hydrogenation.