A series of bis(phosphanes) (SP,SP′)-R1(Me)PCH2CH2P′R2R3 (1a−k; R1, R2, and R3 = 1-adamantyl, tert-butyl, cyclohexyl, cyclopentyl, isopropyl, methyl, phenyl; abbreviated as unsymmetrical BisP*) has successfully been synthesized, by coupling of the (RP)-configured tosylates 5a−d or mesylates 6a−g with lithiated (SP)-R1(Me)PH−BH3 adducts. Asymmetric hydrogenations catalyzed by rhodium complexes of the unsymmetrical BisP* moieties as ligands revealed extremely high enantioselectivities − 99% (9b) and 98% (9e) − when the trisubstituted and tetrasubstituted dehydro-α-amino acid derivatives 8b and 8e, respectively, were used as substrates. It was found that unsymmetrical BisP* species tended to exhibit higher enantioselectivity than C2-symmetrical BisP* species in the Rh-catalyzed hydrogenation of (Z)-dehydro-β-amino acid and enamide derivatives. These results implied that the differentiation between the chiral environments at the two phosphorus atoms could effectively achieve higher enantioselectivity. For each substrate, moreover, it was possible to improve the enantioselectivity by changing the combination of substituents on the two phosphorus atoms. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)