Structural requirements in chiral diphosphine-rhodium complexes. VII. Use of Z-methyl-α-acylaminocinnamtes as structural probes for Diop-rhodium(I) complexes.

(+)-(25,3S)-DIOP was used in neutral rhodium(I)-diphosphime complexes to catalyze the asymmetric hydrogenation of Z-methyl-u-acylaminocimnamates. Increasing steric bulk in the acyl function (NHCOR, where R is a hydrocarbon moiety) resulted in a decrease in optical purity of the N-acylphenylalanine methyl ester products. The optical purity decreased from 69 % ee (S) [Me], 15 % ee (S) [i-Pr], to 0 % ee [t-Bu and 1-adamantyl]. The α-formamido substrate decreased in optical purity [58 % ee (S)] relative to the Me analogue. The a-trifluoroacetamido analogue gave a reversal in chirality [22 % ee (R)].