Structural requirements in chiral diphosphine-rhodium complexes. IV. Use of Z-α-acetamidocinnamate esters as structural probes for diop-rhodium(I) complexes

(-)-2,3-Isopropylidene-2,3-dihydroxy-l,4-bis (diphenylphosphino)butane (DIOP) was used in neutral rhodium(I)-diphosphine complexes to catalyze the asymmetric hydrogenation of Z-α-acetamidocinnamate esters. Increasing the steric bulk of the alcohol moiety in the unsaturated esters resulted in enhancement of the optical purity in the N-acetylphenylalanine ester products without changing the chirality of the major product. In the series Me, Et, i-Pr and t-Bu, the optical purity increased from 69% ee (Me) to a maximum of 77% ee (t-Bu), while 1-adamantyl gave 71% of the R-isomer.