Structural requirements in chiral diphosphine-rhodium complexes—XI: Asymmetric homogeneous hydrogenation of z-α-acylaminocinnamic acids and esters with (1s, 2s)-trans-1,2-bis(diphenylphosphinomethyl) cyclohexane/rhodium(i) complexes

Z-α-acylaminocinnamate esters were hydrogenated with neutral rhodium(I) complexes containing (1S, 2S)-trans-1,2-bis(diphenylphosphinomethyl)cyclohexane. Increasing the steric bulk of the alcohol moiety of the ester function results in increased enantioface differentiation in favor of the re-si prochiral face to yield an excess of the S-amino acid derivatives. In the series of N-acetylphenylalanine ester products (resulting from hydrogenation of Z-α-acetamidocinnamate esters) the optical purity increased from 1% ee-(R) Me]; 20% ee-(S) Et]; 47% ee-(S) i-Pr]; to 58% ee-(S) t-Bu]. Increasing the steric bulk of the acyl function (NHCOR, where R is an alkyl moiety) favors the reduction of the si-re prochiral face in the methyl ester substrates] to yield an excess of the R-amino acid derivatives. In the series of N-acylphenylalanine methyl ester products (resulting from hydrogenation of Z-methyl α-acylaminocinnamates) the optical purity increased from 1% ee-(R) Me]; 13% ee-(R) i-Pr]; to 15% ee-(R) t-Bu and 1-adamantyl]. The α-formamido and α-benzamido substrates gave hydrogenation products having 22% ee-(R) H] and 35% ee-(R) Ph]. In the corresponding free acids, increasing the steric bulk of the acyl function (NHCOR, where R is an alkyl moiety) results in almost no change in the optical purity of the reduction products. In the series of N-acylphenyl-alanine products (resulting from hydrogenation of Z-α-acylaminocinnamic acids) the optical purity was 35% ee-(S) Me]; 31% ee-(S) i-Pr]; 33% ee-(S) t-Bu]; and 35% ee-(S) 1-adamantyl]. The α-benzamido substrate gave a hydrogenation product having 8% ee-(S).