Substrate effects on the mechanism of enantioselective hydrogenation using ruthenium bis(phosphine) complexes as catalyst:: A mechanistic investigation of the hydrogenation of α,β-unsaturated acids and esters based on deuterium labeling studies

The mechanism of ruthenium-bis(phosphine) catalyzed enantioselective hydrogenation of olefins was examined using [Ru((R)BINAP)(H)(MeCN)(n)(sol)(3-n)] BF(4) (n = 0-3, sol = solvent used in reaction) as catalyst. Tiglic and angelic acids were used as standard alpha,beta-unsaturated acid substrates; (Z)-methyl alpha-acetamidocinnamate and dimethyl itaconate were used as standard alpha,beta-unsaturated ester substrates. Isotopic labeling studies (deuterium scrambling) indicate that two distinct mechanisms are in operation for alpha,beta-unsaturated acids versus alpha,beta-unsaturated esters. In each case, 5-membered metallocycle intermediates are formed via olefin-hydride insertion. The mechanisms, however, deviate primarily in the activation of dihydrogen, which is strongly affected by the nature of the substrate. Hydrogenation of alpha,beta-unsaturated acids proceed via heterolytic cleavage of dihydrogen, whereas hydrogenation of alpha,beta-unsaturated esters proceed via homolytic cleavage of dihydrogen. A full discussion of the mechanisms is presented. (c) 2005 Elsevier B.V. All rights reserved.