KINETICS AND MECHANISM OF CATALYSIS OF THE ASYMMETRIC HYDROGENATION OF ALPHA-BETA-UNSATURATED CARBOXYLIC-ACIDS BY BIS(CARBOXYLATO)(2,2'-BIS(DIPHENYLPHOSPHINO)-1,1'-BINAPHTHYL)-RUTHENIUM(II), [RUII(BINAP)(O2CR)2]

The rate law for the hydrogenation of alpha,beta-unsaturated carboxylic acids in methanol, catalyzed by bis(carboxylato){(R)- or (S)-2,2'bis(diphenylphosphino)-1,1'binaphthyl}ruthenium(II) ([Ru(II)(BINAP)(O2CR)2]), at near ambient conditions and in the absence of added product, has the following form: rate = k(obs)[Ru]t[substrate][H2]/[substrate]0. The rate law and the results of labeling studies support the conclusion that the catalytically active species in the turnover-limiting step of the reaction is a substrate-containing bis(carboxylato)ruthenium complex that splits H2 heterolytically to form a metal hydride. Subsequent insertion of the C = C bond of the coordinated substrate yields a five-membered heterometallacyclic species; protonolysis of the resulting Ru-C bond gives the final product. The catalytic rate is very sensitive to the presence of strong acid; slightly more than 1 equiv (with respect to catalyst) of triflic acid inhibits hydrogenation completely. In contrast, the rate of hydrogenation is unaffected by base. The enantiomeric excess, greater than 90% when the substrate is tiglic acid, is unaffected by addition of acid or base.