The mechanism of the Ru-II(BINAP)-catalysed hydrogenation of vinylcarboxylic acids has been investigated via detailed deuterium labelling studies. The activation of H-2 by the Ru catalyst was found to be via a heterolytic: splitting route. The regioselectivity of the hydride migration step strongly correlates to the stability of the resulting metal-alkyl intermediate. When an alpha, beta-unsturated carboxylic acid such as 2-(6-methoxy-2-naphthy])aclylic acid is used as the substrate, the hydride migrates exclusively to the alpha position, forming a metal-primary alkyl intermediate. In the case of a beta, gamma-unsaturated carboxylic acid with an electron-withdrawing group attached to the beta-carbon, the hydride migrates exclusively to the gamma-carbon, forming a five-membered ring metal-alkyl intermediate with the beta-carbon coordinated to the metal. The product formation step involves two competing routes: the hydrogenolysis and the solvolysis of the metal-alkyl intermediates. The choice of each route is highly dependent on the reaction conditions. The solvolysis route is significant if the reaction is carried out under low H-2 pressure and the reaction medium is more acidic. Under high H-2 pressure, hydrogenolysis becomes the dominant route. Under basic conditions, the solvolysis route is essentially shut off and only the hydrogenolysis product is obtained. A unified mechanism which explains all of the known experimental results is proposed.
