The role of functionalized phosphines in the hydrogenation of carboxylic acids in the presence of phosphine substituted hydride ruthenium complexes

Hydride ruthenium carbonyl complexes substituted by functionalized phosphin es such as H4Ru4(CO)(8)[P(CH2OCOR)(3)](4) have been synthesized and tested as catalysts in the hydrogenation of carboxylic acids. These complexes are more active than those reported previously, containing trialkyl- or triaryl phosphines. On the basis of their behavior, their different activity has be en explained in terms of an involvement of the phosphine ligand in the cata lytic cycle. The ester group present in the phosphine P(CH2OCOR)(3) is hydr ogenated to produce an alcohol (RCH2OH) and a P(CH2OH) group which, in turn , reacts with the free acid present in solution to restore the P(CH2OCOR) g roup. This hypothesis has been confirmed by the reactivity of the possible intermediate H4Ru4(CO)(8)[P(CH2OH)(3)](4) with acetic acid. Another support to this statement is the almost equal catalytic activity, displayed by H4R u4(CO)(8)[P(CH2OCOR)(3)](4) complexes, whatever the R group present, in the phosphine ligand, in the hydrogenation of carboxylic acids. These complexe s, on the other hand, are less active than the corresponding tributylphosph ine substituted ones in the hydrogenation of alkenes and ketones. Finally w hen the phosphine ligand is P(CH2CH2COOCH3)(3) the ester group is not reduc ed and consequently the catalytic activity of this complex in the hydrogena tion of carboxylic acids is very low. (C) 1999 Elsevier Science S.A. All ri ghts reserved.