HYDROGENATION OF BENZYLIDENEACETONE CATALYZED BY OSHCL(CO)(PR3)2 (PR3 = P-I-PR3, PME-TERT-BU2) - NEW ROLES OF DIHYDROGEN COMPLEXES IN HOMOGENEOUS CATALYTIC-HYDROGENATION

The five-coordinated hydrido carbonyl complexes OsHCl(CO)(PR3)2 (PR3 = P-i-Pr3 (1), PMe-t-Bu2 (12)) catalyze the selective hydrogenation of benzylideneacetone to 4-phenylbutan-2-one. In 2-propanol solutions, selectivities close to 100% are achieved. In the presence of 1, the reaction is first-order with respect to the concentrations of catalyst and substrate and independent of the hydrogen pressure. For the reaction catalyzed by 12, the kinetic experimental data are in accordance with an expression of the form -d[benzylideneacetone]/dt = k[12]2[benzylideneacetone](P(H-2)). The mechanisms deduced for these reactions, on the basis of the rate laws and spectroscopic observations, illustrate new roles of the dihydrogen complexes in homogeneous catalytic hydrogenation. 1, initially nonactive, is activated as a result of the formation of trans(hydride,dihydrogen)-OsHCl(eta2-H-2)(CO)(P-i-Pr3)2 (6), which isomerizes to cis(hydride,dihydrogen)-OsHCl(eta2-H-2)(CO)(P-i-Pr3)2 (14) and subsequently dissociates molecular hydrogen. The reaction catalyzed by 12 is proposed to go by the intermediate trans-[OsCl(CO)(PMe-t-Bu2)2]2H4 (18), which could be formed by reaction of 12 with cis(hydride,dihydrogen)-OsHCl(eta2-H-2(CO)(PMe-t-Bu2)2 (20). According to the theoretical works of Burdett and Pourian, this binuclear intermediate could contain a planar 4-gon of cyclically bound hydrogen atoms.