Abstract
The carboxylate pincer complexes trans-[Ru(η(1)-OCOR)(2)(SNS)(PPh(3))] (R = CH(3) 1, (t)Bu 2; SNS = bis(2-(ethylthio)ethylamine) are obtained from [Ru(η(2)-OCOR)(2)(PPh(3))(2)] (R = CH(3), (t)Bu) and HN(CH(2)CH(2)SEt)(2) in acetone by elimination of PPh(3). Substitution of PPh(3) in 1 with PPh(2)Me gives trans-[Ru(η(1)-OAc)(2)(SNS)(PPh(2)Me)] (3) in toluene at 105°C. The cationic complexes cis-[Ru(η(1)-OAc)(SNS)(dppb)]OAc (4) and [Ru(η(1)-OAc)(CO)(SNS)(PPh(3))]OAc (5) are synthesized from [Ru(η(2)-OAc)(2)(dppb)] and [Ru(η(2)-OAc)(2)(CO)(PPh(3))(2)], respectively, by treatment with the SNS ligand in toluene. Hydride trans-[RuH(η(1)-OAc)(SNS)(PPh(3))] (6) is obtained by reaction of 1 with NaOiPr in a mixture of iPrOH/toluene. The carboxylate complexes 1-6 catalyze the homogeneous hydrogenation of methyl decanoate to 1-decanol with NaOMe under mild reaction conditions (T = 40°C, H(2) pressure 27.5 bar) and extremely low catalyst loading (S/C 10,000-100,000). Complexes 1 and 2 display remarkably high catalytic activity for the reduction of fatty acid methyl and ethyl esters at S/C of 50,000-100,000, with high selectivity for the reduction of the carboxylate group in the presence of C ═ C functionalities. An easy scale-up of this process (100 mmol of substrate) has been demonstrated. Mechanistic studies show that 1 reacts with H(2) (5 bar), affording the monohydride 6, that by further reaction with H(2) and an alkoxide gives the dihydride cis-[RuH(2)(SNS)(PPh(3))], which reduces methyl benzoate to benzyl alcohol.