Productive Homogeneous Hydrogenation of Fatty Esters with Carboxylate SNS Ruthenium Catalysts

The carboxylate pincer complexes trans-[Ru(η1-OCOR)2(SNS)(PPh3)] (R = CH3 1, tBu 2; SNS = bis(2-(ethylthio)ethylamine) are obtained from [Ru(η2-OCOR)2(PPh3)2] (R = CH3, tBu) and HN(CH2CH2SEt)2 in acetone by elimination of PPh3. Substitution of PPh3 in 1 with PPh2Me gives trans-[Ru(η1-OAc)2(SNS)(PPh2Me)] (3) in toluene at 105°C. The cationic complexes cis-[Ru(η1-OAc)(SNS)(dppb)]OAc (4) and [Ru(η1-OAc)(CO)(SNS)(PPh3)]OAc (5) are synthesized from [Ru(η2-OAc)2(dppb)] and [Ru(η2-OAc)2(CO)(PPh3)2], respectively, by treatment with the SNS ligand in toluene. Hydride trans-[RuH(η1-OAc)(SNS)(PPh3)] (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, H2 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 H2 (5 bar), affording the monohydride 6, that by further reaction with H2 and an alkoxide gives the dihydride cis-[RuH2(SNS)(PPh3)], which reduces methyl benzoate to benzyl alcohol.