Phosphine-free pincer-ruthenium catalyzed biofuel production: high rates, yields and turnovers of solventless alcohol alkylation

Phosphine-free pincer-ruthenium carbonyl complexes based on bis(imino)pyridine and 2,6-bis(benzimidazole-2-yl) pyridine ligands have been synthesized. For the β-alkylation of 1-phenyl ethanol with benzyl alcohol at 140 °C under solvent-free conditions, ( Cy2 NNN)RuCl 2 (CO) (0.00025 mol%) in combination with NaOH (2.5 mol%) was highly efficient ( ca. 93% yield, 372 000 TON at 12 000 TO h −1 ). These are the highest reported values hitherto for a ruthenium based catalyst. The β-alkylation of various alcohol combinations was accomplished with ease which culminated to give 380 000 TON at 19 000 TO h −1 for the β-alkylation of 1-phenyl ethanol with 3-methoxy benzyl alcohol. DFT studies were complementary to mechanistic studies and indicate the β-hydride elimination step involving the extrusion of acetophenone to be the overall RDS. While the hydrogenation step is favored for the formation of α-alkylated ketone, the alcoholysis step is preferred for the formation of β-alkylated alcohol. The studies were extended for the upgradation of ethanol to biofuels. Among the pincer-ruthenium complexes based on bis(imino)pyridine, ( Cy2 NNN)RuCl 2 (CO) provided high productivity (335 TON at 170 TO h −1 ). Sterically more open pincer-ruthenium complexes such as ( Bim2 NNN)RuCl 2 (CO) based on the 2,6-bis(benzimidazole-2-yl) pyridine ligand demonstrated better reactivity and gave not only good ethanol conversion ( ca. 58%) but also high turnovers ( ca. 2100) with a good rate ( ca. 710 TO h −1 ). Kinetic studies indicate first order dependence on concentration of both the catalyst and ethanol. Phosphine-free catalytic systems operating with unprecedented activity at a very low base loading to couple lower alcohols to higher alcohols of fuel and pharmaceutical importance are the salient features of this report. High TONs and TOFs are observed for the β-alkylation of alcohols using phosphine-free pincer-ruthenium catalysts at a very low base loading. Kinetic studies and DFT calculations were complementary and provide a clear understanding on the mechanism.