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 beta-alkylation of 1-phenyl ethanol with benzyl alcohol at 140 degrees C under solvent-free conditions, ((NNN)-N-Cy2)RuCl2(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 beta-alkylation of various alcohol combinations was accomplished with ease which culminated to give 380 000 TON at 19 000 TO h(-1) for the beta-alkylation of 1-phenyl ethanol with 3-methoxy benzyl alcohol. DFT studies were complementary to mechanistic studies and indicate the beta-hydride elimination step involving the extrusion of acetophenone to be the overall RDS. While the hydrogenation step is favored for the formation of alpha-alkylated ketone, the alcoholysis step is preferred for the formation of beta-alkylated alcohol. The studies were extended for the upgradation of ethanol to biofuels. Among the pincer-ruthenium complexes based on bis(imino)pyridine, ((NNN)-N-Cy2)RuCl2(CO) provided high productivity (335 TON at 170 TO h(-1)). Sterically more open pincer-ruthenium complexes such as ((NNN)-N-Bim2)RuCl2(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.