Phosphine-Free Pincer-Ruthenium Catalyzed Biofuel Production: High Rate, 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, (Cy2NNN)RuCl2(CO) (0.00025 mol %) in combination with NaOH (2.5 mol %) was highly efficient (ca. 93% yield, 372000 TONs at 12000 TO/h). These are the highest reported values hitherto for a ruthenium based catalyst. The β-alkylation of various alcohol combinations were accomplished with ease which culminated to give 380000 TONs at 19000 TO/h 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 complex based on bis(imino)pyridine, (Cy2NNN)RuCl2(CO) provided high productivity (335 TONs at 170 TO/h). Sterically more open pincer-ruthenium complexes such as (Bim2NNN)RuCl2(CO) based on 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). Kinetic studies indicate first order dependence on concentration of both 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.