Catalytic value addition of glycerol to lactic acid by chromium chloride and its corresponding pincer complex

Abstract

The 3d-metal salt chromium(III) chloride (CrCl₃·6H₂O) and its corresponding pincer-Cr complexes were utilized to bring about the catalytic transfer dehydrogenation of glycerol (GLY) to lactic acid (LA) in the presence of a sacrificial hydrogen acceptor, acetone. Among the catalysts that were screened, the (^Cy2NNN)CrCl₃ (0.5 mol%) catalyzed net transfer dehydrogenation of glycerol in the presence of 1.1 equivalents of NaOH at 160 °C afforded 94% lactic acid at 98% selectivity along with another value-added product isopropanol (IPA). Homogeneous molecular nature of the active catalyst in the reaction mixture was evident from the retention of catalytic activity in the presence of catalyst poisons such as PPh₃, CS₂ and Hg. These homogeneous molecular species are likely to be based on Cr(II) as inferred from detailed mechanistic investigations involving EPR analysis, HRMS studies and Evan's magnetic moment experiments. Isotope labelling experiments indicate a KIE of 1.3.