Ruthenium(II)-catalyzed base-free transfer hydrogenation in air using N-alkyl substituted 1-methylimidazole-2-carboxamido ligands: Insight into the role of [(p-Cymene)RuCl]2(µ-Cl)(µ-H)

Abstract

Four new half-sandwich ruthenium(II) complexes featuring N-alkyl substituted 1-methylimidazole-2-carboxamido ligands were synthesized, characterized, and evaluated as pre-catalysts for the base-free transfer hydrogenation of acetophenone in isopropanol at 85 °C in air with 1 mol% catalyst loading. Based on the alkyl substitution at the carboxamido nitrogen, the catalytic activity follows the order tert‑butyl > isopropyl > methyl > benzyl, in which yields of 1-phenylethanol up to 86 % after one hour and 92 % after three hours were observed. NMR analysis revealed the formation of [(p-cymene)RuCl]₂(µ-Cl)(µ-H) during catalytic reactions. This hydride dimer was found to be an effective catalyst for transfer hydrogenation, although experiments suggest that it is unlikely to be the sole catalyst in the reactions described herein.