Pd-PEPPSI Catalyzed Dehydrogenative Oxidation of Alcohols to Carboxylic Acids: An Experimental and Theoretical Study

Herein, we report syntheses of two new PEPPSI-type (PEPPSI: pyridine enhanced precatalyst preparation stabilization and initiation) palladium N-heterocyclic carbene (NHC) complexes, [PdBr2(NHC)Py] (1,2) [NHC = 3-(3,5-dimethoxybenzyl)-1-(4-methoxyphenyl)-1H-imidazol-3-ium-2-ide (L1); 1-(4-methoxyphenyl)-3-(naphthalen-1-ylmethyl)-1H-imidazol-3-ium-2-ide (L2)], and explored their catalytic potentials for alcohol dehydrogenation (AD) reactions. The molecular structures of the complexes were determined by single-crystal X-ray analysis. Both the complexes could catalyze the AD reactions; however, under similar conditions, the sterically bulky naphthyl-containing complex 2 showed slightly better activity than the 3,5-dimethoxy benzyl-containing complex 1. A range of alcohols, including relatively challenging aliphatic and biomass-based alcohols, were converted to corresponding carboxylates in moderate-to-excellent yields. Interestingly, our catalyst is also effective in gram-scale synthesis of benzoic acid with a catalyst-to-substrate ratio of 1:3333, exhibiting a turnover number (TON) of 2466. A comprehensive mechanistic investigation using density functional theory (DFT) has been made. Four possible reaction pathways were computationally examined: one dissociative pathway where both Py and Br– ligands leave the coordination site, two associative pathways where either Py or Br– ligand leaves the coordination site, and another where the reaction proceeds through intermediary ester formation. The calculated energy profile studies revealed that the ester formation pathway is the most favorable among the four pathways.