Computational mechanistic study on oxidative esterification of alcohols to esters catalyzed by palladium complex

Mechanistic details of palladium-catalyzed oxidative esterification of methanol to methyl formate have been studied by density functional theory (DFT) calculations without using system simplification. A two-step mechanism involving oxidation of methanol to formaldehyde and further oxidation of formaldehyde to methyl formate and four potential reaction pathways for the later step have been proposed and fully characterized. This mechanistic study provides evidence that each oxidation step proceeds via a deprotonation followed by β-H transfer. The results indicate that methoxymethanol oxidation is the most favorable pathway kinetically and thermodynamically in the formation of methyl formate. Comparative studies on the structural and electronic properties of three Pd complexes indicate that the acetate and the acetonitrile ligands play key ligand-accelerated catalytic roles in the reactions, that is, the former acts as a nucleophilic site to facilitate the deprotonation and the latter provides a coordination site to facilitate the β-H transfer.