Bases Tuning Selective Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde Catalyzed by Cu²⁺/TEMPO

Promotional role of basic ligands on the combination of cupric ions and 2,2,6,6-tetramethyl-piperidine-N-noxyl (Cu²⁺/TEMPO) was tried in the previous publications for selective aerobic oxidation of benzyl alcohol but a lack of a detailed investigation encourages us to perform further work. Here, Cu²⁺/TEMPO was used to catalyze selective aerobic oxidation of benzyl alcohol to benzaldehyde in the presence of bases, and triethylamine was found to be the best additive. A conversion of benzyl alcohol of 92.9% and a selectivity to benzaldehyde of 100% was observed in the presence of Cu²⁺/TEMPO/triethylamine. The promotion of triethylamine on Cu²⁺/TEMPO can be related to its capability in electron-donor and deprotonation. The speciation of the ternary catalyst combination was partially observed by the means of high-resolution mass spectroscopy (HRMS) and some key intermediates such as N-hydroxyl-2,2,6,6-tetramethylperidine (TEMPOH) and ethyl-substituted ammonium ions were confirmed. This study might provide some insights into the highly efficient transformation from benzyl alcohol to benzaldehyde and the promotion of bases on this transformation.