Aerobic oxidation of secondary benzyl alcohols catalyzed by phosphinite-based palladium pincer complexes

Abstract

Aerobic oxidation of organic compounds is receiving attention because molecular oxygen is an abundant, cheap and comparatively safe reagent. Several catalyst systems have been developed so far, but the priority is efficiency and the use of more sustainable reaction media. Here the synthesis of two bis-phosphinite-based palladium(II) complexes has been improved to overcome the lack of reproducibility from previous reports. The improvement relied on solvent modification, strictly inert conditions, monitoring by phosphorus-31 nuclear magnetic resonance, quick filtration and solvent removal steps. We show that both palladacycles are excellent palladium sources for the oxidation of secondary benzyl carbinols in the presence of molecular oxygen at atmospheric pressure. Oxidation of secondary benzyl carbinols carried out in the presence of a very small amount of such palladium complexes, of 0.01 mol%, provided the corresponding ketones in 35–91% yield. High turnover number values, of 3500–9100, substoichiometric amounts of base, of 10 mol%, and the environmentally friendly solvent polyethylene glycol 400 are additional advantages.