Simple copper/TEMPO catalyzed aerobic dehydrogenation of benzylic amines and anilines

Abstract

CuBr₂ with 2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) has been successfully employed for the aerobic oxidation of primary and secondary benzyl amines in aqueous acetonitrile. Such catalytic systems have previously been used extensively in alcohol oxidation reactions. The same catalyst system was also used for oxidative cross-couplings of benzylamines with anilines. The electronic and steric properties of the aniline partner were found to be of crucial importance in determining reactivity or lack thereof. A mechanism for these reactions is proposed based on the data obtained to date. In the absence of benzyl amines, electron-rich anilines were found to undergo dehydrogenative coupling and yields of the resulting azo products could be increased by replacing CuBr₂ with CuBr. No ligand (e.g.pyridine) is required for either reaction to proceed and presumably water and acetonitrile solvate the copper-containing intermediates.