Copper-catalyzed homolytic and heterolytic benzylic and allylic oxidation using tert-butyl hydroperoxide

Abstract

Allylic and benzylic alcohols were oxidized in good yields to the respective ketones by tert-butyl hydroperoxide (TBHP) in the presence of copper salts under phase-transfer catalysis conditions. This dehydrogenation was found to proceed via a heterolytic mechanism. CuCl2, CuCl, and even copper powder were equally facile as catalysts, as they were all transformed in situ to Cu(OH)Cl which was extracted into the organic phase by the phase-transfer catalyst (PTC). Deuterium labeling experiments evidenced the scission of the benzylic C–H bond in the rate-determining step. Nonproductive TBHP decomposition was not observed in the presence of the alcohol substrates. Conversely, the oxygenation of π-activated methylene groups in the same medium was found to be a free radical process, and the major products were the appropriate tert-butyl peroxides. Catalyst deactivation, solvent effects, and extraction effects are discussed. By applying Minisci’s postulations concerning the relative reactivity of TBHP molecules towards tert-butoxyl radicals in protic and nonprotic environments, the coexistence of the homolytic and the heterolytic pathways can be explained. A complete reaction mechanism is proposed, wherein the free-radical oxidation obeys Kochi’s mechanism, and the heterolytic dehydrogenation is based on either a high-valent CuIVO species or a [Cu(OH)Cl]2 species.