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

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 Cu^IV=O species or a [Cu(OH)Cl]₂ species.