Intermediacy of Copper(I) under Oxidative Conditions in the Aerobic Copper-Catalyzed Decarboxylative Thiolation of Benzoic Acids

An experimental mechanistic study of the aerobic copper-catalyzed decarboxylative thiolation of benzoic acids with arenethiols is reported. For the model reaction, the findings support the corresponding disulfide (PhSSPh) of the arenethiol (PhSH) as the active thiolating source under reaction conditions. Synthesis and reactivity studies along with kinetic measurements support the chemical and kinetic competence of catalytically active well-defined Cu complexes: (phen)CuI(O2CC6H4-o-NO2) (2), [(phen)CuI(μ-SC6H5)]2 (3), (phen)CuI(C6H4-o-NO2) (4), and (phen)CuII(O2CC6H4-o-NO2)2 (5) (phen = 1,10-phenanthroline). The presence of an induction period in the stoichiometric reaction of the copper(II) complex (phen)CuII(O2CC6H4-o-NO2)2 (5) with PhSSPh and the absence of an induction period in the analogous stoichiometric reaction of the copper(I) complex (phen)CuI(O2CC6H4-o-NO2) (2) suggest that a copper(I) carboxylate is a more likely intermediate than a copper(II) carboxylate. The observation of in situ reduction of CuII to CuI further supports CuI as the primary active catalytic species, and spectroscopic studies also indicate the catalyst resting state to be a CuI species. The catalytic reaction exhibits a first-order dependence on [CuI] and [2-nitrobenzoic acid] and a zero-order dependence on [PhSSPh] and p(O2), suggestive of turnover-limiting decarboxylation of a copper(I) carboxylate. Oxygen was found to promote the essential oxidative cleavage of the copper(I) thiolate intermediate [(phen)CuI(μ-SC6H5)]2 (3) to regenerate a catalytically active [(phen)CuII] (Cuox) species with concomitant formation of PhSSPh. On the basis of these findings, a reaction pathway is proposed for the C–S coupling reaction that includes the key CuI-based intermediates (phen)CuI(O2CC6H4-o-NO2) (2) and (phen)CuI(C6H4-o-NO2) (4). The pathway accounts for the role of O2 in generating the active thiolating source, PhSSPh, as well as enabling catalytic turnover of in situ generated [(phen)CuI(μ-SC6H5)]2 (3).