A comparative mechanistic study of Cu-catalyzed oxidative coupling reactions of N-phenyltetrahydroisoquinoline with different nucleophiles was conducted. Two previously reported combinations of catalyst and oxidant were studied, CuCl2 center dot 2H(2)O/O-2 and CuBr/tert-butyl hydroperoxide (TBHP). On the basis of a synthetic study with different nucleophiles, the electrophilicity of the intermediate iminium ion was estimated and differences between the two methods were revealed. The key intermediate in the aerobic method is shown to be an iminium ion, formed through oxidation by copper(II), which can react with any nudeophile of sufficient reactivity. The role of oxygen is the reoxidation of the reduced catalyst. In the CuBr/TBHP system, an alpha-amino peroxide is proposed as a true intermediate within the catalytic cycle, formed from the amine and TBHP by a Cu-catalyzed radical reaction pathway and acting as a precursor to the iminium ion intermediate.