Competitive Hydrogen Atom Transfer to Oxyl- and Peroxyl Radicals in the Cu-Catalyzed Oxidative Coupling of N-Aryl Tetrahydroisoquinolines Using Tert-Butyl Hydroperoxide

The question of whether hydrogen atom transfer (HAT) or electron transfer (ET) is the key step in the activation of N-aryl tetrahydroisoquinolines in oxidative coupling reactions using CuBr as catalyst and tert-butyl hydroperoxide (tBuOOH) has been investigated. Strong indications for a HAT mechanism were derived by using different para-substituted N-aryl tetrahydroisoquinolines, showing that electronic effects play a minor role in the reaction. Hammett plots of the Cu-catalyzed reaction, a direct time-resolved kinetic study with in situ generated cumyloxyl radicals, as well as density functional calculations gave essentially the same results. We conclude from these results and from kinetic isotope effect experiments that HAT is mostly mediated by tert-butoxyl radicals and only to a lesser extent by tert-butylperoxyl radicals, in contrast to common assumptions. However, reaction conditions affect the competition between these two pathways, which can significantly change the magnitude of kinetic isotope effects.