Reactions Catalysed by a Binuclear Copper Complex: Aerobic Cross Dehydrogenative Coupling of N-Aryl Tetrahydroisoquinolines

Binuclear copper complex [{Cu(Sal)(2)(NCMe)}(2)] (Sal=salicylate) was found to be an active catalyst for the aerobic oxidation of N-aryl tetrahydroisoquinolines to the corresponding iminium ions, which could be trapped by a wide range of nucleophiles to form coupled products. The reactions took place under 1 bar of O-2 at room temperature with 1 mol% of the copper catalyst being sufficient in most cases, and are considerably accelerated by catalytic chloride anions. Mechanistic studies show that the Cu-II dimer oxidizes the amine to the iminium ion, and this two-electron process requires O-2, whereby the resulting Cu-I is concomitantly reoxidised back to Cu-II. Various lines of evidence suggest that the oxidative coupling reaction is turnover-limited by the step of iminium formation, and it is this step that is promoted by the chloride anion. Since it is more efficient than and mechanistically distinct from the well-studied simple copper salts such as CuBr and CuCl2, the binuclear copper catalyst provides a new tool for oxidative coupling reactions.