The oxidative coupling of N-aryl tetrahydroisoquinolines with nucleophiles has inspired the development of novel C-H functionalization reactions as well as mechanistic studies. Here, we investigate the oxidation step that forms iminium ions as key intermediates in the method using CuCl2 as the catalyst and oxygen as the terminal oxidant. A strong electronic effect of substituents in the N-aryl ring was found by synthetic studies and a Hammett plot analysis, supporting initial electron transfer from the amine to Cu(II). The importance of the mechanism of oxidation on the substrate scope with differently substituted tetrahydroisoquinolines is discussed.