Dual Role of Acetate in Copper(II) Acetate Catalyzed Dehydrogenation of Chelating Aromatic Secondary Amines: A Kinetic Case Study of Copper-Catalyzed Oxidation Reactions

Copper(II) acetate is a frequent empirical choice of the copper source in copper(II)-mediated redox reactions. The effect of the acetate counterion appears crucial but has not been adequately investigated. Herein, we report that copper(II) acetate catalyzes the aerobic dehydrogenation of chelating aromatic secondary amines. The chemoselectivity of acetate and chelating amines in this reaction provides a unique opportunity for a mechanistic study. The progression of this homogeneous reaction is monitored by using electron paramagnetic resonance spectroscopy, UV/Vis absorption spectroscopy, and manometry. The kinetic dependence on the amine substrate, copper(II), and acetate counterion, together with the results of kinetic isotope and substituent effect experiments, suggests that acetate acts both as a bridging ligand of a dinuclear catalytic center for mediating two-electron transfer steps and as a base in the turnover-limiting C-H bond-cleavage step. Upon including 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a surrogate base, DBU and acetate act in a complementary manner to enable a rapid, catalytic dehydrogenation reaction of a chelating secondary amine substrate. Finally, the contrasting reactivities between copper(II) acetate (promoting two-electron transfer) and copper(II) perchlorate (promoting single-electron transfer) underscores how a counterion could completely alter the mechanistic pathway of a copper-mediated oxidation reaction.