Chan-Evans-Lam Couplings with Copper Iminoarylsulfonate Complexes: Scope and Mechanism

Copper(II) pyridyliminoarylsulfonate complexes with chloride or triflate counteranions were employed in ChanEvansLam (CEL) couplings of N-nucleophiles and arylboronic acids. The complexes avoided typical side reactions in CEL couplings, and an excess of boronic acid was not required. Water was tolerated, and addition of neither base nor other additives was necessary. Primary amines, acyclic and cyclic secondary amines, anilines, aminophenol, imidazole, pyrazole, and phenyltetrazole can be quantitatively arylated at either 25 or 50 degrees C with 2.5 mol % of the catalyst. Reaction kinetics were investigated in detail. Kinetic and spectroscopic studies provide evidence for the formation of unproductive coppersubstrate complexes. Formation of an anilinephenylboronic acid adduct was responsible for the zero-order dependence of reaction rates on phenylboronic acid concentration. Kinetic evidence indicates that the order of reaction steps is transmetalation, nucleophile coordination, and oxidation. Couplings performed poorly with electron-deficient arylboronic acids, due to a slower Cu(II)/Cu(III) oxidation in the catalytic cycle. Photoredox catalysis partially resolved this problem, but addition of copper acetate as an external oxidant proved to be more efficient.