Kinetico-Mechanistic Studies on a Reactive Organocopper(II) Complex: Cu–C Bond Homolysis versus Heterolysis

Organocopper(II) reagents are an unexplored frontier of copper catalysis. Despite being proposed as reactive intermediates, an understanding of the stability and reactivity of the Cu II –C bond has remained elusive. Two main pathways can be considered for the cleavage mode of a Cu II –C bond: homolysis and heterolysis. We recently showed how organocopper(II) reagents can react with alkenes via radical addition, a homolytic pathway. In this work, the decomposition of the complex [Cu II LR] + [L = tris(2- dimethylaminoethyl)amine, Me 6 tren, R = NCCH 2 – ] in the absence and presence of an initiator (RX, X = Cl, Br) was evaluated. When no initiator was present, first-order Cu II –C bond homolysis occurred producing [Cu I L] + and succinonitrile, via radical termination. When an excess of the initiator was present, a subsequent formation of [Cu II LX] + via a second-order reaction was found, which results from the reaction of [Cu I L] + with RX following homolysis. However, when Brønsted acids (R′–OH: R′ = H, Me, Ph, PhCO) were present, heterolytic cleavage of the Cu II –C bond produced [Cu II L(OR′)] + and MeCN. Kinetic studies were undertaken to obtain the thermal (Δ H ⧧, Δ S ⧧ ) and pressure (Δ V ⧧ ) activation parameters and deuterium kinetic isotopic effects, which provided an understanding of the strength of the Cu II –C bond and the nature of the transition state for the reactions involved. These results reveal possible reaction pathways for organocopper(II) complexes relevant to their applications as catalysts in C–C bond forming reactions.