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.