Kinetic and Spectroscopic Studies of Aerobic Copper(II)-Catalyzed Methoxylation of Arylboronic Esters and Insights into Aryl Transmetalation to Copper(II)

We previously reported a preliminary mechanistic study of aerobic Cu(OAc)(2)-catalyzed methoxylation of 4-tolylboronic ester (King et al. J. Am. Chem. Soc., 2009; 131, 5044-5045), which revealed that aryl transmetalation from the boronic ester to Cu-II is the turnover limiting step. In the present study, more thorough kinetic and spectroscopic studies provide additional insights into the transmetalation pathway and identity of the Cu-II catalyst resting state(s). EPR spectroscopic studies show that at least two copper(II) species are present under catalytic Conditions, and their relative populations vary as a function of reaction time and acidity of the arylboronic ester and are influenced by addition of acetic acid or acetate to the reaction Mixture. Analysis of kinetic data and B-11 NMR and EPR-spectra under diverse reaction conditions suggests that aryl transmetalation occurs from a tetracoordinate, anionic boranate to a cationic Cu-II species, mediated by a methoxide bridge.