Direct Observation of Alkyl Group Transmetalation from Boron to Copper: Impact of Structure Modification and the Critical Role of Copper–Oxygen Preassociation in Stereospecificity

Mechanistic investigations of prospective intermediates in copper-catalyzed cross-coupling reactions with alkyllithium-activated alkylboronic esters are described. Complexes between (NHC)copper and the borate salts were characterized by X-ray crystallography and NMR, and the rate of intramolecular transmetalation of an alkyl group from boron to copper was measured. Derived activation parameters were correlated with those determined by DFT analysis and give insight into the mechanism of stereospecific transmetalation.