Organocuprate Conjugate Addition: The Square-Planar “CuIII” Intermediate

Abstract

Rapid-injection NMR experiments with 2-cyclohexenone, TMSCl, and Me2CuLi•LiCN by the Bertz and Ogle team (Bertz, S. H.; Cope, S.; Murphy, M.; Ogle, C. A.; Taylor, B. J. J. Am. Chem. Soc. 2007, 129, 7208−7209) have led to the observation of a new product-forming conjugate addition intermediate characterized by 1H and 13C NMR at −100 °C. By employing conformational analysis, density functional theory (DFT) optimization, and prediction of NMR observables (δ (TMS), 2J), the structure of the observed species is confirmed to be the elusive and long sought tetracoordinate square-planar “Cu(III) intermediate”. Additional calculations suggest that symmetrical or unsymmetrical tetraalkyl analogues ([R4Cu]-1) are inherently more stable than the CN analogues ([R3CuCN]-1) and may be isolable.