Rapid injection NMR in mechanistic organocopper chemistry. Preparation of the elusive copper(III) intermediate

A "copper(III) intermediate" has been proposed to be crucially involved in many reactions of organocopper reagents, for example, with alpha-enones, alkylhalides, and allylic carboxylates. By using rapid injection NMR (RI-NMR), we have been able to prepare the first example of this elusive species under conditions where it is stable for extended periods of time. Extensive NMR investigations establish its structure and include two-bond C-13-C-13 coupling constants (2)J across copper, measured on C-13-labeled material. Especially noteworthy is the extremely deshielded nature of the methyl C atoms attached to Cu, as indicated by the downfield C-13 NMR shifts of 12.43 and 25.31 ppm. Large trans couplings, (2)J = 38.1 Hz (upfield methyl-ring methine) and (2)J = 35.4 Hz (downfield methyl-cyano), are consistent with a square planar complex.