Rapid Injection NMR in Mechanistic Organocopper Chemistry. Preparation of the Elusive Copper(III) Intermediate

Abstract

A “copper(III) intermediate” has been proposed to be crucially involved in many reactions of organocopper reagents, for example, with α-enones, alkyl halides, 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 13C−13C coupling constants 2J across copper, measured on 13C-labeled material. Especially noteworthy is the extremely deshielded nature of the methyl C atoms attached to Cu, as indicated by the downfield 13C NMR shifts of 12.43 and 25.31 ppm. Large trans couplings, 2J = 38.1 Hz (upfield methyl−ring methine) and 2J = 35.4 Hz (downfield methyl−cyano), are consistent with a square planar complex.