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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 13 C− 13 C coupling constants 2 J across copper, measured on 13 C-labeled material. Especially noteworthy is the extremely deshielded nature of the methyl C atoms attached to Cu, as indicated by the downfield 13 C 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.
Bertz et al. (Thu,) studied this question.