A study on an unusual S N 2 mechanism in the methylation of benzyne through nickel-complexation.

In this study, three reaction mechanisms of a benzyne-nickel (Ni) complex ([Ni(C6H4)(dcpe)]) with iodomethane during the methylation process were investigated, namely (a) S(N)2 reaction of the benzyne-Ni complex with iodomethane, (b) concerted sigma-bond metathesis during the bond breaking/ forming processes, and (c) oxidative addition of iodomethane to the Ni-center and the subsequent reductive elimination process. DFT calculations revealed that the reaction barrier of the S(N)2 reaction is slightly lower than those of the other mechanisms. The results of orbital analyses suggest that [Ni(C6H4)(dcpe)] forms a metallacycle structure between benzyne and the Ni-II (3d(8)) center instead of the eta(2)-structure with the Ni-0 (3d(10)) center. The metallacycle structures became inappropriate as the intermediates of oxidative addition in the formation of the Ni-II-Me bond, avoiding further oxidation to the high-valent Ni-IV. The high free energy along s-bond metathesis was generated from the steric hindrance, thus invoking methylation and Ni-I bond formation concertedly.