Tuning carbanion reactivity by complexing with boranes: γ-elimination reaction as a model

The anion of 3-chloro-3-phenylselenocyclobutanecarbonitrile undergoes in DME, in the presence of alcohols, competitive protonation and γ-elimination reactions [Eqn (1)]. With highly acidic alcohols such as trifluoroethanol the protonation is diffusion controlled and the rate for the cross ring elimination reaction is greater by a factor of three (at an alcohol concentration of 1 M). However, in the presence of BH₃ or Bu ₃B, the protonated product becomes dominant. This result is attributed to the binding of the carbanion by the boranes. Experimental data as well as ab initio calculations show that the preferred complexation site is the ring carbon and not the cyano nitrogen of the complex. The results also show that direct protonation of the complex is not a viable rationalization of the findings. It is suggested that the enhanced protonation results from the formation of a hydrogen bond between the alcohol and the cyano nitrogen, thus increasing the effective molarity of the proton donor which migrates to the α-carbon upon dissociation of the C - B bond.