Nucleophilic “Addition-Elimination” Displacements on Activated Bicyclobutanes

In MeOH CN^- reacts with 3-chlorobicyclobutanecarbonitrile (1Cl) to give both chloride substitution and addition of HCN across the central bond. The bromo derivative (1Br) undergoes only substitution. An addition mechanism is offered for the halide substitution reactions. In Me₂SO with an excess of 1Cl, the major product is 1,3-dicyanobicyclobutane. In the presence of excess CN^-, 1,1,3-tricyanocyclobutane is obtained. With the carbanion NCH₂C^- with HCH = 90° as a model, the barrier for inversion was calculated at a 3-21G⁺ level to be 2.6 kcal/mol. The relative rates of carbanion inversion, ring flip, protonation, and Γ carbanionic halide displacement are discussed.