Cyclobutane-bicyclobutane system. 2. Protonation, deprotonation, and intermediate carbanion stereochemistry in the 1, 3-hydrochloric acid elimination from 3-chlorocyclobutanecarbonitrile

The elimination of HC1 from trans-and cis-3-chlorocyclobutanecarbonitrile (3 and 4, respectively) in t-BuOH-t-BuOK was studied. The reaction of the two isomers proceeds via a common carbanion or rapidly interconverting isomeric carbanions, with the trans isomer 3 having an additional effective elimination mechanism in which the potassium cation electrophilically assists in the expulsion of the leaving group. Contrary to expectations, in the presence of crown ether the product distribution from the two isomers differs markedly, indicating that the intermediate carbanions derived from the two isomers arenot identical. Addition of crown ether also causes inversion of selectivity of the carbanion with respect to the elimination andreprotonation reactions. This is explained by a change in the acidity of the solvation shell as a function of the removal of thepotassium cation. In t-BuOK-t-BuOH, the intermediate carbanion is protonated faster to give the less stable isomer. This isinterpreted in terms of the least hindered approach principle. It is suggested that protonation and deprotonation proceed fromthe equatorial position in the trans isomer and from the axial position in the cis isomer.