CYCLOBUTANE-BICYCLOBUTANE SYSTEM - 2. PROTONATION, DEPROTONATION, AND INTERMEDIATE CARBANION STEREOCHEMISTRY IN THE 1,3-HCl ELIMINATION FROM 3-CHLOROCYCLOBUTANECARBONITRILE.

Article abstract of DOI:10.1021/ja00503a038

In the reported experiments, the elimination of HCI from trans- and -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 are not identical. Addition of crown ether also causes inversion of selectivity of the carbanion with respect to the elimination and reprotonation reactions. This is explained by a change in the acidity of the solvation shell as a function of the removal of the potassium cation. In t-BuOK-t-BuOH, the intermediate carbanion is protonated faster to give the less stable isomer. This is interpreted in terms of the least hindered approach principle. It is suggested that protonation and deprotonation proceed from the equatorial position in the trans isomer and from the axial position in the cis isomer.