1,3-Diphenylsulphonylcyclobutane: An unprecedented case of a higher stability of the trans isomer of 1,3-disubstituted cyclobutane

Equilibration of 1,3-diphenylsulphonylcyclobutane in t-BuOK/t-BuOH shows that the trans isomer is more stable than is the cis isomer by 2.1 kcal/mol. This is in contradistinction to the general case of 1,3-disubstituted cyclobutanes, where the bulky substituent prefers the cis geometry in order to avoid 1,3-diaxial interactions. The effect was found to be enthalpic in nature. Ab initio 3-21G calculations resulted in an energy difference of 2.6 kcal between the two isomers, in favor of the trans isomer. The cyclobutane rings were found to be nearly planar. While this may explain the higher stability of the trans isomer, it is not clear why the system avoids steric interaction this way, rather than by adopting the puckered conformation with the two bulky substituents at equatorial positions.