Differential Activation Energy and NBO Interaction Approaches to Torquoselectivity and Its Dependence on the Conformational Profile of the Substituent

The torquoselectivity of ring opening of 3-CF₃-cyclobutene, 3-CHF₂-cyclobutene, 3-CH₂F-cyclobutene, 3-CF₃-oxetene and perfluoro-3-CH₃-cyclobutene have been studied at the MP2/cc-pVTZ level of theory and the results analysed by using the differential activation energy approach and also differential NBO interactions of the breaking ring bond with the substituent bonds. The outward or inward ring opening that has lower activation energy in the differential activation energy approach or larger interaction in the differential NBO interaction approach constitutes the preferred mode. The predictions from the two approaches are largely found to be contradicting each other, specifically when the substituent is electron-deficient. The CHF₂ and CH₂F substituents on cyclobutene and oxetene can adopt three distinct conformations with respect to the cleaving ring bond. It has been discovered that each conformer exhibits a distinct level of torquoselectivity and some may even contribute to the overall selectivity substantially. The conformational profile of the substituent, therefore, is recommended to be taken into account in any serious treatment of the subject. The experimental torquoselectivity, if otherwise, is a likely consequence of secondary reactions, specifically equilibration through reaction reversal while honouring the relative thermodynamic stabilities of the ring opened products.