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-fluoromethylcyclobutenes, 2-fluoromethyl-3-oxetenes and perfluoro-3-methyl-cyclobutene have been studied at the MP2/cc-pVTZ level of theory and the results analysed by using the activation energy approach and also the NBO interactions of the breaking ring bond with the substituent bond. The outward or inward opening that has lower activation energy in the activation energy approach or larger interaction in the NBO approach constitutes the preferred mode. 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 higher lying conformer may even significantly contribute to the overall selectivity. The conformational profile of the substituent, therefore, is recommended for taking into consideration in any serious treatment of the subject. The experimental selectivity, if otherwise, is likely to be a consequence of secondary reactions such as the reaction equilibration, which honours the relative thermodynamic stabilities of the ring opened products.