Activation Energy and NBO Interaction Approaches to Torquoselectivity and its Dependence on the Conformational Profile of the Substituent
2021
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.
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