Mechanism of trans⇔cis isomerization reaction in 3-substituted thietane-1-oxide

Abstract

The trans⇔cis isomerization reaction in 3-chloro thietane-1-oxides (TOX) has been studied by ab initio methods. To predict the thermodynamics of this molecular system, various basis sets and levels including second, third and fourth Møller-Plesset perturbation theory have been used. The QCISD/6-31+G∗∗ level has also been explored. The calculations predict, in good agreement with the experimental data, that in the gas phase the cis concentration is ca. 85%. In solution of polar and non-polar solvents the ‘equilibrium’ is displaced to the trans formation. Thus, on solution the cis concentration is 25% in the chloro derivative. The transition state (TS) for this isomerization reaction has been found and fully characterized. The heterocyclic ring in the TS possesses a quasi-planar structure with a puckering angle of ca. 4°. The TS for the methyl derivative isomerization reaction has also been found. The puckering angle in this case is just 1.5°. The activation energies (Ea) for the chloro and methyl derivatives are ca. 49 and 45kcal/mol in the gas phase, decreasing to ca. 37 and 33kcal/mol in solution, respectively. The mechanisms for the trans⇔cis isomerization reactions reported here are most likely also to occur in the ethyl and t-butyl-TOX derivatives.