Mechanism of trans double left right arrow cis isomerization reaction in 3-substituted thietane-1-oxide

The trans double left right arrow cis isomerization reaction in 3-chloro th ietane-1-oxides (TOX) has been studied by ab initio methods. To predict the thermodynamics of this molecular system, various basis sets and levels inc luding second, third and fourth Moller-Plesset perturbation theory have bee n used. The QCISD/6-31+G** level has also been explored. The calculations p redict, 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 solve nts the 'equilibrium' is displaced to the trans formation. Thus, on solutio n the cis concentration is 25% in the chloro, derivative. The transition st ate (TS) for this isomerization reaction has been found and fully character ized. The heterocyclic ring in the TS possesses a quasi-planar structure wi th a puckering angle of ca. 4 degrees. The TS for the methyl derivative iso merization reaction has also been found. The puckering angle in this case i s just 1.5 degrees. The activation energies (E-a) for the chloro and methyl derivatives are ca. 49 and 45 kcal/mol in the gas phase, decreasing to ca. 37 and 33 kcal/mol in solution, respectively. The mechanisms for the trans double left right arrow cis isomerization reactions reported here are most likely also to occur in the ethyl and t-butyl-TOX derivatives. (C) 2001 El sevier Science B.V. All rights reserved.