Detection and kinetics of the single-crystal to single-crystal complete transformation of a thiiranium ion into thietanium ion

The conversion of a di-tert-butyl-methylthiiranium ion into thietanium ion, that is reported in the literature as taking place spontaneously at 25 C in a CD2Cl2 solution, has been discovered to occur quantitatively at room temperature (RT) also in the crystalline state. The ring enlargement reaction is accompanied, in the solid phase, by a modest deterioration of the quality of the sample under investigation, and all three specimens here studied by in situ crystallography maintained their single-crystal nature up to 100% conversion. The rearrangement reaction implies the breaking of a C-S bond and the formation of a new bond of the same type, together with the migration of a methyl group. The extent of the corresponding atomic displacements has been measured by comparing the initial and final crystal structures. Several intermediate stages of the process have been investigated and characterized by the site occupancy factor of the episulfonium ion. The RT temporal evolution of this factor and that of the unit-cell volume indicate multi-step kinetics, with processes of simple molecular reorientation or displacement before and after the main, central stage, where the conversion reaction takes place. The overall kinetics is well described by an Avrami-Erofeev equation, with exponent m = 1.75(3) and rate coefficient k = 10.4(3) x 10-8 s-1 at 25 C. Ab initio calculations in the gas phase predict a three-step mechanism resulting in a slightly spontaneous reaction with an overall decrease of entropy.