Geometry and Conformation of Thietanium Ions from Diffraction Data and Ab Initio Calculations

Four-membered ring thiosulfonium ions may be obtained quantitatively and under mild conditions by anionotropic rearrangement of C-(tert-butyl)-substituted thiiranium ion precursors. Thus, t-4-(tert-butyl)-r-1,2,2,c-3-tetramethylthietanium tetrafluoroborate or hexachloroantimonate (2a or 2b, resp.) were formed from thiiranium ion 1. The thietanium salts 2a and 2b were characterized by X-ray crystal-structure analysis. Their cation geometry was also optimized by ab initio calculations at the RHF/6-31G*//RHF/6-31G* level, as were those of its stereoisomer 3 and of the unsubstituted S-methylthietanium ion 5. Comparison of 2, 3, and 5 with 4 – the only other thietanium ion studied by XRD, where the C-atoms of the thioniacyclobutane ring are part of a trinorbornane skeleton – indicates that, in these systems, relief from substituent overcrowding is easily achieved by a folding of the four-membered ring along the line connecting the two opposite C-atoms. The corresponding ring-deformation normal mode has a calculated frequency as low as 109 cm−1 in ion 5, to be compared with a frequency of 138 cm−1 in methylcyclobutane. For thietanium ion 2, the frequencies of the two normal modes involving such ring deformation have calculated values of 61 and 85 cm−1.