α-Sulfur or α-fluorine—Which is more stabilizing for a carbocation? A computational study of electrophilic addition to HFCCH(SMe) and FC(R1)CR2(SMe) and related model systems

Carbocations derived from protonation, methylation, and bromine addition to HFCCH(SMe) (1) were studied by DFT [B3LYP/6-31G*, B3LYP/6-311G(3df,p), and B3LYP/aug-cc-pVTZ] and by MP2/6-31G* to examine relative carbocation stabilizing effects of α-SMe versus α-fluorine. The α-SMe carbocations 1aE+ and the unsymmetrical thiiranium ions 1cE+ were found to be considerably more stable than the corresponding α-fluorocarbenium ions 1bE+. Study of protonation and methylation of FC(R1)CR2(SMe) [R1 = H, R2 = SMe (2); R1 = F, R2 = H (3); R1 = F, R2 = SMe (4); R1 = Me, R2 = H (5); R1 = Me, R2 = SMe (6)] by B3LYP/6-31G* identified the corresponding unsymmetrical thiiranium cations as lowest energy minima, followed closely by the α-thiocarbenium ions. With 2, 4, and 6, skeletally rearranged α-thiocarbenium ions are formed by SMe migration. The α-SMe and the thiiranium cations are also favored relative to α-fluorocarbenium ions in protonation and methylation of the cyclic analogs 8 and 9. Computed NPA charges and the GIAO-derived 13C and 19F NMR chemical shifts underscore the significance of α-thiocarbocations and thiiranium ions in electrophilic addition to FC(R1)CR2(SMe).