Reactions of Thiiranium and Sulfonium Ions with Alkenes in the Gas Phase.

Ion-molecule reactions between thiiranium ion ( 213) and cyclohexene and -cyclooctene resulted in the formation of addition products and ( 295 and 323, respectively) via an electrophilic addition pathway. Associative π-ligand exchange involving direct transfer of the PhS moiety, which has been observed for analogous seleniranium ions in the gas phase, did not occur despite previous solution experiments suggesting it as a valid pathway. DFT calculations at the M06-2X/def2-TZVP level of theory showed high barriers for the exchange reaction, while the addition pathway was more plausible. Further support for this pathway was provided with Hammett plots showing the rate of reaction to increase as the benzylic position of thiiranium ion derivatives became more electrophilic (ρ = +1.69; = 0.974). The more reactive isomeric sulfonium ion was discounted as being responsible for the observed reactivity with infrared spectroscopy and DFT calculations suggesting little possibility for isomerization. To further explore the differences in reactivity, thiiranium ion and sulfonium ion were formed independently, with the latter ion reacting over 260 times faster toward -cyclooctene than the thiiranium ion rationalized by calculations suggesting a barrierless pathway for sulfonium ion to react with the cycloalkene.