THE THIO-WITTIG REARRANGEMENT OF DEPROTONATED ALLYL METHYL SULFIDE - A GAS-PHASE UNIMOLECULAR ISOMERIZATION PROBED WITH A VARIABLE-TEMPERATURE FLOWING AFTERGLOW-TRIPLE QUADRUPOLE DEVICE

The thio-Wittig rearrangement of deprotonated allyl methyl sulfide has been examined in the gas phase with a variable temperature flowing af terglow-triple quadrupole device. Collision-induced dissociation studi es of a series of thiolate anions (RS(-)) reveal that methyl deprotona tion leads to 3-butene-1-thiolate (2a), the [2,3]-Wittig product, whil e 1-thiomethylallyl anion (1a) isomerizes to 1-butenyl thiolate (4a), the [1,4]-Wittig product, at elevated temperatures. Activation energie s for these processes have been estimated using the Arrhenius equation and are compared to high-level (G2) calculations for the homolytic an d heterolytic bond dissociation energies. Stepwise and concerted [1,4] pathways are found to have similar energy requirements, which account s for some of the mechanistic controversy regarding these transformati ons in solution. The observed selectivity, [1,2] vs [1,4], is most eas ily accommodated by a concerted process but can be explained in terms of a stepwise mechanism by considering the spin density and charge loc ation in a radical anion intermediate (6a). Frontier molecular orbital theory, however, leads to the wrong prediction. The [2,3]-Wittig rear rangement appears to proceed via a concerted pathway in the gas phase as has been invoked in the liquid phase. Heats of formation for acrole in (Delta H degrees(f298) = -15.6 kcal mol(-1)), thioacrolein (Delta H degrees(f298) = 37.9 kcal mol(-1)), and their radical anions (Delta H degrees(f298) = -17.3 kcal mol(-1) and Delta H degrees(f298) = 16.3 k cal mol(-1), respectively) are also provided.