The Thio-Wittig Rearrangement of Deprotonated Allyl Methyl Sulfide. A Gas-Phase Unimolecular Isomerization Probed with a Variable Temperature Flowing Afterglow-Triple Quadrupole Device

Abstract

The thio-Wittig rearrangement of deprotonated allyl methyl sulfide has been examined in the gas phase with a variable temperature flowing afterglow-triple quadrupole device. Collision-induced dissociation studies of a series of thiolate anions (RS - ) reveal that methyl deprotonation leads to 3-butene-1-thiolate ( 2a ), the [2,3]-Wittig product, while 1-thiomethylallyl anion ( 1a ) isomerizes to 1-butenyl thiolate ( 4a ), the [1,4]-Wittig product, at elevated temperatures. Activation energies for these processes have been estimated using the Arrhenius equation and are compared to high-level (G2) calculations for the homolytic and heterolytic bond dissociation energies. Stepwise and concerted [1,4] pathways are found to have similar energy requirements, which accounts for some of the mechanistic controversy regarding these transformations in solution. The observed selectivity, [1,2] vs [1,4], is most easily accommodated by a concerted process but can be explained in terms of a stepwise mechanism by considering the spin density and charge location in a radical anion intermediate ( 6a ). Frontier molecular orbital theory, however, leads to the wrong prediction. The [2,3]-Wittig rearrangement appears to proceed via a concerted pathway in the gas phase as has been invoked in the liquid phase. Heats of formation for acrolein (Δ H ° f298 = −15.6 kcal mol -1 ), thioacrolein (Δ H ° f298 = 37.9 kcal mol -1 ), and their radical anions (Δ H ° f298 = −17.3 kcal mol -1 and Δ H ° f298 = 16.3 kcal mol -1, respectively) are also provided.