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 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
(ΔH°f298 = −15.6
kcal mol-1), thioacrolein
(ΔH°f298 = 37.9 kcal mol-1),
and their radical anions (ΔH°f298 =
−17.3 kcal mol-1 and
ΔH°f298 = 16.3 kcal mol-1,
respectively) are also provided.