Experimental and Computational Studies on the [3,3]- and [3,5]-Sigmatropic Rearrangements of Acetoxycyclohexadienones: A Non-ionic Mechanism for Acyl Migration

Flash vacuum pyrolysis studies of
substituted 6-acetoxy-2,4-cyclohexadienones
(3 and 10) from 300 to 500 °C provide
strong experimental evidence that direct [3,5]-sigmatropic rearrangements
in these molecules are favored over the more familiar [3,3]-sigmatropic
rearrangements. The preference holds when the results are extrapolated
to 0.0% conversion, indicating that this is a concerted process. Pyrolysis
of 6,6-diacetoxy-2-methyl-2,4-cyclohexadienone (9) at
350 °C gives a modest yield of the initial [3,5]-sigmatropic
rearrangement product, 2,6-diacetoxy-6-methyl-2,4-cyclohexadienone
(11). Qualitative arguments and electronic structure
theory calculations are in agreement that the lowest energy pathway
for each [3,5]-sigmatropic rearrangement is via an allowed, concerted
pseudopericyclic transition state. The crystal structures of compounds 3, 9, and 10 prefigure these transition
states. The selectivity for the [3,5] products increases with an increasing
temperature. This unexpected selectivity is explained by a concerted,
intramolecular, and pseudopericyclic transition state (TS-5) that forms a tetrahedral interemediate (ortho-acid
ester 4′), followed by similar ring openings to
isomeric phenols, which shifts the equilibrium toward the phenols
from the [3,5] (but not the [3,3]) products.