Experimental and Computational Studies on the [3,3]- and [3,5]-Sigmatropic Rearrangement 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 °C to 500 °C provide strong experimental evidence that direct [3,5]-sigmatropic rearrangements in these molecules are favored over the more familiar [3,3]-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]-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]-rearrangement is via an allowed, concerted pseudopericyclic transition state. The crystal structures of 3, 9, and 10 prefigure these transition states. The selectivity for the [3,5]-products increases with increasing temperature. This unexpected selectivity is explained by a concerted, intramo