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

Abstract

Flash vacuum pyrolysis studies of substituted 6-acetoxy-2,4-cyclohexadienones (<b>3</b> and <b>10</b>) 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 (<b>9</b>) at 350 °C gives a modest yield of the initial [3,5]-sigmatropic rearrangement product, 2,6-diacetoxy-6-methyl-2,4-cyclohexadienone (<b>11</b>). 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 <b>3</b>, <b>9</b>, and <b>10</b> 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 (<b>TS-5</b>) that forms a tetrahedral interemediate (<i>ortho</i>-acid ester <b>4′</b>), 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