title: 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 (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.
authors: Department of Chemistry and Biochemistry, Texas Tech University , Lubbock, Texas 79409-1061, United States.
authors: Shikha
authors: S
authors: Sharma
authors: Trideep
authors: T
authors: Rajale
authors: David B
authors: DB
authors: Cordes
authors: Fernando
authors: F
authors: Hung-Low
authors: David M
authors: DM
authors: Birney