Experimental and computational studies on the [3,3]- and [3,5]-sigmatropic rearrangements of acetoxycyclohexadienones: A non-ionic mechanism for acyl migration

Shikha Sharma, Trideep Rajale, David B. Cordes, Fernando Hung-Low, David M. Birney

Research output: Contribution to journalArticle

18 Scopus citations

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.

Original languageEnglish
Pages (from-to)14438-14447
Number of pages10
JournalJournal of the American Chemical Society
Volume135
Issue number38
DOIs
StatePublished - Sep 25 2013

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