Experimental and Computational Studies on the [3,3]- and [3,5]-Sigmatropic Rearrangements of Acetoxycyclohexadienones: A Non-ionic Mechanism for Acyl Migration
| Autor: | David B. Cordes, David M. Birney, Fernando Hung-Low, Trideep Rajale, Shikha Sharma |
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| Rok vydání: | 2013 |
| Předmět: | |
| Zdroj: | Journal of the American Chemical Society. 135:14438-14447 |
| ISSN: | 1520-5126 0002-7863 |
| DOI: | 10.1021/ja4077364 |
| Popis: | 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. |
| Databáze: | OpenAIRE |
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