Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 135, Issue 38, Pages 14438-14447Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja4077364
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Funding
- Robert A. Welch Foundation [D-1239]
- NSF CRIF MU Grant [CHE-1048553]
- CRIF MU Instrumentation Grant from the National Science Foundation [CHE-0840493]
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Flash vacuum pyrolysis studies of substituted 6-acetoxy-2,4-cyclohexadienones (3 and 10) from 300 to 500 degrees 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 degrees 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.
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