Abstract
A quantum chemical study of [1,j] sigmatropic proton shifts in polyenyl anions and related conjugated systems has been performed. We found that the Woodward-Hoffmann rules can be applied to understand the stereochemical outcome of these sigmatropic rearrangements, showing that [1,j] sigmatropic proton shift occurs antarafacially when j = 4n + 2, while suprafacial proton shift is symmetry-allowed when j = 4n. The activation barriers for [1,j] proton shifts in polyenyl anions CjHj+3- are 48.2 (j = 2), 32.8 (j = 4), 21.0 (j = 6), 40.5 (j = 8), and 49.1 (j = 10) kcal mol-1, respectively. This trend can be explained by the trade-off between stereoelectronic requirement and ring strain in the proton shift transition structure. Among these reactions, only the [1,6] proton shift with the lowest activation barrier can occur intramolecularly under mild reaction conditions. The others are unlikely to take place in a direct manner. Consequently, proton shuttles are generally required to facilitate these sigmatropic proton shifts through a protonation/deprotonation mechanism.
Published Version
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