Reinvestigation of the Isotope Effects for the Claisen and Aromatic Claisen Rearrangements: The Nature of the Claisen Transition States

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Abstract

The aliphatic Claisen rearrangement of allyl vinyl ether and the aromatic Claisen rearrangement of allyl phenyl ether are investigated in a combined experimental and calculational study. Theoretically predicted kinetic isotope effects (KIEs) at all levels disagree with about half of the literature experimental heavy-atom isotope effects. New experimental 13C and 2H isotope effects were determined by multisite NMR methodology at natural abundance, and 17O isotope effects were determined by novel NMR methodology. These new experimental isotope effects are inconsistent with the literature values and agree well the high-level predicted KIEs, suggesting that the prior theory/experiment disagreement results from inaccuracy in the experimental KIEs. A one-dimensional tunneling correction is found to improve kinetic isotope effect predictions in a number of reactions and is found to be sufficient to provide differences between predicted and experimental heavy-atom isotope effects on the order of the experimental uncertainty in the reactions studied. The best agreement between experimental and predicted isotope effects is seen for the highest-level calculations. On the basis of the experimentally supported transition state geometries, the nature of the Claisen and aromatic Claisen transition states is discussed.

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