Consideration of the role of dynamic trajectories in [1,2]- and [2,3]-sigmatropic rearrangements suggests a counterintuitive approach to controlling the selectivity. In our hypothesis, [2,3] selectivity can be promoted by reaction conditions that thermodynamically disfavor the [2,3] rearrangement step and thereby make the transition state later. The application of this idea has led to a successful prescription for dictating the selectivity in Stevens/Sommelet-Hauser rearrangements of ammonium ylides. A combination of kinetic isotope effects, crossover experiments, and computational dynamic trajectories support the idea that the selectivity is controlled through control of the path of trajectories.

The [2,3]- and [1,2]-sigmatropic rearrangements of ammonium ylides are studied by a combination of experimental, standard computational, and dynamic trajectory methods. The mixture of concerted [2,3] rearrangement and bond cleavage observed experimentally is accounted for by the outcome of trajectories passing through the formal [2,3] rearrangement transition state. In this way the bond cleavage is promoted by the pericyclic stabilization of the [2,3] transition state. It is proposed that this dynamic effect is responsible for the pervasive co-occurrence of the two rearrangements.

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