Concerted [2,3]-sigmatropic rearrangements are allowed pericyclic reactions while concerted [1,2]-rearrangements of this kind are forbidden. The conventional mechanistic explanation of this competition is that [1,2]-sigmatropic rearrangements occur via stepwise cleavage-recombination mechanism and the extent of competition depends on the difference in the free energies of the corresponding transition states. Though the cleavage-recombination paths are energetically less favored over pericyclic [2,3]-processes those have an entropic advantage over the pericyclic processes, as the first step is associated with cleavage. We studied base mediated sigmatropic rearrangements of allyl ammonium salts to understand the origin of the competition between [2,3]- and [1,2]-sigmatropic rearrangements via combination of experiments and theoretical calculations. Theoretical calculations suggest a very loose transition state for [2,3]-sigmatropic rearrangements. The experimental Kinetic isotope effects (KIEs) matched with theoretically predicted KIEs. Based on a loose transition state, dynamics showed that few trajectories lead to cleavage of the molecule to produce diradicals. Those diradicals can combine together to form [1,2]-product. Overall, based on this single transition state dynamic trajectories successfully predicted the extent of competition between [2,3]- and [1,2]-sigmatropic rearrangement.References:Biswas, B.; Collins, S. C.; Singleton, D. A. J. Am. Chem. Soc. 2014 .

Insights into the facility of �forbidden� [1,2]-sigmatropic rearrangements

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.