The origin of the common competition between [2,3]- and [1,2]-sigmatropic rearrangement has been investigated. The [2,3]-rearrangement is an allowed concerted pericyclic reaction; the [1,2]- with retention is forbidden and should occur by a stepwise process. We studied the mechanism of base-induced [2,3]- and [1,2]-sigmatropic rearrangement of allyl ammonium salts by a combination of kinetic isotope effects (KIEs), temperature dependence of product distribution, cross-over experiments, theoretical calculations and dynamic trajectories. The theoretical calculations predicted a loose transition state for such reactions. Experimental KIEs matched with the theoretically predicted KIEs. Based on a loose transition state dynamic trajectories showed that from a single transition state there is a bifurcation on the free energy surface. A concerted path leads to only the [2,3]- product and an alternative path leads to diradicals that can combine to give both [1,2]- and [2,3]-products. The diradical path is enthalpically disfavored but entropically favored compared to concerted path. An increase in [1,2]-product was observed at higher temperatures. Cross-over experiments showed greater cross-over in [1,2]-product over [2,3]-product which also supports predictions by dynamic trajectories. Overall, the unexpected competition between an allowed and forbidden product is due to the fact that both products come from the same transition state and there is a bifurcation on the free energy surface.

Origin of common competition between "allowed" [2,3]- and "forbidden" [1,2]-sigmatropic rearrangements: A mechanistic investigation

246th National Meeting (2013)

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