Evidence for a Sigmatropic and an Ionic Pathway in the Winstein Rearrangement

Packard, M. H.; Ott, A. A.; Ortuño, M. A.; Johnson, A.; Suding, V. P.; Cramer, C. J.; Topczewski, J. J.
J. Org. Chem. 2018, 83, 8214 (doi:10.1021/acs.joc.8b00961).

The spontaneous rearrangement of allylic azides is thought to be a sigmatropic reaction. Presented herein is a detailed investigation into the rearrangement of several allylic azides. A combination of experiments including equilibrium studies, kinetic analysis, density functional theory calculations, and selective ¹⁵N-isotopic labeling are included. We conclude that the Winstein rearrangement occurs by the assumed sigmatropic pathway under most conditions. However, racemization was observed for some cyclic allylic azides. A kinetic analysis of this process is provided, which supports a previously undescribed ionic pathway.