In today’s reaction seminar, the Wharton transposition made an appearance. In our practice case it started from the epoxide as in scheme 1 below as on Wikipedia; Kürti/Czakó (page 482) begin from the enone which is oxidised by hydrogen peroxide to give the same epoxide.
The first steps are clear to me, I am interested in the mechanism of the final one. Our problem set’s solution key (taken from a Japanese book that I cannot reference for lack of understanding) draws a concerted [1,3] migration as depicted in scheme 2a. Kürti/Czakó depict a sequential release with an intermediate vinyl anion. Wikipedia makes no note on that step and does not discuss mechanistic proposals.
Kürti/Czakó also point out that:
Alternatively, the formation of a vinyl radical has been proposed.6
6: G. Stork, P. G. Willard, J. Am. Chem. Soc. 1977, 99, 7067–7068.
The concerted mechanism seems slightly unreasonable due to the strained four-membered ring that would be required in the transition state. Likewise, the stepwise mechanism requires the formation of a vinyl anion which — while possible — is generally not very stable. In its defence, the step would be rendered irreversible by the liberation of nitrogen gas and the subsequent anion immediately captured by any acidic proton around.
Which mechanism does the reaction most probably follow?
L. Kürti and B. Czakó: Strategic Applications of Named Reactions in Organic Synthesis. Background and Detailed Mechanisms, Elsevier Academic Press, Burlington, MA, USA, 2005, page 482.