Wednesday night’s problem featured a variant of the venerable Staudinger reaction, which can be used to reduce azides to amines or generate iminophosphoranes which can participate in aza-Wittig reactions, which form imines rather than alkenes. The reactivity of azides is apparently more subtle than one might think, if this reaction has anything to say about it.
In the presence of base and electrophilic phosphorus, substitution first takes place to generate a phosphite. The phosphite then attacks the “hanging” nitrogen of the azide, which neutralizes the central nitrogen and leaves the phosphorus poised for attack by the negatively charged azide nitrogen, which generates the heterocycle shown (what would the name of this heterocycle be…?). Retro [2+2] cycloaddition from this dihydrotriazaphosphete releases nitrogen gas and gives the “curious intermediate” alluded to, an iminophosphorane. [3,3] sigmatropic rearrangement then generates the product.
The selectivity for the E olefin can be rationalized by a six-membered transition state that places the methyl group equatorial.
Interesting stuff! Tune in next week for another Wednesday Night Problem Session…