Thermal rearrangement of oxaziridines

Lattes and co-workers (23) have studied the photochemical and thermal rearrangement of oxaziridines into amides (79 80). 

They have further pointed out that the lower regioselectivity observed in the thermal rearrangement of oxaziridines does not rule out the above stereoelectronic requirement. In this case, the energy required for the reaction in sufficient to induce both nitrogen inversion in the starting oxaziridine and migration of the C-substituents. 

A potentially important industrial application of this reaction involves the thermal rearrangement of 3,3-pentamethylene-oxaziridine 67 to e-caprolactam, a reaction that is strongly catalyzed by transition-metal ions. ... 

There are a number of thermal and photochemical reactions for which oxaziridine intermediates have been proposed but never isolated. These include, among others, the photochemical Beckmann rearrangement of oximes, many photochemical reactions of aromatic A -oxides, and the thermal rearrangement of nitrones to amides. A brief discussion of the first two seems warranted in this review because they have been studied extensively and some strong inferential evidence for oxaziridine intermediates has been obtained. 

The reported synthesis and properties of an oxaziridine by the thermal isomerization of the C7-trityl ester of 3-ac/nitrocamphor bears reinvestigation, particularly in view of the recent report that irradiation of nitronate salts yields hydrox-amic acids but that oxaziridine intermediates could not be detected.) A -Sulfonyl-oxaziridines also rearrange, thermally to nitrones that further decompose to products, but photochemically they yield amides. ... 

In contrast, perfluorooxaziridines 30, containing a fluorine substituent on the ring carbon, are more stable than oxaziridines 28 and also rearrange thermally by a different pathway to give imidates 31 in quantitative yield.57 The mechanism presumably involves initial homolytic cleavage of the N-0 bond, as is the case for oxaziridines 28, followed by a fluoroalkyl shift from carbon to oxygen.56 57 Thus, the fluorine at C3 in oxaziridines 30 seems to have a dramatic effect on the mode of rearrangement. 

The remarkable stability reportedfor 2-phenyl-3,3-dibenzoyloxaziridine in the face of Padwa s inability to isolate oxaziridines but only amides upon oxidation of jV-phenylacylimine derivatives and the lability of nitrones 80 and 82 has recently been shown to be in error.The product from the thermal or photochemical isomerization of A -phenyl-C,C-dibenzoylnitrone is the expected rearrangement product, A -benzoylphenylglyoxanilide. 

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