Azirines bond rupture

The photochemically induced isomerization of isoxazole (279) to oxazole (281) is known to proceed via the intermediate keto-azirine (280). An M.O. study of the isomerization reactions of the azirine that are involved has been reported. Irradiation of (280) at >300 nm gives n tt excitation of the CO chromophore to 5i, intersystem crossing to Ti, and then C—N bond rupture, to form (279). Irradiation at 254 nm, however, gives n - n excitation of the C=N chromophore to S2, intersystem crossing to T, and then C—C bond rupture, to form (281). Such an isomerization reaction resulted in the photochemical conversion of isoxazolophane (282) into (283) (40%). Irradiation of (283) at 254 nm produced a quantitative yield of the expected oxazolophane. A new rearrangement was found, however, on irradiation of (283) at >300 nm, when the acyl-ketenimine (284) was formed, which could be converted into (285) by acid hydrolysis. 

The condensation of amines with 2-formyl-3-phenyl-2/f-azirine (350) yields the imines 352, which undergo thermal reorganization to pyrazoles 353 via 1,5-dipolar cyclization of the resulting vinylnitrene intermediate derived from CN bond rupture. The thermal reactivity of 350 and 352 parallels completely that of the 2-vinyl-substituted 2/f-azirines. 

Although acid- and thermally induced rearrangements of azido-quinones have been known for several decades, only in the last few years have efforts been made to elucidate the mechanism of these reactions. Unlike most other vinyl azides discussed in this chapter, azidoquinone chemistry generally seems not to involve azirines. Instead tiae products result from expulsion of molecular nitrogen usually followed by rupture of a carbon-carbon bond. 

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