Photochemical induced 2/7-azirine

The mechanism of the photochemically induced conversion of vinylazides into 2H-azirines has been examined using ab initio MO calculations. A new approach to the elusive thionitrosobenzene system has been reported. Direct irradiation of the 3-azido-2,l-benzisothiazole (88) affords a transient 2-cyanothionitrosobenzene (89) which can be trapped as the cycloadduct (90) with cyclopentadiene. Nitrene cyclization is the preferred pathway on irradiation of the 5-formyl- or 5-benzoyl-6-azidouracils (91) and yields the isoxazolo[3,4-d]pyrimidines (92). In contrast, 5-phenyl- and 5-benzyl-6-azidouracils were converted into pyrimido[4,5-b]indoles and pyrimido[4,5-ti Jquinolines, respectively, by a pathway involving photochemically induced loss of nitrogen and intramolecular nitrene insertion. 

Photochemically induced 1,3-rearrangements of 2-aroyl-3-aryl-2//-azirines result in the formation of either oxazoles or isoxazoles, depending on the wavelength of light used <72JA1199>. Padwa and co-workers have studied the photochemical rearrangement of 2Ff-azirines which bear an unsaturated... 

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. 

Section 6.4.4 already presented several examples of photocatalysts such as metalloporphyrins and other organic metal complexes. In another example, a singlet excited photocatalyst, 1,4-dicyanonaphthalene (DCN), initiates the photochemical electron transfer-induced ring opening of an azirine 568 (Scheme 6.272).1473 Subsequent addition of an intermediate 569 to acrylonitrile leads to 570 in two steps, while DCN is regenerated.883... 

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