Azirines, photoreactions

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

A selective sampling of the photochemical cycloaddition and cyclization chemistry of 2H-azirines has been outlined in this chapter. Some photochemical sequences increase molecular complexity more than others, but each seems to provide complex heterocyclic structures in a very efficient manner. Indeed, many of these photoreactions rapidly construct hetero-polycyclic systems that are difficult to produce in other ways. In contrast to their photochemical behavior, the major thermal reaction of 2H-azirines generally involves C(2)-N bond cleavage to form vinyl nitrenes which further react by either insertion into an adjacent C-H bond or else undergo addition across a neighboring rc-bond. The 27i-electrons of the carbon-nitrogen double bond of 2H-azirines can also participate in thermal symmetry-allowed [4- -2]-cycloadditions with a variety of substrates. It is clear from the above discussion that the chemistry of 2H-azirines is both mechanistically complex and... 

Gakis, N., Heimgartner, H., and Schmid, H., Photoreactions. Part 34. Photochemical cycloadditions of 3-phenyl-2//-azirines with benzoyl-, ethoxycarbonyl-, and vinylphosphonates, Helv. Chim. Acta, 58, 748, 1975. 

The photoreactions of azides can in most cases be rationalized in terms of initial loss of nitrogen with the formation of nitrenes which undergo rearrangement, insertion or addition reactions. Further examples of rearrangement of the nitrenes derived from vinyl azides to 2H-azirines have been reported ... 

The energetics of the photoreaction of 2if-azirine as well as the thermal ground-state reaction as obtained from CASPT2 calculations are summarized in Figure 6.14. From this diagram it can be concluded that the photolysis of 2H-azirine to form nitrile ylide occurs from the nji -excited state by way of an S -Sq conical intersection. Because the reaction paths on the surface from the Franck-Condon geometry to the conical intersection as well as on the Sq surface from the conical intersection to the nitrile ylide are... 

Streith and Nastasi have reviewed the photoreactions of three-membered rings. A study of the photo-ring-opening reactions of the azirines (125) has been reported. A CIDNP study of photoelectron transfer from cis- and trans-, 2-diphenylcyclopropane to chloranil has been carried out. The evidence collected from this study indicates that the intermediate involved is the radical cation (126), since no polarized rearrangement product was observed. The failure to observe reaction is in marked contrast to the behaviour when the cyclopropane is irradiated in the presence of 1,4-dicyanonaphthalene. ... 

Further examples of the preparation of 2i/-azirines by irradiation of a,j5-unsaturated azides have been reported.The azirine (112) has been proposed as an intermediate in the photoreaction of 6-azido-l,3-dimethyluracil (113) with... 

Many photoreactions of aryl azides can be interpreted as arising via unstable ring-fused azirines. The formation of the 6,7-diaminobenzothiazoles (117) and the 6-amino-8//-thiazolo[5,4-c]azepines (118) on irradiation of the corresponding 6-azidobenzothiazoles (119) in diethylamine can be explained in this way, as shown in Scheme 14. An alternative explanation for the photodecomposition of phenyl azide (120) at low temperatures has recently been advanced and involves the formation of an intermediate l-azacyclohepta-l,2,4,6-tetraene (121). Further evidence for this pathway is to be found in a study of the photodecomposition of phenyl azide in acetic acid the azepin-2-one (122) is believed to arise in this way, as shown in Scheme 15. The formation of the same azepin-2-one on irradiation of phenyl azide in the presence of naked acetate anion has been reported the nature of the intermediate in this transformation is still open to question, but both benzazirine and l-azacyclohepta-l,2,4,6-tetraene intermediates are considered possible. In contrast, irradiation of the azide (123) in an argon matrix at lOK affords, on the basis of spectral evidence, the cyclic carbodi-imide (124). " 2-... 

The usual photoreaction of a vinyl azide is intramolecular cyclization to yield a 2H-azirine. Although this process is a well-established path to these compounds, the details of the mechanism by which the reaction occurs is unresolved. In some instances the formation of the azirine is thought not to involve the nitrene intermediate. However, other studies have shown that azirines and vinyl nitrenes are in thermal equilibrium and thus it seems likely that a vinylnitrene is the key intermediate in such reactions. 

A photoreaction similar to that observed with 2i/-azirines and acyl... 

Photoreactions have also been observed between 3-phenyl-2/f-azirines and ketenes, carbodiimides, isocyanates as well as isothiocyanates. The isocyanates and their thio analogs react with the C-O and C-S double bonds, respectively, to give 79, 80, and 81, and not with the C-N double bond. ... 

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