Photochemistry of Type A Meso-ionic Heterocycles

The investigation of the photochemistry of meso-ionic heterocycles is still rather limited. However, some common mechanistic features can be appreciated in the pathways that have been put forward to account for the products of a number of photoreactions. A summary of the results obtained for various classes of meso-ionic heterocycles follows. Although the decision has been taken not to discuss in detail in this review the general chemistry of sydnones (1) and sydnone imines (2), it is nevertheless appropriate to include in this section the results obtained in the study of their photochemistry. 

The photochemistry of iVC-diphenylsydnone (1, R = R = Ph) has been the subject of almost simultaneous study by a number of groups. Some variations of isolated reaction products occurred, but the formation of 2,4,5-triphenyl-l,2,3-triazole (320, R = R = Ph) 

Irradiation of A -cyclohexyl-C-phenylsydnone (1, R = cyclohexyl, R = phenyl) in benzene under a nitrogen atmosphere gave the 1,3,4-oxadiazolin-5-one (317, R = cyclohexyl, R = phenyl 15%), the bisazoethylene (318, R = cyclohexyl, R = phenyl 15%), and the triazole (320, R = cyclohexyl, R = phenyl 12%). When the nitrogen atmosphere was replaced by carbon dioxide, then the yield of the 1,3,4-oxadiazolin-5-one was increased from 15% to 47%. These results provide good support for the general mechanistic proposals given in Fig. 4. 

Photochemical cycloaddition reactions between sydnones (1) and 1,3-dipolarophiles take place to give products which are different from, but isomeric with, the thermal 1,3-dipolar cycloaddition products. These results are directly interpreted in terms of reactions between the 1,3-dipolarophiles and Ae nit mine (316). The photochemical reactions between sydnones and the following 1,3-dipolarophiles have been reported dicyclopentadiene, dimethyl acetylene dicarboxylate, dimethyl maleate, dimethyl fumarate, indene, carbon dioxide, and carbon disulfide.  

The photochemical isomerization of -acylsydnone imines (2, R = COR) to give meso-ionic l,2,3-triazol-4-ones (176) has been reported.  

Hansen, and H. Schmid, Helv. Chim. Acta 54, 1275 (1971). 

Huseya, A. Chinone, and M. Ohta, Bull. Chem. Soc. Jap. 45, 3202 (1972). 

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