Undesirable side-reactions of diazirines

Like diazo compounds and aryl azides, diazirines give rise in most instances to unwanted photolysis products besides the desired carbenes, including long-lived electrophilic intermediates. The most serious problem is the generation of diazo compounds (e.g. Fig. 3.13). For example, 3-H-3-aryl-diazirines form diazo isomers to the extent of 30 to 70% when irradiated. These isomers are themselves photolysed to form carbenes, but relatively slowly at wavelengths at which the diazirines absorb (Smith and Knowles, 1973, 1975). 

The rearrangement of phenyl carbene to cyclohepta-1,2,4,6-tetraene has been detected at 10 K (Westetal., 1982 cf. Fig. 3.3), but the relevance of this to photochemistry in solution at higher temperatures is not yet clear. Ortho-substituents that might react with photogenerated aryl carbenes should be avoided as they are with arylazides (Section 3.2.4 Fig. 3.4). 

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