Pivaloyl azide nitrenes from

The results with carboethoxynitrene are quite different from those obtained with pivaloyl azide 7 and the aroylnitrenes. Furthermore, photolysis of azidoesters 13 and 14 in contrast to pivalolyl and aroylazides at cryogenic temperatures produces persistent nitrene EPR signals. Thus, it is clear that the nitrene esters have triplet ground states. 

Nitrene addition reactions have somewhat more scope when the reactions are carried out by azide photolysis for example, the acyinitrene pivaloylnitrene adds in moderate yield and with good stereoselectivity to cj>alkenes when it is generated from pivaloyl azide by photolysis in dichloromethane." Similarly p-cyanobenzoylnitrene, when generated by photolysis of the azide 21 in the presence of 2,5-dihydrofuian, gives the aziridine 22 in moderate yield (Scheme 6.11)." This nitrene and other p-substituted benzoylnitrenes also react with Cgo to give fulleroaziridines such as 23." ... 

It has been shown that the yield of isocyanates, formed upon photolysis of a series of carbonyl azides (R-CO-N3, R = -Butyl, Aryl), remains constant in the presence and in the absence of nitrene traps.For example, the yield of isocyanate 33a produced on photolysis of pivaloyl azide (R = r-Butyl, 31a) in methylene chloride (an inert solvent) is 40%. Photolysis of 31a in cyclohexene leads to a 45% yield of aziridine adduct 34a and a 41% yield of isocyanate 33a. Trapping the nitrene does not depress the yield of isocyanate. Hence, isocyanate 33a and adduct 34a cannot be derived from the same reactive intermediate, but instead the isocyanate must be formed from the excited state of the azide or from the electronically or thermally excited nitrene. 

Pivaloyl nitrene, generated photolytically from pivaloyl azide, adds to olefins stereospecifically in its singlet state and stereoselectivily in its triplet state. Dichloro-... 

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