Aziridines via Nitrene Intermediates

An outline of azides that have been shown to give aziridines via nitrenes either thermally or photochemically and in synthetically useful yields is given in Table 6.2. 

A further restriction on the synthetic utility of the nitrene addition reaction is its unpredictable stereochemistry in reactions with disubstituted alkenes such as cis- and trans-but-2-ene. Nitrenes can exist in a singlet or triplet state. For most nitrenes the triplet (diradical) state is the ground state. Nitrenes that are generated thermally or by direct photolysis are initially in the singlet state and their (concerted) addition to alkenes is stereospeciflc. If the alkenes are relatively unreactive the nitrene can convert into its ground triplet state either partially or completely before addition. The resultant aziridines are produced with varying degrees of stereoselectivity because the addition of the triplet nitrene is a stepwise process. The triplet species can also be produced directly by photosensitized addition.  

Diphenylphosphoryl azide 30 is another useful reagent for catalyzed aziridination. Several substituted styrenes have been converted into the corresponding iV-phosphory- 

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The Nagata group also developed a new approach to the construction of bridged aziridines via nitrene intermediates that led to the total synthesis of the indole alkaloids, ibogamine, velbanamine, and coronaridine (1968-1971). 

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