Vinyl azides conversion into 1-azirines

As indicated in Table 6.1 some 2/f-aziiines are isolated from one-pot reactions in which the vinyl azide precursors are produced in situ. For example, 3-dialkylamino-2//-azirines 4 have been prepared from tertiary amides 3 the vinyl azide intermediates are generated by successive chlorination and reaction with sodium azide (Scheme 6.2). The conversion of these vinyl azides into azirines takes place at room temperature and it is clear that the nature of the substituents on the double bond of vinyl azides influences their temperature of decomposition. Fluoro substituted vinyl azides also decompose at room temperature. ... 

A typical high-yield synthesis of a vinyl substituted 2/f-azirine is shown by the conversion of the azide 624 into 625". Similar behaviour is seen for the isomeric azidoeneynes 626 and 627 that are converted into the alkynyl substituted azirine 628 while irradiation of the azidoallylphosphonates 629 yields the corresponding azirines 630 and 631 is transformed into 632 at 0 This route to azirines has been extended to the synthesis of the bis-2/f-azirines 633 and 634, which are formed by irradiation of the bis azides 635 and 636", respectively. The formation of these products arises by sequential loss of nitrogen. Thus the bis azide 635 is converted to 637 prior to the ultimate formation of the final product 633. Methylene azirines such as 638 are formed on photoelimination of nitrogen from the azides 639. In this instance, however, the presence of the azirine derivative was established by trapping experiments with HCN when the adducts 640 are formed . 

Table 6.1 Examples of the conversion of vinyl azides into 2H-azirines...

For example, 2-arylazirines 13 can rearrange to indoles 14 when heated in xylene (Scheme 6.6) and the direct thermal conversion of alkyl 2-azidocinnamates into indoles has been exploited as a route to indole-2-carboxylic acid esters. The temperature has to be controlled when these vinyl azides were decomposed in solution in order to isolate 2H-azirines. ° Dilute solutions are also usually preferable in order to avoid bimolecular reactions. Photolysis is sometimes preferable to thermolysis for the generation of thermally unstable azirines. ... 

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