Photoelimination from azides

Aryl and heteroaryl nitrene insertion processes have also been employed in the synthesis of heterocycles. The azide 459, for example, is converted on irradiation into the imidazole 460,383 and dihydro-10-thiaisoalloxazines are obtained in good yield on photoelimination of nitrogen from 6-(2-azido-phenylthio)uracils.384... 

Scheiner has shown that triazolines undergo both direct and sensitized photoelimination of molecular nitrogen to yield aziridines.463 With the triazolines prepared from phenyl azide and cis- or trans-fi-methylstyrene, direct photolysis produces a mixture of aziridines in which the starting geometry has been largely retained. Sensitized photolysis, however, is totally nonstereospecific. 

The photoelimination of nitrogen from azo compounds, diazo compounds, and azides is a topic that has been thoroughly reviewed elsewhere. These transformations are found in oxygen and sulfur heterocycles, but as they are not unique to these systems, they will not be discussed in this article. Examples can be found in a review on the photochemistry of nitrogen-containing heterocycles.1... 

In contrast with these ring expansion processes, photoelimination of nitrogen from ethyl o-azidobenzimidate (114) is followed by cyclization with the formation of 3-ethoxy-IH-indazole (115), and irradiation of the 2-azidopyrazines (116) yields the ring-contracted 1-cyanoimidazoles (117), presumably via the nitrenes (118). Products derived by insertion of (p-toluenesulphonyl)nitrene are obtained on irradiation of p-toluenesulphonyl azide in p-xylene or cyclohexane. "... 

Photoelimination of nitrogen from fluoroalkyl azides gave the... 

The photoreactions of azides can in most cases be rationalized in terms of intermediate nitrenes. Irradiation of t-butyl azide (106) in nitrogen matrices at 12 K gave the imine (107), but no evidence was obtained for a nitrene intermediate.Photoelimination of nitrogen from 4-azido-2-pyrolinones provides a new synthetic route in high yield to 3-cyano-2-azetidines, as shown, for example, for the pyrrolinone (108) in Scheme 12. ° Zwitterionic intermediates have been... 

Flynn and Michl have obtained o-xylylene from the low-temperature photoelimination of nitrogen from 1,4-dihydrophthalazine. On the basis of trapping experiments with a secondary amine, DeGraff, Gillespie, and Sundberg have suggested that the species shown below is formed in the flash photolysis of phenyl azide it had a lifetime of 5 ms in the absence of the amine. 

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 . 

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