Azides, photolysis

Figure 13.30. Primary nitrenium ions through azide photolysis-nitrene protonation.

Enmein was converted to 20-hydroxykaur-6-en-15a-pyranylether (382), which was oxidized with chromium trioxide in pyridine to afford the aldehyde 383. The latter was converted with hydroxylamine to the oxime 384. The nitrone 385 was prepared by treatment of 384 with bromine azide. Photolysis of 385 gave the desired compound 381 in 46% yield. This intermediate possesses several useful functionalities (e.g., carbinolamine ether linkage), which may be of interest for synthesis of C20-diterpenoid alkaloids after minor changes in this scheme. 

Moriarity has suggested that ground state conformation is important in determining migrator> aptitudes in alkyl azide photolysis. We can extend this concept to develop a theory to explain the results obtained so far °. It is first necessary to consider the likely non-bonded interactions involved in the various possible conformations of the ground state of alkyl azides. The structure of methyl azide has been determined to be as in 339. The a-nitrogen atom is considered to be sp hybridized, with two of the sp hybrid orbitals forming... 

The photolysis of benzoyl azide can be sensitized efficiently by benzophenone" . Other conventional sensitizers such as naphthalene, triphenylene and anthraquinone have been reported to have only a small effect on the photoreaction, diazo-iso-butyronitrile and fluo-rene to have none. In the benzophenone-sensidzed reaction, where triplet nitrenes are formed directly, the only reaction product in alcohol soludon is benzamide, which is obtained in quandtadve yield. This demonstrates again the different chemistry of singlet and triplet nitrene routes in azide photolysis. 

Horner has undertaken a systematic study of aryl azide photolysis . He found that substituted phenyl azides produce on irradiation in solution high yields of the corresponding azo compounds. / -Methoxyphenyl azide (124) gave 18% of the azo compound (125)... 

A by-product of the study of aryl azide photolysis was the solution of a problem of long standing in heterocyclic chemistry. Wolff ° , in 1910, had pyrolysed phenyl azide in the presence of aniline and obtained a product which he formulated as dibenzamil (136). 

Aziridine yields in intermolecular additions are good addition of ethoxycarbonylnitrene to cyclohexene gave the aziridine in 56% yield at room temperature, and in 75% yield at Dry Ice temperature (azide photolysis) . At a 0-2 mole% concentration of cyclohexene (in dichloromethane) a 35% yield of aziridine was obtained . Other olefins give similar yields. The good yields at low olefin concentrations are possible because both the singlet and the triplet nitrenes will add readily, and the intersystem crossing of the nitrene does not interfere with the reaction (except for its stereospecificity, see above ). With isoprene , the singlet ethoxycarbonylnitrene does not measurably discriminate between the two double bonds, while the triplet species prefers the more substituted double bond by a factor of two. 

Nitriles add carbonylnitrenes to form 1,3,4-oxadiazoles . Alkoxycarbonylnitrenes (made either by azide photolysis, azide thermolysis or by the a-elimination route) give the 2-alkoxy-1,3,4-... 

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