Azides, decompositions

The most reliable method of preparing benzofuroxans is by decomposition of o-nitrophenyl azides. Decomposition can be achieved by irradiation, or more usually by pyrolysis temperatures between 100° and 1.50° are commonly used. Refluxing in glacial acetic acid is the recommended procedure for 4- or 5-sub-stituted 2-nitrophenyl azides, but with 3- or 6-substituted compounds higher boiling solvents are usually necessary. Quantitative studies on the reaction rate have been made, and a cyclic transition state invoked, an argument which has been used to account for the greater difficulty of decomposition of the 6-substituted 2-nitrophenyl azides. Substituent effects on the reaction rate have also been correlated with Hammett a constants, ... 

The structure of the tar, formed in many azide decompositions, consists of polyazepine units,103 and arises by attack of arylamine, formed from triplet arylnitrene, on the azepine precursor 30 to give the l//-azcpin-2-amine 31, which acts as a nucleophile towards more azepine precursor to yield ultimately the polyazepine 95. 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

For a detailed and authoritative review of these, and related, azide decompositions reference (81AHC(28)23l) should be consulted. 

Related to the azide decompositions reported in Section 5.16.4.1.2(ii) are the formation of 3-acyl-2-alkoxy-3Z/-azepines and -3//-azepin-2-ones by the photoinduced ring expansions of 3-substituted 2,1-benzisoxazoles (Scheme 22), in alcohol and aqueous alcohol solution, respectively (81AHC(29)l), and the preparation of 3- and 7-acetyl-3//-azepin-2-ones by photolyses of 2,3-dimethylindazoles in dilute sulfuric acid (73HCA1852). 

Diazo compounds have previously been prepared by a variety of methods. Some of these methods Include hydrazone oxidations, the reaction of diazomethane with acid chlorides,4 the reaction of activated methylene compounds with tosyl azide, decomposition of N-nitroso compounds, ... 

Raman and infrared (IR) spectra of pentazoles are difficult to measure. The main problem is the separation of the pentazole from impurities of the azide decomposition products that are almost impossible to remove quantitatively in the synthesis and can be formed even during the measurement at low temperatures. In Table 7, an overview is given over the principal vibration modes of the pentazole anion at the CCSD level of theory. 

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