Photochemical and thermal decomposition of cyclic azo compounds

Cyclic azo compounds also undergo elimination of nitrogen when subjected to photolysis. The photodecomposition can be carried out directly, or photosensitization techniques can be used.  

Cyclization is sometimes accompanied by formation of alkenes in thermal decomposition. For the eight-membered azo compound 19, for example, alkene formation is the major reaction pathway. An intramolecular disproportionation via transition state F may account for the large amount of alkene formed in this particular case.  

The cyclic nature of the transition states in these reactions dictates that elimination will proceed with syn stereochemistry. A cyclic transition state involving only five or six atoms cannot accommodate an anti stereochemical relationship at the site of elimination. For this reason, this family of reactions is often referred to as thermal syn eliminations. 

Amine oxide pyrolysis occurs under milder conditions than any of the other 

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Scheme 6.12. Photochemical and Thermal Decomposition of Cyclic Azo Compounds...

Equation II-C. However, as pointed out earlier (Chapter 4), such electro-cyclic processes occur with conservation of symmetry, and the process (and its reverse) is thermally disrotatory to the singlet. The triplet might then be produced by a process that occurs in a stepwise fashion, such as the low-temperature photochemical decomposition of the azo compound 2,3-diazabicyclo[2.2.1]hept-2,3-ene, shown in Equation II-D. 

The most widely studied example is decomposition of azo compounds, where —X—Y— is —N=N—,220 The elimination of nitrogen from cyclic azo compounds can be carried out either photochemically or thermally. Although the reaction generally does not proceed by a concerted mechanism, there are some special cases in which concerted elimination is possible. We will consider some of these cases first and then consider the more general case. 

The question remains will it be possible to isolate the cycloadduct between 1,3,4-oxadiazole and the dienophiles from the reaction mixture Let us examine the case that is likely to be very easy experimentally the addition of cyclopropene to 3,5-bis(trifluoromethyl)-l,3,4-oxadiazole. This cycloadduct actually represents a cyclic azo compound (Scheme 10). Azo compounds are known to decompose very easily producing nitrogen molecules and the corresponding radicals. The energy for the decomposition can be either thermal or photochemical [60]. In... 

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