1.2.4.5- Tetrazines photodecomposition

No photolyses of hexahydro-1,2,4,5-tetrazines have been published. The kinetics of the photodecomposition of 1,3,5-triphenylverdazyl (61) was studied by Soviet authors in various hydrocarbons. The rate depends on the solvent. The excited radical is consumed by H extraction from the solvent and by dissociation at the N—N bond, forming the product 1,3-diphenyl-l,2,4-triazole (138), which catalyzes the decomposition of the verdazyl radical (74IZV2204). The dipole moment of (61) in the excited state is 9 1 D in the ground state it is 2.94 D. 

The photodissociation of the parent 1,2,4,5-tetrazine was found to be exothermic by more than 100 kcal mol- 1.263 The activation energy for the photodecomposition of 1,2,4,5-tetrazine is 39.7 kcal mol-1 and 40.1 kcal mol-1 for 3,6-dimethyl-1,2,4,5-tetrazine.107 Using the results obtained a mechanism has been suggested wherein the tetrazines first absorb a photon, then decay to an intermediate species which under certain conditions requires absorption of a second photon to achieve decomposition to nitrogen and nitriles.267,273... 

Calculations on the photodecomposition of 1,2,4,5-tetrazines are in agreement with experimental data.122... 

Irradiation of 5-phenyltetrazole (73) at 254 nm resulted in evolution of one molecule of N2 and the formation of compound 74 along with lesser yields of the other products indicated in Eq. (I).135,136 Further photodecomposition of compound 74 was the origin of the benzonitrile and the 3,5-diphenyl-1,2,4-triazole in the mixture.133,136 The reaction is considered to involve loss of N2 from the 3- and 4-positions of the ring38 137 to yield a nitrilimine intermediate, Ph-C=N+—NH-, which dimerizes to the dihydrotetrazine 74, some of which is subsequently oxidized to the tetrazine form. 

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