Tetrazines, decomposition

Methods have been described that involve microwave-assisted graphite-supported dry media for the cycloaddition of anthracene, 1-azadienes and 1,2,4,5-tetrazines with several C-C dienophiles and carbonyl compounds in hetero-Diels-Alder reactions [35], This technique leads to a shortening of reaction times, a situation that enables work to be undertaken at ambient pressure in an open reactor to avoid the formation of unwanted compounds by thermal decomposition of reagents or products. 

Decomposition of 1,2,3,4-benzotetrazine 1-oxides 282 involves opening of the tetrazine ring to afford ortho-azidonitroso derivatives, followed by their cyclization with the evolution of the N2 molecule to give benzofurazans 283 (Equation 56) <2002EJO3435>. 

Hydrazine was first obtained by hydrolytic decomposition of bis-diazoacetic acid . The latter, a tetrazine derivative, is obtained from ethyl diazoacetate in the form of alkali salt by the (catalytic) action of strong alkali. Two molecules of the acid simply unite and, at the same time, the ester group is hydrolysed ... 

Like hexazine, s-tetrazine (39) and pentazine (40) are thermodynamically unstable to decomposition into HCN and N2 (see Table VII). 

The pyrido[2,3- ]- and pyrido[3,4- ]-l,2,3,4-tetrazines 9-11 are new heterocyclic systems since the appearance of CHEC-II(1996) <1996CHEC-II(7)785> and are reported as stable yellow solids which recrystallize from ethanol and melt without decomposition in the range 180-230 °C. These three compounds are stable to chromatographic separation on silica with ethyl acetate-chloroform as eluent. 

In the area of reaction energetics. Baker, Muir, and Andzehn have compared six levels of theory for the enthalpies of forward activation and reaction for 12 organic reactions the unimolecular rearrangements vinyl alcohol -> acetaldehyde, cyclobutene -> s-trans butadiene, s-cis butadiene s-trans butadiene, and cyclopropyl radical allyl radical the unimolecular decompositions tetrazine -> 2HCN -F N2 and trifluoromethanol -> carbonyl difluoride -F HF the bimolecular condensation reactions butadiene -F ethylene -> cyclohexene (the Diels-Alder reaction), methyl radical -F ethylene -> propyl radical, and methyl radical -F formaldehyde -> ethoxyl radical and the bimolecular exchange reactions FO -F H2 FOH -F H, HO -F H2 H2O -F H, and H -F acetylene H2 -F HC2. Their results are summarized in Table 8.3 (Reaction Set 1). One feature noted by these authors is... 

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. 

Although thermal decomposition of 2-trityl-5-aryltetrazoles 197 occurs specifically, the final result of the reaction may be ambiguous. For instance, these compounds on heating in benzonitrile give 3,6-diaryl-l,3,4,5-tetrazines 198. Apparently the N2 elimination in a polar solvent (benzonitrile) occurs with cleavage of triphenylmethane followed by dimerization of the intermediate arylnitrile imine (Equation 19) <2002RJ01360>. 

The intermediate formation of acenaphtho[l,2-e][l,2,3,4]tetrazine (8) was postulated in the base-catalyzed decomposition of accnaphthoquinone bis(sulfonylhydrazones) 7.35... 

Azolo-l,2,3,5-tetrazin-4(3//)-ones(e.g.,9,12and compounds 12, Min Section 4.1.)decompose partly at their melting points with gas evolution.30 A similar decomposition occurs on treatment with aqueous base or in boiling water.30,36,41... 

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... 

The l,2,3,5-tetrazin-4-ones (29) decompose at about 120°C to give nitrogen, dimethylcyanamide and aryl isocyanates <85JCS(P1)415>. The thermal (sometimes spontaneous) decomposition of many of the compounds whose preparations are described in the previous section (6.22.3) has already been mentioned. 

Papers dealing with photochemical decomposition of 1,2,4,5-tetrazines continue to resolve and clarify dilemmas outlined previously in CHEC-II(1996) <1996CHEC-II(6)901>. They cover experimental <1995JPC1733> and/or theoretical <2002PCP2554, 1997MI1> aspects of photochemical decomposition of 1,2,4,5-tetrazines. 

Usually electropolymerization of these monomers occurs without any problems, and the properties of the subsequent polymers are detailed in Section 18.3.3. However, in very few occasions, the classical electropolymerization reaction does not work. This has been especially demonstrated in the case of tetrazines [121], where probably the too strong electron attracting character of the tetrazine impedes the classical cation-radical coupling, favoring another type of decomposition reaction, e.g., a proton loss. However, while even EDOT functionalized tetrazine does not polymerize, the same tetrazine ring can be incorporated into an ECP with the help of a bithienyl functionalized moiety [97]. 

Oxidation of aminopyrazoles or decomposition of azidopyrazoles usually leads to ring-opened products,but recently the diaminoimidazole 86 has been found d to yield a mixture of triazine (87) and triazole (88) on oxidation. The intermediacy of a C-nitrene (89) was proposed but an 7V-nitrene (90) was not ruled out. This reaction is similar to the ring expansion of triazoles to tetrazines. ... 

Tetrazine Derivatives. 1,4,5,6-Hexahydro-l, 4-dimethyM, 2,3,4-tetrazine (396) has been prepared in 5 % yield by oxidation of the bishydrazine (397) with sodium hypochlorite in basic conditions. Its chemical properties as a c -tetra-alkyl-2-tetrazene were compared with those of the long-known rra/ij-2-tetrazene (398). The expected decrease in activation energy for thermal decomposition of (396) compared with the trans-mo6 (398) was not observed, and reaction products of low volatility were predominant. In contrast, the only organic product from room-temperature photolysis was the triazine (399), presumably arising from cleavage of the biradical (400). 

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