1,2,4,5-Tetrazine structure

At Los Alamos National Laboratory, USA, Chavez et al. still continue their work on the development and discovery of new high nitrogen content-HEMs for various applications [281, 285-287] and recently reported the preparation of several novel nitroguanyl-substituted tetrazines including 3,6-bis(nitroguanyl)-l,2,4,5-tetrazine [Structure (2.71)] and bis (triaminoguanidinium) salt [Structure (2.72)] as shown in Figure 2.6. 

Only about 20 compounds with the tetrahydro-l,2,3,4-tetrazine structure are known, but two X-ray crystallographic analyses have been reported. The structure of 1,4-dimethyl-l,4,5,6-tetrahydro-l,2,3,4-tetrazine 2-oxide (6) is a twisted boat conformation with non-planar terminal nitrogen atoms, which reflects the repulsion in the four-atom six-electron N—N=N—N system. The bond distances and angles are given in Figure 1 (78HCA1622). A similar study of dimethyl 2-phenyl-2,3,4,4a,5,6,7,8-octahydrobenzo-l,2,3,4-tetrazine-... 

When cyanamide was electrochemically oxidized at a platinum anode in 2M potassium hydroxide, and the anolyte was neutralized with hydrochloric acid or carbon dioxide, several products were isolated, and to three of these were assigned 1,2,3,5-tetrazine structures. These were the 4-amino-6-cyanamino- (27), 4-amino-6-cyanoguanidino- (28) and 4-ureido-6-cyanoguanidino-l,2,3,5-tetrazines (29) (76BCJ1339). The structures were consistent with the IR spectral data and elemental analyses, but it would be desirable to have unambiguous evidence on these compounds, particularly as their method of synthesis is not an obvious one for forming the connection of three adjacent nitrogen atoms. 

Further reports on 1,4-dihydro-1,2,4,5-tetrazine structures by x-ray diffraction are found for the 3-(p-chlorophenyl)-6-trifluoromethyl derivative <85JHC643>, l,3,6-triphenyl-l,4-dihydro-l,2,4,5-tetrazine <90JCS(Pl)2527>, 6-bromo-l,4-dihydro-l,4-di(o-tolyl)-l,2,4,5-tetrazin-3(27/)-one, and 3,6-dibromo-1,4-dihydro-l,4-bis(p-methoxyphenyl)-1,2,4,5-tetrazine <94AX(C)l78l>. Finally, there are reports on the crystal structures of the heterocyclic betaine l,4-dimethyl-6-oxo-1,2,4,5-tetrazin-l-ium-3-olate (17) <84TL629> and tetramethyl-p-urazine (18), from which cation radicals could be formed <9UOC1045>. 

Figure 9.46 Rotational structure of the Ojj bands in the fluorescence excitation spectra of s-tetrazine dimers at about 552 run. Bottom Ojj band of planar dimer. Middle Ojj band of T-shaped dimer with transition in monomer unit in stem of T. Top Ojj band of T-shaped dimer with transition in monomer unit in top of T. (Reproduced, with permission, from Haynam, C. A., Brumbaugh, D. V and Levy, D. H., J. Chem. Phys., 79, f58f, f983)...

Although all the rings in Figure 1 contain six tt-electrons, the accumulation of electronegative nitrogen atoms in the polyaza structures leads to hydrolytic as well as thermal instability. This is noticeable in pyrimidine, and marked in the triazines and tetrazine. Some stability can be conferred by appropriate substitution, as we shall outline later. 

A particularly interesting system where nitrogen is lost cheletropically after formation of the initial [4 + 2] cycloadduct involves the thermal reaction of azirines with tetrazines (82) (74CC45, 74TL2303, 74CC782, 75JHC183). A variety of heterocyclic products are produced depending on the structure of the azirine and tetrazine used and the reaction conditions. 

Prolonged heating (1 -14 d) of equimolecular amounts of 1,2,4,5-tetrazines with cyclopropenes in refluxing benzene gives 4//-1,2-diazepines directly, which, however, were assigned the 5//-structure.106... 

Hartung et al. [120] determined the crystal structure of the mesogenic 6-hexylamino-l,2,4,5-tetrazin-3-yl 4-pentyloxybenzoate. The plane of the bridging carboxylic group is inclined to those of the phenyl and the tetrazine rings by 12.1 and 76.3°, respectively. The molecules are arranged in sheets parallel to (021). 

The structure of coordination polymers formed with 3,6-bis(pyridin-3-yl)-l,2,4,5-tetrazine and zinc salts can be controlled by the choice of alcoholic solvents. Infinite lattice compounds of the form [Zn2L2(N03)4(Me0H)2(//-L)] and [Zn2(/U-L)3(N03)4](CH2C12)2) have been structurally characterized. The former structure shows an alternating single- and double-bridged species whereas the latter exists as a non-interpenetrated ladder complex.273... 

Bertinotti, F., G. Giacomello, and A. M. Liquori. 1956. The Structure of Heterocyclic Compounds Containing Nitrogen. I. Crystal and Molecular Structure of s-Tetrazine. Acta Cryst. 9, 510. 

A new electrofluorescent switch was prepared with an electroactive fluorescent tetrazine blend of polymer electrolyte <06CC3612>. The structure and magnetic properties of the stable oxoverdazyl free radical 6-(4-acetamidophenyl)-1,4,5,6-tetrahydro-2,4-dimethyl-... 

The Boger pyrrole synthesis based on a heterocyclic azadiene Diels-Alder strategy (1,2,4,5-tetrazine to 2,2-diazine to pyrrole) was employed by the author for the total synthesis of ningalin B . Thus a Diels-Alder reaction of the electron-rich acetylene 52 with the electron deficient 1,2,4,5-tetrazine 53 proceeded to give the desired diazine 54 which underwent subsequent ring contraction to afford the core pyrrole structure 55. 

Stable derivatives of the quusi-fullerene 40 were obtained in a one-pot reaction upon treating C50 with the tetrazine 41 (Scheme 11.10) [40]. The sequential rearrangement and fragmentation of the primary adducts, involving intermediates 42 and 43, afforded 43 in yields between 14 and 22%. Unambiguous structural proof was obtained by X-ray crystallography. 

Only those compounds which do not have tautomeric aromatic triazole structures will be considered here, the others having been treated as triazoles. The triazolines are unstable and have been subjected to little study. Compounds which are disubstituted at the C(3) or C(5) atom are more stable than the mono- or unsubstituted analogues. The equilibrium has been observed by NMR spectroscopy between the six-membered tetrazine (75) and the triazolinethione (76) via the open-chain form, thus mirroring monosaccharide equilibria (Scheme 12) <90TL3927>. 

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