Heterocycles with three nitrogen atoms

Among the five-membered heterocycles with three nitrogen atoms, triazoles and benzotriazoles are of significant practical importance. In the case of ben-zotriazole, the aromaticity of the benzenoid 1H-benzotriazole (90A, R = H) has been considered to be greater than that of the quinonoid 2/7-benzotria-zole (90B, R = H) (Scheme 43).134 However, the difference in aromaticity between tautomers is greatly dependent on the dielectric constant of the medium. 

There are three main classes of heterocycles that can be classified rmder this heading. These are heterocycles with three nitrogen atoms and with two nitrogen atoms in addition to one oxygen or sulfur atom. 

Scheme 9.74. Synthesis of seven-membered heterocycles with three nitrogen atoms.

At the beginning of the chapter we mentioned the three six-membered aromatic heterocycles with two nitrogen atoms—pyridazine, pyrimidine, and pyrazine. In these compounds both nitrogen atoms must be of the pyridine sort, with lone pair electrons not delocalized round the ring. 

In order to stabilize nitrogen-rich molecules, n aromatic delocahzation may be used. In this context five-membered heterocycles with three N atoms - triazoles -, four N atoms - tetrazoles -, and five N atoms - pentazoles - play an important role (Fig. 7). While triazoles often do not possess enough intrinsic energy to be suitable candidates for high explosives, pentazole derivatives are usually either far too sensitive and unstable for any application or need to be kinetically stabilized by bulky groups, which reduce the nitrogen con-... 

Five-Membered Ring Heterocycles Containing Three Nitrogen Atoms. Starting with Rink amide resin 341, 4 equiv of Fmoc-amino acid 342 was used in the presence of 4 equiv of ED AC (1 -ethyl-3-(3-dimethylaminopropyl)carbodiimide) and HOBt in DMF overnight (Scheme 9.40). After removal of the Fmoc group with 20%... 

B-Aminoborazines are of particular interest for fundamental studies. In these compounds, boron is bonded to three nitrogen atoms with two different types of environment. B-Aminoborazines are also useful precursors for the synthesis of thermally stable polymers. Quite a few polycondensates of aminoborazines and copolymerisates with organic difunctional molecules have been described 4>. Of major interest are difunctional borazines yielding linear polycondensates. The condensation of l,3,5-tris(2,6-dimethylphenyl)-2,4-dichloroborazine (cf. Section II.2.5) with aliphatic, aromatic, and heterocyclic diamines, as well as the preparation of the same linear polyborazines by transamination of 1,3,5-tris(2,6-dimethylphenyl)2,4-bis(diethyl-amino)borazine with diamines was studied 139). 

Three-membered heterocycles are an extensively studied class of compounds. Those with two nitrogen atoms, diazirine (unsaturated) (1) and diaziridine (saturated) (2) systems are fairly readily... 

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