Pentazoles intermediates

The known pentazoles are aryl derivatives which have the structure (3). They are obtained from the reaction of aryldiazonium ions with azide ion (Equation (2)). No alkylpentazoles are known and the reaction of diazomethane with HN3 which gives methyl azide has been shown by N-labelling not to pass through a pentazole intermediate <58HCA1823>. The parent NH-pentazole is unknown. The numbering system shown in (3) will be used in this chapter. 

In the above investigations of the main features of this reaction it was tacitly assumed, though not unequivocally established, that the pentazene and pentazole intermediates form independently. Ugi ° has considered an alternative process in which phenylpentazole arises by cyclization of the initially formed pentazene intermediate. This is shown in Scheme 5 where the subscripts ch, r,f, d and i denote chain, ring, formation, decomposition and isomerization respectively. 

The effects of cyclic 6n electron conjugation have been found in the optimized geometries of pentazole 17 [102] and hexazine 18 [97], The N=N bond is longer than the isolated double bond in NH=NH. The N-N single bond in the tetrazadiene moiety is shorter than the single bond in NH NH. The bond lengths in 18 are nearly intermediate between those in NH NH and NH=NH. The aromatic character of pentazoles was supported by the effect of electron donating substituents on the thermodynamic and kinetic stabilization [103],... 

Although pentazoles have often been claimed, the only authentic isolated examples now accepted are aryl pentazoles. These are unstable, explosively so above an initial decomposition temperature, —5°C for phenylpentazole, 50°C for the 4-dimethylamino analogue. The structure is stabilised by electron donors [1]. Pentazole itself has lately been detected as an intermediate in solution. Pentazoles decompose to nitrogen and azides, which latter explosive materials were presumably previously claimed as pentazoles. 

Quantum-chemical calculation of the decomposition of methylpentazole has shown that methylpentazole is as stable as aromatic pentazole derivatives <2003CEJ5511>. In the reaction of 1SN-Iabeled diazomethane with hydrazoic acid at — 80 °C, which leads to methyl azide, no methylpentazole intermediate has been detected <1932G716, 1958HCA1823>. Therefore, methylpentazole can only be synthesized by using a different route. Other substituents like CN, F, or NH2 lead to pentazoles with lower activation barriers (Table 5) than the known arylpentazoles. 

Scheme 1 The transition from an organic homocycle (cyclopentadiene) to an inorganic homocycle (pentazole) through intermediate heterocycles...

These conclusions were subsequently confirmed by Huisgen, Ugi and collaborators who isolated the intermediate pentazoles and... 

It appears, therefore, that the formation of phenylpentazole proceeds independendy of the pentazene (Scheme 4) possibly as a one-step, four centre process through intermediate 262. The decomposition of the pentazole is also thought to involve a similar intermediate (263). This is supported by the inverse relationship between the rate... 

Organically substituted pentazoles form from arenedlazonium salts and alkali azides see [11] for a review. The reactions In aqueous solution at ambient temperature are strictly first order in each of the reactants Intermediates were not detected [12]. An almost planar Ng ring in p-(dimethylamino)phenyl pentazole was deduced from a single-crystal X-ray investigation. Internuclear distances are r(N,-N2) = 1.321, r(N2-N3) = 1.309, and... 

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