Pentazoles formation

Huisgen, Ugi and co-workers have offered cogent evidence for formation of cyclic pentazoles (I) in the reaction of dlazonium salts with azide ioi. A sinple (but not necessarily correct) mechanism for cyclic pentazole formation in these reactions would involve ring closure of cpen-chain diazoazides (ll) ... 

Noelting and Michel, as well as A. R. Hantzsch, had observed around 1900 that benzenediazonium cations evolved dinitrogen on treatment with azide anions, but only R. Huisgen and I. Ugi s kinetic reinvestigation of this reaction at low temperature revealed in 1956—1957 the formation of 1-phenyl-pentazole, and allowed later the isolation of stable arylpentazoles when the phenyl group had electron-donating substituents.39... 

A key feature of pentazole chemistry is the formation of the pentazole ring and the route by which this might arise. The history of the discovery and the involvement of laboratories in Munich and Zurich has been thoroughly described by Ugi <84CHEC-I(5)839>. 

In almost all experimentally known pentazoles, the pentazole ring is attached to a substituted phenyl ring. Calculations using these well-known pentazoles were used to determine the mechanism of the formation and decomposition of the pentazoles (see Section 6.18.2.1). Here the theoretical data can be compared to experimental data for verification. As the calculated data for the known pentazole derivatives model their characteristics quite well, calculations can also be used to extrapolate the stability of yet unknown pentazole derivatives. The high sensitivity of these compounds makes calculations a fast and safe route for the evaluation of the properties of potential new compounds. 

Scheme 5 Formation and decomposition of the pentazole ring system...

The first experimental evidence for the existence of a pentazole derivative came from Clusius and Hurzeler (54HCA789) in Zurich. They studied the formation of phenyl azide... 

After the formation of an arylpentazole from an arenediazonium chloride and sodium azide, the accompanying aryl azide is soluble in the petroleum ether, and the sodium chloride stays in the aqueous methanol, whereas generally the arylpentazole is not soluble in any one of the two solvent phases (56AG705, 57CB2914, 58CB2324). The pentazole can be collected by filtration. Note that the crystalline arylpentazoles tend to explode violently at temperatures above their decomposition points (see Table 3) they should be handled with great care, in small amounts and at low temperatures. 

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. 

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... 

Its first observation was reported in 1915," although that proved to be false. After the long-sought pentazole anion is now observed in negative ion mass spectra. However, the NMR observation has been controversial. Butler et asserted that the formation of the pentazole anion Ns" can... 

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