Ugi, I., Pentazoles

The third volume of this series covers three specific groups of compounds the carbolines (reviewed by R. A. Abramovitch and I. D. Spenser), the thiatriazoles (K. A. Jensen and C. Pedersen), and the pentazoles (I. Ugi). The remaining four chapters deal with topics of general chemical interest from the heterocyclic viewpoint the quaternization of heterocyclics (G. F. Duffin), carbene reactions (C. W. Rees and C. E. Smithen), applications of the Hammett equation (H. H. Jaffe and H. Lloyd Jones), and some aspects of the nucleophilic substitution of heterocyclic azines (G. Rluminati). 

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 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. 

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) ... 

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