Pentazoles

The classical age of preparative organic chemistry saw the exploration of the extensive field of five-membered heterocyclic aromatic systems. The stability of these systems, in contrast to saturated systems, is not necessarily affected by the accumulation of neighboring heteroatoms. In the series pyrrole, pyrazole, triazole, and tetrazole an increasing stability is observed in the presence of electrophiles and oxidants, and a natural next step was to attempt the synthesis of pentazole (1). However, pentazole has eluded the manifold and continual efforts to synthesize and isolate it. 

Arylpentazoles can be prepared by adding an aqueous solution of azide to a mixture of an aryldiazonium chloride, aqueous methanol, and petroleum ether at —40 to —20° with stirring. The pure aryl-pentazole (see Table I) crystallizes from the two-phase reaction mixture the inorganic impurities remain in the aqueous methanol and the organic impurities in the petroleum ether. 

Considering the low energy level of elemental nitrogen, the decomposition enthalpy ofp-ethoxyphenylpentazole (5.4 kcal/mole) indicates the high resonance energy of the pentazole system. 

Unsubstituted pentazole (1) would be expected to be a strong acid with a highly aromatic anion (11) which could possibly form ferrocene analogs such as M (N5)2, where Misrepresents a divalent metal... 

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). 

T) (N, N ) mode complexes. Suspected T) -mode of coordination still remains unconfirmed. The organometallic chemistry of pentazole is the challenge for the further developments. 

It is worth noting that as early as 1903 Hantzsch felt that the possibility of forming the pentazole ring lay in the phenyl azide synthesis. 

Sympatocard (Boehringer lng.)-comb. wfm Desintex-Pentazol (M. Richard) wfm Cardiazol (Knoll) wfm Cardiazol Paracodina (Knoll)-comb. wfm... 

Cardiazol (Sankyo) Pentazol (Yashima) Analeptone (Reed Carnrick) wfm Benizol (lCl)-comb. with nicotinic acid wfm... 

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

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],... 

The kinetic stability of pentazole has been estimated by the activation energy of decomposition or retro-[3 -i- 2]-cycloaddition reaction of 19.8 kcal moL [107] and 19.5 kcal mol- [108] with a half-life of only 14 s at 298 K [108]. 

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