Azole anions

Azole anions are derived from imidazoles, pyrazoles, triazoles or tetrazoles by proton loss from a ring NH group. In contrast to the neutral azoles, azole anions show enhanced reactivity toward electrophiles, both at the nitrogen (Section 3.4.1.3.6) and carbon atoms (Section 3.4.1.4.1.i). They are correspondingly unreactive toward nucleophiles. 

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Table 7 H NMR Spectral Data for Ring Hydrogens of Azole Anions...

Reactivity of neutral azoles Azolium salts Azole anions... 

Orientation in azole rings containing three or four heteroatoms Effect of azole ring structure and of substituents Proton acids on neutral azoles basicity of azoles Proton acids on azole anions acidity of azoles Metal ions... 

Similar ambiguities arise in the reactions of azole anions. At least as regards alkylation reactions in the 1,2,3-triazole series (79), the product appears to depend on the reagent used. In the 1,2,4-triazole series (80) a single product is formed, whereas tetrazole (81) gives mixtures. 

Proton acids on azole anions acidity of azoles... 

Many examples are known of complexes between metal cations and both neutral azoles and azole anions. Overlap between the cf-orbltals of the metal atom and the azole rr-orbitals is believed to increase the stability of many of these complexes. 

Oxidation of azole anions can give neutral azole radicals which could, in principle, be tt (139) or a- (140) in nature. ESR spectra indicate structure (141 hyperfine splittings in G) for imidazolyl radicals, but both tt- and cr-character have been observed for pyrazolyl radicals. Tetrazolyl radicals (142 4 143) are also well known (79AHC(25)205). Oxidation of 2,4,5-triarylimidazole anions with bromine gives l,l -diimidazolyls (144) which are in equilibrium with the dissociated free radical (145) (70AHQ 12)103). 

If the fV-aryl group is strongly activated, then it can be removed in nucleophilic substitution reactions in which the azole anion acts as leaving group. Thus l-t2,4-dinitrophenyl)pyrazole reacts with N2H4 or NaOMe. 

Pyrazole and indazole anions, in a manner similar to other azole anions, show the expected inversion of reactivity when compared with the cations. They are more reactive towards electrophiles, both at the nitrogen and carbon atoms, and less reactive towards nucleophiles than the corresponding neutral molecules. For practical purposes most of the N -alkylated pyrazoles and indazoles are prepared from the corresponding anions. 

Simple complexes. Many examples are known of complexes between metal cations and both neutral azoles and azole anions. Azoles can form stable compounds in which metallic and metalloid atoms are linked to nitrogen. For example, pyrazoles and imidazoles N-substituted by B, Si, P, Ga, Ge, Sn, and Hg groups are made in this way. Overlap between the d-orbitals of the metal atom and the azole -orbitals is believed to increase the stability of many of these complexes. 

H and C NMR studies on neutral azoles and pyridinium quaternary salts is by now a well-documented subject, and to a lesser extent, azolium quaternary salts. In contrast, only few studies have been devoted to azo-late ions and practically all the reported data for the anion species have been generated in situ using the appropriate NMR solvent in basic medium, often because the azolate anions themselves are unknown. 

Different physical properties in both the ground and the excited states should provide deeper insight into the high dipolar nature of compounds of general type 1. When the acid-base equilibria of these heterocyclic betaines are discussed, two situations must be considered (i) there is resonance interaction between the pyridinium (azolium) cation and the azolate anion and (ii) the two moities are independent. 

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