Azolium ions

Hydrogen exchange at ring carbon in azolium ions and dimerization... 

Ring hydrogen atoms can be abstracted from the a-carbon atoms of azolium ions b strong bases, as demonstrated in base-catalyzed hydrogen exchange (Section 4.02.1.7.2... 

These compounds are usually written in the unionized form as in (8 Z = NH, NR, O, S). Canonical forms of types (9) or (10) are important, i.e. these compounds can also be considered as betaines formally derived from azolium ions. Many compounds of this type are tautomeric and such tautomerism is discussed in Section 4.01.5.2. 

Reclosure to form a new heterocyclic or homocyclic ring can occur in azolium ions carrying suitable substituents these reactions are considered under the appropriate substituents. 

Most azolium ions are sufficiently reactive to be attacked by amines. Sometimes the initial adducts are stable ammonia and primary and secondary amines add to 1,3-dithiolylium salts at the 2-position to give compounds of the types NT3, RNT2 and R2NT, respectively, where T = the l,3-thiol-2-yl group (80AHC(27)l5l). 

Hydrogen atoms in azolium ions can be removed easily as protons (e.g. 230—>232) exchange with deuterium occurs in heavy water. The intermediate zwitterion (e.g. 231) can also be written as a carbene, and in some cases this carbenoid form can be trapped or isolated as a dimer. 

The reactivity order for the azoles with different heteroatoms (X) should be O > S = NH, and this has been confirmed in deuteriation of the azolium ion at 33°C (Scheme 7.4) (64JA1865 69JA1113). The rather... 

Scheme 7.4. Relative rates of deuteriation of azolium ions at 33°C (64JA1865 69JAI113).

Hydrogens attached to ring carbon atoms of neutral azoles, and especially azolium ions, are acidic and can be removed as protons by bases. Reaction follows the orientations shown in 322 and 323. The anions from neutral azoles can be obtained as lithium derivatives, except in isoxazoles where ring cleavage occurs. The zwitterions from azolium rings can react as carbenes. 

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