Azolium

Curiously enough, bulky substituents on nitrogen increase this reactivity towards methyl iodide (119). This has been related to a steric decompression of the thiocarbonyl group in the transition state. Furthermore, knowledge of the ratio of conformers in the starting 4-alkyl-3-i-Pr-A-4-thiazoline-2-thiones and in the resulting 4-alkyl-3-i-Pr-2-methylthiothi-azolium iodides combined with a Winstein-Holness treatment of the kinetic data indicates that in the transition state, the thiocarbonyl bond is approximately 65% along the reaction coordinate from the initial state... 

Reactivity of neutral azoles Azolium salts Azole anions... 

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

All neutral azoles possess positively charged azolium counterparts. In addition, as di cussed in Chapter 4.01, certain olylium species exist which have no neutral counterpart for example dithiolylium salts. 

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. 

Azole iV-oxides, iV-imides and iV-ylides are formally betaines derived from iV-hydroxy-, iV-amino- and iV-alkyl-azolium compounds. Whereas iV-oxides (Section 4.02.3.12.6) are usually stable as such, in most cases theiV-imides (Section 4.02.3.12.5) andiV-ylides (Section 4.02.3.12.3) are found as salts which deprotonate readily only if the exocyclic nitrogen or carbon atom carries strongly electron-withdrawing groups. 

Because of the increased importance of inductive electron withdrawal, nucleophilic attack on uncharged azole rings generally occurs under milder conditions than those required for analogous reactions with pyridines or pyridones. Azolium rings are very easily attacked by nucleophilic reagents reactions similar to those of pyridinium and pyrylium compounds are known azolium rings open particularly readily. 

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). 

Some azoliums give open-chain products primary and secondary amines with 1,2-dithiolyliums generally give (188) (80AHC(27)l5l). 

Chloride ions are comparatively weak nucleophiles and do not react with azoles. In general, there is also no interaction of halide ions with azolium compounds. 

Azolium rings react readily with organometallic compounds. With a Grignard reagent, conversion (193) -> (194) is known in the benzothiazolium series, and 1,3-benzodithioly-liums give products of type (195). 

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. 

In azolium cations, the electron-pull of the positively charged heteroatom is strong, and substituents attached a or y to positive poles in azolium rings show correspondingly enhanced reactivity. 

The 2-position in imidazoles, thiazoles and oxazoles is electron deficient, and substituents in the 2-position (332) generally show the same reactivity as a- or y-substituents on pyridines. 2-Substituents in azoliums of this type, including 1,3-dithiolyliums, are highly... 

Proton loss from alkyl groups a or 7 to a cationic center in an azolium ring is often easy. The resulting neutral anhydro bases or methides (cf. 381) can sometimes be isolated they react readily with electrophilic reagents to give products which can often lose another proton to give new resonance-stabilized anhydro bases. Thus the trithione methides are anhydro bases derived from 3-alkyl-l,2-dithiolylium salts (382 383) (66AHC(7)39). These... 

Amino groups on azolium rings can lose a proton to form strongly basic azolinimines, e.g. (445) yields (446). 2-Iminobenzothiazoline with acrylic acid yields (447). 

Condensation Reaction of Cyclopentadienides with Azolium Salts. 130... 

Reaction of Cyclopentadienides or Their Nitrogen Analogs with Azolium and Dithiolium Salts... 

As described in Section II,A,1, the condensation of azolium salts with aza derivatives of cyclopentadienide (pyrroles, imidazoles, pyrazoles) also plays... 

Stable heterocyclic five-member azomethine imines, azolium fV-imides, tria-zolium and pyrazolidinium ylides 98H(49)587. 

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