Quaternary 1,2-Azolium Salts

A -acylimidazolium salts will react with allylstannanes, by addition of the equiv alent of an allyl anion.  

Azolium salts are readily attacked by nucleophiles for example, with hydroxide, addition at C-2 is followed by ring opening.  

The C-2-exchange of azolium salts via an ylid mechanism has already been discussed (section 21.1.2.1). Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and in these its mode of action also depends on the intermediacy of a 2-deprotonated species. For example, in the later stages of alcoholic fermentation, which converts glucose into ethanol and carbon dioxide, the enzyme pyruvate decarboxylase converts pyruvate into ethanal and carbon dioxide, the former then being converted into ethanol by the enzyme alcohol dehydrogenase. It is believed that, in the operation of the former enzyme, the coenzyme, thiamin pyrophosphate, adds as its ylid to the ketonic carbonyl group of pyruvate this is followed by loss of carbon dioxide, then the release of ethanal by expulsion of the original ylid. 

In the laboratory, thiazolium salts (3-benzyl-5-(2-hydroxyethyl)-4-methylthi-azolium chloride is commercially available) will act as catalysts for the benzoin condensation, and in contrast to cyanide, the classical catalyst, allow such reactions to proceed with alkanals, as opposed to aryl aldehydes the key steps in thiazolium ion catalysis for the synthesis of 2-hydroxyketones are shown below. Such catalysis, which also finds other applications, provides acyl anions, in effect. 

3-dimethylimidazolium ylid, generated using sodium hydride, allows the introduction of electrophiles to C-2, and the first reported isolable, recrys-tallisable carbene is an imidazolium ylid it was prepared by C-2-deprotonation of the highly hindered 1,3-diadamantylimidazolium chloride.  

3-Dialkyl-imidazolium salts, for example l-n-Butyl-3-methylimidazolium hexafluorophosphate (or tetra-fluoroborate) are a major group of ionic liquids (for a fuller discussion, see 31.5). 

Amino-l,3-azoles exist as the amino tautomers. 2-Amino-l,3-azoles tend to be more stable than other isomers. All amino-1,3-azoles protonate on the ring nitrogen. 2-Aminothiazole has a pA an of 5.39, which compares with the value for 2-aminoimidazole of 8.46, reflecting, in part, the symmetry of the resonating guanidinium type system in the latter. 

The amino-l,3-azoles behave as normal arylamines, for example undergoing carbonyl condensation reactions, easy electrophilic substitutions and diazotisation, though 2-amino-oxazoles cannot be diazo-tised, presumably due to the greater electron withdrawal by the oxygen. 

The base-catalysed degradation of the ring of isoxazolium salts is particularly easy, requiring only alkali metal carboxylates to achieve it. The mechanism, illustrated for the acetate-initiated degradation of 2-methyl-5-phenylisoxazolium iodide, involves initial 3-deprotonation with cleavage of the N-O bond subsequent rearrangements lead to an enol acetate which rearranges to a final keto-imide. 

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Synthesis of [1,2,3]triazolo[1,5-c]pyrimidines and [1,2,4]triazolo[1,5-c]pyrimidines A novel approach to [l,2,3]triazolo[l,5-c]pyrimidines is shown in Scheme 55. Batori and Messmer - in the course of their investigations on fused azolium salts - described a synthetic pathway to l,3-disubstituted[l,2,3]triazolo[l,5-c]-pyrimidinium salts <1994JHC1041>. The cyclization was accomplished by transformation of the hydrazone 436. This compound was subjected to an oxidative ring closure by 2,4,4,6-tetrabromo-2,5-cyclohexadienone to give the bicyclic quaternary salt 437 in acceptable yield. 

Methods (1) and (2) are classical routes for the preparation of pyridinium quaternary salts [74HC( 1)309], which together with route (3) are almost general approaches, and each has its own area of application. Alternatively, condensation reactions (4) offer a rather specific methodology, and as will be seen, they may be useful for the synthesis of vinylogues 36-38. Other more or less unusual methods (5) may be applied for the preparation of specific azolylpyridinium(azolium) salts. 

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. 

Dequatemization of azolium quaternary salts initially involved pyrazol-ium compounds, which could be pyrolyzed in vacuum at ca. 200°C (66AHC417). The use of thiophenolate anion under phase transfer catalysis proved to be an excellent method of obtaining pyrazoles and indazoles in high yield from their corresponding quaternary salts [78CR(C)439]. The thermal descomposition of imidazolium quaternary salts has been studied by Grimmett et al. (77AJC2005). 

Dealkylation, of azolium quaternary salts and betaines, 60, 244 Deazaflavines, bent, 55, 192 5-Deazalumazines, see Pyrido[2,3-(/]pyrimidine-2,4-diones Deaza-purines, -flavines, etc., see the systematically-named ring systems 4-Deazatoxoflavins, syntheses, 55, 182 Dechlorination, catalytic, of chloro-[ 1,2.4]triazolo[ 1,5-a]pyrimidines, 57, 125... 

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