Quaternary thiazolium salts

One may find many publications in the literature on the theoretical aspects of thiazolium quaternary salts, because of the biological importance of thiamine and their use as catalysts for benzoin condensation. 

Thiazolium quaternary salts. 30. See also Thiazolium salts... 

It is noteworthy that some catalysts convert thioethers to quaternary salts where the reactive electrophilic center is no longer one of the two C centers but the C sp center of the thiazolium salt (284. 285). Thus... 

Besides the well-known lower basicity of ethanol, these data illustrate the greater acidity of benzoxazolium compared with benzothiazolium. The relative pK. values of the quaternary salts obtained in acetonitrile when treated with tetrabutylammonium hydroxide are 18.3 and 17.6, respectively (25). Those of 2-methyl 4-phenyl thiazolium and 2.4-dimethyl thiazolium are 20.5 and 21.8 under the same conditions (25). 

Anhydro bases resulting from the proton abstraction by a base at an activated a -methyl group of a quaternary salt (see Section 4.19.2.3.3(iv)(a)) are active C-nucleophiles. These attack the C-2 position of a thiazolium salt affording adducts whose further reaction may lead to thiacyanines. Scheme 28 summarizes the successive steps in the reaction resulting from the addition of sodium ethoxide to a fairly concentrated ethanolic solution of 2,3-dimethylbenzothiazolylium salt (45) (c =0.1 moll-1). The initially formed anhydro base (46) cannot be isolated, it reacts as a nucleophile with a second molecule of benzothiazoly-lium salt yielding an adduct (47) which is deprotonated by ethoxide anion affording the dimeric anhydro base (48) whose reactivity will be discussed later (see Section 4.19.2.3.3.i). Monocyclic thiazolylium salts react similarly. 

The use of thiazolium salts enables the benzoin condensation to proceed at room temperature. It can also be performed in dipolar aptotic solvents or under phase transfer conditions. Thiazolium salts such as vitamin Bi, thiazolium salts attached to y-cyclodextrin, macrobicyclic thiazolium salts, thiazolium carboxylate, ° naphtho[2,l-d]thiazolium and benzothiazolium salts catalyze the benzoin condensation and quaternary salts of 1-methylbenzimidazole and 4-(4-chlorophenyl)-4//-1,2,4-triazole are reported to have similar catalytic activity. Alkylation of 2-hydroxyethyl-4-methyl-l,3-thiazole with benzyl chloride, methyl iodide, ethyl bromide and 2-ethoxyethyl bromide yields useful salts for catalyzing 1,4-addition of aldehydes to activated double bonds. Insoluble polymer-supported thiazolium salts are catalysts for the benzoin condensation and for Michael addition of aldehydes. Electron rich al-kenes such as bis(l,3-dialkylimidazolidin-2-ylidenes) bearing primary alkyl substituents at the nitrogen atoms or bis(thiazolin-2-ylidene) bearing benzyl groups at the nitrogen atoms are examples of a new class of catalyst for the conversion of ArCHO into ArCHOHCOAr. 

Quaternary oxazolium salts are easily cleaved by alkali iV-methylbenzoxazolium iodide affords iV-methyl-2-formamidophenol (135). iV-Methyloxazolium fluorosulfonates react with ethanolic ammonia to give imidazoles with sodium hydrogen sulfide, thiazolium salts are formed. Hydrolysis or aminolysis of 2-amino-lV-phenacyloxazolium salts (136) is followed by cyclization to yield imidazolones (137) or imidazoimidazoles (138), respectively (Scheme 5). 

Oxidation to an azolone is an expected reaction for a pseudo base, but little appears to be known of such reactions. Most commonly, pseudo bases suffer ring fission. Estimated rates of ring opening of 246 are in the ratio 109 104 5 1 for X = 0, S, and NMe, respectively. Thiazolium salts 247 consume two equivalents of OH on titration because the pseudo bases 248 lose a proton to give 249, which then form anions 250. Quaternized oxazoles 251 are readily attacked by hydroxide to give open-chain products such as 252, and quaternized 1,3,4-oxadiazoles behave similarly. Quaternary isothiazoles (e.g., 253) are cleaved by hydroxide, as are 1,2,4-thiadiazolium salts (254 255). 

In yet another study the thiazolium ring was attached to a macrotricyclic quaternary ammonium ion 9, bearing several positive charges to determine if rate accelerations of pyruvate decarboxylation could be observed46. Such rate accelerations could indeed be observed, especially for phenylpyruvic acid as a substrate. In addition, lumiflavin-3-acetic acid as a potential oxidant of the intermediate (see the oxidative decarboxylation pathway in Scheme 1) was shown to be reduced by the pyruvic acid analog in the presence of DBU in ethanol and the macrotricyclic quaternary ammonium salt. 

Nakamura T, Hara O, Tamura T, Makino K, Hamada Y (2005) A facile synthesis of chroman-4-ones and 2,3-dihydroquinolin-4-ones with quaternary carbon using intramolecular Stetter reaction catalyzed by thiazolium salt. Synlett 2005 155-157... 

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