Thiazolium ion

TABLE 1-51. THERMODYNAMICS OF THE AQUEOUS IONIZATION OF THIAZOLIUM IONS (322). 

The reaction corresponds to a proton transfer and not to a net formation of ions, and thus the AS is of minor importance in the whole series, especially for the two t-Bu derivatives. This last effect is believed to be due to a structure-promoting effect of the bulky alkyl groups in the disordered region outside the primary hydration sphere of the thiazolium ion (322). 

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

Hydrogen kinetic acidity ol thiazolium ions and thiazole has received much attention since Breslow (419,420) and Ingraham and Westheimer... 

Tschitschibabin amination of pyridine, the mechanism of which has been established as involving an intermediate charge distribution of the thiazole molecule as well as of the thiazolium ion. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

The electronic densities on the thiazole nucleus or on the thiazolium ion suggest that electrophilic substitution should occur preferentially at the 5-position. The order of tt electronic densities is 2<4<5 (163,164). 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Although the catalysis of the dimerization of aldehydes to acyloins by thiazolium ion has been known for some tlrae, the development of procedures using anhydrous solvents which give satisfactory yields of acyloins on a preparative scale was first realized in the submitters laboratories. The mechanism proposed by Breslow - for the thiazolium ion-catalyzed reactions is similar to the Lapworth mechanism for the benzoin condensation with a thiazolium ylide replacing the cyanide ion. Similar mechanisms are involved... 

ACYLOINS PREPARED BY THIAZOLIUM ION-CATALYZED CONDENSATION OF ALDEHYDES ... 

Evaluation of kinetic data. Rate constants were determined for 2-H exchange from 3-R-4-methylthiazolium ions, catalyzed by D2O (pseudo first order) and DO- (second order).154 The observed rate constants for the pD-independent exchange reaction were corrected for the solvent isotope effect ( h2o/ d2o = 2.6), and the reverse protonation of the carbene by H30+ was assumed to be diffusion-controlled (k = 2 x 1010 M-1 s-1). A similar analysis was performed for the exchange catalysed by DO-. The results agreed nicely, giving pAfa = 18.9 for 213 and p/sfa = 18.0 for thiamine.154 The thiazolium ion 213 seems to be less acidic in water154 than in DMSO152 (Ap/fa = 2.4). Aside from the... 

Haake et al reported on the H/D exchange from the 2-position of 1,3,4-trimethylimidazolium ion (6) and found the predominant exchange pathway to involve 0D as base. The rate is approximately 3 x 10 times slower than that at which the deuteroxide ion abstracts a proton from structurally similar thiazolium ions, in contrast with previous work where a diphenylimidazolium ion was found to exchange at the... 

Hydrogen exchange of thiazole and thiazolium ions has received much attention since Breslow s observation that thiamine (12), which in the form of its pyrophosphate, cocarboxylase, is the coenzyme for a... 

Thiazolium ion based ionic liquids (OIL) have been used to promote the benzoin condensation of benzaldehyde. 4- And 5-methylthiazoles are readily alkylated with n-butyl bromide to give the corresponding bromide salt. Anion exchange with sodium tetrafluoroborate gave the tetrafluoroborate salt 53 as a stable yellow orange oil. When activated with a small quantity of triethylamine (5 mol%) the oil promotes the coupling of benzaldehyde to benzoin <99TL1621>. 

The oxidation of thiazolium ions 199 with 3-methyllumiflavine was studied kinetically in aqueous buffer solutions (80BCJ2340). The rate-determining step is carbanion formation, which is followed by rapid oxidation to 200. 

Use of thiazolium ion catalysis allows the benzoin condensation of aldehydes with a-protons. 

Hydrogen atoms in azolium ions can be removed easily as protons (e.g. 300— 302) exchange with deuterium occurs in heavy water. The intermediate zwitterion (e.g. 301) can also be written as a carbene. The pKa values of thiazolium ions range from 16 to 20 (87CRV863, 88B5044). 

The mechanism of the thiazolium ion-catalyzed conjugate addition reactions5 1s considered to be analogous to the Lapworth mechanism for the cyanide-catalyzed benzoin condensation, the thiazolium ylide playing the role of cyanide. The resulting intermediate carbanion is presumed to be the actual Michael donor. After conjugate addition to the activated olefin, the thiazolium ylide is eliminated to form the product and regenerate the catalyst. 

In 1957, Breslow (13) showed that the hydrogen atom in the 2-position of the thiazolium-ion portion of thiamin is ionized readily the electronic structure of the anion imitates that of cyanide ion. The chemistry of thiamin can then be explained the decarboxylation of pyruvate and the acetoin condensation are processes that follow conventional mechanisms in modern language, thiamin allows an acyl group to become an anion equivalent. Subsequent to Breslow s initial discovery, he and McNelis (14) synthesized 3,4-dimethyl-2-acetylthiazolium ion, and showed that in fact it is hydrolyzed rapidly. 

Acylation, of ethyl (trlphenylphosphoranylldene)acetate, 62, 202 ACYLOIN CONDENSATION BY THIAZOLIUM ION CATALYSIS, 62, 170 Acyloins, table of, 62, 177... 

The heterocyclic thiazolium ring contains six n electrons. Each carbon contributes one electron, nitrogen contributes one electron, and sulfur contributes two electrons to the ring n system. The thiazolium ion is aromatic because it has 6 n electrons in a cyclic, conjugated system. 

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