Thiamine, deprotonated

The pentagon stabilization has been found in a biochemical phenomenon [80], The hydrogen on the thiazolium ring 9 (Scheme 7) is easily ionized to afford the corresponding carbene 10, a key catalyst in enzymatic reactions for which thiamine (vitamin B-1,11) pyrophosphate is the cofactor. The pentagon stability is expected to contribute to this unusual deprotonation. A lone pair generated on the carbon atom in 10 can similarly delocalize through the vicinal C-N and C-S a bonds in a cyclic manner. 

The finding that thiamine, and even simple thiazolium ring derivatives, can perform many reactions in the absence of the host apoenzyme has allowed detailed analyses of its chemistry [33, 34]. In 1958 Breslow first proposed a mechanism for thiamine catalysis to this day, this mechanism remains as the generally accepted model [35]. NMR deuterium exchange experiments were enlisted to show that the thiazolium C2-proton of thiamine was exchangeable, suggesting that a carbanion zwitterion could be formed at that center. This nucleophilic carbanion was proposed to interact with sites in the substrates. The thiazolium thus acts as an electron sink to stabilize a carbonyl carbanion generated by deprotonation of an aldehydic carbon or decarboxylation of an a-keto acid. The nucleophilic carbonyl equivalent could then react with other electro-... 

Breslow and co-workers elucidated the currently accepted mechanism of the benzoin reaction in 1958 using thiamin 8. The mechanism is closely related to Lapworth s mechanism for cyanide anion catalyzed benzoin reaction (Scheme 2) [28, 29], The carbene, formed in situ by deprotonation of the corresponding thiazolium salt, undergoes nucleophilic addition to the aldehyde. A subsequent proton transfer generates a nucleophilic acyl anion equivalent known as the Breslow intermediate IX. Subsequent attack of the acyl anion equivalent into another molecule of aldehyde generates a new carbon - carbon bond XI. A proton transfer forms tetrahedral intermediate XII, allowing for collapse to produce the a-hydroxy ketone accompanied by liberation of the active catalyst. As with the cyanide catalyzed benzoin reaction, the thiazolylidene catalyzed benzoin reaction is reversible [30]. 

Results of a kinetic study of enamine formation by C(2a)-proton abstraction from 2-benzylthiazolium salts (88) have implications for mechanistic studies of the thiamin diphosphate-dependent enzymes which feature protonation of the enamine/C(2a)-carbanion.151 The primary isotope effect for deprotonation of (88a) is kiw/kro = 4-6 and the values estimated for C(2a)—H pXa are 15.0-15.5 and 15.7 for (88a) and (88b), respectively. A minimum effective molarity of 4500 M has been estimated for reprotonation of the enamine (89b) derived from (88b) by benzoylformate decarboxylase. Directed aromatic metallation reactions have been reviewed.152... 

In the thiazolium cation the proton in the 2-position is acidic and its removal gives rise to the ylide/carbene 227. This nucleophilic carbene 227 can add, e.g., to an aldehyde to produce the cationic primary addition product 228. The latter, again via C-deprotonation, affords the enamine-like structure 229. Nucleophilic addition of 229 to either an aldehyde or a Michael-acceptor affords compound(s) 230. The catalytic cycle is completed by deprotonation and elimination of the carbene 227. Strictly speaking, the thiazolium salts (and the 1,2,4-triazolium salts discussed below) are thus not the actual catalysts but pre-catalysts that provide the catalytically active nucleophilic carbenes under the reaction conditions used. This mechanism of action of thiamine was first formulated by Breslow [234] and applies to the benzoin and Stetter-reactions catalyzed by thiazolium salts [235-237] and to those... 

When, in 1832, Wohler and Liebig first discovered the cyanide-catalyzed coupling of benzaldehyde that became known as the benzoin condensation , they laid the foundations for a wide field of growing organic chemistry [1]. In 1903, Lapworth proposed a mechanistical model with an intermediate carbanion formed in a hydrogen cyanide addition to the benzaldehyde substrate and subsequent deprotonation [2]. In the intermediate active aldehyde , the former carbonyl carbon atom exhibits an inverted, nucleophilic reactivity, which exemplifies the Umpo-lung concept of Seebach [3]. In 1943, Ukai et al. reported that thiazolium salts also surprisingly catalyze the benzoin condensation [4], an observation which attracted even more attention when Mizuhara et al. found, in 1954, that the thiazolium unit of the coenzyme thiamine (vitamin Bi) (1, Fig. 9.1) is essential for its activity in enzyme biocatalysis [5]. Subsequently, the biochemistry of thiamine-dependent enzymes has been extensively studied, and this has resulted in widespread applications of the enzymes as synthetic tools [6]. 

On the basis of rapid-reaction studies, Hopmann and Brugnoni237 have claimed pKa = 12.7 for the deprotonation of C-2 of thiamine (70) (i.e., ylid formation). This value is much lower than pKa 20 that has been estimated for this deprotonation by Crosby and Lienhard238 by two different approaches. It seems likely that this pKa = 12.7 is actually pKR + for pseudobase formation at C-2 in the thiazolium ring of the thiamine cation. Such a pKR + value is consistent with pKR+ = 8.3 for the 3-methylbenzothiazolium cation.83... 

The deprotonation and addition of a base to thiazolium salts are combined to produce an acyl carbanion equivalent (an active aldehyde) [363, 364], which is known to play an essential role in catalysis of the thiamine diphosphate (ThDP) coenzyme [365, 366]. The active aldehyde in ThDP dependent enzymes has the ability to mediate an efScient electron transfer to various physiological electron acceptors, such as lipoamide in pyruvate dehydrogenase multienzyme complex [367], flavin adenine dinucleotide (FAD) in pyruvate oxidase [368] and Fc4S4 cluster in pyruvate ferredoxin oxidoreductase [369]. 

Thiazoles can be quaternized at nitrogen by reaction with a range of alkylating agents. These salts can form an ylide by deprotonation at C-2. This thiazolium 2-ylide is markedly stable because of the ability of sulfur to stabilize an adjacent carbanion. The reaction is of considerable importance due to the occurrence of thiazolium-2-ylides as intermediates in classical biochemical (thiamine action) and chemical (Stetter reaction) processes (see Section 3.06.12). Desilylation at C-2 can lead to a thiazolium 2-ylide as well. Thus, the formation of this type of intermediate has been formulated as a key step along the reaction pathway involving a 2-trialkylsilylthiazole and C-electrophiles (Dondoni reaction, see Section 3.06.12.12). Thiazolium salts are also susceptible to be oxidized by a variety of oxidants (see Section 3.06.5.4.8). 

The kinetic behavior of the C2aH of 2-a-hydroxyethylthiazolium salts, including thiamin, was extensively reinvestigated in water by Stivers and Washabaugh using triiodide trapping rates of the enamine as a measure of the deprotonation rate constant. Based on the magnitude of the Bronsted constant, a diffusion-controlled reprotonation... 

Deprotonation Rate of the C2-H of Thiamin Diphosphate in Pyruvate Decarboxylase... 

Suggested Mechanism of the C2-H Deprotonation of Thiamin Diphosphate in Enzymes... 

Similar results were obtained in our studies on the thiamin-dependent indole-pyruvate decarboxylase, where protonation and deprotonation reactions are also catalyzed by a Glu-cofactor proton shuttle and a His-Asp-Glu relay (Schiitz et al., 2005). 

The facility with which 1,3-azoUum cations form yUdes by 2-deprotonation is central to the biological activity of thiamine pyrophosphate (32.2.4). Such ylides have a neutral carbene resonance contributor. 

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