Stabilized Carbanions, Enamines and Ylides

The author believes that students are well aware of the basic reaction pathways such as substitutions, additions, eliminations, aromatic substitutions, aliphatic nucleophilic substitutions and electrophilic substitutions. Students may follow undergraduate books on reaction mechanisms for basic knowledge of reactive intermediates and oxidation and reduction processes. Reaction Mechanisms in Organic Synthesis provides extensive coverage of various carbon-carbon bond forming reactions such as transition metal catalyzed reactions use of stabilized carbanions, ylides and enamines for the carbon-carbon bond forming reactions and advance level use of oxidation and reduction reagents in synthesis. 

ThDP is used by various enzymes as a cofactor to perform a wide range of catalytic functions, including the decarboxylation of a-keto acids and transketolation. The thi-azolium C-2 atom is deprotonated to form a nucleophilic ylide. The latter attacks the a-carbonyl group of an a-keto acid, forming the first covalent tetrahedral intermediate. Decarboxylation of this intermediate leads to a C-2-a-carbanion which is resonance-stabilized by its enamine form. C-protonation of this enamine intermediate yields a second tetrahedral intermediate. Deprotonation of this intermediate with concomitant cleavage of C-2 of ThDP and C-a of the intermediate Anally forms the aldehyde and regenerates the nucleophilic ylide (Figure 1.37). 

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