Organocatalysis acyl anions

Acyl anion free N-heterocycHc carbene organocatalysis 13CSR4906. Advances in bis(N-heterocycHc carbene) chemistry New classes of structurally dynamic materials 12JP0531. 

Another important class of covalent catalysis is carried ont by chiral iV-heterocyclic carbenes [15] that react with carbonyl componnds forming chiral acyl anion eqniva-lents whose reaction with the corresponding electrophile complete the catalytic cycle (F, Fig. 2.2). Finally, it is worthy to mention the ability of certain bifnnctional organo-catalysts to perform simultaneous activation of the nncleophile and the electrophile. The nse of bifunctional organocatalysis has been shown to be very snccessfnl in conjngate additions. 

Reactions employing NHCs as organocatalysts have emerged as an exceptionally prolific research area in organic chemistry in recent years [36]. The phenomenal success of NHCs in organocatalysis can be attributed to their electronic properties, which, as illustrated in Scheme 5.32, allow different types of substrate activation. For instance, the pronounced nucleophilicity of NHCs allows the addition to electrophiles such as aldehydes leading to the formation of acyl anion... 

An overview of organocatalysis based on NHC yet avoiding the formation of a Breslow intermediate (acyl anion equivalent) has appeared. It encompasses the conjugate addition of carbenes, their addition to carboxylic acid derivatives as well as ketenes and analogues and illustrates the wide chemical diversity that can be achieved through these processes. 

The product alcohol of step 2 contains a thiazolium unit as a substituent. This group is electron withdrawing and increases the acidity of the adjacent proton. Deprotonation leads to an unusually stable masked acyl anion. Nucleophilic attack by this anion on another molecule of aldehyde, followed by loss of the thiazolium substituent, liberates the a-hydroxyketone. The thiazolium moiety thus released may initiate another catalytic cycle. This process represents another example of organocatalysis—catalysis utilizing exclusively organic species (Real Life 18-1). 

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