Imidazolium organocatalysts

The recent development of polymers containing imidazolium, thiazolium and related structures as supported equivalents of ionic liquids (i.e. 148 and 149, Scheme 10.23) is an interesting development of this field [356-361], The corresponding salts containing basic counter-anions can act as basic catalysts but can also be transformed, depending on the exact structure, into N-heterocydic car-benes that can act as nucleophilic organocatalysts [362]. As an example, resins 148... 

Due to the unique bivalent carbene and diversity of the N-heterocyclic motif, NHCs have been demonstrated to be efficient organocatalysts for various enantioselective reactions. In addition to the traditional thioazolium and imidazolium NHCs, triazolium NHCs have become the most successful organocatalysts. Recently, NHC/Lewis acid cocatalysis and bifunctional NHCs have shown a very promising future. Beyond the classic NHC-catalysed umpolung of aldehydes, the extended umpolung of functionalised aldehydes are extremely successful. A series of NHC-catalysed reactions of ketenes have been developed for the synthesis of various enantioriched heterocycles. Esters, anhydrides, carboxylic acids and even Michael acceptors are useful alternative substrates for NHC-catalysed reactions. With increasing interest and rapid development of NHC catalysis, new structures of the catalysts, new reaction modes, and synthetic applications can be expected in the near future. 

In the same context, Trombini et al. developed other organocatalysts of this type bearing an imidazolium. Under solvent-free conditions, these catalysts were shown to be highly efficient at a remarkably low catalyst loading of 0.1 mol % to induce excellent stereoselectivities in the aldol reaction of cyclohexanone with aldehydes in the presence of water (Scheme 2.8). Moreover, exceptionally high values of TON (up to 930) were achieved in the case of the most reactive aromatic aldehydes. 

Hence, it would be possible to consider imidazolium-based ionic liquids as hydrogen bond activator organocatalysts, which will be discussed in another chapter. 

Figure 22.7 Pyrrolidine-derived organocatalysts bearing the imidazolium unit attached to a polymeric matrix or triazolyl fragment.

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