Azolium pre-catalysts

One of the most common questions concerning reactions catalyzed by N-het-erocyclic carbenes is which azolium pre-catalyst to use. One consideration is the... 

In order to separate structural effects from the electronic differences of these two catalyst classes. Bode synthesized chiral imidazolium salt 57 (Scheme 14.28). This allowed direct comparison of imidazolium versus triazolium precatalysts across a number of different reaction manifolds including those involving the catalytic generation of homoenolate equivalents, ester enolate equivalents, and acyl anions. These studies conclusively demonstrated that imidazolium-derived catalysts are superior for homoenolate reactions with less reactive electrophiles, while the triazolium-derived pre-catalysts are preferred for all other reactions. Interestingly, from the currently published body of the work, it does not appear to be any effects from the counterion of the azolium pre-catalysts in the presence of bases. 

In many cases of organocatalytic reactions an azolium (e.g., imidazolium) salt is used as pre-catalyst, which is in sim deprotonated by the added external base to release the NHC catalyst itself [19-22]. In some work these salts have also been used as an EL solvent for the reaction, defining therefore a pre-catalytic solvent [13,16, 60-63]. It is also worth mentioning that not only can additional bases be used to prepare the NHC catalyst firom the IL solution, but also the electrochemical reduction of the imidazolium cation to the corresponding NHC is a viable possibility [12, 17, 64—66]. It is worth noting that, apart from the apparent carbene preciusor... 

N-heterocyclic carbene catalysis has become one of the major categories in orga-nocatalysis. Azolium salts are ready deprotonated by weak bases to generate a carbene, which then adds to an aldehyde to form an acyl anion equivalent, generally called the Breslow intermediate. The reactive acyl anion attacks an electrophile to promote the various transformations such as benzoin, Stetter, and redox reactions [107]. Recently, an interesting approach for NHC-catalyzed generation of an enol/enolate intermediate was reported. Enantio-enriched (i-amino acid derivatives (217) are formed by the reaction between the a-aryloxyaldehyde 214 and N-tosyl-imines (215) in the presence of phenyalanine-derived azoUum salt 216 as a pre-catalyst and aryl phenoxide as a base (Scheme 28.28) [108]. 

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