Knoevenagel condensation 456 Subject

The basic nature of N-heterocyclic carbenes, stabilized by the presence of two adjacent N atoms, has been emphasized by several authors [38], Therefore, the yield and the selectivity of any organic synthesis (driven by the reactivity of a substrate vs. appropriate bases and carried out in imidazohum salts as solvents) can be affected by side reactions related to the acidity of C2-H unit in 1,3-dialkylimidazo-lium cation and to the basicity and nucleophilicity of the resulting N-heterocychc carbene. On this subject, noticeable work (related to the Baylis-Hillman, Knoevenagel and Claisen condensation, Homer reaction, etc.) carried out in RTlLs has been reported and discussed [37]. 

The n-system can be extended with methylene structures at the 3- and 5-positions. As expected, methyl groups at the 3,5-positions can be subjected to chemical modifications by the Knoevenagel reaction, owing to their acidic characteristics, generating styryl-bridged BODIPY derivatives by condensation with aromatic aldehydes (Scheme 7.5). " ... 

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