Claisen-Schmidt condensations with substituted benzaldehydes

Aldol and related condensation reactions such as Knoevenagel and Claisen-Schmidt condensations are also widely used in the fine chemicals and specialty chemicals, e.g. flavors and fragrances, industries. Activated hydrotalcites have been employed as solid bases in many of these syntheses. Pertinent examples include the aldol condensation of acetone and citral [107, 108], the first step in the synthesis of ionones, and the Claisen-Schmidt condensation of substituted 2-hydroxyacetophenones with substituted benzaldehydes [109], the synthetic... 

Hydrotalcite-derived materials also show good performances in analogous reactions, such as the Claisen-Schmidt condensation of substituted 2-hydroxyacetophe-nones with substituted benzaldehydes, the synthetic route to flavonoids and the condensation of 2,4-dimethoxyacetophenone with p-anisaldehyde to synthesize Vesidryl, a diuretic drug [270]. Another similar class of reactions in which HT-based materials give good results are Knoevenagel condensations [271]. An example is the synthesis of citronitrile, a perfumery compound with a citrus-like odor, which can be prepared by HT-catalyzed condensation of benzylacetone with ethyl cyanoacetate, followed by hydrolysis and decarboxylation (Figure 2.42b) [272]. 

The Claisen-Schmidt condensation of 2 -hydroxyacetophenone and different chlorinated benzaldehydes over MgO has been investigated through kinetic and FTIR spectroscopic studies. The results indicate that the position of the chlorine atom on the aromatic ring of the benzaldehyde substantially affects the rate of this reaction. In particular, the rate increases in the following order p-chlorobenzaldehyde < m-chlorobenzaldehyde < o-chlorobenzaldehyde. The difference between the meta and para-substituted benzaldehyde can be attributed to electronic effects due to the difference in the Hammett constants for these two positions. Steric effects were found to be responsible for the higher rate observed with the o-chlorobenzaldehyde. 

The Claisen-Schmidt condensation of /r-substituted benzaldehydes with p-substituted acetophenones gives the corresponding a,jd-ketones (chalcones) in hydrotropic media in the presence of NaOH as catalyst (Sadvilkar, 1995). 

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