Schmidt Condensation

The activated Ba(OH)2 was used as a basic catalyst for the Claisen-Schmidt (CS) condensation of a variety of ketones and aromatic aldehydes (288). The reactions were performed in ethanol as solvent at reflux temperature. Excellent yields of the condensation products were obtained (80-100%) within 1 h in a batch reactor. Reaction rates and yields were generally higher than those reported for alkali metal hydroxides as catalysts. Neither the Cannizaro reaction nor self-aldol condensation of the ketone was observed, a result that was attributed to the catalyst s being more nucleophilic than basic. Thus, better selectivity to the condensation product was observed than in homogeneous catalysis under similar conditions. It was found that the reaction takes place on the catalyst surface, and when the reactants were small ketones, the rate-determining step was found to be the surface reaction, whereas with sterically hindered ketones the adsorption process was rate determining. 

The condensation of 2-hydroxyacetophenone with benzaldehyde yielded exclusively 2 -hydroxy-chalcone, and the cyclization to flavanone was not observed. An investigation of the species adsorbed on the catalyst (289) suggested that CS condensation on the Ba(OH)2 surface occurs via a very rigid transition state, whereby the OH group of 2-hydroxyacetophenone is bonded to the catalyst surface and placed at great distance from the carbonyl carbon atom of the aldehyde, making the cyclization of 2 -hydroxy-chalcone to flavanone difficult. Deactivation of the catalyst was not observed in the presence of moderate amounts of organic acids, such as benzoic, acrylic, or trichloroacetic acid. 

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Mixed aldol condensations in which a ketone reacts with an aromatic aldehyde are known as Claisen-Schmidt condensations... 

Claisen-Schmidt condensation (Section 18 10) A mixed al dol condensation involving a ketone enolate and an aro matic aldehyde or ketone... 

Ba.se Catalyzed. Depending on the nature of the hydrocarbon groups attached to the carbonyl, ketones can either undergo self-condensation, or condense with other activated reagents, in the presence of base. Name reactions which describe these conditions include the aldol reaction, the Darzens-Claisen condensation, the Claisen-Schmidt condensation, and the Michael reaction. 

Piperazine, N-alkylation with benzyl chloride, 42, 19 Piperazine, 1-benzyl-, 42,19 Piperidine, addition to ethylene, 43, 45 as catalyst for Claisen-Schmidt condensation, 41, 40 Piperidine, 1-ethyl-, 43, 45 Piperidine, 1-(2-naphthyl)-, 40,74 Pivalic acid, oxidative coupling to a,a,-a, a -tetramethyladipic add, 40, 92... 

Investigation of Chlorine Substitution Effects in the Claisen-Schmidt Condensation of 2 -Hydroxyacetophenone with Chlorobenzaldehydes Over MgO... 

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. 

Substitution effects have been observed for both the homogeneously and heterogeneously base-catalyzed Claisen-Schmidt condensation of ketones and aldehydes with functional groups substituted in the para-position [5 -12], In this... 

Crossed aldol condensations, where both aldehydes (or other suitable carbonyl compounds) have a-H atoms, are not normally of any preparative value as a mixture of four different products can result. Crossed aldol reactions can be of synthetic utility, where one aldehyde has no a-H, however, and can thus act only as a carbanion acceptor. An example is the Claisen-Schmidt condensation of aromatic aldehydes (98) with simple aliphatic aldehydes or (usually methyl) ketones in the presence of 10% aqueous KOH (dehydration always takes place subsequent to the initial carbanion addition under these conditions) ... 

The readily prepared support was then used for dihydropyrimidine and chalcone synthesis (Scheme 7.29). Thus, the modified support was activated prior to reaction by treatment with tosyl chloride. Solutions of the appropriate acetophenones were then spotted onto the membrane and the support was submitted to microwave irradiation for 10 min [45]. In the next step, several aryl aldehydes were attached under microwave irradiation to form a set of corresponding chalcones through a Claisen-Schmidt condensation. 

We recently reported a modified Meerwein-Ponndorf-Verley reduction in which low-boiling alcohols such as EtOH and w-PrOH, but preferably i-PrOH, were used at temperatures near 225 °C in the absence of aluminum alkoxides [42]. The carbonyl moiety of an olefinic aldehyde such as cinnamaldehyde was reduced selectively to the alcohol without the carbon-carbon double bond being affected (Scheme 2.7). Since base was not present, aldol and Claisen-Schmidt condensations were avoided. 

Methylcydopent-2-enone typically is prepared by an intramolecular Claisen-Schmidt condensation from 2,5-hexanedione. The product is usually required free from traces of starting material but these substances co-distil. Accordingly, literature methods have employed strong aqueous base in 2-3% concentration at reflux and have been continued until all of the starting material has been consumed. Under those circumstances, the product also has undergone aldol and Claisen-Schmidt condensations severely lowering the yield. 

Figure 2.6 Claisen-Schmidt condensation-asymmetric epoxidation reactions over nanocrystalline aerogelpre pared AP-MgO catalysts.

Choudary, B.M. Kantam, M.L. Ranganath, K.V.S. Mahendar, K. Sreedhar, B. Bifunctional nanocrystalline MgO for chiral epoxy ketones via Claisen-Schmidt condensation-asymmetric epoxi-dation reactions. J. Am. Chem. Soc. 2004,126, 3396-3397. 

The catalyst surface contains both reducing and basic sites and experiments involving selective poisoning of the former by m-dinitrobenzene demonstrated the high probability of a SET mechanism. The same group also concluded that an SET mechanism was probably involved for Michael, Wittig-Horner, and Claisen-Schmidt condensations in the presence of the same catalyst [139,140]. 

Claisen-Schmidt condensation org chem A reaction employed for preparation of unsaturated aldehydes and ketones by condensation of aromatic aldehydes with aliphatic aldehydes or ketones in the presence of sodium hydroxide. klas-on jshmit kand-on sa-shon ... 

In the Claisen-Schmidt condensation at the same temperature and with ethanol solvent present, lower yields of a-enones were observed. The best yield corresponds to condensation of the most reactive furfural with acetophenone, giving 95% a-enone after 1 h in a batch reactor. A comparison of the results characterizing the two reactions led to the conclusion that the W-H reaction provides the more efficient and selective synthesis of a-enones however, the CS condensation provides the more economic approach. 

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