The present study aims to understand crossed aldol condensation., Crossed aldol reaction have been widely used by synthetic chemists due to its ability to form new carbon-carbon bonds. In the critical review, Dr. Alcaide discussed a formidable synthetic challenge regarding an asymmetric cross-aldol reaction of aldehydes. Dr. Alcaide argued that the classic aldol addition faces the problems of chemo- and regioselectivity as well as that of asymmetry. For the cross aldol reactions of aldehyde enolates with aldehydes, good yields were obtained; however, stereoselectivity was low and dependent on enolate geometry. Dr. Alcaide stated that the direct catalytic asymmetric cross aldol reaction of aldehydes constitutes a significant advancement of synthetic methodology, which allows straightforward enantioselective access to important synthons in polypropionate and polyacetate natural products.1 In addition to classic aldol addition, traditional acid- or base-catalyze reactions resulted in some reverse and side reactions, and thereby the yields of the desired product were decreased. For example, the typical methods for the preparation of alpha,alpha’-bis(substituted benzylidene)cycloalkanones generally involve the cross aldol condensation of cycloalkanone with aldehydes under strong acids or bases. Dr. Zirani studied the reaction in the presence of SiO2-Pr-SO3H under solvent free conditions, and resulted in excellent yields with short reaction time without any side reactions. The SiO2-Pr-SO3H was proved to be an efficient heterogeneous solid acid catalyst, which could be easily handled and removed from the reaction mixture by simple filtration and also recovered and reused without loss of reactivity.2
The aldol reaction requires a hydrogen atom on the alpha-carbon of one of starting materials and an acid or base catalyst. In this experiment, a traditional strong base catalyst removes the alpha-hydrogen of one of the two starting materials, forming an enolate anion. This enolate attacks the carbonyl carbon of the other reactant, generating a beta-alkoxycarbonyl intermediate. Protonation of this intermediate gives an aldol (Figure 1).
Figure 1: The formation of an aldol via reacting aldehydes, ketones, or a combination of both
If the aldol product has other remaining alpha-hydrogens, dehydration occurs, producing an alpha, beta-unsaturated carbonyl compound. This process refers to an aldol condensation reaction, which is the synthesis of an alpha, beta-unsaturated carbonyl from the aldol reaction between two carbonyls followed by the dehydration of the beta-hydroxycarbonyl product (Figure 2).3
Figure 2: The formation of an alpha, beta-unsaturated carbonyl
because there are two different carbonyl compounds presented in this reaction, a crossed-aldol condensation reaction was performed. In the presence of a base catalyst NAOH, a crossed aldol reaction between acetone and benzaldehyde is carried out. Then, this is followed by a formation of an unsaturated carbon-carbon double bond with a removal of water (Figure 3).The balanced equation for this specific reaction is shown in Figure 3.
Figure 3: The cross aldol condensation reaction of acetone and benzaldehyde
By reacting with NaOH, acetone (2) will form an enolate. Then, this enolate will react with benzaldehyde (1) to produce the benzylideneacetone (3). Finally, another alpha-hydrogen between the carbonyl and the beta-hydroxy group leads to dehydration, giving a mono-substituted intermediate benzylideneacetone (3). There are more alpha-hydrogens presented on benzylideneacetone (3); one of these will react with acetone to give the disubstitued product (4). Based on the stoichiometry, dibenzylideneacetone (4) was confirmed to be the major product and benzylideneacetone (3) was determined to be the minor product. The mechanism of the formation of dibenzylideneacetone is illustrated in the scheme 1:
Scheme 1: Mechanism