Carbonyl compounds, condensation reactions crossed aldol dehydration

Tables 1 and 2 show some examples of self- or cross-aldol condensation reactions followed by dehydration to obtain a,/3-unsaturated carbonyl compounds. Also in Table 1, a review of methyl isobutyl ketone (a saturated ketone) synthesis in the presence of hydrogen is included (10). In Table 2, some examples of fine chemicals obtained by aldol-type reactions are shown.

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 aldol condensation involves the reaction of two molecules of an aldehyde or ketone that has alpha hydrogens. Abstraction of an alpha hydrogen by base produces a carbanion which attacks the carbonyl carbon of the other molecule by base-initiated nucleophilic addition an alcohol group is formed. Often the alcohol dehydrates to form the final product, an unsaturated aldehyde or ketone. In a crossed aldol condensation, a carbonyl compound with alpha hydrogens reacts with one without alpha hydrogens. 

The second reaction step is the addition of the carbanion, usually called eno-late anion, to the carbonyl group of the other reactant molecule with formation of a new C—C bond. Then, after desorption of the S-hydroxy carbonyl product, the catalyst surface becomes available for the reaction to continue. As stated above, the /9-hydroxy carbonyl product will be obtained provided that the reaction temperature is sufficiently low to avoid rapid dehydration to an Q, )S-unsatm ated compound. Scheme 4 summarizes the steps involved in a base-catalyzed cross-aldol condensation. 

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