Formation of a, -Unsaturated Carbonyl Compounds

The general sequence for the formation of ,0-unsaturated carbonyl compounds by condensation reactions may be illustrated as follows  

In this sequence, the steps are identical with those which were postulated for aldolization except that they are followed by a dehydration step. Consequently, these reactions are facilitated by reaction mixtures which are basic but contain a certain amount of acid (mostly present, of course, as its salt). This conclusion becomes clear if we ask ourselves how water is lost in the process. In Chapter 5 we saw that dehydration reactions usually require acid and proceed through the steps  

It is well known that the dehydration of aldol to crotonaldehyde is readily effected in the presence of a trace of mineral acid. Since aldols also undergo dehydration in the presence of base, there may be the possibility of a base-catalyzed dehydration  

Normally alcohols are stable to strong bases, but with /3-hydroxy carbonyl compounds (where the removal of an a-hydrogen atom occurs readily [III to IV]) the subsequent elimination of a hydroxyl group might follow. 

The important point, however, is that no reasonable mechanism for the elimination of water can be written until step 5 is complete, and step 5 requires the presence of a proton donor (an acid). These considerations have been nicely verified in the Knoevenagel reaction (pp. 182, 183), where it has been found that a base alone is not as effective a catalyst as it is in the presence of acetic acid. 

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Elimination of phenyltellurol from the telluroesters intermediate explains the formation of a, -unsaturated carbonyl compounds. 

Palladium catalysts are best known for oxidizing alkenes to ketones or vinyl derivatives. However, formation of a, unsaturated carbonyl compounds by UV irradiation of oxygenated solutions of alkoies in the presence of catalytic amounts of palladium s ts has been observed by Muzait - This reaction is believed to proceed through a ir-allylpalladium trifluoroacetate complex, e.g. (77). 

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