Dehydration, of aldol product

SO no condensation reaction could take place. We would then immediately add an alkyl halide to complete the alkylation reaction. 

On the other hand, carhonyl condensation reactions require only a catalytic amount of a relatively weak base rather than a full equivalent so that a small amount of enolate ion is generated in the presence of unreacted carhonyl compound. Once a condensation has occurred, the basic catalyst is regenerated. To carry out an aldol reaction on propanal, for instance, we might dissolve the aldehyde in methanol, add 0.05 equivalent of sodium methoxide, and then warm the mixture to give the aldol product. 

The j8-hydroxy aldehydes or ketones formed in aldol reactions can be easily dehydrated to yield a,)8-unsaturated products, or conjugated enones. In fact, it s this loss of water that gives the carbonyl condensation reaction its name, because water condenses out of the reaction when the enone product forms. 

Most alcohols are resistant to dehydration by base (Section 13.4) because hydroxide ion is a poor leaving group, but aldol products dehydrate easily 

CHAPTER 17 CARBONYL ALPHA-SUBSTITUTION AND CONDENSATION REACTIONS 

---

The ate complex LiAlHBu"Bu 2 is prepared from DIBAL-H and n-butyllithium in either THF or toluene-hexane. This reagent is more effective for selective 1,2-reduction of enones to the corresponding allylic alcohol than is DIBAL-H alone. The reagent also reduces esters, lactones and acid chlorides to the corresponding alcohols, and epoxides to the respective alcohols. a,p-Unsaturated ketones derived from dehydration of aldol products from I-(arylthio)cyclopropanecarbaldehydes and ketones were selectively reduced by this ate complex or by DIBAL-H itself, yielding the allylic alcohols with minor... 

Enolates may also be alkylated with a,p-unsaturated carbonyl substrates. The enolate adds in 1,4-fashion to give a unit extended by three carbon atoms in a process known as the Michad reaction. Many a,p-unsaturated carbonyl systems may be prepared by the dehydration of aldol products. Examples of the Michael reaction using methyl vinyl ketone and (vinylsulfinyl)benzene, two common units in the reaction, are shown in Scheme 3.17. 

It is believed that this reaction involves the following consecutive steps dehydrogenation of ethanol to acetaldehyde, condensation of acetaldehyde into acetaldol, dehydration of aldol product to crotonaldehyde, and deoxygenation of crotonaldehyde. This mechanism is illustrated here. 

Aldol Condensation with an aromatic aldehyde, followed by the dehydration of aldol product to give the derivative of cinnamic acid. ... 

Formation of (x,/3-Unsaturated Compounds by Dehydration of Aldol Products. As previously stated, the /9-hydroxy carbonyl product of the aldol condensation is thermod5mamically unfavored at high temperatures. Under these conditions, the /9-hydroxy carbonyl is hardly found among the reaction products because it is readily converted to the corresponding o, y0-unsaturated carbonyl compound by dehydration on acid and bases. However, the dehydration step will only take place if the reactant molecule that supplies the carbanion initially contains a second a-hydrogen, in other words, a methylenic group. Thus, in Scheme 1, at least one of substituents R4 and R5 is required to be a hydrogen atom. 

---