Ketones cross condensation with esters

The other two unenolizable esters we mentioned on p. 728 undergo cross-condensations with ketones. Unlike formaldehyde, formate esters are well behaved—no special method is necessary to correspond with the Mannich reaction in aldol chemistry, Here is what happens with cyclohexanone. 

In addition to crossed aldol additions and crossed Claisen condensations, a ketone can undergo a crossed condensation with an ester. If both the ketone and the ester have a-hydrogens, then LDA is used to form the needed enolate ion and the other carbonyl compound is added slowly to the enolate ion to minimize the chance of its forming an enolate ion and reacting with another molecule of its parent ester. 

A crossed Claisen is die reaction of an ester enolate with an aldehyde or ketone to produce a /3-hydroxy ester. This works well because aldehydes and ketones are more reactive electrophiles than esters thus the ester enolate reacts faster with die aldehyde or ketone than it condenses with itself, avoiding product mixtures. Moreover, die aldehyde or ketone should not have a hydrogens so that proton transfer to die more basic ester enolate is avoided. This would lead to the formation of an aldehyde or ketone enolate in the mixture, and an aldol reaction would be a major competing reaction. 

Among the enolates of carboxylic acid derivatives, esters are the most widely used. Ester enolates cannot be used in crossed aldols with aldehydes because the aldehyde is both more enolizable and more electrophilic than the ester. It will just condense with itself and ignore the ester. The same is true for ketones. A specific enol equivalent for the ester will therefore be needed for a successful ester aldol reaction. 

We find that we need a crossed aldol condensation between two ketones so tve chemoselectivity. We also need to make one enol(ate) from an unsymmetrical ketone so v. r regioselectivity too. The obvious solutions are a lithium enolate, a silyl enol ether, or a come with an extra ester group. 

In addition, unsymmetrical 1,4-diketones and 7-keto esters can be obtained by cross-coupling of enolates of two different methyl ketones and of the enolate of a methyl ketone with that of an alkyl acetate, respectively. Only traces of condensation products are formed. 

Hydrophobic membranes can be used to extract organic solvents or volatile organic compounds (VOCs) from water. The membranes are made of hydrophobic cross-Unked polymers. The membrane chemistry is designed to attract VOC molecules to the surface of the membrane. Pervaporation takes place in the membrane. VOCs diffuse through the membrane and evaporate on the permeate side with the help of a vacuum. These VOCs later condense in a condenser. Examples of these membrane applications include extraction of numerous types of VOCs from water, extraction of aromatics from water, ketones from water, esters from water, and many more. 

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