Acid-catalyzed halogenation, of aldehydes

As m the acid catalyzed halogenation of aldehydes and ketones the reaction rate is mde pendent of the concentration of the halogen chlorination brommation and lodmation all occur at the same rate Formation of the enolate is rate determining and once formed the enolate ion reacts rapidly with the halogen... 

Hydrolysis of acetals yields aldehydes, which are intermediates in the biochemical /3-oxidation of hydrocarbon chains. Acid catalyzed hydrolysis of unsubstituted acetals is generally facile and occurs at a reasonable rate at pH 4-5 at room temperature. Electron-withdrawing substituents, such as hydroxyl, ether oxygen, and halogens, reduce the hydrolysis rate, however [50]. Anionic acetal surfactants are more labile than cationic [40], a fact that can be ascribed to the locally high oxonium ion activity around such micelles. The same effect can also be seen for surfactants forming vesicular aggregates. 

The halogenation of an aldehyde or ketone is a typical a-substitution reaction that proceeds by acid-catalyzed formation of an enol intermediate, as shown in Figure 22.4 (p. 908). 

Halogenation of Aldehydes and Ketones 827 [ A MECHANISM FOR THE REACTION ] Acid-Catalyzed... 

It is not the aldehyde or ketone itself that is halogenated, but the corresponding enol or enolate ion. The purpose of the catalyst is to provide a small amount of enol or enolate. The reaction is often done without addition of acid or base, but traces of acid or base are always present, and these are enough to catalyze formation of the enol or enolate. With acid catalysis the mechanism is... 

In a series of reactions similar to those described for amino acid complexes, pyruvic acid undergoes metal-catalyzed condensation with aldehydes and also halogenation (Scheme 14). Again, the metal ion presumably functions by accelerating the formation of the methylene carbanion. 

With benzaldehyde 144 or halogenated derivatives (Cl, F) as acceptors the yeast-PDC-catalyzed addition proceeds with almost complete stereoselectivity to furnish the corresponding (R)-configurated 1-hydroxy-1-phenylpropanones 145 [447]. For practical reasons, whole yeast cells are most often used as the catalyst, with only small loss of enantioselectivity [423,424]. The conversion of benzaldehyde in particular has gained industrial importance because the acyloin is an important precursor for the synthesis of L-(-)-ephedrine [448]. Otherwise, the substrate tolerance is remarkably broad for aromatic aldehydes on the laboratory scale, however, yields of acyloins are usually low because of the prior or consequent reductive metabolism of aldehyde substrate and product, giving rise to considerable quantities of alcohol 146 and vicinol diols 147, respectively [423,424,449], The range of structural variability covers both higher a-oxo-acids (e.g. -butyrate, -valerate) as the donor component, as well as a,/J-un-saturated aldehydes (e.g. cinnamaldehyde 148) as the acceptor [450]. 

Mechanism 22-6 Base-Promoted Halogenation 1054 Mechanism 22-7 Final Steps of the Haloform Reaction 1056 Mechanism 22-8 Acid-Catalyzed Alpha Halogenation 1058 22-6 Alpha Bromination of Acids The HVZ Reaction 1059 22-7 The Aldol Condensation of Ketones and Aldehydes 1060... 

Addition of an Enolate to Ketones and Aldehydes (a Condensation) 1046 Substitution of an Enolate on an Ester (a Condensation) 1046 Base-Catalyzed Keto-EnolTautomerism 1047 Acid-Catalyzed Keto-EnolTautomerism 1047 Base-Promoted Halogenation 1054 Final Steps of the Haloform Reaction 1056 Acid-Catalyzed Alpha Halogenation 1058 Acid-Catalyzed Aldol Condensation 1063 1,2-Addition and 1,4-Addition (Conjugate Addition) 1085... 

A Lewis acid-catalyzed one-pot sequential transformation of /3-ketoesters, aromatic aldehydes, and NCS was reported. The reaction proceeds by way of JCnoevenagel condensation/Nazarov cyclization/halogenations to give a-chloro-)3-ketoesters in moderate yields with high diastereoselectivities. ... 

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