Carboxylic acids, unsaturated Perkin reaction

Because of the mild reaction conditions, and its broad applicability, the Knoevenagel reaction is an important method for the synthesis of a ,/3-unsaturated carboxylic acids. Comparable methods are the Reformatsky reaction, the Perkin reaction, as well as the Claisen ester condensation. The Knoevenagel reaction is of greater versatility however the Reformatsky reaction permits the preparation of a ,/3-unsaturated carboxylic acids that are branched in a-position. 

The aldol-like reaction of an aromatic aldehyde 1 with a carboxylic anhydride 2 is referred to as the Perkin reaction. As with the related Knoevenagel reaction, an o ,/3-unsaturated carboxylic acid is obtained as product the /3-aryl derivatives 3 are also known as cinnamic acids. 

Reaction xxklll. (d) Condensation of Aldehydes with the Sodium Salts of certain Adds in the presence of Add Anhydrides (Perkin). (A., 100,126 227, 48 B., 10,68 14,1826 J. C. S., 21, 53 J 1877, 789.) —This is a reaction of very wide application, and one much used in the preparation of unsaturated aromatic carboxylic acids. It consists in heating together—usually to 180°—an aldehyde, the sodium salt of a fatty acid with at least one oc-hydrogen atom, and an acid anhydride. The following reactions then occur —... 

Perkin reaction Condensation of aromatic aldehydes with the anhydrides of aliphatic carboxylic acids to afford a,P-unsaturated carboxylic acids. 338... 

Perkin reaction. Formation of a, (3-unsaturated carboxylic acids by aldol condensation of aromatic aldehydes and acid anhydrides in the presence of an alkali salt of the acid. 

The Perkin reaction involves the condensation of a carboxylic acid anhydride and an aldehyde in the presence of a weak base, often the sodium or potassium salt of the acid or triethylamine, to give unsaturated carboxylic acids (equation 1). The first example of this reaction was described by Perkin in 1868 and involves a synthesis of coumarin by heating the sodium salt of salicylaldehyde with acetic anhydride (equation 2). The reaction is generally applicable only to aromatic aldehydes and is particularly useful for the preparation of substituted cinnamic acids, as illustrated by equation (3). ... 

In 1874 Perkin retired from manufacturing and returned to chemical research. He discovered a reaction (the Perkin reaction) for producing unsaturated carboxylic acids. He also synthesized coumarin, an accomplishment that laid the foundation for the synthetic perfume industry. Perkin died on July 14, 1907, at the age of sixty-nine, see also Dyes. 

The above discourse of the Perkin Reaction is self-explanatory in which benzaldehyde and acetic anhydride interacts to form an anion (I) that undergoes molecular rearrangement to give another anion (II). The resulting restructured anion (II) reacts with acetic anhydride to form an intermediate which subsequently undergoes hydrolysis in the presence of a base to give rise to the formation of an a, P-unsaturated carboxylic acid i.e., cinnamic acid. 

The classic Perkin reaction is the base-catal) ed condensation of an aromatic aldehyde with a carboxylic acid anhydride to yield and an a,p-unsaturated carboxylic acid. The initial stages of the condensation can be viewed as an aldol-type reaction (see Experiments [20], [SAadv], [3Badv], and [A3aj. 

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