Perkin's reaction

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The Perkin reaction is a name reaction and named after the English chemist Sir William Henry Perkin (1838–1907). The first compound he synthesized in this way was cinnamic acid . In general, the Perkin reaction is a condensation of aromatic aldehydes with acetic anhydride in the presence of sodium acetate to form α, β - unsaturated carboxylic acids . Aromatic derivatives of these compounds are also known as cinnamic acids . The Perkin reaction proceeds on the principle of aldol condensation and is also closely related to the Knoevenagel reaction .

Gross equation of the Perkin reaction

mechanism

The mechanism of the Perkin reaction is basic, initially the anion of the acid anhydride is formed. The reason for this is the CH acidity of the acid anhydride in the α position. In the next step, the anion adds nucleophilically to the carbonyl carbon of the aldehyde.

Mechanism of the Perkin reaction

The simplest case, which was also carried out by Perkin, is the reaction with methyl groups in the α position (R = H) and a phenyl group as the aryl residue . Of benzaldehyde and acetic anhydride there was cinnamic acid (and acetic acid ) was synthesized. If only one α - hydrogen atom is present, the reaction stops with the β -hydroxycarboxylic acid. The ( E ) -diastereomer is formed preferentially (but not only) . Another variant of the Perkin reaction is the Erlenmeyer-Plöchl synthesis for the synthesis of amino acids and α- keto acids.

practice

This reaction is a so-called one-pot reaction , ie aldehyde, acid anhydride and base are mixed and heated to 170-200 ° C for several hours. The base used is usually the sodium salt of the carboxylic acid corresponding to the anhydride. However, it has been found that cesium salts sometimes drastically improve the reaction yield and shorten the necessary reaction time. If crown ethers were used to complex the cation , the reaction did not occur at all. This suggested that it is not the increased basicity due to weaker contact ion pairs in the heavier alkali cations that is decisive for better enolization and thus acceleration of the reaction, but rather that these ions precomplex or catalyze the starting materials better.

literature

  • House, HO Modern Synthetic Reactions (WA Benjamin, Menlo Park, California, 2nd Edition, 1972) pp. 660-663.
  • Rosen, T .; Comp. Org. Syn. 1991, 2, 395-408.
  • E. Koepp, F. Vögtle: Perkin synthesis with cesium acetate . In: Synthesis . No. 2 , 1987, pp. 177-179 , doi : 10.1055 / s-1987-27880 .

See also

Knoevenagel reaction

Individual evidence

  1. ^ WH Perkin: XXIII. — On the hydride of aceto-salicyl . In: Journal of the Chemical Society . tape 21 , 1868, p. 181-186 , doi : 10.1039 / JS8682100181 .
  2. ^ WH Perkin: XI. — On the formation of coumarin and of cinnamic and of other analogous acids from the aromatic aldehydes . In: Journal of the Chemical Society . tape 31 , 1877, p. 388-427 , doi : 10.1039 / JS8773100388 .
  3. ↑ JR Johnson: oA In: Org. React. , 1942, 1, 210-266.
  4. ↑ R. Brückner: reaction mechanisms: organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Spectrum, 2004, ISBN 3-8274-1579-9 , pp. 568 .