Prileshayev reaction
The Prileschajew reaction (also Prilezhaev reaction or Prilezhaev epoxidation ) is a name reaction from the field of organic chemistry . The reaction was discovered by the Russian chemist Nikolai Alexandrowitsch Prileschajew (1877–1944) in 1909.
In the Prileschajew reaction, alkenes are reacted with peroxycarboxylic acids (RCO 3 H) to form the corresponding epoxides (oxiranes), with the associated carboxylic acid (RCO 2 H) also being formed.
Overview reaction
Alkenes are reacted with peroxycarboxylic acids to give the corresponding epoxides (oxiranes), the peroxy oxygen atom being formally transferred to the double bond of the alkene.
Meta- chloroperbenzoic acid ( m -CPBA) is often used in the laboratory for this purpose , as this compound is relatively stable compared to other peracids and can be mixed well with most organic solvents. Other peracids such as peroxyacetic acid or peroxyformic acid are so unstable that they have to be prepared in situ . For this purpose, acetic acid is usually used as a solvent and peroxyacetic acid is obtained by adding hydrogen peroxide . The method is often used in technology as a more atom-economical variant.
Reaction mechanism
The peroxy oxygen atom of a peroxycarboxylic acid ( 2 ) is partially positively charged and thus functions as an electrophilic center that can be attacked by the π bond of the alkene ( 1 ). This step works particularly well and quickly when the electron density in the alkene is high. The Prilezhaev epoxidation is a typical pericyclic reaction that takes place simultaneously via a five- and three-membered transition state ( 3 ) ("spiro transition state") in which a total of four electron pairs are shifted in concert. Due to the shape and spatial arrangement, the transition state is often referred to as the "butterfly transition state" . The electrophilic oxygen atom of the peracid is added to the double bond and the proton is transferred to the carbonyl group of the peroxyacid. Finally, the epoxide ( 4 ) and the corresponding carboxylic acid ( 5 ) are formed, which must be removed from the product mixture by a subsequent, slightly basic work-up.
The reaction proceeds stereospecifically , as from cis -alkenes only cis epoxides and from trans -alkenes exclusively trans -epoxides are formed. It is also a syn addition of the oxygen to the double bond. However, the epoxidation does not proceed enantioselectively; instead, a racemic mixture is always obtained. If stereogenic centers are already present in the alkene, the epoxidation usually gives good diastereoselectivities .
In general, the Prileschajew reaction takes place under mild conditions (at or below room temperature ) in an inert organic solvent such as chloroform , dichloromethane , benzene , diethyl ether or dioxane . Usually a slightly basic buffer such as hydrogen carbonates or hydrogen phosphates is added so that the epoxide that is formed is not opened by the carboxylic acid that is also formed.
literature
- Beyer-Walter: Textbook of Organic Chemistry , 23rd Edition, S. Hirzel Verlag Stuttgart Leipzig, 1998, ISBN 3-7776-0808-4 .
- Reinhard Brückner: reaction mechanisms - organic reactions, stereochemistry, modern synthesis methods . 3. Edition. Springer Spectrum, Berlin-Heidelberg 2015, ISBN 978-3-662-45683-5 , p. 120 f .