Pinacol rearrangement

The pinacol rearrangement is a chemical reaction, in which di- tert - glycols under the action of a protic acid to α-tertiary ketones rearrange. The name of the reaction, first described in 1860, comes from the common name of the simplest such glycol, pinacol . The reaction was elucidated in 1874 by the Russian chemist Alexander Michailowitsch Butlerow (1828–1886) as a rearrangement of the carbon structure.

The reaction consists of four sub-steps:

1. Protonation of one of the two hydroxyl groups
2. Elimination of water (H ₂ O) and formation of a carbenium ion
3. [1,2] rearrangement of the carbenium ion
4. Deprotonation

Mechanism of a symmetrical pinacol rearrangement

First a hydroxyl group of pinacol 1 is protonated , then a molecule of water is split off, forming a tertiary carbenium ion 2 . This then rearranges in the third step via the [1,2] shift of a methyl group to the more stable carbenium ion (carboxonium ion), which then leads to ketone 3 by deprotonation .

Mechanism of an unsymmetrical pinacol rearrangement

In the case of asymmetrical di- tert- glycols, the rearrangement always takes place via the more stable carbenium ion under the same acidic conditions as above. Thus, the methyl group moves and not the phenyl residue :

Both intermediate compounds are tertiary carbenium ions. The carbenium ion 1 is, however, more stable than the carbenium ion 2 , as the benzyl position enables mesomerism stabilization :

Synthesis of spiro compounds

If the pinacol coupling is carried out with a cyclic ketone (e.g. cyclopentanone ) 1 , a spiro compound 2 is synthesized by means of the pinacol rearrangement .

literature

• Reinhard Brückner : reaction mechanisms , 2003, 2nd edition, 602 f.
• Roberts, Frederick H .: The pinacol-pinacolone molecular rearrangement: the rearrangement of pinacol dibromide 1937 .
• Stefan F. Kirsch: Long known - but successful: the pinacol rearrangement , Nachrichten aus der Chemie , 2008 , 56 , 1228–1231.

See also

• Name reaction
• Pinacol coupling
• Wagner-Meerwein rearrangement

Individual evidence

1. ↑ R. Fittig: 41. About some derivatives of acetone. In: Annals of Chemistry and Pharmacy. 114, 1860, pp. 54-63, doi : 10.1002 / jlac.18601140107 .
2. ↑ A. Butlerov, Justus Liebigs Ann. Chem. 1874 , 174 , 125.
3. ^ Siegfried Hauptmann : Organic Chemistry , 2nd Edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 331, ISBN 3-342-00280-8 .
4. ↑ Stuart Warren: Designing Organic Syntheses - A programmed introduction to the Synthon Approach . John Wiley & Sons, 2007, ISBN 978-0-471-99612-5 , pp. 49 .