Collins reagent
The Collins reagent, also known as Sarett's reagent, is a complex of pyridine and chromium (VI) oxide in dichloromethane .
It is used for the selective oxidation of primary alcohols to aldehydes . Oxidation reactions with the Collins reagent are also known as Collins oxidation . The reagent was first introduced in 1968 and represents an advance on previous methods as it uses dichloromethane as a solvent . This allows the oxidizing agent to be used for a wider range of base- sensitive substrates.
The Collins reagent tolerates the presence of many functional groups . It is usually used in excess of four to six equivalents.
- Gross reaction equation of the Collins oxidation.
To avoid losses of aldehyde through adduct formation with precipitated reduced chromium species, silica gel or kieselguhr can be added before the Collins reagent is formed. For the oxidation of sensitive substrates (for example carbohydrates or nucleosides ), milder reaction conditions can be created by adding acetic anhydride . In this way, β- eliminations in the presence of heteroatoms in the β-position are avoided.
Other chromium (VI) complexes with pyridine that are used as oxidizing agents are pyridinium chlorochromate and the Cornforth reagent .
Manufacturing
To prepare the Collins reagent, chromium (VI) oxide is added in a stoichiometric ratio to a solution of pyridine in anhydrous dichloromethane.
Individual evidence
- ↑ a b c G. Tojo, M. Fernández: Oxidation of alcohols to aldehydes and ketones , 2nd edition, pp. 20-27, Springer Verlag, Berlin, ISBN 0-387-23607-4 .
literature
- JC Collins, WW Hess: Aldehydes from Primary Alcohols by Oxidation with Chromium Trioxide: Heptanal In: Organic Syntheses . 52, 1972, p. 5, doi : 10.15227 / orgsyn.052.0005 ; Coll. Vol. 6, 1988, p. 644 ( PDF ).