Dihydroxylation
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The dihydroxylation refers to a chemical reaction , with the double bond of an alkene two vicinal hydroxy groups are added. Since the oxidation number of the carbon atoms increases, it is an oxidation reaction . In the organic chemistry dihydroxylation reactions are widely used, the best known is the so-called Upjohn dihydroxylation with catalytically inserted osmium tetroxide (OsO 4 ) and N -Methylmorpholin- N oxide (NMO). The 2001 Nobel Prize in Chemistry was awarded for the stereoselective Sharpless dihydroxylation , named after its discoverer, Barry Sharpless .
Reagents
The oxidizing agents required for the reaction are usually oxides of transition metals such as osmium tetroxide (OsO 4 ), permanganate (MnO 4 - ) or ruthenium tetroxide (RuO 4 ). While permanganate is inexpensive enough to be used stoichiometrically , i.e. in a ratio of 1: 1 with the starting material ( starting material ), osmium and ruthenium tetroxide are often only used in catalytic amounts. In order to make this possible, the reaction product, the osmium (VI) or ruthenium (VI) acid, is oxidized again to the tetroxide with the aid of a cooxidant and can thus convert a further starting material molecule. The cooxidant used for the oxidizing agent is OsO 4 N -methylmorpholine- N -oxide (NMO) and for RuO 4 sodium periodate (NaIO 4 ).
With the help of a special mixture of oxidizing agents and suitable chiral catalysts, a stereoselective dihydroxylation is also possible.
Reaction mechanism
The mechanism for dihydroxylation has been extensively investigated and confirmed for osmium tetroxide as an oxidizing reagent. It is assumed that permanganate and ruthenium tetroxide react analogously. The reaction begins with a 1,3-dipolar cycloaddition of the oxidizing agent to the double bond. The resulting five-membered ring is a diester of osmium (VI) acid, which is split ( hydrolyzed ) in the presence of water and releases the end product. Due to the five-membered intermediate , only the cis -addition product is created. The resulting osmium (VI) acid OsO 2 (OH) 2 is then converted back into the octavalent osmium tetroxide by NMO.
The course of the reaction differs depending on the type of oxidizing agent: RuO 4 is the only one capable of selectively dihydroxylating double bonds in an aromatic . OsO 4 does not attack these double bonds, but if permanganate is used, the product is further converted directly and undergoes oxidative cleavage . This secondary reaction can also occur under certain reaction conditions during the dihydroxylation of alkenes with permanganate.
Ligand acceleration
The reaction can be accelerated by adding a base such as pyridine . Here, coordinates the free electron pair of the base at the metal center, thus generating a negative charge on the oxygen ligand . This accelerates the cycloaddition to the double bond and thus the entire course of the reaction. However, since the base does not intervene directly in the reaction, but acts as an additional ligand on the catalyst, it is referred to as ligand acceleration.
literature
- Reinhard Brückner : reaction mechanisms. Organic reactions, stereochemistry, modern synthetic methods. 2nd, updated and expanded edition. Spectrum - Akademischer Verlag, Heidelberg et al. 2003, ISBN 3-8274-1189-0 , pp. 750-758.