Prévost-Woodward hydroxylation
The Prévost-Woodward-hydroxylation (also: Woodward cis -hydroxylation or Woodward variant ) is a chemical reaction , in which two under mild chemical conditions hydroxyl groups to alkenes can add up. In contrast to the Prévost hydroxylation , in this reaction both hydroxyl groups are added syn to one another.
This reaction is named after Charles Prévost and Robert Burns Woodward , who published this reaction in the Journal of the American Chemical Society in 1958 .
mechanism
In the first step, the alkene reacts with elemental iodine to form a cyclic iodonium ion. This is then opened by the silver benzoate and trans- 1,2-iodine carboxylate is formed . After an intermolecular rearrangement, the iodine is split off and a cationic cyclic intermediate is formed. In the presence of water, a hydroxide ion is added to the positive carbon atom of the ester group. The benzoate group is re-formed by rearrangement and a hydroxyl group is formed in the syn position . In the final steps the ester group is hydrolyzed . Since the hydrolysis does not change the configuration of the molecule , the resulting hydroxyl groups remain syn to one another.
The chemical yield can be increased by changing the starting substances (other halogens , thallium (I) acetate or benzoate) .
application
The Prévost-Woodward hydroxylation is very well suited as an alternative to the usual dihydroxylation reactions with potassium permanganate or osmium tetroxide , since the sterically less favorable product is formed here, in contrast to those. Here, the halogen attacks first on the sterically more favorable side of the alkene, then the benzoate is added on the sterically less favorable side.
literature
- Michael B. Smith, Jerry March, March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure , 5 th Edition, Wiley-Interscience, New York, 2001 , 1049-1050
- Woodward, RB ; Brutcher, FV J. Am. Chem. Soc. 1958 , 80 , 209. ( doi : 10.1021 / ja01534a053 )
- ChemieOnline: Name Reactions - Woodward-cis-Hydroxylation
- László Kürti & Barbara Czakó: Strategic Applications of Named Reactions in Organic Synthesis , 2005, Elsevier Academic Press