As you can probably guess from the name, the dihydoxylation consists in introducing two hydroxyl groups (−OH) in a molecule.
We're going to see two different methods to carry on this particular reaction.
Dihydroxylation with osmium tetroxide
Osmium tetroxide is a quite common substance, formule OsO4. Treating an alkene with osmium tetroxide and subsequently with water H2O we can obtain a molecule bearing adjacent hydroxyl−OH groups, oriented on the same side with respect of the plane of the molecule. From the reaction a diastereomer is obtained as mixture of two enantiomers but not other eventual diastereomers (n.b. diastereomers are stereoisomers with two or more stereogenic centers which differ for the configuration of one stereogenic center).
For example from this reaction aren't obtainable the products with configuration R,S and S,R of the two stereogenic carbons.
This synthesis gives products that have the groups oriented on the same side, for geometric reasons. Indeed, osmium tetroxide is obligated to react with the double bond from above or from below the molecule plane.
Actually the reaction isn't currently carried on just with osmium tetroxide, but different catalysts can be added:
The reaction is very efficient, but osmium tetroxide is very expensive and in addition very toxic, especially for the eye (oxidize the retinal). For this and other reasons different new methods have been invented and allow to use OsO4 in catalytic amount. s The reaction is relatively complicated, but is anyway a redox, where the osmium tetroxide is reduced. At the same time are used reagents that allow to re-oxidize it immediately so that we can use it actually in catalytic amount.
For the first step of the reaction we can guess a mechanism like this, that shows how the molecule can attack just from above or below the plane:
Dihydroxylation with iodine and silver acetate