Alkene Reactions: 1,2-diol formation via dihydroxylation with osmium tetroxide (OsO4)
Osmium tetroxide (OsO4), in combination with H2O and NaHSO3, react with alkenes to form vicinal diols (1,2-diol):
//image3.png)
The shape of OsO4 makes it so that the most favorable bonding for the intermediate has syn stereochemistry branching from the alkene to the OsO4 moiety. This means that if the carbons that the OsO4 bonds to become stereocenters (check out our lesson on stereochemistry), the new “OH” bonds will have the same stereochemistry configuration i.e. they will both be R or S. They will never have anti configuration where one will be R and the other S. Lastly, if after binding to the OsO4 molecule the alkene does not generate new stereocenters, there will be no stereochemistry configuration:
Syn Addition
//image2.png)
Partial syn Addition
No stereocenters
//image5.png)
NEVER ANTI!
//image4.png)
The reaction mechanism is depicted below:
//image1.png)
In the first step, the osmium tetroxide (OsO4) reacts with the alkene bond, shifting electrons and creating bonds, creating a cyclic osmium compound.
In the second step, NaHSO3 and H2O enter the reaction and hydrolyze the two bonds to form a diol.
The acids commonly used for this reaction (but not exclusive to) are: NaHSO3, KHSO3, and H2S.