Alkene Reactions: 1,2-diol formation via dihydroxylation with potassium permanganate (KMnO4)
Potassium permanganate (KMnO4), in combination with a strong base such as sodium hydroxide (NaOH), reacts with alkenes to form vicinal diols (1,2-diol) in cold temperatures. If this reaction is run at higher temperatures, the diol will not form and instead a carbonyl group would form instead:
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The shape of KMnO4 makes it so that the most favorable bonding for the intermediate has syn stereochemistry branching from the alkene to the KMnO4 moiety. This means that if the carbons that the KMnO4 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 KMnO4 molecule the alkene does not generate new stereocenters, there will be no stereochemistry configuration:
Syn Addition
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Partial syn Addition
No stereocenters
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NEVER ANTI!
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The reaction mechanism is depicted below:
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In the first step, the potassium permanganate (KMnO4) reacts with the alkene bond, shifting electrons and creating bonds, creating a cyclic manganese compound.
In the second step, NaOH enters the reaction and hydrolyzes the two bonds to form a diol as long as the reaction is held under cold conditions..
The bases commonly used for this reaction (but not exclusive to) are: NaOH, and OH-.