Description: Treatment of alkenes with cold, dilute basic KMnO4 leads to 1,2-diols (vicinal diols).
- The reaction proceeds with “syn” stereochemistry of the alkene, meaning that the two alcohols end up on the same side of the alkene
- Also note that osmium tetroxide (OsO4) does exactly the same reaction.
- The purpose of the NaOH is to assist in breaking up the intermediate manganate ester that forms after dihydroxylation. If this manganate ester is allowed to sit around, oxidative cleavage of the diol may occur. This is also why the temperature is kept cold.
Notes: The reaction works well so long as it is kept cold. If higher temperatures are used, cleavage of the diol to give carbonyl compounds is observed. Note that the reaction does not occur with alkynes.
Mechanisms: Potassium permanganate adds to one face of the alkene through a cycloaddition reaction (Step 1, arrows A, B and C) to give a cyclic manganese compound (“manganate ester”). The manganate ester is then reduced with NaHSO3 or KHSO3 and hydrolyzed to the diol through a very long process that is excruciatingly boring to write out and generally not bothered with in Org 1/ Org 2
Notes: The boring part goes something like this: water attacks Mn, transfer proton to O, break Mn–O bond, then add second equivalent of water to Mn, transfer proton to O, break Mn–O bond. This gives the free diol.
Reference: For those curious about the mechanism I strongly suggest you read this paper: “Permanganate Peroxidation of Cyclohexene: Hydroxide Ion And Salt Effect Studies. Taylor, J. E. ; Green, R. Can. J. Chem. 63, 2777 (1985).
“Good yields of cis-l,2-cyclohexanediol were formed by the reaction of cyclohexene and aqueous potassium permanganate under turbulent stirring conditions only in the presence of low concentrations of sodium hydroxide. Larger amounts had no further benefit or were deleterious.”
Also, apparently under acidic or neutral conditions, more highly oxidized products such as alpha-hydroxy ketones can be formed “without going through the glycols”. See J. Am. Chem. Soc. 1981, 103, 938.