Dihydroxylation of alkenes with cold, dilute KMnO4 to give vicinal diols
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.
(Advanced) References and Further Reading
- G. Wagner, J. Rus. Phys.-Chem. Soc., 27, 219 (1895)
The first publication describing the dihydroxylation of olefins by KMnO4. Unfortunately, the old Russian literature can be difficult to search and so a DOI is unavailable. In this paper, Wagner proposed that the reaction involved the formation of a cyclic ester between the permanganate ion and the olefin. Decomposition of the intermediate with cleavage of the manganese-oxygen bonds leads to the formation of the glycol.
- Improved Preparation of 9(10),10(9)-Ketohydroxystearic Acids by Oxidation of Oleic Acid with Potassium Permanganate in Neutral Solution
Joseph E. Coleman, C. Ricciuti, and Daniel Swern
Journal of the American Chemical Society 1956, 78 (20), 5342-5345
- The Mechanisms of Permanganate Oxidation. IV. Hydroxylation of Olefins and Related Reactions
Kenneth B. Wiberg and Klaus A. Saegebarth
Journal of the American Chemical Society 1957, 79 (11), 2822-2824
Mechanistic studies of dihydroxylation of olefins with KMnO4. The first paper is by the same Swern of Swern oxidation fame! The disadvantage with KMnO4 is that it can overoxidize the substrate, giving ketols. Carrying out the oxidation in basic solution can minimize this.
- dl-GLYCERALDEHYDE ETHYL ACETAL
J. Witzemann, Wm. Lloyd Evans, Henry Hass, and E. F. Schroeder
Org. Synth. 1931, 11, 52
Typical procedure for dihydroxylation of an alkene with KMnO4. This procedure highlights one of the difficulties with using KMnO4 for oxidation; even though it is cheap, using it in stoichiometric quantities results in the formation of an equivalent amount of MnO2, which can be difficult to separate from the desired product.
- Phase-transfer catalysis. I. Heterogeneous reactions involving anion transfer by quaternary ammonium and phosphonium salts
Charles M. Starks
Journal of the American Chemical Society 1971, 93 (1), 195-199
KMnO4 reactions are typically performed in aqueous media, which restricts the array of suitable subtrates to those that are water-soluble. This issue was partially solved by the use of phase-transfer catalysts with KMnO4.A number of reviews and articles have been published on KMnO4 oxidations and dihydroxylations; here is a sample:
- Permanganate: A Green and Versatile Industrial Oxidant
Nirmal Singh and and Donald G. Lee
Organic Process Research & Development 2001, 5 (6), 599-603
- Osmium-free direct syn-dihydroxylation of alkenes
Carole J. R. Bataille and Timothy J. Donohoe
Chem. Soc. Rev. 2011, 40, 114-128