Syn dihydroxylation: OsO₄ and cold KMnO₄ template

OsO₄ adds to cyclohexene in one concerted step to give a cyclic osmate ester, hydrolysis releases cis-cyclohexane-1,2-diol and NMO reoxidises Os(VI) to OsO₄ (Upjohn); cold, dilute, basic KMnO₄ works the same way through a cyclic manganate ester.

Mechanism step by step

Both oxygens add to the same face through a cyclic ester: cyclohexene gives cis-cyclohexane-1,2-diol

1. Osmate ester – A concerted [3+2] cycloaddition: both C–O bonds form on the same face; Os(VIII) → Os(VI)
2. Hydrolysis – Water cleaves the two Os–O bonds; the C–O bonds are untouched, so the diol stays cis
3. Reoxidation (Upjohn) – N-Methylmorpholine N-oxide turns Os(VI) back into OsO₄, so only a catalytic amount of OsO₄ is needed
4. Cold dilute KMnO₄ – Permanganate (cold, dilute, basic) gives a cyclic manganate(V) ester the same way; hydrolysis gives the same cis-diol and MnO₂

Key points

• Syn addition is stereospecific: cyclohexene gives only cis-cyclohexane-1,2-diol (meso); cis-but-2-ene gives meso-butane-2,3-diol, trans-but-2-ene the racemic (2R,3R)/(2S,3S) diol.
• OsO₄ is toxic, volatile and expensive, so it is used in catalytic amounts with a co-oxidant: NMO (Upjohn) or K₃Fe(CN)₆ (Sharpless asymmetric dihydroxylation with chiral ligands).
• KMnO₄ must be cold, dilute and basic (Baeyer test: purple → brown MnO₂); hot or acidic KMnO₄ cleaves the diol to carbonyl compounds and acids.
• For the anti (trans) diol use an epoxide then aqueous acid: epoxidation is syn, but ring opening is backside attack.

Drawn with 110 atoms, 11 arrows, 10 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 19 Textbook
• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 8 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 15 Textbook