1,2-Diol formation via syn dihydroxylation of alkenes with osmium(VIII) oxide (OsO4); sodium hydrogensulfite (NaHSO3, H2O) workup

Osmium(VIII) oxide (OsO4; retained name "osmium tetroxide") converts alkenes to vicinal (1,2-) diols by syn addition through a cyclic osmate(VI) diester formed in a concerted [3+2] cycloaddition. Because the pathway is closed-shell, no carbocation forms and rearrangements do not occur. Reductive workup with sodium hydrogensulfite (NaHSO3) in water cleaves the O–Os bonds, liberating the diol with retention of configuration and reducing osmium (commonly to osmium(IV) oxide, OsO2). In practice, OsO4 is routinely used catalytically with co-oxidants such as N-methylmorpholine N-oxide (NMO), tert-BuOOH, or K3Fe(CN)6 to regenerate Os(VIII) in situ (H2O2 variants typically require an additional catalyst) while minimising stoichiometric osmium exposure.

OsO4 is highly toxic and volatile (it sublimes readily); always perform manipulations in a fume hood with strict PPE and capture osmium-bearing waste for proper reduction/disposal.

Quick Summary

• Reagents/conditions: OsO4, then NaHSO3, H2O; typical Upjohn conditions use acetone/H2O or 2-methylpropan-2-ol/H2O at 0–25 °C with catalytic OsO4 plus co-oxidants such as NMO, tert-BuOOH, or K3Fe(CN)6 (H2O2 variants usually require additional catalysts).
• Outcome: Syn dihydroxylation -> vicinal syn-diol; no rearrangements (closed-shell).
• Mechanism: Concerted [3+2] osmylation gives a cyclic osmate(VI) diester; NaHSO3/H2O reductively cleaves O–Os bonds while retaining stereochemistry.
• Stereochemistry: Symmetric (Z) alkenes -> meso diols; symmetric (E) alkenes -> racemic (R,R + S,S); cyclic alkenes -> cis-1,2-diols.
• Common pitfalls: Forgetting the reductive workup (needed to release the diol), confusing syn OsO4 addition with anti diol pathways, underestimating OsO4 hazards.

Mechanism (OsO4 Syn Dihydroxylation)

Class: Polar, closed-shell addition via a bridged osmate(VI) diester.

The alkene π-bond engages OsO4 in a [3+2] cycloaddition, installing both C–O bonds on the same face (syn) and forming a five-membered osmate(VI) diester. No discrete carbocation is generated; 1,2-shifts are not observed.

Aqueous sodium hydrogensulfite reduces osmium and cleaves both O–Os bonds, retaining the configuration established during osmylation. Water is shown for simplicity to demonstrate the the proton transfer and subsequent OH addition. In catalytic variants a co-oxidant re-forms OsO4 from reduced osmium species.

Hydration and ligand reorganisation position the osmate(VI) diester for bisulfite reduction, keeping both C–O bonds syn.

Sodium hydrogensulfite reduces osmium and cleaves both O–Os bonds, releasing the vicinal syn diol while the reduced osmium is quenched or re-oxidized under catalytic conditions.

Cold, basic KMnO4 also yields syn diols via a cyclic manganate ester, but is more prone to over-oxidation/oxidative cleavage. See the KMnO4 syn dihydroxylation guide for comparison.

Mechanistic Checklist (Exam Focus)

• Closed-shell [3+2] osmylation → cyclic osmate(VI) diester (no radicals).
• Depict syn delivery of both oxygen atoms to the alkene.
• No free carbocation → no rearrangements.
• NaHSO3/H2O workup reduces Os and releases the diol with stereochemical retention.
• Contrast with anti diols formed via epoxide formation followed by anti ring opening.

Worked Examples (IUPAC names)

Example A — Cyclohex-1-ene -> cyclohexane-1,2-diol (cis on the ring)

• Reagents: OsO4; then NaHSO3, H2O (0–25 °C)
• Outcome: Syn dihydroxylation furnishes cyclohexane-1,2-diol as the enantiomeric pair (1R,2R)/(1S,2S) with cis-1,2 relationship on the ring.

When Multiple Alkenes Are Present

More electron-rich, less hindered C=C bonds osmylate fastest. Longer reaction times, higher temperatures, or aggressive re-oxidants (keeping OsO4 active longer) can erode site-selectivity; control stoichiometry, temperature, and time accordingly.

Practical Tips & Pitfalls

• Always include the reductive workup. NaHSO3/H2O (or sulfite) is required to cleave the osmate and quench residual OsO4.
• Catalytic OsO4 is standard. Pair with NMO, tert-BuOOH, or K3Fe(CN)6 to regenerate OsO4 in situ (H2O2 variants usually need additional catalysts); typical media include acetone/H2O or tert-butanol/H2O.
• Safety first. OsO4 is highly toxic and volatile; work in a hood with appropriate PPE and segregate osmium waste for reduction to OsO2.
• Not an anti pathway. Anti 1,2-diols originate from epoxide formation followed by anti ring opening, not from OsO4/NaHSO3.

Exam-Style Summary

OsO4 adds syn across alkenes to form a cyclic osmate(VI) diester; NaHSO3/H2O reduces osmium and releases the syn vicinal diol with retention. Symmetric (Z) alkenes -> meso diols; symmetric (E) alkenes -> racemic diols; cyclic alkenes -> cis-1,2-diols. No rearrangements occur.

Interactive Toolbox

• Compare OsO4/NaHSO3 syn diols with epoxide → anti diol routes in the Reaction Solver.
• Use the Mechanism Solver to export osmate(VI) diester and reductive workup frames (SVG-ready).

FAQ / Exam Notes

• Why is the addition syn? The [3+2] osmylation delivers both oxygen atoms from the same face via a cyclic osmate(VI) intermediate.
• What does NaHSO3 do? It reduces osmium, cleaving the O–Os bonds and releasing the diol while quenching residual OsO4.
• Are rearrangements possible? No; the pathway is closed-shell and does not generate a carbocation.
• Can this be catalytic in OsO4? Yes; co-oxidants such as NMO, tert-BuOOH, or K3Fe(CN)6 regenerate OsO4 in situ (H2O2 variants generally employ additional catalysts).