Alkene Reactions: Sharpless Asymmetric Epoxidation

Sharpless asymmetric epoxidation converts a primary or secondary allylic alcohol into an enantioenriched 2,3-epoxy alcohol using titanium isopropoxide, tert-butyl hydroperoxide, and a chiral tartrate ester ((+)/(–)-DET or DIPT). The free allylic OH binds titanium and organizes a chiral pocket that selects the alkene face according to the Sharpless mnemonic: orient the C=C vertically with the allylic alcohol group at the lower-right corner—(+)-DET delivers oxygen to the bottom face, (–)-DET to the top face. Typical procedures employ dry CH₂Cl₂, molecular sieves, −20→0 °C, and rigorously anhydrous reagents to maintain high ee.

Want to explore variations? Open the Reaction Solver, Mechanism Solver, or IUPAC Namer for supported tasks. These general links do not preload the substrate or reagents; handling of this asymmetric example has not been verified.

Quick Summary

• Reagents/conditions: Ti[Oi-Pr]₄ (5–10 mol %), (+)/(–)-DET (or DIPT), t-BuOOH (1.1–1.5 equiv), CH₂Cl₂, 3 Å or 4 Å molecular sieves, −20→0 °C.
• Outcome: Enantioenriched 2,3-epoxy alcohol; ee depends on the substrate, catalyst and conditions, including exclusion of water.
• Mnemonic: Draw the C=C vertically with the allylic alcohol group at the lower-right. (+)-DET adds from the bottom face; (–)-DET from the top. Position flips if the OH is drawn on the lower-left.
• Scope caveat: Requires a free allylic OH; tertiary allylic alcohols and non-allylic alkenes react sluggishly.
• Alternatives: Jacobsen–Katsuki or Shi epoxidations address alkenes lacking an allylic OH.

Mechanism (5 Steps)

Class: Titanium-mediated oxygen transfer; closed-shell (no radicals or carbocations).

The schematic stages below use (E)-4-methylpent-2-en-1-ol, the same C6 allylic alcohol as the worked example. Ti* abbreviates the chiral titanium–tartrate complex; other ligands and catalyst aggregation are omitted. The illustrated (+)-DET choice gives the (2S,3S) epoxy alcohol.

Ligand exchange binds tartrate to titanium and releases isopropanol, setting the chiral environment for face selection. The diagram abbreviates the catalyst rather than assigning a complete coordination structure.

The free allylic OH participates in ligand exchange; its alkoxide binds Ti and positions the alkene relative to the tartrate.

Hydroperoxide binding positions the O–O bond for oxygen delivery within the chiral complex. The simplified drawing omits the remaining ligands; it is not a complete structure of the active catalyst.

The peroxide supplies one oxygen to the alkene as its O–O bond breaks. The chiral Ti–tartrate environment favors one face. TBHP is consumed, with tert-butanol as the net coproduct after proton transfer; an alkoxide cannot donate an O–H proton, and the hydroperoxide is not regenerated by oxygen transfer.

Protic workup breaks the Ti–alkoxide link, releasing (2S,3S)-2,3-epoxy-4-methylpentan-1-ol for the illustrated (+)-DET choice. Catalyst turnover by ligand exchange during the reaction is distinct from the final aqueous workup; hydrolysis does not regenerate pristine Ti(OiPr)₄.

Mechanistic Checklist

• A free primary or secondary allylic OH provides the directing group for the standard asymmetric reaction; protected, tertiary or non-allylic substrates are outside its reliable scope.
• Catalyst trio: Ti[Oi-Pr]₄ + t-BuOOH + (+)/(–)-DET (or DIPT). Molecular sieves and cold, dry solvent preserve ee.
• Mnemonic: Vertical C=C, allylic alcohol group lower-right → (+)-DET attacks from below, (–)-DET from above. Drawn lower-left? Flip the assignment.
• Closed-shell pathway: No radicals, no carbocations—oxygen transfer is polar and concerted.
• Kinetic resolution is possible for secondary allylic alcohols with sub-stoichiometric t-BuOOH.

Worked Examples

Multiple Alkenes & Selectivity

• The allylic OH dictates reactivity: distant, non-allylic alkenes are usually untouched under standard conditions.
• If multiple allylic alcohols are present, steric accessibility and competing coordination can affect site selection and ee; one site is not guaranteed to bind or react first.
• For substrates lacking a coordinating OH, switch to Jacobsen–Katsuki or Shi epoxidation protocols instead.

Practical Tips & Pitfalls

• Keep it dry and cold: Dry CH₂Cl₂, 3 Å/4 Å sieves, and −20→0 °C are standard to maintain high ee.
• Use anhydrous t-BuOOH: Dry solutions in solvents such as toluene, isooctane or decane are used; aqueous TBHP can reduce rate and enantioselectivity.
• DET vs DIPT: Diisopropyl tartrate (DIPT) often improves ee for hindered substrates—test both enantiomers if needed.
• Free OH required: Protecting the allylic OH removes its directing interaction with Ti; do not assume the same conversion or enantioselectivity.
• Avoid over-oxidation: Excess peroxide or warmer temperatures can trigger allylic oxidation instead of epoxidation.

Exam-Style Summary

• Reagents: Ti[Oi-Pr]₄, t-BuOOH, (+)/(–)-DET (or DIPT), sieves, cold CH₂Cl₂.
• Mechanism: Ti–tartrate assembly → allylic alkoxide binding → hydroperoxide activation → enantioselective oxygen transfer → hydrolysis.
• Mnemonic: Vertical C=C, allylic alcohol group lower-right → (+)-DET gives bottom-face epoxide, (–)-DET top-face; flip if OH drawn lower-left.
• Scope: Primary/secondary allylic alcohols with free OH are the standard scope. Protected or absent OH removes the directing interaction needed for the usual facial-selectivity rule.

Interactive Toolbox

• Practice the mnemonic by sketching the C=C vertically with the allylic alcohol group lower-right, then assign (+)/(–)-DET faces.
• Compare to our peracid epoxidation/workup sequence for alkene epoxidation without chiral induction.

FAQ / Exam Notes

Why must the allylic OH be free? The hydroxyl anchors the substrate to titanium and positions its alkene within the chiral catalyst. Without that directing interaction, the usual Sharpless facial-selectivity rule cannot be assumed.

Can I run the reaction warm or with aqueous peroxide? Moisture and higher temperatures rapidly erode ee—use sieves, dry solvent, and anhydrous t-BuOOH.

What if my substrate lacks an allylic OH? Switch to Jacobsen–Katsuki (salen Mn) or Shi (dioxirane) epoxidations tailored for unfunctionalized alkenes.

How do I predict the product configuration? Apply the Sharpless mnemonic: draw the C=C vertically with the allylic alcohol group at the lower-right, (+)-DET attacks from below, (–)-DET from above; flip the rule if the OH sits lower-left in your drawing.