Sharpless Epoxidation

Under construction

This page is a draft outline. It is visible to admins and still needs a full lesson.

Klein 13.5. The Reboxetine page mentions this reaction in one paragraph. This page teaches the method.

What it does: enantioselective epoxidation of an allylic alcohol.
The ingredients at the level students need: titanium(IV) isopropoxide, tert-butyl hydroperoxide, and diethyl tartrate.
Which tartrate enantiomer delivers oxygen to which face. A mnemonic is enough. The full transition-state model is optional.
Why an allylic alcohol is required, and what the reaction does not do to an ordinary alkene.
One worked example, then point back to the Reboxetine application.

Sharpless Asymmetric Epoxidation

While standard peroxyacid epoxidations (like MCPBA) yield racemic mixtures of epoxides from chiral or achiral alkenes, the Sharpless Asymmetric Epoxidation provides a powerful method to install an epoxide enantioselectively. Developed by K. Barry Sharpless, this reaction converts an allylic alcohol into an optically active epoxide with exceptional enantiomeric excess.

[ Insert Image: Overview of Sharpless Asymmetric Epoxidation ]

Figure 1. Enantioselective epoxidation of an allylic alcohol using the Sharpless catalytic system.

1. Reagents and Catalytic Ingredients

The Sharpless epoxidation relies on a precisely tuned catalytic reagent system composed of four components:

Titanium(IV) Isopropoxide [$\text{Ti}(\text{O}i\text{-Pr})_4$]: The Lewis acid metal catalyst that coordinates the reactants.
Diethyl Tartrate (DET): A chiral ligand (either (+)-DET or (-)-DET) that forms a complex with titanium to dictate which face of the alkene receives the oxygen atom.
tert-Butyl Hydroperoxide (TBHP): The terminal oxidizing agent that delivers the oxygen atom.
An Inert Solvent: Typically dichloromethane ($\text{DCM}$) maintained under strictly anhydrous conditions.

[ Insert Image: Sharpless Catalyst Complex Architecture ]

Figure 2. Titanium-tartrate catalytic complex coordinating the oxygen transfer.

### 2. Facial Selectivity: The Tartrate Mnemonic

The choice of diethyl tartrate enantiomer controls the three-dimensional outcome of the epoxide:

(-)-DET: Delivers oxygen to the bottom face (re-face or si-face depending on orientation) of the allylic alcohol.
(+)-DET: Delivers oxygen to the top face.

The Sharpless Mnemonic (The "Drawing" Trick)

To easily predict the major enantiomer without memorizing complex transition-state models: orient your allylic alcohol horizontally in the xy-plane with the hydroxyl ($\text{OH}$) group positioned in the lower-right quadrant.

* (-)-DET will always deliver the epoxide oxygen from the bottom (away from you / dashed).
* (+)-DET will always deliver the epoxide oxygen from the top (toward you / wedged).

[ Insert Image: Sharpless Epoxidation Facial Mnemonic ]

Figure 3. Visual mnemonic for predicting epoxide stereochemistry using (+)- and (-)-DET.

3. Why an Allylic Alcohol is Required

The Sharpless epoxidation has a strict structural requirement: the substrate must be an allylic alcohol (an $\text{OH}$ group attached to a $\text{sp}^3$ carbon adjacent to the $\text{C}=\text{C}$ double bond).

Coordination is Mandatory: The titanium metal does not just float freely; it must coordinate simultaneously to both the hydroperoxide oxygen and the hydroxyl group of the allylic alcohol. This precise bidentate binding locks the substrate into a fixed geometry.
Unreactive Ordinary Alkenes: If you subject an ordinary isolated alkene (lacking a nearby $\text{OH}$ group) to Sharpless conditions, no reaction occurs because the molecule cannot anchor itself to the titanium catalyst.

[ Insert Reaction: Substrate Scope: Allylic Alcohols vs. Ordinary Alkenes ]

Figure 4. Mandatory bidentate coordination required for titanium activation.

4. Worked Example

Consider the asymmetric epoxidation of (E)-2-methyl-3-penten-2-ol:

Reaction Breakdown

Substrate: An allylic tertiary alcohol containing a trans disubstituted double bond.
Reagents: $\text{Ti}(\text{O}i\text{-Pr})_4$, (-)-DET, and TBHP in DCM at -20°C.
Outcome: The titanium-((-)-DET) catalyst delivers oxygen stereoselectively to one face of the double bond. The resulting epoxide is formed in >95% enantiomeric excess (ee), ready for downstream ring-opening or synthetic manipulations.

[ Insert Reaction: Worked Example of Sharpless Epoxidation ]

Figure 5. Enantioselective synthesis of an epoxy-alcohol using (-)-DET.

5. Real-World Connection: The Reboxetine Synthesis

As noted in our discussion of pharmaceutical targets, the Sharpless epoxidation is not just an academic exercise—it is a critical industrial tool.

During the enantioselective synthesis of reboxetine (a selective norepinephrine reuptake inhibitor used as an antidepressant), a key chiral epoxide building block is constructed using a Sharpless asymmetric epoxidation. Setting this single stereocenter with high enantiomeric purity early in the synthetic route ensures that the final drug molecule possesses the correct three-dimensional shape required for biological efficacy.

[ Insert Reaction: Sharpless Epoxidation Key Step in Reboxetine Synthesis ]

Figure 6. Application of the Sharpless epoxidation in the synthesis of reboxetine.