Hydride transfer — no external reductant
Meerwein-Ponndorf-Verley reduction
How the Meerwein-Ponndorf-Verley reduction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Al(OiPr)₃ (cat. to stoich.), iPrOH (excess). Conditions: Reflux; distil off acetone. Bonds formed: C–H (carbinol); C=O of acetone; bonds broken: C=O π of ketone; C–H of iPrO. Stereochemistry: Chair-like TS; reversible (Oppenauer is the reverse). Rate and selectivity: Driven by distilling acetone / excess iPrOH.
At a glance
Step 1: Ketone binds aluminium
An aluminum isopropoxide coordinates both the ketone's oxygen and delivers its isopropoxide ligand nearby, setting up a six-membered cyclic transition state.
Step 2: Hydride shuttles through a six-membered TS
A hydride transfers directly from the isopropoxide's carbon to the ketone's carbonyl carbon — intramolecularly, through the aluminum, with no external hydride reagent.
Step 3: Alkoxide and acetone
The ketone becomes an alcohol while the isopropoxide is oxidized to acetone — a mild, chemoselective reduction that leaves other reducible groups (like alkenes) untouched.
Abbreviations and terms
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Meerwein-Ponndorf-Verley reduction | Oppenauer oxidation | Acid-catalyzed glycoside hydrolysis | |
|---|---|---|---|
| Reagents | Al(OiPr)₃ (cat. to stoich.), iPrOH (excess) | Al(OiPr)₃ (or Al(OtBu)₃), acetone (large excess) | Dilute HCl or H₂SO₄ (H₃O⁺), H₂O |
| Conditions | Reflux; distil off acetone | Toluene/acetone, reflux | Heat 60–100 °C; excess water |
| Stereochemistry | Chair-like TS; reversible (Oppenauer is the reverse) | Stereocentre at carbinol carbon destroyed; other centres untouched | Both anomers form (planar oxocarbenium) |
| Rate / selectivity | Driven by distilling acetone / excess iPrOH | Equilibrium: driven by excess acetone; hydride transfer rate-limiting | Aglycone departure from the protonated glycoside is rate-limiting (A-1 mechanism) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Meerwein-Ponndorf-Verley reduction?
Al(OiPr)₃ (cat. to stoich.), iPrOH (excess). Conditions: Reflux; distil off acetone.
Which bonds are formed and which are broken?
Formed: C–H (carbinol); C=O of acetone. Broken: C=O π of ketone; C–H of iPrO.
What is the stereochemical outcome?
Chair-like TS; reversible (Oppenauer is the reverse)
What controls the rate or the selectivity?
Driven by distilling acetone / excess iPrOH
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- Curved-arrow notation: rules and the mistakes students make