Meerwein–Ponndorf–Verley reduction and Oppenauer oxidation template

The ketone binds to aluminium isopropoxide and a hydride moves from the isopropoxide C–H to the carbonyl carbon in a six-membered cyclic transition state, releasing acetone. Every step is reversible: excess isopropanol (and distilling off acetone) reduces the ketone (MPV); excess acetone oxidises an alcohol (Oppenauer).

Mechanism step by step

Al(OiPr)₃ moves a hydride between two carbonyl partners through a six-membered ring

1. Coordination – The carbonyl O binds to the Lewis-acidic Al
2. Hydride transfer – Six-membered cyclic transition state (Al, O, C, H, C, O): H moves from the isopropoxide carbon to C=O
3. Acetone leaves – The Al–O bond to acetone breaks (acetone is distilled off)
4. Alcohol exchange – Isopropanol releases cyclohexanol and regenerates Al(OiPr)₃
5. Oppenauer (reverse) – The same steps backwards: excess acetone takes the hydride

Key points

• No free hydride: H⁻ moves directly from carbon to carbon inside the aluminium complex.
• An equilibrium: Le Chatelier sets the direction (distil off acetone for MPV; use excess acetone or cyclohexanone for Oppenauer).
• Very chemoselective: C=C, NO₂, halides and esters are untouched (crotonaldehyde → crotyl alcohol).
• Oppenauer oxidises secondary alcohols under mild, non-acidic conditions; used for steroids (cholesterol → cholest-4-en-3-one).

Drawn with 165 atoms, 11 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005, Meerwein–Ponndorf–Verley reduction; Oppenauer oxidation Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 19 Textbook