Química Orgânica 2 · Ferramentas de oxidação e redução
Meerwein–Ponndorf–Verley reduction of cyclohexanone
Condições: Al(OiPr)3 (0.3 equiv), iPrOH (solvent), reflux with slow distillation of acetone
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Cyclohexanone coordinates aluminium
A lone pair of the ketone oxygen bonds to the Lewis-acidic aluminium of aluminium isopropoxide. This activates the carbonyl carbon and holds it next to an isopropoxide ligand.
As setas curvas
- A ketone oxygen lone pair forms the O–Al bond.
Cyclic hydride transfer
In one concerted step through a six-membered ring of atoms (Al, two oxygens, two carbons and the moving hydrogen), the isopropoxide C–H pair carries the hydrogen to the ketone carbon, the C=O π pair returns to the cyclohexyl oxygen, and the Al–O pair of the isopropoxide becomes the C=O π bond of acetone.
As setas curvas
- The C–H pair of the isopropoxide carries the hydrogen to the ketone carbon.
- The C=O π pair moves onto the oxygen bound to aluminium.
- The Al–O pair of the isopropoxide becomes the C=O π bond of acetone.
Where did the hydride come from?
The hydrogen delivered to cyclohexanone is the C–H hydrogen of an isopropoxide ligand, so that ligand leaves as acetone.
Isopropanol binds aluminium
Solvent isopropanol coordinates the aluminium through an oxygen lone pair, preparing to exchange with the cyclohexyloxide ligand.
As setas curvas
- An isopropanol oxygen lone pair forms a bond to aluminium.
Proton transfer to the cyclohexyl oxygen
The proton on the coordinated isopropanol moves to the neighbouring cyclohexyloxide oxygen on the same aluminium.
As setas curvas
- A cyclohexyl oxygen lone pair takes the proton.
- The O–H pair stays on the isopropoxide oxygen.
Cyclohexanol leaves aluminium
The O–Al bond to the now-protonated cyclohexyl oxygen breaks with its pair returning to oxygen, releasing cyclohexanol and regenerating aluminium isopropoxide for the next turnover.
As setas curvas
- The O–Al pair returns to the cyclohexyl oxygen.
Cyclohexanol and acetone
Cyclohexanone has been reduced to cyclohexanol by hydrogen taken from isopropanol, which ends up as acetone; aluminium isopropoxide is regenerated.
O que você vai praticar
Draw the Meerwein–Ponndorf–Verley reduction of cyclohexanone: coordination to aluminium isopropoxide, the cyclic hydride transfer that releases acetone, and the exchange with isopropanol that frees cyclohexanol.
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