Meerwein–Ponndorf–Verley reduction
| Meerwein–Ponndorf–Verley reduction | |
|---|---|
| Named after |
Hans Meerwein Wolfgang Ponndorf Albert Verley |
| Reaction type | Organic redox reaction |
| Identifiers | |
| Organic Chemistry Portal | meerwein-ponndorf-verley-reduction |
| RSC ontology ID | RXNO:0000089 |
The Meerwein–Ponndorf–Verley (MPV) reduction in organic chemistry is the reduction of ketones and aldehydes to their corresponding alcohols utilizing aluminium alkoxide catalysis in the presence of a sacrificial alcohol. The beauty of the MPV reduction lies in its high chemoselectivity, and its use of a cheap environmentally friendly metal catalyst.
The MPV reduction was discovered by Meerwein and Schmidt, and separately by Verley in 1925. They found that a mixture of aluminium ethoxide and ethanol could reduce aldehydes to their alcohols. Ponndorf applied the reaction to ketones and upgraded the catalyst to aluminium isopropoxide in isopropanol.
The MPV reduction is believed to go through a catalytic cycle involving a six-member ring transition state as shown in Figure 2. Starting with the aluminium alkoxide 1, a carbonyl oxygen is coordinated to achieve the tetra coordinated aluminium intermediate 2. Between intermediates 2 and 3 the hydride is transferred to the carbonyl from the alkoxy ligand via a pericyclic mechanism. At this point the new carbonyl dissociates and gives the tricoordinated aluminium species 4. Finally, an alcohol from solution displaces the newly reduced carbonyl to regenerate the catalyst 1.
Each step in the cycle is reversible and the reaction is driven by the thermodynamic properties of the intermediates and the products. This means that given time the more thermodynamically stable product will be favored.
Several other mechanisms have been proposed for this reaction, including a radical mechanism as well as a mechanism involving an aluminium hydride species. The direct hydride transfer is the commonly accepted mechanism recently supported by experimental and theoretical data.
One of the great draws of the Meerwein–Ponndorf–Verley reduction is its chemoselectivity. Aldehydes are reduced before ketones allowing for a measure of control over the reaction. If it is necessary to reduce one carbonyl in the presence of another, the common carbonyl protecting groups may be employed. Groups, such as alkenes and alkynes, that normally pose a problem for reduction by other means have no reactivity under these conditions.
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