Oppenauer oxidation

Oppenauer oxidation
Named after	Rupert Viktor Oppenauer
Reaction type	Organic redox reaction
Identifiers
Organic Chemistry Portal	oppenauer-oxidation
RSC ontology ID	RXNO:0000047

Oppenauer oxidation, named after , is a gentle method for selectively oxidizing secondary alcohols to ketones.

The reaction is the opposite of Meerwein–Ponndorf–Verley reduction. The alcohol is oxidized with aluminium isopropoxide in excess acetone. This shifts the equilibrium toward the product side.

The oxidation is highly selective for secondary alcohols and does not oxidize other sensitive functional groups such as amines and sulfides. Though primary alcohols can be oxidized under Oppenauer conditions, primary alcohols are seldom oxidized by this method due to the competing aldol condensation of aldehyde products. The Oppenauer oxidation is still used for the oxidation of acid labile substrates. The method has been largely displaced by oxidation methods based on chromates (e.g. pyridinium chlorochromate) or dimethyl sulfoxide (e.g. Swern oxidation) or Dess–Martin oxidation due to its use of relatively mild and non-toxic reagents (e.g. the reaction is run in acetone/benzene mixtures). The Oppenauer oxidation is commonly used in various industrial processes such as the synthesis of steroids, hormones, alkaloids, terpenes, etc.

In the first step of the mechanism, the alcohol (1) coordinates to the aluminium to form a complex (3), which then, in the second step, gets deprotonated by an alkoxide ion (4) to generate an alkoxide intermediate (5). In the third step, both the oxidant acetone (7) and the substrate alcohol are bound to the aluminium. The acetone is coordinated to the aluminium which activates it for the hydride transfer from the alkoxide. The aluminium-catalyzed hydride shift from the α-carbon of the alcohol to the carbonyl carbon of acetone proceeds over a six-membered transition state (8). The desired ketone (9) is formed after the hydride transfer.

...
Wikipedia