Oppenauer-Oxidation

The Oppenauer oxidation is a name reaction in organic chemistry. It was named after its discoverer Rupert Viktor Oppenauer ( 1910-1969 ). The Oppenauer oxidation is a method for the synthesis of ketones from the corresponding secondary alcohols.

It is the inversion of the Meerwein - Ponndorf -Verley reduction. The alcohol is oxidized in an excess of acetone with aluminum alcoholate. In this way the equilibrium is on the right side.

Since the advent of on chromates (eg PCC) or dimethyl sulfoxide -based oxidation methods (eg Swern oxidation) is the Oppenauer oxidation of reduced preparative importance. However, due to the basic reaction conditions, it is suitable for the oxidation of acid-sensitive substrates. A variant that uses trichloroacetaldehyde and alumina, it allows selective secondary to oxidize primary alcohols in addition to unchanged.

Mechanism

In this mechanism the radical R again represents a tert -butyl group.

The mechanism described as using the Oppenauer oxidation, a secondary alcohol is 1 is oxidized to a ketone 4. First, the alcohol reacts with the aluminum alkoxide catalyst to the second molecule added in excess acetone then acts as Hydridakzeptor and thus as an oxidizing agent for the secondary alcohol. The reaction of the aluminum alcoholate 2 with acetone to give the desired ketone 4, and 3 molecule passes over a six-membered transition state:

Finally molecule 3 reacts with the split at the beginning of alcohol to iso -propanol and the aluminum alkoxide, which is now again available as a catalyst.