Oppenauer Oxidation

Oppenauer oxidation has been one of the most essential questions that come in Chemistry. This topic covers all the bases and the concepts that revolve around the oxidation of a given material on the surface of the earth and how the following materials are affected by the process of oppenauer oxidation. Further, this topic also covers the disadvantage of the process besides explaining the reason behind the same. Today, we take a look at the different aspects of the oppenauer oxidation mechanism and understand the reaction in detail. Following which, we will also understand the various favourable aspects for oxidation.

What is Oppenauer Oxidation?

Oppenauer Oxidation is a chemical conversion process in which, the secondary alcohols that are present in a given composition turn to ketones under a controlled atmosphere with the help of selective oxidation. One of the most useful reactions in modern chemistry, the reaction is named after Rupert Viktor Oppenauer. Further, the oxidation under the oppenauer reaction takes place with sufficient [Al(i-Pro)3], provided there is an excess of acetone.

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Oppenauer oxidation is the process where an aluminium alkoxide has catalysed the oxidation of the secondary alcohol that is present over the corresponding ketone. The oppenauer oxidation is the reverse process of Meerwein Ponndorf Verley reduction. The oppenauer oxidation reaction is one of the most reliable methods that help oxidise allylic alcohols to α, β- unsaturated ketones.

Oppenauer Oxidation Reaction Mechanism

Now that we finally understand the process of oppenauer oxidation reaction, let’s have a look at its reaction mechanism to understand better:

• Step 1: In the initial stage of the oxidation reaction mechanism, the alcohol present in the reaction coordinates with the aluminium isopropoxide that is present over the solution, to form a complex.

• Step 2: Further, the resultant complex then reacts with the ketone, to result in the creation of a six-membered transition complex.

• Step 3: In the following stage of the preparation of the transition complex, the alpha-carbon that is present in the alcohol, converts to the carbonyl carbon, procured from the aluminium-catalyzed hydride shift.

• Step 4: Now that the carbonyl carbon is formed, the acetone proceeds over a six-membered transition state.

• Step 5: The result is the formation of ketone, created post the process of hydride transfer.

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The oppenauer oxidation reaction is used in the process to oxidise alcohols to carbonyl compounds.

Let’s understand the same with an example. Simple ketone acetone or cyclohexanones is often used as a hydride acceptor that is usually used in the presence of aluminium alkoxide, traditionally given as isopropoxide or t-butoxide.

The oxidation reaction comes as an exact result of the reverse of Meerwein-Ponndorf-Verley reduction. It involves the process of deprotonation of the alcohol by equilibration over the alkoxide, concluded by a hydride transfer. This process of equilibrium is generally achieved by displacing to the right, using a large excess of the hydride acceptor.

Advantages of oppenauer oxidation

The basic advantage of the oppenauer oxidation is that it uses non-toxic reagents and is relatively inexpensive. Since the substrates are generally heated in acetone/benzene mixtures, the reaction conditions are mild and gentle. Another advantage of the oppenauer oxidation that is making it an unique oxidation method as compared to the other oxidation reactions like  pyridinium chlorochromate (PCC) and Dess–Martin periodinane is that the secondary alcohols are much likely to get oxidized much quicker than the primary alcohols. That is the reason that chemoselectivity is achieved. Moreover, there is no further oxidation reaction where the aldehyde is getting converted into the carboxylic acid as it happens in many of the oxidation processes like that of John's oxidation.

Disadvantages of Oppenauer Oxidation and its Mechanism

The traditional Oppenauer Oxidation is a highly chemoselective oxidation process. However, the reactions and its mechanism do come with their own set of disadvantages.

To begin with, the reaction method makes use of high temperatures that is generally achieved using large quantities of ketone hydride acceptors. The result is the production of aldol condensation products that are formed with hydride acceptors. While there has been extensive research that is focused around the development of more efficient Oppenauer type Oxidations, we can perform some of these oxidation processes under milder conditions.

Let’s understand the same with an example:

For example, aluminium compounds involved in the oxidation are primary; consequently, prototropic shifts that can easily take place within the product. Thus, while the process of oxidation of cholesterol is being processed, the C=C migrates to give α, β – unsaturated ketones.