1.7.1 Introduction
Oxygen is the most abundant element of Earth’s crust. It is tasteless, odorless and very pale blue diatomic gas. Oxygen is important for oxidation reactions though molecular oxygen itself is not a good oxidizing agent. Advantage of using molecular oxygen is that it is abundant in nature and eco-friendly giving water as by product. A large number of catalysts have been used to activate oxygen for the oxidation of different functional groups. This lecture will discuss some of the important catalysts and methods.
1.7.2 Palladium Catalyzed Reactions
1.7.2.1 Wacker Oxidation
Wacker oxidation of alkenes to aldehydes or ketones is one of the most important industrial processes. In Wacker process, Pd catalyst is used in combination with O2 and copper salt to transform alkenes to aldehydes or ketones (Scheme 1). In these reactions, copper(II) oxidizes the reduced Pd(0) to palladium (II) to regenerate the catalyst, while the
reduced copper(I)
is reoxidized to copper(II) by oxygen. The oxygen incorporated in the alkene to give the carbonyl compound is obtained from water.
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1.7.2.2 Oxidation of Alcohols
Palladium based catalysts have been extensively studied for the aerobic oxidation of alcohols to carbonyl compounds. For some examples, Pd(OAc)2 with molecular oxygen in the presence of NaHCO3 has been shown as effective protocol for the oxidation of allylic and benzylic alcohols to afford the corresponding carbonyl compounds (Scheme 2).
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Scheme 2 |
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Scheme 3 |