Oxidative Addition and C — H Bond Activation

These reactions involve an increase in both the oxidation state and the coordination number of the metal. Oxidative addition (OA) reactions are essential steps in many catalytic processes. The reverse reaction, designated reductive elimination (RE), is also very important. These reactions are described schematically by the following  

Oxidative addition and reductive elimination reactions play key roles in C—H activation reactions, where a strong C—bond is cleaved by a transition-metal complex. These are important reactions because they permit unfunctionalized hydrocarbons to be transformed into complex molecules. Bergman reported the following classic C—H reductive elimination/oxidative addition sequence.  

The first step involves reductive elimination of cyclohexane from a six-coordinate Ir(in) complex (ML3X3 via the CBC method in Section 13.7) to afford a four-coordinate Ir(I) intermediate (ML3X). Like most reductive eliminations, this first step involves a decrease by two in both the oxidation state and coordination number of the metal. The second step results in oxidation from Ir(I) to Ir(III) as benzene oxidatively adds to the iridium center, resulting in activation of a C(sp )—H bond. Like most oxidative additions, this second step involves an increase by two in both the oxidation state and the coordination number of the metal. 

FIGURE 14.5 Cyclometallation Reactions. The first reaction is an orthometallation where the iridium coordinates to the carbon ortho to the phosphorus atom. 

Another class of reaction that can be classified as oxidative addition, even though it does not strictly meet the requirements of the scheme at the beginning of this section, is nucleophilic displacement. Negatively charged organometallic complexes often behave as nucleophiles in displacement reactions. For example, [(tj -C5H5)Mo(CO)3] can displace iodide from methyl iodide  

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