The four most common methods of transition metal catalyzed methane activation are the Shilov system, sigma bond metathesis, oxidative addition, and 1,2 addition reactions.
The Shilov system involves platinum based complexes to produce metal alkyls. It was first discovered when a hydrogen-deuterium exchanged was observed in a deuterated solution with the platinum tetrachloride anion. Shilov et al. then was able to catalytically convert methane into methanol or methyl chloride when a Pt(IV) salt was used as a stoichiometric oxidant. The process is simplified down into three main steps: (1) C-H activation, (2) a redox reaction to form an octahedral intermediate, followed by (3) the formation of the carbon-oxygen bond to form methanol ().
Sigma bond metathesis involves the formation of new C-H and metal-carbon bonds, where the metals are typically in the d configuration. Starting with a metal alkyl, a C-H bond coordinates with the metal complex via sigma bonding. A four-member transition state is created, where a new metal-carbon bond is formed, and the former C-H linkage is broken ().
In oxidative addition, the metal center's oxidation state increases by 2 units during the process. First, the metal center coordinates with a sigma C-H bond to form an intermediate called a sigma-methane complex. The C-H linkage is then broken, as the metal becomes covalently bonded each to the carbon and the hydrogen ().
Similar to sigma bond metathesis is the 1,2 addition reaction, where a four-membered transition state is also formed. However, a polarized double or triple metal-ligand bond is required in order to favor the formation of the desired product ().