0—Bond metathesis methane exchange reaction

Silica-supported Ta hydride (=SiO)2Ta-H (93a) presents unusual properties in the activation of alkanes. It catalyzes the metathesis reaction of alkanes to give higher and lower molecular weight alkanes, and the hydrogenolysis of alkanes such as ethane to methane. This hydride also activates the C H bonds of cycloalkanes to form the corresponding surface metal-cycloaUcyl complexes, and catalyses the H/D exchange reaction between CH4 and CD4, prodncing the statistical distribution of methane isotopomers. ... 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

In this reaction, a C—H bond in methane is made, and a C—H bond of benzene is cleaved, but the oxidation state of the scandium center remains +3. This class of reaction, which is not hmited to early transition metals, is called sigma-bond metathesis. In this mechanism, the metal is first postulated to coordinate the bond to be activated in an rf- fashion, followed by formation of a four-centered transition state that leads to an exchange of ligands at the metal (Figure 14.8). 

One of the most striking early discoveries (1982) in alkane activation chemistry was Watson s exchange reaction between a coordinated methyl group and ftee methane, via a bond metathesis, discovered by isotope labeling of the methane carbon.. ... 

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