Methane functionalization
Its ideal use would be as a raw starting material for methanol or acetic acid synthesis, with plants built at the source to eliminate the issue of transportation.
[2] 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.
However, a polarized double or triple metal-ligand bond is required in order to favor the formation of the desired product (Figure 6).
In general, alkanes of various lengths have typically been functionalized by a number of more commonly known reactions: electrophilic activation (Shilov system, see above), dehydrogenation, borylation, hydrogen-deuterium exchange, and carbene/nitrene/oxo insertion.
The resulting mercury complex Hg2(OSO3)2 is re-oxidized by sulfuric acid to regenerate the catalyst and restart the catalytic cycle (Figure 7).
This method of functionalizing methane preceded the 1998 discovery by the same group of the so-called Catalytica system, the most active cycle to date in terms of turnover rate, yields, and selectivity.
T. Don Tilley and coworkers were able to use the process of sigma-bond metathesis to design catalytic systems that work by the formation of carbon-carbon bonds.
The scandium complex then transfers the methyl group to the silane by sigma-bond metathesis to form the product and the Cp*2ScH intermediate.
The favorable formation of hydrogen gas combined with methane will regenerate the methyl complex from the hydride derivative (Figure 10).
The general mechanism for this cycle begins with the reaction of an electron-poor metal center with a diazocompound to form a metallo-carbene intermediate.
The reaction then proceeds in a concerted manner, where the C-H bond of the incoming molecule coordinates with the carbene carbon of the metallocarbene complex.
The hydrocarbon then dissociates from the metal center to regenerate the catalyst and free the newly formed carbon-carbon bond (Figure 11).
This route is very successful for higher order alkanes due to the fact that there is no formation of strong metal-carbon or metal-hydrogen bonds that could prevent any intermediates from reacting further.
Therefore, Pérez, Asensio, Etienne, et al. developed a solution to use supercritical carbon dioxide as the solvent, which is formed under the critical pressure of 73 bar and a temperature of 31 °C.
| The mechanism for the Shilov system | ( Figure 3 ) |
| Methane activation via sigma-bond metathesis | ( Figure 4 ) |
| Simple mechanism of using oxidative addition for methane activation | ( Figure 5 ) |
| A simplified mechanism of 1,2 addition of methane to unsaturated metal-ligand bonds. | ( Figure 6 ) |
| Electrophilic functionalization of methane with Hg(II). | ( Figure 7 ) |
| The Catalytica system with a platinum catalyst. | ( Figure 8 ) |
| Synthesis of acetic acid from methane using palladium salts. | ( Figure 9 ) |
| Silation of methane using a scandium catalyst. | ( Figure 10 ) |
| Metal-catalyzed carbene insertion to functionalize alkanes from diazocompounds. | ( Figure 11 ) |