text: Carbene insertion use a different strategy for the functionalization of methane. A strategy using metallocarbenes has been shown with several linear and branched alkanes with rhodium, silver, copper, and gold-based catalysts. With a carbene ligand attached to a metal center, it can be transferred from the coordination sphere and inserted into an activated C-H bond. In this case, there is no interaction between the metal center and the alkane in question, which separates this method from the other methods mentioned above. 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. In order for this reaction to occur, the diazocompound must be very electrophilic, since the C-H bond is such a poor nucleophile as well as being an unactivated alkane. 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 (). 
  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. The reactions also take place in room temperature under mild conditions. However, when applying this method to methane specifically, the gaseous nature of methane requires an appropriate solvent. Reactions with other alkanes usually have the alkane in question be the solvent itself; however, any C-H bond with a lower BDE or higher polarity than methane will react first and prevent methane functionalization. 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. In these conditions, scCO behaves as a liquid, and since fluorinated compounds can dissolve easily in scCO, highly fluorinated silver-based catalysts were developed and tested with methane and ethyl diazoacetate. However, under the reaction conditions, only 19% yield of ethyl propionate was able to be achieved. The reaction depends on a delicate balance between methane pressure and catalyst concentration, and consequently more work is being done to further improve yields.