Recent advancement in homogeneous functionalizations of methane

Abstract

Methane is one of the most abundant and inexpensive carbon-based feedstocks on earth. It is the major component of natural gas, which is considered as a greener fuel and promising alternative to petroleum for energy and chemicals production. However, the prevalent methane utilization was largely limited by its chemical properties and existing processes for methane upgrading. On one side, the low boiling point of methane (109 K at 1 atm), rendered the liquefaction of methane for long-distance transportation economically impractical. On the other side, the direct conversion of methane to value-added liquid chemicals is rewarding, but challenging processes due to its inert chemical properties, namely, high bond dissociation energy (~105 kcal/mol), ionization potential (12.6 eV), relatively low solubility in solvents. Moreover, the functionalized products (e.g., methanol, acetic acid) and the solvents (e.g., hexane, acetonitrile) show the higher reactivities than methane, resulting the selectivity issues, such as the overfunctionalizations and solvent activations. Accordingly, the development of efficient selective processes to directly convert methane into value-added liquid products under economical and green reaction conditions has been of significant importance and remains one of the grand challenges in synthetic chemistry. In the 20th century, considerable efforts have been devoted to the selective oxidation of methane to methanol and methanol derivatives. In the initial stage, the processes were dominated by energy intensive systems with harsh reaction conditions, such as high temperature, high pressure and high catalyst loading. Therefore, new catalytic modes have been highly demanding for the sake of more sustainable and affordable methane conversion process. During the last decade, a variety of direct C-H functionalizations of methane has been achieved in the realm of synthetic organic chemistry. Nevertheless, in comparison to the advancement in heterogeneous functionalization of methane, the homogenous approach remains relatively underdeveloped, thusly intriguing opportunities lies therein. The recent advancement of transition metal homogenous catalysis has greatly facilitated the direct activation of those symmetric C-H bonds of methane. Amongst the reported transformations, strategies including electrophilic activation, carbene insertion and oxidative addition have been frequently employed in homogeneous functionalizations of methane. In some cases, main-group metals could also function in a similar pathway. Notably, photocatalysis that can directly utilize light energy for chemical activation, has also been applied in the homogenous functionalization of methane. As reactions largely proceed by free radicals, the intermolecular hydrogen atom transfer process should be the crucial step in this catalytic pattern. Pleasingly, the mild reaction conditions and high selectivity of these photo-induced functionalizations have paved a way for developing more cost-effective and environmentally benign functionalizations of methane. Herein, we aim to highlight the recent advance in homogeneous functionalizations of methane, in particular, underline the novel transition metal catalytic and photocatalytic manifolds that we would anticipate spur more brilliant studies towards the versatile functionalizations of methane. The context was thus arranged in three parts according to the mechanism of homogenous functionalization endeavors, including the transition metal catalyzed methane activation, maingroup metals mediated electrophilic activation of methane and photocatalytic functionalizations of methane. We believe this classification of existing literatures would shed some light on the future progress in homogenous functionalizations of methane. © 2019, Science Press. All right reserved.