甲烷均相转化研究进展

Recent advancement in homogeneous functionalizations of methane

在绿色、经济的反应条件下实现甲烷的直接转化一直是化工能源领域面临的重大挑战之一.在过去的10年中,有机合成化学家们相继发展了多种甲烷均相转化方法,为利用丰富的甲烷作为化工原料来合成高附加值的化工产品提供了新的转化途径.本文从反应作用机制方面总结了近年来甲烷均相转化领域的研究进展,包括过渡金属和主族金属参与的,以及光促进的甲烷均相转化等.在这些研究进展中,过渡金属催化的甲烷转化方法占据主要,涌现了对甲烷碳氢键进行亲电活化、氧化加成和卡宾插入等多种不同活化机制.引人注意的是,光催化模式利用清洁的光能为反应提供能量,为甲烷的均相转化提供了一种更加绿色可持续的化学转化方案.文末对发展更多高效的光促甲烷转化反应及其应用前景进行了展望.

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 20 th 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.