Transition-metal complex-catalyzed alkane functionalization

Abstract

The selective conversion of simple alkane feedstocks to high-value organic compounds constitutes one of major challenges in organic synthesis. There are three general catalytic methods for alkane functionalizations: reactions via radical intermediates and through carbene/nitrene insertion pathways typically occur preferentially at the secondary and tertiary C−H bonds, whereas transition metal-mediated C−H bond activation holds promise for the functionalization of primary C−H bonds. From a practical point of view, the installation of a functional group at the terminal position of alkanes is of particular attractive for large-scale synthesis of specialty or commodity chemicals. This review focuses on the area of molecular transition-metal-catalyzed alkane transformations through catalytic alkane dehydrogenation. In the first section, the development of iridium pincer and related complexes for alkane dehydrogenation is summarized. Among various catalysts, the PCP type Ir pincer complexes have proven to be most active toward transfer dehydrogenation of alkanes. α-Olefins are the kinetic products at the early stage of the dehydrogenation process, but they can be rapidly isomerized to internal alkenes. Recently, other non-phosphorus-ligated Ir and non-Ir metal dehydrogenation catalysts have also been developed, but they are generally less efficient than the PCP Ir catalysts. The second section first describes two dehydrogenation-based alkane transformations without the incorporation of heteroatom-containing functionality. The dehydroaromatization reaction involves multiple steps of dehydrogenation of linear alkanes to form conjugate trienes, which undergo electrocyclization and another step of dehydrogenation to generate aromatic products. The alkane-alkene coupling reaction employs an Ir pincer catalyst for dehydrogenation and a tantalum catalyst for alkene/alkene coupling, thus providing a method for upgrading light alkanes to higher alkanes that may be suitable to transportation fuel. Following that, the selective installation of functional groups at the terminal positions of alkanes through a dehydrogenation-alkene isomerization-hydrofunctionalization strategy using a dual catalyst system is described. Several combinations between a PSCOP Ir pincer dehydrogenation catalyst and molecular Fe or Rh catalysts for tandem isomerization and hydrofunctionalization of terminal alkenes have been developed for terminal selective alkane silylation, borylation, carbonylation, and aminomethylation. The third section covers alkane metathesis and its application to polyethylene (PE) degradation. The alkane metathesis reaction described here consists of two catalysts, one Ir dehydrogenation catalyst and one olefin metathesis catalyst. The metathesis process can find potential application in upgrading low carbon number n -alkanes to higher-molecular-weight fuel alkanes. A cross-alkane metathesis strategy has been developed for PE degradation. Using excess of low-value light alkanes as the reagent/solvent, PE with molecular weight up to 1.7 million undergoes multiple times of cross-alkane metathesis at 175℃ to produce liquid fuels or high-quality waxes. The catalysts can tolerate the commercial HDPE, LDPE and LLDPE, and enable the efficient degradation of postconsumer PE plastic wastes. Finally, the review discusses the limitations of the known catalytic approaches and future opportunities in this field. Catalyst development will be the central theme of research, and insights into the factors controlling the activity and selectivity gained in mechanistic studies will guide the design of more efficient dehydrogenation catalysts. The cooperative catalysis involving enantioselective catalysts will provide a protocol for synthesis of valuable fine chemicals from simple saturated hydrocarbons. Lastly, we should not overlook the potential of cooperation between photo- or electro-catalysis and transition-metal catalysis, which may allow the exploitation of alkane feedstocks more cleanly and efficiently.