Abstract:

Normal alkyl sp³C―H bonds are ubiquitous in compounds such as methane, linear alkanes, and cycloalkanes that are not linked directly to heteroatoms or other functional groups. These unactivated bonds are not broken readily under mild conditions because their bond dissociation energy values are high and acidity values are low. Moreover, in the radical processes at high temperatures, reaction selectivity is not good for an alkane substrate with various alkyl sp³C―H bonds, which is commonly methyl < 1° < 2° < 3°. In the past five decades, C―H activation by transition-metal species to give C-metal bonds under mild conditions was intensively studied; all efforts were undertaken to provide new methods that can be applied in both chemical synthesis and chemical industry. However, the effective transformations of inert C―H bonds, particularly alkyl sp³C―H bonds, without the assistance of directing groups have been rarely investigated. This review focuses on the functionalization of normal alkyl sp³C―H bonds, such as methyl and primary sp³C―H bonds, via electrophilic activation or oxidative addition by using homogenous transition-metal catalysts, which are two main strategies in the study of inert C―H activation. The selectivity on C―H bond is methyl > 1° > 2° > 3° in both the reactions. Neither heterogeneous catalysis nor biocatalysis is mentioned in this review. Some remarkable progress is described on the study of reaction mechanisms and the establishment of novel reactions. For example, several selective oxidations of methane or linear alkanes have been introduced to afford new C―O, C―Cl, or even C―C bonds in the presence of Pt or Pd catalysts. The Shilov chemistry, which combines electrophilic activation of the C―H bond by the transition-metal complex, oxidation of the transition-metal intermediate, and nucleophilic substitution of organometallic species, has been emphasized in these reactions. Other transition-metal catalysts including Rh, Ir, Re, and W have been employed successfully in the carbonylation, borylation, and dehydrogenation of alkanes at moderate temperatures. The reaction pathways normally involve oxidative addition of the C―H bond with the transition-metal complex followed by insertion-elimination, reductive elimination, or β-H elimination. In the cascade reactions consisting of dehydrogenation of alkanes and addition of alkenes, new C―C or C―Si bonds can also be formed at terminal sites of linear alkanes. However, most of the above-mentioned reactions are still under investigation because of limited scope of the substrate, excess loading of the alkane, low efficiency of the catalyst, and high cost of the reaction operation. Breakthroughs in this promising field of alkane functionalization are possible when new concepts and technology are realized and applied.

Key words: Alkane, C―H activation, Functionalization, Selectivity, Transition metal