C-H bond activation is a series of mechanistic processes by which highly stable carbon-hydrogen bonds in organic compounds are cleaved. The purpose is to enable functionalization of these molecules, leading to the synthesis of more complex intermediate or product compounds often containing C-0, C-C and C-N bonds. The ability to cleave the C-H bond enables inexpensive feedstock molecules to be transformed into commercially valuable molecules. Directed C-H activation enables selectivity and specificity in the synthesis of more complex molecules of importance in pharmaceutical and fine chemical applications.
There are a number of mechanisms by which C-H bonds are activated including oxidative addition, σ-Bond metathesis, electrophilic substitution, etc. By definition, C-H activation occurs via catalytic mechanisms. As an example, C-H activation occurs when transition metals such as Pt, Rh, Ir, etc are used in catalytic oxidative addition reactions. A proton attached to a carbon atom on the substrate hydrocarbon molecule coordinates with the transition metal to form an intermediate organometallic species. This intermediate can the react with other species to form the functionalized carbon bond. C-H cleavage and functionalization can proceed stoichiometrically, for example using Friedel-Craft chemistry via electrophilic aromatic substitution. C-H activation/borylation of substrate molecules via transition metals is particularly useful to create C-B bonds. Organoboron compounds are used in cross-coupling reactions, and therefore are important in the formation of C-C bonds.
Biocatalytic, electrocatalytic and photocatalytic reactions, as well as hybrid techniques including photobiocatalysis and photoelectrocatalysis, are increasingly used to activate C-H bonds. This is associated with the interest in C-H activation under milder conditions and using Earth-abundant metal catalysts for more sustainable approaches to chemical synthesis. Furthermore, C-H activation under milder conditions enables the synthesis of molecular functionality that would not be tolerant of the harsher reaction conditions of thermal chemocatalysis.