Carbon–hydrogen bond activation

Carbon–hydrogen bond functionalization (C–H functionalization) is a type of reaction in which a carbon–hydrogen bond is cleaved and replaced with a carbon-X bond (where X is usually carbon, oxygen, or nitrogen). The term usually implies that a transition metal is involved in the C-H cleavage process. Reactions classified by the term typically involve the hydrocarbon first to react with a metal catalyst to create an organometallic complex in which the hydrocarbon is coordinated to the inner-sphere of a metal, either via an intermediate "alkane or arene complex" or as a transition state leading to a "M−C" intermediate. The intermediate of this first step (known as C-H activation and sometimes used interchangeably with C-H functionalization) can then undergo subsequent reactions to produce the functionalized product. Important to this definition is the requirement that during the C–H cleavage event, the hydrocarbyl species remains associated in the inner-sphere and under the influence of "M".

Mechanisms for C-H activations fall under three general categories: (i) oxidative addition, in which a metal center inserts into a carbon-hydrogen bond, which cleaves the bond and oxidizes the metal, producing an intermediate that can undergo reductive elimination to yield the organometallic reactive intermediate (ii) electrophilic activation, in which a generally electron rich substrate undergoes an SeAr-type mechanism (iii) Sigma-bond metathesis, which proceeds through a "four-centered" transition state in which bonds break and form in a single step: the target hydrocarbon bond breaks as the carbon bonds to the metal and the hydrogen bonds to one of the metal’s ligands, which causes bond breakage between the ligand and the metal.

C–H bonds, which are traditionally considered unreactive, can be cleaved by coordination. Much research has been devoted to the design and synthesis of new reagents and catalysts that can effect C–H activation. C-H activation chemistry has the potential to transform the chemical world through the development of novel synthetic methods. C-H activation could enable the conversion of cheap and abundant alkanes into valuable functionalized organic compounds and the efficient structural editing of already complex molecules (i.e. natural product synthesis). Selective activation of a specific C-H bond poses a great challenge. In addition to a high bond dissociation energy, C-H bonds have very low polarity because these two elements have similar electronegativities.

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