Carbon–hydrogen bond activation 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.
Carbon–hydrogen bond activation substitution at the carbon also greatly affects selectivity; the secondary methylene position is preferred over the terminal methylene position because the former has a secondary carbon while the latter has a primary one. In the oxidation of cyclohexane compound, the tertiary site on the ring is preferential over the tertiary site of the isopropyl substituent. The two methyl groups hinder the oxidation on the isopropyl group, making the less hindered cyclohexyl site more favorable. In contrast to innate selectivity, guided selectivity results from external reagents or directing groups affecting the nature of specific C-H bonds. The figure above demonstrates the