Privileged strategies for direct transformations of inert aliphatic carbon-hydrogen bonds
Functional group transformations are central to organic synthesis. Traditionally, the functionalities used in such transformations are highly active organic groups such as halogens, ester groups and hydroxyl groups. Carbon–hydrogen bonds are ubiquitous structural motifs in organic compounds, but they are not considered to be functional groups because (1) in general, the bond dissociation energy of a C–H bond is high, and therefore, such bonds are thermodynamically hard to break; and (2) the selective activation of one C–H bond among many similar and different C–H bonds in one organic molecule is difficult. However, direct C–H transformations, which could be used to perform synthetic chemistry in a greener and more atom-economical way, are highly appealing. The importance and challenges of this field make it one of the 'Holy Grails' of chemistry.