Transient directing ligands for selective metal-catalysed C–H activation

C–H activation is a ‘simple-to-complex’ transformation that nature has perfected over millions of years of evolution. Transition-metal-catalysed C–H activation has emerged as an expeditious means to expand the chemical space by introducing diverse functionalities. Notably, among the strategies to selectively cleave a particular C–H bond, the catalytic use of a small molecule as co-catalyst to generate a transient directing group, which provides a balance between step economy and chemical productivity, has gained immense attention in recent years. This allows one to convert a desired C–H bond irrespective of its geometrical or stereochemical configuration. This Review describes the various transient directing groups used in C–H activation and explains their mechanistic significance. Transient directing groups enable selective metal-catalysed C–H functionalization reactions to give diverse products. These directing groups form and dissociate in situ, such that their use is an efficient route to complex organics, examples of which are summarized in this Review.