Traceless directing groups: a novel strategy in regiodivergent C–H functionalization
The use of functional groups as internal ligands for assisting C–H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C–C and C–X bonds from inert C–H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C–H functionalization reactions in the last decade.