The Evolving Enantioselective C─H Functionalization of Tertiary Allylic Systems

The direct functionalization of tertiary and other inert allylic C─H bonds represents one of the most attractive strategies in the construction of complicated molecules with quaternary carbon centers. Unlike approaches that rely on pre-functionalized allylic substrates, such as halides, acetates, and carbonates, allylic C─H activation provides an atom- and step-economical route to forge diverse and valuable C─C and C─X bonds. Although the functionalization of secondary allylic C─H bonds has been extensively studied and well established in recent years, the corresponding transformation of tertiary allylic C─H bonds remains considerably underdeveloped. This concept summarizes the progress and mechanistic pathways-including transition-metal insertion (via two-electron transfer) and radical-involving single-electron transfer processes-enabling the functionalization of various tertiary allylic C─H bonds. Future directions in this long-standing and challenging field are also discussed.