C-H activation followed by addition to alkenes is challenging in the current C-H activation/functionalization field. We report herein an unprecedented diene-assisted transition-metal-catalyzed activation of allylic C-H bonds and their subsequent insertion into the alkene moieties of conjugated dienes. This novel C-H activation/alkene insertion reaction provides an efficient way to synthesize polysubstituted tetrahydropyrroles, tetrahydrofurans, and cyclopentanes with quaternary carbon centers. Preliminary mechanistic studies using D-labeling experiments have revealed that allylic C-H activation and alkene insertion are reversible steps, whereas the reductive elimination step, which is the final and the rate-determining step of the catalytic cycle, is irreversible.