An efficient synthesis of substituted 3,4-dihydroisoquinolinones through [4+2]-annulation ofN-chlorobenzamides/acrylamides having a monodentate directing group with alkylidenecyclopropanes in the presence of a less expensive, highly abundant and air stable Co(iii) catalystviaa C-H activation is demonstrated. In this reaction, the N-Cl bond ofN-chlorobenzamide serves as an internal oxidant and thus an external metal oxidant is avoided. The 3,4-dihydroisoquinolinone derivatives are converted successfully into the highly useful imidoyl chloride derivatives. The deuterium labeling and kinetic isolabelling studies reveal that the C-H activation is a rate-determining step in this cyclization reaction. © The Royal Society of Chemistry 2021.