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The metalloradical activation of o-aryl aldehydes with cobalt(II) porphyrin complexes as catalysts produces cobalt(III)-carbene radical intermediates, providing a novel and powerful strategy for the synthesis of medium-sized ring structures. Herein we make use of the intrinsic radical-type reactivity of cobalt(III)-carbene radical intermediates in the [CoII(TPP)]-catalyzed (TPP = tetraphenylporphyrin) synthesis of two types of 8 membered ring compounds; novel dibenzocyclooctenes and unique monobenzo-cyclooctadienes. The method was successfully applied to a variety of substrates, producing several 8-membered ring compounds in good yields and with excellent substituent tolerance. DFT calculations and experimental results suggest that the reactions proceed via initial hydrogen atom transfer from the bis-allylic/benzallylic C-H bond to the carbene radical moiety, followed by two divergent processes for ring-closure to the two different types of 8-membered ring products. While the dibenzocyclooctenes are formed by dissociation of o quinodimethanes (o-QDMs) from the catalyst that undergo an uncatalyzed ring-closure reaction involving 8-pi-cyclisation, DFT calculations suggest that ring-closure to the monobenzocyclooctadienes involves a radical-rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring-closure reactions to chiral benzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt-porphyrin catalyst.