One-electron oxidation of borepin derivatives that consists of a boron-containing seven-membered ring has been reported to cause deborylation/C-C coupling, yielding aromatic compounds. The reaction can be achieved not only by transition metal compounds but also by oxidants without transition metal such as O2 and other organic compounds. Despite numerous experimental attempts, the mechanism of this peculiar reaction as well as the fate of the BCl part eliminated from borepin remain unclear to date. Based on theoretical approaches using the artificial force induced reaction method, here we address the mechanism of the unusual boron-mediated C-C coupling. For this purpose, two borepin derivatives (1 and 35), bearing ethyl and phenyl groups, respectively, were used as reactants, and FeCl3/MeNO2 and O2 were chosen as oxidants. The calculations revealed reaction pathways that provided an overall picture of the mechanism of the target reaction, which features four key steps, namely, (i) quaternization of the boron atom by the coordination of oxidant, (ii) intersystem crossing, (iii) skeletal rearrangement to form a six-membered ring, and (iv) elimination of a boron moiety. The intrinsic nature of boron, i.e., a strong tendency to accept a coordination ligand even under oxidative conditions, is responsible for the oxidative deborylation/C-C coupling of borepin.