DFT study. As aforementioned, a disubstituted alkene is preferable to its monosubstituted counterpart in this four-membered heterocycle formation. From a mechanistic standpoint, this preference merits discussion. For this purpose, we focused on the cyclization of a cationic alkylcobalt(IV) complex. According to the literature, a disubstituted alkene forms a sterically hindered reactive carbon, which is generally unfavorable for SN2-type processes. In classic nucleophilic substitution, this is more likely to be an SN1-type process, i.e., dissociation of the Co-C bond, which generates a highly electrophilic tertiary carbocation intermediate, followed by ring formation. To verify our hypothesis regarding the ring-formation step, we performed DFT calculations by using a simple model with the minimum structural requirements for oxetane synthesis to reduce the computational cost (Figure ). Activation energies were estimated for three types of cyclizations with alkene (mono-or di-) substitution and ring size (four or five). As is intuitive, when reactions with the same substituted alkene were compared, the energetic barrier for the four-membered ring formation was higher than that for the five-membered ring formation (TS1a and TS1c). More importantly, when reactions involving the same ring size were compared, the activation energies for disubstituted alkenes were clearly lower than those for monosubstituted alkenes, which was consistent with the experimental results (TS1a and TS1b). A slight vibration of the cobalt atom in the transition states of TS1a and TS1c was observed according to the frequency analysis results. Notably, TS1a had the longest Co-C bond distance (3.53 Å) and had a transition state most similar to an SN1-type transition state. The orbital overlap, which is required for SN2-type transition states, may be structurally restricted during the four-membered ring formation. Instead, the prior extension of the Co-C bond led to the formation of a more electrophilic carbon atom (closer to a tertiary carbocation), before enabling the following C-O bond formation. For the monosubstituted alkenes, the C-O bond extension was even more difficult due to the generation of the more unstable secondary carbocation, which suggested that product formation was inefficient. The DFT calculations gave similar results for azetidine formation (TS2a-TS2c). In conclusion, we developed an unprecedented cycloisomerization for the synthesis of four-membered heterocycles via a cobalt-catalyzed MHAT/RPC mechanism. The reaction conditions were mild, and the starting materials (homomethallyl alcohol or homomethallyl amine derivatives) could be easily prepared by conventional methods. This synthetic method successfully generated a broad range of polysubstituted four-membered heterocycles. The products were derivatized to demonstrate the applicability of our method. DFT studies confirmed that a fourmembered heterocycle could be formed via an SN1 displacement-like reaction. In the future, we hope to expand the substrate scope to include monosubstituted alkenes by redesigning the cobalt catalyst and optimizing the reaction conditions. Further studies should focus on the design of enantioselective variants. Additionally, from the standpoint of medicinal chemistry, the physicochemical and metabolic properties of the oxetane and azetidine modules produced by this method should be evaluated systematically.