The recent trends in drug discovery encouraged us to develop a simple and general synthetic method for four-membered heterocycles by harnessing the metal hydride hydrogen atom transfer (MHAT) /radical polar crossover (RPC) reaction (Figure ). This catalytic process is realized by the combination of a cobalt catalyst, an oxidant, and a silyl hydride that generates key cobalt hydride and a highly reactive carbocationic intermediate (cationic alkylcobalt(IV) complexes) from alkenes via carbon radicals under near-neutral conditions. Following our report on cobalt-catalyzed MHAT/RPC reactions, this catalytic system has been extensively utilized in cycloisomerizations, deprotective cyclizations, intermolecular additions, rearrangements, asymmetric variants, and polymerizations. Notably, our group, and Zhu have reported various cycloisomerizations to furnish numerous cyclic ethers and cyclic amines except for the synthetically challenging four-membered heterocycles. Four-membered heterocycles has not been formed by cycloisomerization, even in other catalytic reactions. Two points must be considered when attempting oxetane or azetidine formation via MHAT/RPC-mediated cycloisomerization. First, the general rate of formation of saturated cyclic compounds by intramolecular displacement reactions follows the order 5 > 6 > 3 > 7 > 4 > 8-10 rings. 24 The ring-closure step in the MHAT/RPC process is an intramolecular displacement reaction (Figure ); therefore, there should be no significant deviation from the general order of the reaction rate. One concern with oxetane is that ring formation is more challenging than epoxide formation. 19a,20a However, less favored eight-and nine-membered ring formation has been achieved, albeit with limited success. Therefore, there was an opportunity to realize four-membered heterocycle formation via the MHAT/RPC mechanism. Second, there was a concern that the high ring strain of the product may stimulate ring opening during product formation. However, this seemed unlikely considering the remarkable functional group tolerance observed under MHAT/RPC conditions. For example, Pronin synthesized epoxides with high ring strain (114 kcal/mol), similar to those of oxetane (107 kcal/mol), under MHAT/RPC conditions. Herein, we describe a method for the construction of four-membered heterocycles via the MHAT/RPC process, which is challenging both enthalpically and entropically. The substrate scope, including the spiro structure, which is important in drug discovery research, was investigated. We also examined the reaction mechanism on the basis of the results of deuterium experiments and DFT calculations, focusing on the four-membered ring formation. To the best of our knowledge, this is the first example of cycloisomerization affording four-membered heterocycles despite the numerous synthetic efforts involving the intramolecular hydrofunctionalization of unactivated alkenes. RESULTS AND DISCUSSION Cycloisomerization affording oxetane. We first investigated whether energetically challenging four-membered heterocycles could be formed via MHAT/RPC. We began by searching for a suitable homoallylic alcohol for oxetane formation. Fortunately, the screening of various homoallylic alcohols revealed that 1a could be cycloisomerized to provide the desired oxetane 2a in 42% yield by using a cobalt Schiff base catalyst C1, N-fluoro-2,4,6-collidinium trifluoromethanesulfonate, and 1,1,3,3-tetramethyldisiloxane (TMDSO); [Table . Screening of cobalt catalysts C1-C4 revealed that a higher yield (70%) of 2a was obtained with C4 bearing cyclohexanediamine (entries 2-4). The yield was slightly improved upon substituting a less bulky group, the tert-amyl group (C5), for the tert-butyl group of the ligand (C6 and C7; entries 5-7). Replacing the counteranion of the oxidant was essential for an excellent yield: hexafluorophosphate (PF6) performed better than trifluoromethanesulfonate (OTf) and tetrafluoroborate (BF4) (entries 8 and 9). When 2 equivalents of the oxidant were used, the yield of 2a decreased (entry 10). Alkenyl alcohol 1a appeared to favor intermolecular C-O bond formation. However, the yield decreased when the reaction mixture was diluted (entry 11). The use of the common silane reagent, phenylsilane, did not afford the desired product (entry 12). Other solvents such as tetrahydrofuran, toluene, dichloromethane, and acetone also afforded the desired product, albeit in lower yields than that achieved when using 4-methyltetrahydropyran (MTHP, entries 13-16), a less commonly employed solvent.