Table In an attempt to synthesize a medium-membered cyclic ether, applying the initial conditions to alkenyl alcohol 1l yielded oxepane 2l in only 30% yield due to the recovery of 1l and alkene isomerization (Table ). The yield improved slightly upon changing the diamine moiety to ethylenediamine in the cobalt complex (C2). Changing the size of the substituents on the aromatic ring to methyl (C3) and isopropyl (C4) groups, which are smaller than tert-butyl groups, significantly decreased the yield of 2l. Conversely, using a tert-hexyl group as a substituent successfully increased the yield to 60% (C6). Thus, catalyst structure optimization plays a key role in the effectiveness of the MHAT/RPC system. Table The optimized cobalt complex C6 was then used to examine the substrate generality of the synthesis of medium-membered rings (Table ); 1,4-Oxazepine 2m was synthesized in a similar manner to that of 2l. In the synthesis of the eight-membered ring ether 2n, we again compared the cobalt complexes C1 and C6 and reconfirmed the superiority of C6. For the synthesis of ninemembered rings, 2o was produced in low yield even when C6 was used. In the synthesis of a medium-membered ring lactone, oxepanone 4g, benzodioxooctanone 4h, and benzodioxanone 4i were furnished. Table Additionally, we attempted to construct an oxetane structure. The reaction conditions were optimized using the starting material 1p, which was synthesized in a single step from commercially available 2-indanone (Table ). The initial conditions using C1 produced oxetane 2p in 42% yield (entry 1). Continued optimization of the reaction conditions revealed that C8 improved the yield to 80% (entry 2). The type of oxidant counter anion also affected the yield, with Me3NFPY·PF6 producing 2p in yields of up to 90% (entries 3, 4). Using (Me2SiH)2O proved optimal in terms of both cost and yield. 4-Methyltetrahydropyran (MTHP) was superior to a less-polar solvent such as toluene (entries 5-8). Table Most of the homoallylic alcohols were synthesized by treating aldehydes or ketones with zinc powder and methallyl bromide. MHAT/RPC reactions were performed on various homoallylic alcohols under optimal conditions, and various oxetane-containing compounds were successfully synthesized (Table ). In particular, we focused on the synthesis of spiro compounds, which have attracted attention from the viewpoint of high Fsp 3 (2q-2w). The problematic side reaction in this oxetane synthesis was the retro-allylation reaction, which forms ketones corresponding to the starting materials of homoallylic alcohols. The reaction is limited to that of disubstituted alkenes and is not suited to monosubstituted alkenes. In this regard, the energetic barrier for cyclization from alkyl cobalt(IV) complex was calculated and was approximately 7 kcal/mol higher for the monosubstituted alkene than that for disubstituted alkenes.