[Ir] = [Ir{dF(CF3)ppy}2(dtbbpy)]PF6 (see Scheme 2). Oxetane radical III can also embark on alternative reaction pathways such as dimerization to form 3 or reduction to generate 4. The extent of these side reactions was intriguingly dictated by the choice of radical acceptor (see Supporting Schemes S5 and S6 for full overview). Particularly, less electrophilic and/or more sterically hindered alkenes led to increased amounts of 3 and 4. This is presumably due to an increase in DG and DG ‡ (both more positive) of the Giese addition, shifting the equilibrium towards oxetane radical III. A higher concentration of oxetane radical significantly increases the rate of dimerization (directly proportional to the square of radical concentration), and the formation of reduced oxetane 4. Reduction of oxetane radical III to generate 4 was intriguing, with no explicit reductant or H-atom transfer (HAT) reagent present in the reaction mixture. We investigated the formation of 4 through deuteration studies using cyclohexene methyl carboxylate as acceptor, which had shown elevated amounts of 4 (Figure ). High deuterium incorporation with D2O as additive suggests protons to be the source of "H" in 4. The low deuterium incorporation with DMF-d7 rules out HAT from DMF to the oxetane radical. Reduction of the oxetane radical by [Ir II ] to an oxetane-3-anion followed by protonation is unlikely based on the calculated reduction potential of the oxetane radical (𝐸 !/# 𝐈𝐈𝐈/𝐈𝐈𝐈 - = -1.67 V vs SCE) which lies outside the reduction range of [Ir II ] (Scheme 5a). We hence tentatively propose reduced oxetane 4 to be generated through a concerted multisite proton-coupled electron transfer (PCET) 54 with [Ir II ] as source of electrons. Interestingly, only minimal amounts of 3 and 4 were observed in the absence of acceptor (Figure ), hinting to its involvement in the oxidation of [Ir II ] back to [Ir III ]. However, direct oxidation of [Ir II ] by the alkene is unfeasible based on the alkenes' reduction potentials (e.g. for cyclohexene methyl carboxylate calculated to be 𝐸 !/# %/% •- = -2.60 V vs SCE; Supporting Table ). 55 Thus, speculatively, we propose that the high concentration of strained oxetane radical also increases the rate of irreversible oxetane degradation pathways to generate radicals that can more easily add into the acceptor and form unidentified Giese adducts capable of oxidizing [Ir II ]. In summary, we report the generation of unusual tertiary benzylic strained oxetane and azetidine radicals under photoredox catalysis. We have developed a protocol for the use of 3-aryl-3carboxylic acid oxetanes and azetidines as radical precursors which react with alkenes to form medicinally relevant alkylated 3,3-disubstituted oxetanes and azetidines with previously inaccessible substitution patterns. The reaction is reproducible, easy to set up and insensitive to common deviations from the conditions. The products could be further transformed to reveal free