Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability

4-Membered ring heterocycles oxetanes and azetidines offer exciting potential as small polar molecular motifs in medicinal chemistry but require further methods for their incorporation. Photoredox catalysis is a powerful method for the mild genera-tion of alkyl radicals for C–C bond formation. However, the effect of ring strain on radical reactivity is not well understood and there are no studies that address this question systematically. Furthermore, examples of such reactions that use tertiary and benzylic radicals are rare, and their reactivity is challenging to direct towards productive reaction pathways. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to that of other common benzylic systems. Computational studies indicate that reversible Giese additions of non-strained benzylic radicals into acrylates result in low yields and radical dimerization. Benzylic radicals that are part of a strained ring, however, are both less stable and more 𝜋-delocalized, decreasing dimer formation and increasing Giese product formation. Oxetane substitution, in particular, results in high product yields due to ring strain and Bent’s rule rendering the Giese addition irre-versible. Mechanisms for the formation of side products observed with different substrates are proposed based on experi-mental and computational evidence.