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

Abstract

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 has emerged as a powerful method for the mild generation of alkyl radicals for C–C bond-forming reactions. However, the reactivity of radicals on strained rings is not well understood and there are few studies that address this question systematically. Furthermore, examples that use tertiary and benzylic radicals are rare, and their reactivity is challenging to harness and 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 on the reactivity of medicinally important small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precur- sors for the formation of tertiary benzylic oxetane/azetidine radicals with loss of CO2 and subsequent conjugate addition into activated alkenes. The process is shown to be tolerant of polar functional groups and heterocycles to generate medicinally relevant compounds bearing oxetane or azetidine motifs. We compare the reactivity of the oxetane radicals with that of other common benzylic systems. Computational studies indicate the Giese addition of benzylic radicals into acrylates to be generally reversible, resulting in lower yields and radical dimerization. However, the oxetane structure provides an overall exergonic process for the Giese addition step of the benzylic radical. Furthermore, strained ring structures benefit from lower spin density at the benzylic position, which is crucial to minimize radical dimerization. The lower spin density results from more extensive radical delocalization into the aromatic system which is rationalized on the basis of hybridization. The effect of radical acceptor on the distribution of products is investigated and a mechanism for the formation of side products of reduction is proposed based on experimental and computational evidence.