Cellulose-Stabilized BiOBr Enables Light-Mediated Radical-Polar Crossover of Unactivated Alkenes to Achieve Cyclopropanes

A cellulose-stabilized BiOBr photocatalyst has been developed to enable efficient light-mediated radical-polar crossover cyclopropanation of unactivated alkenes. Embedding highly dispersed BiOBr within a renewable cellulose matrix enhances catalyst stabilization and promotes catalytic performance through combined effects involving interfacial charge transfer, matrix structure, and substrate interaction. Under light irradiation, this composite selectively activates alpha-bromomalonates to generate reactive radical intermediates that engage a diverse range of aliphatic, aromatic, and amide-containing alkenes, delivering cyclopropanes in up to 97% yield with broad functional-group tolerance. The method accommodates complex bioactive substrates, providing a practical approach to late-stage derivatization. Furthermore, mechanistic studies, including kinetic analysis, radical trapping, and radical-clock experiments, support a photoinduced radical-polar crossover pathway featuring alpha-bromomalonate activation, carbocation formation, and subsequent ring closure. This work establishes cellulose-stabilized BiOBr as an environmentally friendly, broadly applicable photocatalyst for light-driven activation of nonactivated organic molecules.