Palladium‐Photocatalyzed C3‐Alkylation of Coumarins With Alkyl Bromides: Development and Mechanistic Insights

A visible‐light‐driven palladium‐catalyzed method for the direct C3‐alkylation of coumarins with alkyl bromides is reported. The reaction uses a Pd(TFA) 2 /XantPhos/PPh 3 catalytic system and proceeds under mild conditions. Primary, secondary, and tertiary alkyl bromides were employed as radical precursors, and the transformation was extended to substituted coumarins. Control experiments, including radical trapping, a radical‐clock experiment, and light on/off studies, support a photoinduced radical pathway. Density functional theory calculations indicate that excitation of the Pd(0) complex is followed by single‐electron activation of the alkyl bromide and regioselective radical addition to the coumarin CC bond. Comparison of four possible product‐forming pathways favors base‐assisted deprotonation of the resulting benzylic radical followed by outer‐sphere single‐electron transfer to the Pd(I) species.