Synergistic Effect of Cerium in Dual Photoinduced Ligand-to-Metal Charge Transfer and Lewis Acid Catalysis: Diastereoselective Alkylation of Coumarins.

We report the dual role of cerium to promote the photoinduced ligand-to-metal charge transfer (LMCT) process for the generation of the alkyl radical and subsequent Lewis acid catalysis to construct stereodefined C-C bonds. This paradigm utilized ubiquitous carboxylic acids as alkyl radical surrogates and offers excellent diastereoselectivity for the formation of C-4 alkylated coumarins in good to excellent yield. UV-vis spectroscopy studies in combination with in situ Fourier transform infrared spectroscopy are consistent with the proposed mechanism, supporting the participation of the Ce-carboxylate complex in photoinduced LMCT and its subsequent homolysis to generate the alkyl radial through the exclusion of CO. Finally, the oxophilicity of cerium enables a two-point complexation with the in situ generated enolate intermediate and facilitates the diastereoselective protonation to form the desired product. Furthermore, this mild and atom-economical catalytic manifolds allow the late-stage modification of pharmaceuticals.