Dynamic Kinetic Sensitization of β‐Dicarbonyl Compounds—Access to Medium‐Sized Rings by De Mayo‐Type Ring Expansion

Herein, we present a photocatalyzed two-carbon ring expansion of β-dicarbonyl compounds with unactivated olefins that provides facile access to medium-sized rings. Selective sensitization of the substoichiometric enol tautomer enables reactivity of substrates incompatible with the classical De Mayo reaction conditions. Key to success is the identification of the metal-based sensitizer fac-[Ir(CF ₃-pmb) ₃], which can be excited using common near-visible LEDs, and possesses a high triplet excited state energy of 73.3 kcal mol ⁻¹. This exactly falls in the range between the triplet energies of the enol and keto tautomer, thereby enabling a dynamic kinetic sensitization. Demonstrating the applicability of fac-[Ir(CF ₃-pmb) ₃] as a photocatalyst in organic synthesis for the first time, we describe a two-step photocycloaddition-ring-opening cascade with β-ketoesters, -diketones, and -ketoamides. The mechanism has been corroborated by time-resolved spectroscopy, as well as further experimental and computational studies.