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

Abstract 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.