Dynamic kinetic sensitization of β-dicarbonyl compounds – Access to medium-sized rings via a De Mayo-type ring expansion

Herein, we present a visible-light-mediated two-carbon ring expansion of β -dicarbonyl compounds with unactivated olefins providing facile access to medium-sized rings. Selective sensitization of the substoichiometric enol tautomer enables reactivity with 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 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 is corroborated by time-resolved spectroscopy, as well as further experimental and computational studies.