title: Studies on the Enantioselective Iminium Ion Trapping of Radicals Triggered by an Electron-Relay Mechanism.
abstract: A combination of electrochemical, spectroscopic, computational, and kinetic studies has been used to elucidate the key mechanistic aspects of the previously reported enantioselective iminium ion trapping of photochemically generated carbon-centered radicals. The process, which provides a direct way to forge quaternary stereocenters with high fidelity, relies on the interplay of two distinct catalytic cycles: the aminocatalytic electron-relay system, which triggers the stereoselective radical trap upon iminium ion formation, and the photoredox cycle, which generates radicals under mild conditions. Critical to reaction development was the use of a chiral amine catalyst, bearing a redox-active carbazole unit, which could rapidly reduce the highly reactive and unstable intermediate generated upon radical interception. The carbazole unit, however, is also involved in another step of the electron-relay mechanism: the transiently generated carbazole radical cation acts as an oxidant to return the photocatalyst into the original state. By means of kinetic and spectroscopic studies, we have identified the last redox event as being the turnover-limiting step of the overall process. This mechanistic framework is corroborated by the linear correlation between the reaction rate and the reduction potential of the carbazole unit tethered to the aminocatalyst. The redox properties of the carbazole unit can thus be rationally tuned to improve catalytic activity. This knowledge may open a path for the mechanistically driven design of the next generation of electron-relay catalysts.
authors.affiliation: ICIQ - Institute of Chemical Research of Catalonia, The Barcelona Institute of Science and Technology , Avinguda Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: Ana
authors.initials: A
authors.lastname: Bahamonde
authors.affiliation: ICIQ - Institute of Chemical Research of Catalonia, The Barcelona Institute of Science and Technology , Avinguda Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: John J
authors.initials: JJ
authors.lastname: Murphy
authors.affiliation: D3 CompuNet, Istituto Italiano di Tecnologia , via Morego 30, 16163 Genova, Italy.
authors.forename: Marika
authors.initials: M
authors.lastname: Savarese
authors.affiliation: D3 CompuNet, Istituto Italiano di Tecnologia , via Morego 30, 16163 Genova, Italy.
authors.forename: Éric
authors.initials: É
authors.lastname: Brémond
authors.affiliation: D3 CompuNet, Istituto Italiano di Tecnologia , via Morego 30, 16163 Genova, Italy.|Dept. of Pharmacy and Biotechnology, via Belmeloro 6, 40126 Bologna, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cavalli
authors.affiliation: ICIQ - Institute of Chemical Research of Catalonia, The Barcelona Institute of Science and Technology , Avinguda Països Catalans 16, 43007 Tarragona, Spain.|ICREA , Passeig Lluís Companys 23, 08010 Barcelona, Spain.
authors.forename: Paolo
authors.identifier: 0000-0001-8722-4602
authors.initials: P
authors.lastname: Melchiorre