title: Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals.
abstract: An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon-carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity, radical transformations have been far less successful. This is despite the fact that open-shell intermediates are intrinsically primed for connecting structurally congested carbons, as their reactivity is only marginally affected by steric factors. Here we show how the combination of photoredox and asymmetric organic catalysis enables enantioselective radical conjugate additions to β,β-disubstituted cyclic enones to obtain quaternary carbon stereocentres with high fidelity. Critical to our success was the design of a chiral organic catalyst, containing a redox-active carbazole moiety, that drives the formation of iminium ions and the stereoselective trapping of photochemically generated carbon-centred radicals by means of an electron-relay mechanism. We demonstrate the generality of this organocatalytic radical-trapping strategy with two sets of open-shell intermediates, formed through unrelated light-triggered pathways from readily available substrates and photoredox catalysts--this method represents the application of iminium ion activation (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity.
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Avda. Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: John J
authors.initials: JJ
authors.lastname: Murphy
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Avda. Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: David
authors.initials: D
authors.lastname: Bastida
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Avda. Països Catalans 16, 43007 Tarragona, Spain.
authors.forename: Suva
authors.initials: S
authors.lastname: Paria
authors.affiliation: Photogreen Laboratory, Department of Chemistry, University of Pavia, viale Taramelli 12, 27100 Pavia, Italy.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Fagnoni
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Avda. Països Catalans 16, 43007 Tarragona, Spain.|Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain.
authors.forename: Paolo
authors.initials: P
authors.lastname: Melchiorre