Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals

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 catalysis5 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 radicaltrapping 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 activation6 (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity.