Highly Enantioselective Radical Cation [2+2] and [4+2] Cycloadditions by Chiral Iron(III) Photoredox Catalysis

Radical cations show a unique reactivity that is fundamentallydifferent from that of conventional cations and have thus attractedconsiderable attention as alternative cationic intermediates for noveltypes of organic reactions. However, asymmetric catalysis to promoteenantioselective radical cation reactions remains a major challengein contemporary organic synthesis. Here, we report that the judiciousdesign of an ion pair consisting of a radical cation and a chiralcounteranion induces an excellent level of enantioselectivity. Thisstrategy was applied to enantio-, diastereo-, and regioselective [2+ 2] cycloadditions, as well as enantio-, diastereo-, and regioselective[4 + 2] cycloadditions, by using chiral iron(III) photoredox catalysis.We anticipate that this strategy has the potential to expand the useof several mature chiral anions to develop numerous unprecedentedenantioselective radical cation reactions.