Carbon nitride photocatalyzes regioselective aminium radical addition to the carbonyl bond and yields N-fused pyrroles

Abstract

Addition of N-centered radicals to C=C bonds or insertion into C–H bonds is well represented in the literature. These reactions have a tremendous significance, because they afford polyfunctionalized organic molecules. Despite the tetrahydroisoquinoline (THIQ) moiety widely occurring in natural biologically active compounds, N-unsubstituted THIQs as a source of N-centered radicals are not studied. Herein, we report a photocatalytic reaction between tetrahydroisoquinoline and chalcones that gives N-fused pyrroles—1,3-disubstituted-5,6-dihydropyrrolo[2,1-a]isoquinolines (DHPIQ). The mechanism includes at least two photocatalytic events in one pot: (1) C–N bond formation; (2) C–C bond formation. In this process potassium poly(heptazine imide) is used as a visible light active heterogeneous and recyclable photocatalyst. Fifteen N-fused pyrroles are reported with 65–90% isolated yield. DHPIQs are characterized by UV–vis and fluorescence spectroscopy, while the fluorescence quantum efficiency of fluorinated DHPIQs reaches 24%.