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%.
Highlights
Addition of N-centered radicals to C=C bonds or insertion into C–H bonds is well represented in the literature
Under light irradiation (λ = 461 nm, 51.7 ± 0.03 mW cm−2, blue LED) at 80 °C with K-PHI as the photocatalyst after 20 h using 3 equivalents of THIQ and acetonitrile as solvent, 87% of the starting chalcone 2a was converted into a mixture of compounds: the desired dihydropyrrolo[2,1-a]isoquinoline (DHPIQ) 3a and ketone 4a
We have shown that DHPIQs, as exemplified by compound 3d, smoothly undergo electrophilic substitution using Nbromosuccine imide (NBS) as a reagent at room temperature (Fig. 5)
Summary
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. 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. Potassium poly(heptazine imide) (K-PHI hereafter) is a member of the carbon nitride family with negatively charged nitrogen atoms[10,11] This structural particularity has a great impact on the band structure and leads to a significantly more positive position (+2.54 eV vs RHE) of the valence band[12]. There are only few works on R2N∙ radicals addition to the polar C=O carbonyl bonds[19,20,21]
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