Insights into the Role of Graphitic Carbon Nitride as a Photobase in Proton-Coupled Electron Transfer in (sp3 )C‒H Oxygenation of Oxazolidinones

Abstract

Graphitic carbon nitride (g-CN) is a transition metal free semiconductor that mediates a variety of photocatalytic reactions. Although photoinduced electron transfer is often postulated in the mechanism, proton-coupled electron transfer (PCET) is a more favorable pathway for substrates possessing X−H bonds. Upon excitation of an (sp2)N-rich structure of g-CN with visible light, it behaves as a photobase—it undergoes reductive quenching accompanied by abstraction of a proton from a substrate. The results of modeling allowed us to identify active sites for PCET—the ‘triangular pockets’ on the edge facets of g-CN. We employ excited state PCET from the substrate to g-CN to selectively cleavethe endo-(sp3)C−H bond in oxazolidine-2-ones followed by trapping the radical with O2. This reaction affords 1,3-oxazolidine-2,4-diones. Measurement of the apparent pKa value and modeling suggest that g-CN excited state can cleave X−H bonds that are characterized by bond dissociation free energy (BDFE) ≈100 kcal mol−1.