Construction of Amide-Linked Covalent Organic Frameworks by N-Heterocyclic Carbene-Mediated Selective Oxidation for Photocatalytic Dehalogenation

Linkage conversion of imine to amide can not only boost the stability of covalent organic frameworks (COFs) but also enhance their functionality. However, current methods for generating amide linkages usually involve the use of inorganic oxidants, such as NaClO2 and oxone, which have a severe drawback of functional group incompatibility. In this study, we introduced a novel synthetic strategy to fabricate amide-linked COFs (Am-COFs) through N-heterocyclic carbene-mediated aerobic oxidation of C═N bonds in the corresponding imine-linked COFs. This protocol not only featured mild reaction conditions, high crystallinity retention, quantitative conversion efficiency, and generality but, most importantly, delivered great compatibility with functional groups due to the umpolung of imine. Accordingly, we successfully synthesized Am-COFD19 and Am-COFR3, which contained oxidant-susceptible dibenzothiophene and vinyl groups, respectively. Am-COFD19 exhibited excellent photocatalytic dehalogenation activity of α-bromoacetophenone, while Am-COFR3 underwent intramolecular hydroamidation to yield dihydroisoquinolinone-linked 2HQ-COFR3 that has never been reported before. The discovery of this strategy is expected to expedite the application of amide-linked COFs.