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

Abstract

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 NaClO 2 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-COF D19 and Am-COF R3 , which contained oxidant-susceptible dibenzothiophene and vinyl groups, respectively. Am-COF D19 exhibited excellent photocatalytic dehalogenation activity of α-bromoacetophenone, while Am-COF R3 underwent intramolecular hydroamidation to yield dihydroisoquinolinone-linked 2HQ-COF R3 that has never been reported before. The discovery of this strategy is expected to expedite the application of amide-linked COFs.

Covalent Organic Frameworks: Opportunities for Rational Materials Design in Cancer Therapy

“…This offers extensive possibilities for synthesizing COFs with novel structures and functionalities (Figure 4A). [40] For example, imine bonds—the most common COF linkages—can be converted to amide bonds through oxidation with NaClO 2 , [41] KHSO 5 , [42] or N ‐heterocyclic carbene, [43] or to C−N bonds through reduction with NaBH 4 in ice methanol [44] . The Ugi reaction involving imine bonds allows two functional groups to be simultaneously introduced into the framework [45] .…”

Covalent Organic Frameworks: Opportunities for Rational Materials Design in Cancer Therapy

“…This offers extensive possibilities for synthesizing COFs with novel structures and functionalities (Figure 4A). [40] For example, imine bonds-the most common COF linkages-can be converted to amide bonds through oxidation with NaClO 2 , [41] KHSO 5 , [42] or N-heterocyclic carbene, [43] or to CÀ N bonds through reduction with NaBH 4 in ice methanol. [44] The Ugi reaction involving imine bonds allows two functional groups to be simultaneously introduced into the framework.…”

N‐Heterocyclic Carbene Promoted Construction of Amide‐Linked COFs via the Oxidation of Imine‐Linked COFs with Molecular Oxygen

“…During the preparation of this manuscript, Xiang et al reported a similar approach for the oxidation of imine-based COFs with small organic molecules. [14] Here we would like to report our progress in this area by using NHC as the efficient catalyst. This oxidation has proven to be a versatile and mild synthetic approach for transforming imine-linked COFs to amide-linked COFs.…”

“…During the preparation of this manuscript, Xiang et al . reported a similar approach for the oxidation of imine‐based COFs with small organic molecules [14] . Here we would like to report our progress in this area by using NHC as the efficient catalyst.…”