Embedding Hydrogen Atom Transfer Moieties in Covalent Organic Frameworks for Efficient Photocatalytic C-H Functionalization.

Abstract

Covalent organic frameworks (COFs) have emerged as efficient heterogeneous photocatalysts for a wide range of relatively simple organic reactions, whereas their application in complex organic transformations, like site-selective functionalization of unactivated C-H bonds, is underexplored, which can be mainly attributed to the lack of highly active organophotocatalytic cores. Herein through bonding oxygen atoms at the N-terminus of quinolines in nonsubstituted quinoline-linked COFs (NQ-COFs), we successfully realized the embedding of active hydrogen atom transfer (HAT) moieties into the skeleton of COFs. This novel designed COF (NQ-COFE5 -O), serving as both an excellent photosensitizer and HAT catalyst, exhibited much higher efficiency in C-H functionalization than the corresponding NQ-COFE5 . Specially, we evaluated the photocatalytic performance of NQ-COFE5 -O on ten different substrates, including quinolines, benzothiazole, and benzoxazole, all of which were transferred to desired products in moderate to high yields (up to 93 %). Furthermore, the as-synthesized NQ-COFE5 -O displayed excellent photostability and could be reused with negligible loss of activity for five catalytic cycles.