Anthraquinone-based covalent organic framework as a recyclable direct hydrogen atom transfer photocatalyst for C-H functionalization

Abstract

Photocatalytic direct hydrogen atom transfer (d-HAT) is a synthetically important strategy to convert C-H bonds to useful C-X bonds. Herein we report the synthesis of an anthraquinone-based two-dimensional covalent organic framework, DAAQ-COF, as a recyclable d-HAT photocatalyst for C-H functionalization. Powder X-ray diffraction, N₂ sorption isotherms, solid-state NMR spectra, infrared spectra, and thermogravimetric analysis characterized DAAQ-COF as a crystalline, porous COF with a stable ketoenamine linkage and strong absorption in the visible region. Under visible light irradiation, DAAQ-COF is photo-excited to cleave C(sp³)-H or C(sp²)-H bonds via HAT to generate reactive carbon radicals, which add to different radical acceptors to achieve C-N or C-C coupling reactions. DAAQ-COF is easily recovered from the reaction mixture via centrifugation or filtration and used in six consecutive reaction runs without any decrease in catalytic efficiency. The ease of catalyst separation allows sequential conversion of the C-N coupling intermediate to synthetically useful amide, ester, or thioester products. Photophysical and isotope labelling experiments support the d-HAT mechanism of DAAQ-COF-catalyzed C-H bond functionalization.

Fig. 1. (a) Synthetic scheme of DAAQ-COF. (b) Experimental (red) and simulated (black) powder X-ray diffraction patterns of DAAQ-COF. (c) TEM image of DAAQ-COF. (d) High-resolution TEM image showing lattice fringes of DAAQ-COF. (e) N₂ sorption isotherms of DAAQ-COF at 77 K.

Fig. 2. (a) Control experiments of DAAQ-COF catalyzed C–N coupling between THF and DEAD. (b) Yields of the C–N coupling product between THF and DEAD in six consecutive reaction runs with the recovered DAAQ-COF as catalyst. (c) A sequential approach to convert the C–H bond in benzaldehyde to C–O, C–S and C–N bonds without rigorous isolation of the intermediates.

Fig. 3. (a) Diffuse reflectance UV-vis spectrum of DAAQ-COF. (b) Tauc plot from the diffuse reflectance UV-vis spectrum of DAAQ-COF. (c) Kinetic isotope experiment of DAAQ-COF-catalyzed C–N coupling reaction between DEAD and a mixture of THF and d₈-THF in a one-pot reaction. (d) Proposed mechanism for DAAQ-COF-catalyzed C–N coupling with DEAD as the radical acceptor.