Rational synthesis of interpenetrated 3D covalent organic frameworks for asymmetric photocatalysis

Abstract

Covalent organic frameworks (COFs) show great promise as heterogeneous photocatalysts, but they have not yet been explored for asymmetric photocatalysis, which is important for the sustainable production of pharmaceuticals and fine chemicals. We report here a pair of twofold interpenetrated 3D COFs adopting a rare (3,4)-connected ffc topology for photocatalytic asymmetric reactions by imine condensation of rectangular and trigonal building blocks. Both COFs containing a photoredox triphenylamine moiety are efficient photocatalysts for the cross-dehydrogenative coupling reactions and asymmetric a-alkylation of aldehydes integrated with a chiral imidazolidinone catalyst. Under visible-light irradiation, the targeted chiral products are produced in satisfactory yields with up to 94% enantiomeric excess, which are comparable to those of reported reactions using molecular metal complexes or organic dyes as photosensitizers. Whereas the COFs became amorphous after catalysis, they can be recrystallized through solvent-assisted linker exchange and reused without performance loss. This is the first report utilizing COFs as photocatalysts to promote enantioselective photochemical reactions.

Integrating single Ni sites into biomimetic networks of covalent organic frameworks for selective photoreduction of CO2

“…The development of covalent organic frameworks (COFs) 18 provides a promising platform for photocatalysis. [19][20][21][22][23][24] The periodic and permanent porosity endow COFs with a naturemimicking architecture, while the diverse compositions and synthetic approaches allow COFs with tunable microstructures and optical and electronic structures. [25][26][27][28][29] The highly conjugated structure in-plane in COFs can ensure the mobility of photoinduced charges.…”

Covalent Organic Frameworks in Catalytic Organic Synthesis

“…Similarly, Liu's group also established a 2D COF (COF‐JLU5, Figure 4, Table 2) to catalyze this reaction (Scheme 7, b) [44] . Besides, Cui and co‐workers illustrated two interpenetrated 3D COFs (COF‐1/2 Figure 4, Table 2) to catalyze the same reaction (Scheme 7, c) [45] . Compared these works with each other, it is not difficult to find that although researchers used various building blocks to establish diverse COFs, they can catalyze the same reaction.…”

“…Besides, we also introduced few works that show us the potential for COFs to substitute transition‐metals in visible‐light‐induced construction of C−C bonds. In the last section, we would like to introduce an asymmetric α‐alkylation of aldehydes catalyzed by COF‐1/2, which were already introduced to catalyze the CDC reaction [45] . Actually, COFs played the same role as before, the different thing was that COFs, for the first time, replaced a noble‐metal photosensitizer (Ru(bpy) 3 Cl 2 ) to finish an asymmetric reaction with good yields (up to 88%) and selectivity (up to 94%) (Scheme 18, 19).…”

The Application of Covalent Organic Frameworks for Chiral Chemistry

“…[38] Copyright 2019, American Chemical Society. 3D COFs@chiral imidazolidinone b) Chiral imidazolidinone Cross-dehydrogenative coupling reaction (53-85%, -) Postmodification 2020 [59] Asymmetric α-alkylation of aldehydes (88-51%, 83-94%) QH-COFs@chiral secondary amine b) Chiral secondary amine Asymmetric α-alkylation of aldehydes (70-91%, 85-94%) Postmodification 2020 [60] molecular so that the reactant can hardly reach the catalytic site. It can be used to explain the phenomenon that these heterogeneous catalysts do not work well when the reaction substrates are larger than the pore size of COFs.…”

“…These COFs could be served as the metal-free photo-catalysts for cross-dehydrogenative coupling reaction (yield for 50-83%) and asymmetric α-alkylation of aldehydes(yield for 50-83%, ee values for 85-94%) after integrating with chiral imidazolidinone. [59] In the same year, Li and Yang et al developed two tetrahydroquinoline-linked COFs (QH-COFs) to employ as semiconductors for organic synthesis (Figure 13). [60] It was the first report about synthesizing COFs via cascade condensation and cycloaddition reaction between aldehydes and amines.…”