ArticleAsymmetric photocatalysis over robust covalent organic frameworks with tetrahydroquinoline linkage
Graphical Abstract
The synthesis of robust COFs with an irreversible tetrahydroquinoline linkage was established, and the potential application of QH-COFs in the photocatalytic asymmetric MacMillan reaction was demonstrated for the first time by merging with the chiral secondary amine.
Introduction
The asymmetric photocatalytic organic synthesis (APOS) process is a sustainable and environmentally benign method for the production of optically active chemicals with sunlight as an energy source [1, 2, 3]. Most APOS processes employ a dual catalysis approach consisting of organic dyes/inorganic semiconductors and chiral catalysts for light absorption and chiral induction, respectively. In general, organic dyes confront problems with narrow absorption bands and low photostability, while the band gap and band edge of inorganic semiconductors are difficult to be tuned [4, 5]. Furthermore, the efficiency of APOS is strongly related to the textural structure and surface properties of the semiconductors, because the interphases of the semiconductors with the asymmetric catalysts and the reactants play an important role in achieving efficient electron transfer and chiral induction [6, 7]. Therefore, the development of semiconductors with designable band structures, textural structures, and surface properties is of extreme importance for APOS.
Covalent organic frameworks (COFs) [7, 8, 9, 10, 11, 12, 13, 14] with periodically ordered structures, high surface areas, and tunable band gaps and band edges are potential organic semiconductors in the field of photocatalysis and have been successfully used for photocatalytic H2 production [7, 15, 16, 17, 18, 19, 20, 21], CO2 reduction [22, 23, 24, 25], and organic synthesis [26, 27, 28, 29, 30, 31, 32]. The COFs with reversible C=N or B-O-B as a linkage are not stable enough in acid/base medium and under light irradiation, which is one of the biggest obstacles to their applications. Intensive efforts have been devoted to improving the stability of COFs by replacing the reversible chemical bond with an irreversible bond [29, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42]. For example, the direct cascade transformation of the imine linkage of COFs with phenolic hydroxyl to oxazole linkage [32] and the post-synthesis conversion of imine-linked COFs to amide-linked and quinoline-linked COFs have been reported [33, 34]. Recently, Yaghi's groups successfully constructed highly stable dioxin-linked and olefin-linked COFs via an irreversible nucleophilic aromatic substitution reaction and Knoevenagel condensation reaction, respectively [35, 36].
However, the reported stable COFs synthesized via the one-pot approach are only limited to some special monomers [32, 36, 37, 38]. This not only makes the monomer synthesis tedious but also limits the universality of the synthesis method. It is highly desirable to use independent transformation reagents instead of attaching them to the monomers. Through a literature survey, we found that tetrahydroquinoline derivatives could be formed by cascade condensation and cycloaddition reactions of amine, aldehyde, and alkene (Povarov cascade reaction, Scheme 1) with Lewis acids as catalysts, such as Yb(OTf)3 and Sc(OTf)3 [43, 44, 45]. More importantly, the Povarov cascade reaction has a wide substrate scope. Previous reports also demonstrated the efficiency of M(OTf)3 (M = Sc, Eu, In, Yb, Y, etc.) for the formation of COFs with imine linkage [46]. Therefore, the cascade condensation and cycloaddition reactions may provide a general method for the synthesis of ultrastable COFs with irreversible linkages via the judicious selection of the Lewis acids.
Herein, we report an efficient and general method for the synthesis of ultrastable tetrahydroquinoline-linked COFs (QH-COFs) via cascade condensation and cycloaddition reactions with Sc(OTf)3/Yb(OTf)3 as the catalysts. As far as we know, this is the first example of the one-pot synthesis of COFs via cascade reactions using aldehydes and amines in the presence of transformation reagents. The successful formation of a tetrahydroquinoline linkage was confirmed by FT-IR and 15N NMR spectroscopies of the QH-COFs. The QH-COFs exhibited extremely high stability under the acidic/basic conditions and light irradiation. More interestingly, the QH-COFs with visible-light absorption expanding to more than 560 nm could efficiently catalyze the visible-light-driven asymmetric alkylation of aldehydes in combination with a chiral secondary amine to afford both high yield and high ee for a large substrate scope.
Section snippets
Synthesis of QH-COF-1
A 10 mL high-pressure flask was charged with 1,3,5-tris(p-formylphenyl)benzene (32.5 mg, 0.083 mmol), benzidine (23.0 mg, 0.125 mmol), ethyl vinyl ether (72 mg, 1 mmol). A mixture of 1,2-dichlorobenzene and n-butyl alcohol (4:1 v/v, 2.5 mL) was added, and the resulting suspension was sonicated at room temperature until the monomers were fully dispersed. Sc(OTf)3 (2.5 mg, 0.005 mmol) and Yb(OTf)3 (8.0 mg, 0.013 mmol) were added, and the resulting suspension was further sonicated for 30 s. The
Synthesis and characterizations
Firstly, benzaldehyde and benzidine were used as model substrates to explore the feasibility of the Povarov reaction for the formation of the tetrahydroquinoline model compound (Scheme 1a). The imine model compound could be facilely obtained by the reaction of benzaldehyde and benzidine in the presence of 0.02 equiv. Sc(OTf)3 under ambient conditions. Next, the imine model compound was reacted with ethyl vinyl ether (EVE) in the presence of Yb(OTf)3 under N2 to afford the tetrahydroquinoline
Conclusions
In summary, we successfully developed an efficient and general method to build QH-COFs as robust photocatalysts via the one-pot Povarov reaction. QH-COFs could endure strong acidic/basic conditions and light irradiation, which is attributed to the irreversible tetrahydroquinoline linkage. The formation of tetrahydroquinoline linkage could effectively narrow the band gap of QH-COFs to widen the absorption spectrum. The QH-COFs exhibited high activity, enantioselectivity, and excellent
Acknowledgments
We are grateful to Prof. X. Feng and Mr. P. Shao for the support on the structural modeling of COFs. H. Li thanks Ms. X. Tao for helpful discussions.
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This work was supported by the National Natural Science Foundation of China (21733009, 21621063), and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020200).
Published 5 August 2020
Dedicated to Prof. Can Li on the occasion of his 60th birthday.