Red-Light-Driven C(sp2)–H Sulfonylation of Anilines Using a Recyclable Benzothiadiazole-Based Covalent Organic Framework

Red-Light-Driven C(sp2)−H Sulfonylation of Anilines Using a Recyclable Benzothiadiazole-Based Covalent Organic Framework Saul Alberca, ∥ Akshay M. Nair, ∥ Paula Escamilla, Pedro Ferreira, Manuel Souto,* and Martín Fananás-Mastral* Cite This: J. Am. Chem. Soc. 2025, 147, 39582−39589 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The limitations of traditional high-energy (NUV or blue) photocatalysis, such as limited penetration in reaction media, off-target reactivity, and health hazards, have spurred the development of seminal red-light-mediated transformations. Despite recent advances, most homogeneous red-light photocatalysts suffer from poor recyclability, and recyclable heterogeneous systems remain underexplored. Herein, we report a red-lightdriven C(sp2)−H sulfonylation of anilines using a highly stable benzothiadiazole-based covalent organic framework (Tp-BT COF) as an efficient, recyclable photocatalyst. The reaction proceeds under exceptionally mild conditions, affording sulfonylated products in good to excellent yields with minimal catalyst loading. Notably, the Tp-BT COF retains its catalytic activity over six consecutive cycles. Comparative studies with structurally related COFs highlight the critical role of the BT core in red-light absorption and the superior performance of the AA stacking mode. This work underscores the potential of rationally designed photoactive COFs for sustainable red-light photocatalysis. ■INTRODUCTION Visible-light photocatalysis has emerged as one of the main pillars of modern organic synthesis. 1,2 Despite significant advances, these reactions still present some limitations. The high-energy near-ultraviolet (NUV) or blue light typically used in these transformations limits scalability due to light attenuation through the reaction medium, as described by the Beer−Lambert−Bouguer law. 3 Moreover, the reliance on high-energy (NUV and blue) light introduces health hazards, functional group incompatibilities, and off-target reactivity due to the competitive absorption of such light by substrates and reaction intermediates. In contrast, low-energy red-lightmediated photocatalysis offers a promising alternative, providing higher permeability and milder reaction conditions (Figure 1a). 4 Consequently, recent years have seen significant interest in red-light photocatalysis, leading to substantial advances in both synthetic and biomedical applications. 5 In this line, elegant examples of photoredox catalysis using red light have been developed. 6 However, most red-light photocatalysts are homogeneous in nature, and therefore exhibit limited or no recyclability (Figure 1b). This limitation could be addressed by developing efficient heterogeneous systems. However, heterogeneous red-light photocatalysis remains a largely underexplored area. 7 To date, these transformations have been limited to the oxidative dimerization of amines 7a−c and the oxidation of sulfides and C(sp3)−H bonds. 7d This is likely due to the scarcity of heterogeneous catalysts that combine red-light absorption with redox activity and long-term stability. Covalent Organic Frameworks (COFs) are crystalline porous polymers that represent an emerging class of highly tunable and recyclable heterogeneous photocatalysts. 8 Their ordered porosity, high thermal and chemical stability, and tunable electronic properties make them excellent candidates for photocatalysis. In general, their broad absorption range, arising from extensive π-conjugation and low band gaps, especially in 2D COFs, make them promising platforms for red-light photocatalysts. However, to the best of our knowledge, only two examples have been reported that document red-light driven COF photocatalysis for the oxidative dimerization of amines. 7a,b In this context, expanding the chemical space of red-light COF photocatalysis, in terms of both catalyst design and development of value-added transformations, is of significant interest. Sulfones are valuable organosulfur compounds having broad industrial and biological significance. 9 Traditional synthesis of sulfones via sulfide oxidation or electrophilic aromatic substitution rely on the use either foul smelling sulfides or exotic reaction conditions. 10 In recent years, elegant alternative protocols for sulfonylations have emerged relying on sulfur dioxide insertions 11 and directing group-assisted C−H Received: July 28, 2025 Revised: October 3, 2025 Accepted: October 5, 2025 Published: October 13, 2025 Articlepubs.acs.org/JACS © 2025 The Authors. Published by American Chemical Society 39582 https://doi.org/10.1021/jacs.5c12697 J. Am. Chem. Soc. 2025, 147, 39582−39589 This article is licensed under CC-BY 4.0 Downloaded via 79.116.140.34 on December 24, 2025 at 10:56:44 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. sulfonylations. 12 In a seminal work, Willis and co-workers reported a homogeneous iridium-catalyzed oxidative C(sp2)− H sulfonylation of anilines under blue-light irradiation. 13 Inspired by this work, we envisioned that a heterogeneous photocatalyst capable of mediating a C−H sulfonylation under red-light irradiation would be highly valuable. To this end, we Figure 1. Overview of the work. Figure 2. Physicochemical characterization of COF photocatalysts. (a) Molecular structures of Tp-BT and Tp-TD COFs. (b) Powder X-ray diffraction (PXRD) patterns of Tp-BT-AA (experimental and simulated) and Tp-TD-AA (experimental). (c) N2adsorption (filled symbols) and desorption (empty symbols) isotherms of Tp-BT-AA and Tp-TD-AA. The inset shows the pore width distributions. (d) Absorbance spectra of TpBT-AA and Tp-TD-AA. (e) Normalized Tauc plot of the Kubelka−Munk-transformed data for Tp-BT-AA and Tp-TD-AA. Dashed lines indicate linear fits to the absorption onsets. Journal of the American Chemical Society pubs.acs.org/JACS Article https://doi.org/10.1021/jacs.5c12697 J. Am. Chem. Soc. 2025, 147, 39582−39589 39583 sought to develop a COF photocatalyst capable of promoting this transformation under red light. In particular, photoactive benzothiadiazole (BT)-based COFs have emerged as promising photocatalysts, especially for hydrogen production 14 and certain oxidative coupling reactions. 15 The red-light absorption and favorable oxidation potentials of BT-based COF (Tp-BT) upon photoexcitation in the presence of an oxidant motivated us to investigate its potential as a catalyst for this reaction. Although the photocatalytic activity of BT-based COFs has been studied for the oxidative dimerization of amines, 15 their application as heterogeneous red-light photocatalysts has not yet been reported. 16 Herein, we report an efficient red-light mediated C(sp2)−H sulfonylation of anilines using a recyclable Tp-BT COF photocatalyst (Figure 1c). The reaction affords the corresponding sulfones in good to excellent yields under mild and energy-efficient conditions, employing a remarkably low catalyst loading (as low as 0.035 mol %). The high functional group tolerance (including compatibility with late stage functionalizations) along with the facile scalability and recyclability of the COF photocatalyst are important features of this transformation. Furthermore, a structure−reactivity comparison study with related COF photocatalysts highlights the significance of the BT core for red-light photocatalysis and the importance of the AA-type stacking in the Tp-BT COF. ■RESULTS AND DISCUSSION At the outset of our study, a series of isoreticular βketoenamine-linked COFs with AA stacking mode were synthesized. Accordingly, 4,7-bis(4-aminophenyl)-2,1,3-benzothiadiazole (BT) and 4,4″-diamino-p-terphenyl (TD) were employed as linkers and condensed with 2,4,6-triformylphloroglucinol (Tp). Tp-BT-AA and Tp-TD-AA COFs (Figure 2a) were obtained by solvothermal reaction at 120 °C using pyrrolidine as the catalyst, in place of aqueous acetic acid, as previously reported, 15b a method known to promote the formation of highly crystalline β-ketoenamine COFs. 17 Powder X-ray diffraction (PXRD) was used to assess the crystallinity and phase purity of the COFs. Both materials exhibited nearly identical diffraction patterns (consistent with isoreticular structures), with the main intense peak at 2θ= 2.7°. Simulated PXRD patterns based on an eclipsed (AA) stacking model closely matched the experimental data (Figure 2b). Furthermore, Pawley refinement of the unit cell parameters yielded an excellent fit with minimal discrepancies (Figures S1 and S2). Fourier transform infrared (FT-IR) spectra were recorded for Tp-BT-AA and Tp-TD-AA COFs and compared with those of the corresponding building blocks (Figure S3). The disappearance of the characteristic −CHO (1645 cm−1) and −NH2(3500−3300 cm−1) stretching frequencies, along with the appearance of a new band at 1620 cm−1, confirmed the successful polymerization of the precursors. The permanent porosity of both COFs was evaluated by nitrogen adsorption−desorption isotherms at 77 K (Figure 2c). The Brunauer−Emmett−Teller (BET) specific surface areas were calculated using the BETSI analysis, 18 yielding values of 1351 and 1241 m2g−1for Tp-BT-AA and Tp-TD-AA, respectively, in agreement with previous reports (Figures S4 and S5). 17,19 Prior to photocatalytic studies, the light-harvesting properties of the COFs were evaluated using diffuse reflectance spectroscopy (Figure 2d). Optical band gaps were estimated by linear fitting of the absorption onsets in Tauc plots derived from Kubelka−Munk-transformed data (Figure 2e). Tp-TDAA exhibited relatively narrow optical absorbance with an Table 1. Reaction Optimization a , b a Reaction conditions: 1a (0.4 mmol), 2a (1.2 mmol), Tp-BT-AA (0.07 mol %), K2S2O8(1.2 mmol), (TBA)HSO4(0.08 mmol) in MeCN/H2O 10:1 (4 mL), LEDs, rt, air. b Isolated yields. Journal of the American Chemical Society pubs.acs.org/JACS Article https://doi.org/10.1021/jacs.5c12697 J. Am. Chem. Soc. 2025, 147, 39582−39589 39584 onset at 520 nm and a calculated optical band gap of 2.29 eV. In contrast, Tp-BT-AA displayed a red shift of over 100 nm in its absorption onset, resulting in a narrower optical band gap of 2.04 eV. Thermogravimetric analysis (TGA) revealed that both COFs are stable up to 320 °C (Figure S6), well beyond the temperature range of the proposed photocatalytic transformation. The optimal band gap and red-light absorption of Tp-BTAA COF made it a promising candidate for our studies. We chose the reaction between N,N-dimethyl-p-toluidine 1a and sodium toluyl sulfinate 2a as the model reaction to evaluate the catalytic activity of this COF. Screening of the reaction parameters (Table 1 and Supporting Information) led to the optimized conditions involving Tp-BT-AA COF (0.07 mol %), K2S2O8(3 equiv) and phase-transfer agent (TBA)HSO4(0.2 equiv) in MeCN:H2O (10:1) under irradiation by two red LED lamps (660 nm) for 12 h. Under these conditions, product 3a was isolated in 85% yield (Table 1, entry 1). Irradiation using a single red LED lamp resulted in lower efficiency (entry 2). The catalytic activity of the Tp-BT-AA COF was efficient across a broad wavelength range (entries 3− 5). Notably, 3a was obtained in 95% yield within 2 h under green LED (525 nm) light irradiation (entry 4). Remarkably, even far-red LED irradiation (740 nm) provided the product, albeit in diminished yield (entry 6). It is important to note that the crystallinity of Tp-BT-AA COF, modulable depending on the modulator used in the synthesis (Figure S7), does not have a significant influence on catalytic performance (see Section 4c in the Supporting Information). Tp-BT COFs with AB and ABC stacking modes were also synthesized (see Supporting Information for details) to investigate the influence of stacking on the catalytic performance. The AA stacking mode for TpBT COF was found to be critical, as the same COF with AB and ABC stackings was catalytically inefficient under the same conditions (entries 7 and 8). Since the optical properties of the three COFs are similar (Figure S10), the lower catalytic performance could be attributed to the reduced porosity of the Tp-BT COFs with AB and ABC packing (Figure S11), which may limit the diffusion of reactants. The chemical structure of the COF photocatalyst also proved to be a key factor. The TpFigure 3. Substrate scope of COF photocatalyzed C(sp2)−H sulfonylation of anilines. aReaction conditions: 1(0.4 mmol), 2(1.2 mmol), Tp-BTAA (0.07 mol %), K2S2O8(1.2 mmol), (TBA)HSO4(0.08 mmol) in MeCN/H2O 10:1 (4 mL), stirred at room temperature, under air atmosphere and irradiated with light (525 nm, 44 W or 2x 660 nm, 44 W). bIsolated yields. Journal of the American Chemical Society pubs.acs.org/JACS Article https://doi.org/10.1021/jacs.5c12697 J. Am. Chem. Soc. 2025, 147, 39582−39589 39585 TD-AA COF, lacking the BT core, delivered 3a in a 93% yield under green light (entry 9) but was ineffective under orange (595 nm) and red (660 nm) irradiation (entries 10 and 11). This highlights the superior red-light harvesting ability of the BT core. Finally, control experiments confirmed that both the COF photocatalyst and light irradiation are essential for the transformation, as no product was formed in their absence (entry 12). Having identified the optimal COF photocatalyst and reaction conditions, we next explored the scope of this heterogeneous photocatalytic transformation (Figure 3). A variety of sulfinates (2) and aniline derivatives (1) were tested under both red and green light irradiations. Phenyl and aryl sulfinates bearing either electron-donating or electron-withdrawing substituents at the para position were well tolerated, delivering the corresponding sulfones 3b−din excellent yields. Both 1and 2-naphthyl sulfinates (3e and 3f) also proved to be efficient substrates. Interestingly, these substrates showed better performance under green or red light compared to blue light, further highlighting the advantage of using lowenergy irradiation. Notably, sterically hindered mesityl sulfinate (3g) was also compatible, although diminished yield was observed, likely due to the more encumbered nature of this reagent. C−H sulfonylation of aniline 1a with alkyl sulfinates (including both primary and secondary derivatives) afforded products 3h−3j in good to excellent yields. We then turned to the evaluation of various aniline derivatives. While the electron-donating OMe group (3k) was well tolerated at the para-position, the presence of a Cl group (3l) at the same position led to a diminished yield. Unsubstituted N,N-dimethylaniline led to product 3m as a 2:1 (ortho:para) mixture of regioisomers. Biphenyl diamine underwent selective monosulfonylation to afford product 3n in good yield. Notably, biologically relevant heterocycles such as N-toluyl morpholine (3o) and N-anisyl piperidine (3p) provided the corresponding sulfones in excellent yields. Triphenyl aniline underwent mono-sulfonylation at the para position to deliver 3r. Interestingly, C−H sulfonylation of the organic photosensitizer methyl-phenothiazine was feasible, selectively providing product 3s. To demonstrate the synthetic versatility of our protocol, late-stage functionalization of naturally occurring and biologically relevant molecules was performed. Camphor sulfinate was found to be suitable, delivering 3t in an excellent yield. Furthermore, C−H sulfonylation of drug molecules, such as Imipramine core (3u and 3v), Mifepristone (3w) and Linezolid (3x), was successfully accomplished. Notably, the present protocol affords consistently higher product yields compared to those reported by Willis, 13 while operating with lower photocatalyst loading and reduced sulfinate equivalents. 20 Moreover, the high calculated apparent quantum yield (AQY = 75.5%) of the reaction further underscores the high catalytic performance of the Tp-BT-AA COF (see Section 10 in the Supporting Information). The reaction could be easily scaled up to a 5 mmol scale under slightly modified reaction conditions (Figure 4a). Remarkably, the COF photocatalyst loading could be further reduced to 0.035 mol %. This corresponds to only 5 mg of TpBT-AA COF required to produce 1.2 g of 3a, highlighting the high catalytic efficiency of this COF. Also noteworthy is that the Tp-BT-AA COF could be easily recovered and reused with no loss in the yield of 3a after six catalytic cycles (Figure 4b, top). Analysis of the recovered COF by PXRD, FT-IR and scanning electron microscopy (SEM) confirmed that its structure and morphology remained unchanged compared to the pristine material (Figures 4b, bottom and S14). Although the BET surface area of the recovered material decreased, the pore size distribution remained unchanged (Figure S15), ruling out pore collapse. This demonstrates the exceptional stability of Tp-BT-AA COF under our reaction conditions and emphasizes the potential for additional recycling and reuse. Preliminary mechanistic studies provide important insight into the reaction pathway (Figure 5). No product formation was observed in the presence of the radical scavenger TEMPO, indicating that the reaction proceeds via stable radical intermediates. The reaction also failed to deliver 3a in the presence of the single-electron quencher (AgNO3) and hole quencher (KI), indicating the formation of electron−hole pairs within the COF. 21 A light on−off study revealed that the reaction shuts off in the dark and only proceeds under light irradiation, indicating the absence of chain propagation steps. Based on these studies and previous reports, 13,15 we propose the mechanism depicted in Figure 5c. Photoexcitation of TpBT-AA COF promotes electron transfer from the valence band (VB) to the conduction band (CB), forming electron−hole pairs. Based on the Mott−Schottky plot 15 and the optical band gap of Tp-BT-AA, the VB and CB were determined to be +1.26 and −0.79 eV vs NHE, respectively. Single-electron transfer from the CB to persulfate occurs, while both the sulfinate (Eox 2a =+0.30 eV vs SCE) 22 and amine (Eox 1d = +0.70 eV vs SCE) 23 undergo single electron oxidation by the Figure 4. Scale-up of the reaction and photocatalyst recyclability studies. Journal of the American Chemical Society pubs.acs.org/JACS Article https://doi.org/10.1021/jacs.5c12697 J. Am. Chem. Soc. 2025, 147, 39582−39589 39586 holes at the VB (+1.26 eV vs SCE), generating radical cation A and sulfonyl radical B. The subsequent coupling of these radical species followed by deprotonation would furnish the product (3a). ■CONCLUSIONS In conclusion, we have developed an efficient protocol for the C(sp2)−H sulfonylation of anilines under red-light irradiation using the Tp-BT-AA COF as a photocatalyst. The reaction proceeds under very mild conditions, delivering the sulfone products in good to excellent yields. Remarkably, the protocol requires extremely low photocatalyst loading. Furthermore, no significant loss in catalytic activity was observed even after six consecutive catalytic cycles. A comparative study of related COFs highlighted the superior red-light harvesting ability of the BT core, as well as the catalytic significance of the AA stacking mode in the Tp-BT COF. ■ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c12697. List of starting materials, experimental procedures, powder X-ray diffraction, FT-IR, TGA, N2adsorption isotherm, photocatalytic optimization studies, and compound characterization data (PDF) ■AUTHOR INFORMATION Corresponding Authors Manuel Souto −Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-393 Aveiro, Portugal; Oportunius, Galician Innovation Agency (GAIN), 15702 Santiago de Compostela, Spain; orcid.org/00000003-3491-6984; Email: [email protected] Martín Fananás-Mastral −Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; Oportunius, Galician Innovation Agency (GAIN), 15702 Santiago de Compostela, Spain; orcid.org/0000-0003-4903-0502; Email: [email protected] Authors Saul Alberca −Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain Akshay M. Nair −Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain Paula Escamilla −Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS), Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; orcid.org/0000-0002-6419-8891 Pedro Ferreira −Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro, 3810-393 Aveiro, Portugal Complete contact information is available at: https://pubs.acs.org/10.1021/jacs.5c12697 Author Contributions ∥ S.A. and A.M.N. contributed equally. Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS Financial support from the European Research Council (ERCCoG 863914-BECAME; ERC-StG 101039748-ELECTROCOFS), Agencia Estatal de Investigación (TED2021132449B−I00; PID2023-152083OA-I00, COFCAT), Xunta de Galicia (ED431C 2022/27; Centro de investigación do Sistema universitario de Galicia accreditation 2023−2027, ED431G 2023/03), Generalitat Valenciana (Prometeo program CIPROM/2022/48) and the European Regional Development Fund (ERDF) is gratefully acknowledged. A.M.N. thanks the European Union’s Horizon Europe research and innovation program for a Marie Skłodowska-Curie postdoctoral fellowship (MePhoCat, No.101150274). P.E. thanks the Agencia Estatal de Investigación for a Juan de la Cierva postdoctoral fellowship (JDC2023-051113-I). P.F. is grateful to FCT for the PhD grant (UI/BD/151049/2021) and from the FCT/MCTES (CICECO-Aveiro Institute of Figure 5. Preliminary mechanistic studies and plausible mechanism. 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