Visible-light-driven graphitic carbon nitride-catalyzed ATRA of alkynes: highly regio- and stereoselective synthesis of (E)-β-functionalized vinylsulfones

Abstract

Heterogeneous photocatalysis has emerged as a powerful and sustainable technique in organic synthesis; however, the application of graphitic carbon nitride (g-C₃N₄) as a heterogeneous photocatalyst for organic transformations is still in its infancy. The development of novel organic transformations catalyzed by g-C₃N₄ is actively being pursued by chemists but it remains challenging. Herein, we describe a visible-light-driven g-C₃N₄-catalyzed atom transfer radical addition (ATRA) of alkynes to synthesize valuable (E)-β-thio/seleno vinylsulfones. This approach features heterogeneous photocatalysis, excellent regio- and stereoselectivities, 100% atom economy, a metal- and additive-free nature, and broad substrate scope covering (hetero)aryl and alkyl alkynes, especially the industrial feedstock propyne. Furthermore, this method can be applied to the late-stage functionalization of complex molecules. Notably, g-C₃N₄ can be recovered and reused in five runs without loss of catalytic activity. Mechanistic studies demonstrate that the reaction occurs via an energy transfer process rather than single electron transfer.

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Introduction

Photoredox catalysis has attracted great interest from chemists not only because of the current global energy and environmental crisis, but also because it is a powerful method for the development of new and valuable organic transformations.¹ The most common photoredox approaches employ homogeneous photocatalysts such as transition-metal complexes based on [Ir] or [Ru]² and organic dyes.³ However, these homogeneous photocatalysts are difficult to recycle and reuse, which may cause high costs and environmental issues. Furthermore, they also suffer from the limitations of incompatibility with reactive radical intermediates, strong nucleophiles and electrophiles, or strong acidic and basic reaction media.⁴ Therefore, growing interest has been focused on using metal-free, recyclable, and stable materials as photocatalysts in visible light-induced reactions.

Polymeric graphitic carbon nitride (g-C₃N₄) as a heterogeneous photocatalyst has opened up new vistas in organic synthesis because of its unique properties as described here (Scheme 1). (1) g-C₃N₄ is easy to prepare from feedstock chemicals and can be recycled multiple times as a photocatalyst.⁵ (2) g-C₃N₄ has excellent stability in the presence of high temperatures (up to 600 °C), reactive radicals, and strong nucleophiles and electrophiles.⁶ (3) g-C₃N₄ is a transition-metal-free organic semiconductor, which can avoid the introduction of transition metals into late-stage functionalization reactions.⁷ (4) g-C₃N₄ possesses a suitable band gap (2.7 eV, λ_Ex ≈ 460 nm) and favorable valence and conduction band positions for the controlled oxidation and reduction of substrates.⁸ (5) g-C₃N₄ is a two-dimensional material with rich surface characteristics and more importantly, the properties of g-C₃N₄ can be readily tuned by synthetic modifications.⁷ Recently, g-C₃N₄ has often been utilized in carbon dioxide reduction,⁹ organic pollutant degradation,¹⁰ and water splitting.¹¹ The application of g-C₃N₄ as a heterogeneous photocatalyst for organic transformations is still in its infancy.¹² Hence, the development of novel organic transformations catalyzed by g-C₃N₄ is highly desired.

Scheme 1 Properties of graphitic carbon nitride.

Vinylsulfones are ubiquitous in pharmaceuticals and bioactive molecules,¹³ and are also important organic building blocks¹⁴ (Scheme 2a). The conventional approaches for the synthesis of vinylsulfones involve addition, oxidation, and elimination reactions¹⁵ (Scheme 2b), which require tedious operations and complex purification processes, and also suffer from poor atom economy. Atom transfer radical addition (ATRA) of alkynes is a straightforward and atom economical way to construct various functionalized vinylsulfones.¹⁶ Nevertheless, the regio- and stereoselectivity control of vinylsulfones via ATRA reaction is difficult and challenging due to the involvement of highly reactive vinyl radical intermediates (Scheme 2c).¹⁷ In order to synthesize the specific configuration of (E)-β-thio/seleno vinylsulfones, Xu developed a gold and photoredox combined ATRA approach.¹⁸ Subsequently, Ji achieved Co- or Cu-catalyzed selenosulfonylation of alkynes to regioselectively synthesize β-seleno vinylsulfones, but with poor stereoselectivity.¹⁹ In the presence of eosin Y and a strong base KOH, Jia altered the regioselectivity of ATRA of alkynes to afford (E)-β-arylsulfonylvinyl sulfides.²⁰ Although some progress in the regio- and stereoselectivity control of vinylsulfones via ATRA reaction has been made, these methods employ complex catalytic systems and extra additives, and also have limited alkyne scope mainly covering aromatic alkynes. Furthermore, the homogeneous nature of catalysts used in the process makes their separation from the product and recycling difficult. The aforementioned aspects would significantly hinder their wide practical application. In light of the unique properties of g-C₃N₄, we wondered if recyclable g-C₃N₄ could be utilized as the sole catalyst for realizing the highly regio- and stereoselective synthesis of valuable thio/seleno vinylsulfones with broader substrate scope via visible light-induced ATRA of alkynes without using metals and additives (Scheme 2d).

Scheme 2 Synthetic strategies for functionalized vinylsulfones.

Results and discussion

Based on the above-mentioned proposal, we began to investigate g-C₃N₄-catalyzed ATRA of phenylacetylene 1a with TsSPh 2a under blue LED irradiation at room temperature. Initially, the reaction was attempted in toluene; however, no product was detected (Table 1, entry 1). Excitingly, when 1,2-dimethoxyethane (DME) was used as the solvent, the reaction exclusively gave (E)-β-sulfonyl addition product 3, although with poor yield (entry 2). The possible competitive products 3′ and 3′′ were not observed. Inspired by the above results, we further screened various solvents and found that DMSO was superior (entries 3–8). It is noteworthy that reducing the amount of g-C₃N₄ or 2a had a remarkable influence on the yield (entries 9 and 10). For comparison, heterogeneous photocatalysts such as BiVO₄ and TiO₂ and homogeneous photocatalysts such as Ru(bpy)₃Cl₂ and Ir(ppy)₃ were also tested. The results showed that 3 was obtained with lower yields (entries 11–14). Control experiments indicated that g-C₃N₄ and visible light were essential to the success of this catalytic transformation (entries 15 and 16, for more optimization details, see Tables S1–S4 in the ESI†).

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst	Solvent	Isolated yield (%)
a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), heterogeneous PC (5 mg) or homogeneous PC (1 mol%) in solvent (1 mL) at room temperature under blue LED (7 W) irradiation for 17 h under argon. b g-C3N4 (3 mg). c 2a (0.12 mmol). d Without light.
1	g-C3N4	Toluene	NR
2	g-C3N4	DME	45
3	g-C3N4	THF	67
4	g-C3N4	CH3CN	Trace
5	g-C3N4	CH2Cl2	73
6	g-C3N4	DMF	88
7	g-C3N4	DMSO	89
8	g-C3N4	Acetone	86
9^b	g-C3N4	DMSO	33
10^c	g-C3N4	DMSO	51
11	BiVO4	DMSO	9
12	TiO2	DMSO	7
13	Ru(bpy)3Cl2	DMSO	77
14	Ir(ppy)3	DMSO	50
15	—	DMSO	11
16^d	g-C3N4	DMSO	NR

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With the optimal reaction conditions established, we explored the generality of the method by testing various substrates 1 and 2 (Scheme 3). Notably, compared with Co- or Cu-catalyzed selenosulfonylation of alkynes with poor stereoselectivity,¹⁹ the newly developed approach could exclusively generate (E)-β-seleno vinylsulfones with a modified procedure. We first investigated different aryl alkynes and found that both electron-donating and electron-withdrawing groups at different positions (ortho, meta, or para) of phenyl rings and a naphthalene ring were well tolerated, giving the desired thio/seleno vinylsulfones 3–24 in good yields with excellent regio- and stereoselectivities. Many potentially reactive groups, such as formyl (18 and 19), nitrile (21) and ester (22), were left untouched under the standard conditions. The structure of 19 was further confirmed by X-ray crystallography. In addition, mono- and double selenosulfonylation of 1,4-diethynylbenzene (24 and 25) were realized by tuning the amount of 2. The reaction with heteroaryl alkynes including medicinally relevant heterocycles, such as pyridine (26), thiophene (27–29) and quinoline (30 and 31), also gave outstanding outcomes. Direct synthesis of complex synthetic intermediates or medicinally relevant compounds from industrial feedstock is a central goal of organic synthesis. As such, the simple commodity chemical propyne was subjected to the standard conditions, providing the corresponding products in good yields with excellent regio- and stereoselectivities (32 and 33). Besides, other acyclic and cyclic aliphatic alkynes with different functional groups such as chloro (44 and 45), acetate (46 and 47), and keto (48 and 49) were also well tolerated (34–49). The configuration of 36 was also confirmed by NOESY spectra. It is worth noting that for 1-ethynylcyclohex-1-ene, the alkyne reacted chemoselectively over the alkene (50 and 51). Subsequently, various thiosulfonates were examined and both aromatic and aliphatic thio reagents were suitable to deliver the thio vinylsulfones (52–56). More significantly, to further demonstrate the value of this protocol, we evaluated the late-stage functionalization of a series of complex alkynes derived from biologically active molecules such as estrone (57 and 58), L-phenylalanine (59), L(−)-borneol (60 and 61), atomoxetine (62 and 63), and diacetonefructose (64). The desired (E)-β-thio/seleno vinylsulfones were exclusively formed in moderate to excellent yields.

Scheme 3 Substrate scope. Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol) and g-C₃N₄ (10 mg) in DMSO (2 mL) at rt for thiosulfonates and DCM (2 mL) at −20 °C for selenosulfonates. Isolated yields are reported. ^a2 (4 equiv.). ^b2 (5 equiv.). ^cPropyne (5 equiv.). ^dReaction time is 72 h.

The potential of this method was further demonstrated by large-scale synthesis of 3 with an 81% yield (Scheme 4a). Furthermore, the resulting vinylsulfones are versatile synthetic building blocks and their synthetic transformations were also explored (Scheme 4b). Upon treatment of 3 with Ni(acac)₂ and MeMgBr, the coupling product 65 was obtained in 73% yield. 3 could be reduced by LiAlH₄ to generate 66 in 65% yield. Noteworthily, the selective oxidation of 3 was realized under different oxidative conditions (67 and 68).

Scheme 4 Synthetic applications.

We next studied the stability of g-C₃N₄ by carrying out a recycling experiment and g-C₃N₄ was recycled five times without losing its catalytic activity, highlighting its high potential for sustainable photocatalysis (Fig. 1a). Furthermore, X-ray powder diffraction (XRD) of recycled g-C₃N₄ displayed a characteristic peak at 27.6 which was consistent with that of fresh g-C₃N₄ (Fig. 1b). Scanning electron microscopy (SEM) showed that there was no obvious difference in the morphology of fresh g-C₃N₄ and recycled g-C₃N₄ (Fig. 1c). The characteristic peaks at 809 cm⁻¹ of triazine and in the range of 1200–1650 cm⁻¹ of recycled g-C₃N₄ were retained when analysed by Fourier-transform infrared spectroscopy (FT-IR) (Fig. 1d). These experiments demonstrated the high sustainability and photostability of g-C₃N₄.

Fig. 1 Evaluation of g-C₃N₄ recycling. ^aRecycling experiment. ^bXRD patterns. ^cSEM images. ^dFT-IR spectra.

To gain insight into the reaction mechanism, control experiments were conducted. When the radical scavenger TEMPO or ethene-1,1-diyldibenzene was added, the reaction was completely inhibited and the corresponding adducts were detected by HRMS, indicating the radical nature of the reaction (Scheme 5a). Furthermore, 69 underwent radical cyclization with 2a to deliver pyrrolidine 70 in 75% yield, which suggested that sulfonyl and sulfenyl radicals were involved in the reaction (Scheme 5b). To further verify the production mechanism of the sulfonyl and sulfenyl radicals, a cross-over experiment with 2b and 2c was performed and scrambled thiosulfonates 2d and 2e were obtained in 19% yield, showing that the sulfonyl and sulfenyl radicals were generated via the homolytic cleavage of the SO₂–S bond rather than single electron transfer (SET) in the presence of g-C₃N₄ (Scheme 5c). Furthermore, the quenching of electrons and valence band holes had almost no influence on the reaction (Scheme 5a). When the standard chemical one-electron reductants (SmI₂ and Zn) or an oxidant (ceric ammonium nitrate) were used instead of g-C₃N₄, 3 was hardly generated. On the other hand, the use of benzil as a triplet sensitizer led to the formation of 3 (Scheme 5d).²¹ These observations further supported that the reaction was initiated via an energy transfer pathway. In order to check if the acidic proton on the alkyne played a role in the reaction, deuterated alkyne [D]-1b was subjected to the standard conditions and 5′ was obtained with no observation of H/D exchange. Furthermore, no H/D-exchange products were detected in the NMR spectrum with D₂O isotopic labeling. These experimental results indicated that the acidic proton did not participate in the reaction, which was in consistent with a mechanism involving radical addition to the triple bond (Scheme 5e). The quantum yield of the model reaction was determined to be 3.67, showing that a free radical chain mechanism might be involved in the reaction. The Hammett plot of our reaction system was constructed and a negative slope (ρ = −0.58) was observed (see Fig. S3 in the ESI†), which indicated that (1) the electron-deficient alpha-alkenyl radical was formed and that (2) this was a stepwise reaction and the radical addition to the triple bond was the rate-determining step.²²

Scheme 5 Mechanistic studies. ^a19F NMR yield with hexafluorobenzene as the internal standard.

Based on the experimental results and literature precedents, a plausible reaction mechanism was proposed (Scheme 6). Initially, an energy transfer to 2a from excited-state g-C₃N₄* occurred to yield excited 2a* and regenerate ground-state g-C₃N₄ under visible-light irradiation. Then, sulfenyl radical A and sulfonyl radical B were generated via the homolytic cleavage of the SO₂–S bond in 2a*. The sulfonyl radical B was intercepted by phenylacetylene 1a to give α-alkenyl carbon radical C. Finally, the target product 3 was formed via the addition of A or 2a to vinyl radical C.

Scheme 6 Proposed reaction mechanism.

Conclusions

In summary, we have developed a novel strategy for ATRA of alkynes to construct (E)-β-thio/seleno vinylsulfones utilizing a heterogeneous photocatalyst g-C₃N₄ under visible-light irradiation. Key features of this method include heterogeneous photocatalysis, excellent regio- and stereoselectivities, 100% atom economy, a metal- and additive-free nature, and broad substrate scope covering (hetero)aryl and alkyl alkynes, especially the industrial feedstock propyne. In addition, the potential of this method is further demonstrated by the late-stage modification of complex molecules and large-scale synthesis. The sustainability and photostability of g-C₃N₄ is also evaluated by multiple recycle experiments. Preliminary mechanistic studies indicate the involvement of an energy transfer process in the reaction. g-C₃N₄-catalyzed valuable organic transformations are currently being explored further in our laboratory.

Author contributions

S.-L. Xie, J.-Z. Yan, M.-J. Xie, X. Li, F. Zhou, M.-Q. Zheng, X.-L. Wang, J. Feng, Y. Zhang and Y.-N. Duan performed the experiments. Y.-D. Niu, D. Li, and H.-D. Xia wrote and revised the paper. All authors discussed the results and commented on the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22301094), the Guangdong Basic and Applied Basic Research Foundation (2023A1515010728, 2022A1515110829 and 2023A1515010873), the Special Fund for the Sci-Tech Innovation Strategy of Guangdong Province (STKJ202209066, 210730166882026) and the Chemistry and Chemical Engineering Guangdong Laboratory (2111008, 2011006, 2132013 and 2211003).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2267691. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3gc02897f

‡ These authors contributed equally to this work.

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