Synthesis of sulfur- and oxygen-bridged cationic [4]-helicenes mediated by Friedel–Crafts-S_NAr tandem reactions for red-light-driven organophotoredox catalysis

Abstract

The synthesis of sulfur- and oxygen-bridged cationic [4]-helicenes via a tandem Friedel–Crafts–S_NAr reaction of a diaryl sulfide or a diaryl ether with a (thio)salicylic acid has been developed. The sulfur-bridged cationic [4]-helicenes are suitable as catalysts for photoredox reactions under low-energy light sources such as red LED light.

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Cationic helicenes, originating from the pioneering studies on cationic triarylmethyl organic dyes by Hofmann, Verguin, and Lauth, have attracted considerable attention in organic chemistry owing to their unique structural and electronic properties.¹ Oxygen- and nitrogen-bridged cationic helicenes are typically synthesized via S_NAr reactions of triarylmethyl cations, which are prepared from 1,3-dimethoxybenzene and diethyl carbonate (Fig. 1(a)).² In contrast to the numerous reports on oxygen- and nitrogen-bridged cationic helicenes, only two examples of sulfur-bridged cationic helicenes have been described, both synthesized via a two-step route starting from thioxanthone (Fig. 1(b)).³ Moreover, the physical properties of sulfur-bridged cationic helicenes remain underexplored; for example, to date, no reports have addressed their application as photoredox catalysts. Therefore, the development of efficient synthetic routes to sulfur-bridged cationic helicenes as well as the investigation of their physical properties and potential applications are highly desirable.

Fig. 1 Representative synthetic routes to cationic helicenes and red-light-activated organophotoredox catalysts.

Visible-light-mediated photoredox catalysis has seen widespread development, particularly in the context of sustainable energy-conversion systems.⁴ Conventional photoredox reactions often rely on the use of high-energy light sources such as blue LED light; however, recent advances in energy-efficient systems have prompted a shift in attention toward lower-energy light sources such as red LED light. Although metal-based photoredox catalysts that are activated by red light have been extensively studied, red-light-activated organophotoredox catalysts remain relatively underexplored (Fig. 1 (c)).⁵ In particular, although Gianetti and co-workers reported a cationic helicene photocatalyst, [ⁿPr-DMQA⁺][BF₄⁻], that functions as a red-light-driven photoredox catalyst in 2020,^5b only a few examples of cationic helicene photocatalysts have been reported to date.

Recently, we have reported the design and synthesis of thioxanthylium-based organophotoredox catalysts (Fig. 1, ‘Our previous work’) which were prepared from diaryl sulfides and aryl-acid chlorides in the presence of trifluoromethanesulfonic acid (TfOH) via a Friedel–Crafts reaction.⁶ These catalysts can be activated by green LED light to efficiently promote a variety of photoredox reactions.⁷ Accordingly, we hypothesized that using (thio)salicylaldehyde instead of aryl-acid chloride could allow performing a Friedel–Crafts–S_NAr tandem reaction to yield cationic helicenes. Here, we report the synthesis of sulfur- and oxygen-bridged cationic helicenes via Friedel–Crafts–S_NAr tandem reactions and their application as organophotoredox catalysts under red-light irradiation (Fig. 1, ‘This work’).

To evaluate the relationship between their structural and physical properties, various cationic [4]-helicenes (TXTH-1–4) were initially synthesized in moderate to good yield via Friedel–Crafts–S_NAr tandem reactions between diaryl (thio)ether 1 and (thio)salicylic acid 2 in chlorobenzene at 120 °C (24 h) in the presence of TfOH (Table 1).⁶ The as-obtained cationic [4]-helicenes TXTH-1–4 exhibit moderate excited-state reduction potentials (E_1/2(C*/C^•−) = +1.51 to +1.76 V vs. SCE) and an absorption band in the visible spectrum. Compared with TXT (Fig. 1), the cyclic voltammograms of TXTH-1 revealed slightly negatively shifted reduction potentials (Fig. 2). Furthermore, the absorption spectrum is markedly red-shifted relative to that of TXT (Fig. 3), which can be attributed to the π-extension of the framework. In particular, TXTH-1 exhibits a broad absorption band extending to the red region of the visible spectrum (λ_abs < 700 nm), suggesting that it could be activated by lower-energy irradiation sources such as red light.

Table 1 Synthesis and physical properties of cationic [4]-helicenes

Catalyst	Yield^a (%)	E1/2(C*/C˙^−)^b (V)	E1/2(C/C˙^−)^c (V)	E0,0 (eV)	Excitation λmax (nm)	Emission λmax (nm)
a All reactions were conducted using diaryl (thio)ether 1 (0.2 mmol), (thio)salicylaldehyde acid 2 (0.6 mmol), and TfOH (0.6 mmol) in chlorobenzene (120 °C; N2 atmosphere; 24 h).b Excited-state oxidation potentials were estimated on the basis of the ground-state redox potentials and the intersection of the absorption and emission bands.c Determined by cyclic voltammetry in CH3CN vs. SCE.d The values for TXT were extracted from previous reports.^6a
TXTH-1	71	+1.51	−0.42	1.93	517	682
TXTH-2	72	+1.63	−0.46	2.09	481	647
TXTH-3	69	+1.54	−0.56	2.10	477	630
TXTH-4	72	+1.76	−0.58	2.34	457	600
TXT^d	—	+1.76	−0.39	2.15	464	630

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Fig. 2 Cyclic voltammograms of TXTH-1 and TXT (10 mM); supporting electrolyte: tetra-n-butylammonium perchlorate (100 mM); in CH₃CN.

Fig. 3 Absorption spectra of TXTH-1 and TXT in CH₃CN (10 µM).

Encouraged by these promising results, we investigated TXTH-1 as a catalyst in photoredox reactions (Scheme 1). When the [2+2] cycloaddition reaction of trans-anethole (3) with styrene (4) was conducted in the presence of TXTH-1 under red-LED-light irradiation, the desired product (5) was obtained in high yield.^7c Since the excited-state reduction potential of TXTH-1 is higher than that of trans-anethole (3), one-electron oxidation efficiently occurs (TXTH-1: E_1/2(C*/C˙⁻) = +1.51 V vs. SCE; 3: E_p/2 = +1.24 V vs. SCE). Notably, TXTH-1 also catalyzed a radical-cation Diels–Alder reaction, affording the desired cyclohexene derivative (7) in high yield.^6a Finally, p-quinone methide (9) underwent efficient trifluoromethylation to give the corresponding product (10) in 85% yield.⁸ This transformation proceeds via oxidation of the Langlois reagent (8; E_p/2 = +1.05 V vs. SCE) by TXTH-1 to generate a trifluoromethyl radical, which subsequently adds to p-quinone methide (9). These results demonstrate that TXTH-1 can serve as an effective catalyst for various photoredox reactions under red-LED-light irradiation.

Scheme 1 Photoredox reactions catalyzed by TXTH-1 under red-LED-light irradiation.

In summary, we have developed a synthetic route to cationic sulfur and oxygen-bridged [4]-helicenes based on Friedel–Crafts–S_NAr tandem reactions. The reactions of a diaryl (thio)ether with a (thio)salicylic acid in the presence of TfOH successfully yield the cationic [4]-helicenes in good yield. These catalysts exhibit moderate excited-state reduction potentials (E_1/2(C*/C^•−) = +1.51 to +1.76 V vs. SCE) and an absorption band in the visible spectrum. Especially TXTH-1 produces an absorption band that extends to the red region of the visible spectrum, thus enabling the efficient promotion of [2+2] cycloaddition, radical-cation Diels–Alder reaction, and trifluoromethylation under red-LED-light irradiation. The present reaction provides a promising platform for the synthesis of cationic helicenes and the development of red-light-driven organophotoredox catalysis.

This work was supported by Murata Science Foundation, Ichiju Industrial Science and Technology Promotion Foundation, Foundation for Interaction in Science & Technology, ENEOS Tonen General Research/Development Encouragement & Scholarship Foundation, The Yakumo Foundation for Environmental Science, Okayama Foundation for Science and Technology, JKA and its promotion funds from KEIRIN RACE, Amano Institute of Technology, Casio Science Promotion Foundation, Wesco Scientific Promotion Foundation, and JSPS Program for Forming Japan's Peak Research Universities (J-PEAKS) Grant Number JPJS00420230010.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5cc07398g.

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