Visible-light-induced radical cascade cyclization of 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl chlorides for the synthesis of sulfonated benzoxepines

Abstract

A visible light photocatalyzed cascade sulfonylation/cyclization of 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl chlorides for the construction of sulfonated benzoxepines is developed. In the presence of Eosin Y (2.0 mol%) as a photocatalyst and Na₂CO₃ as a base, the reactions proceed smoothly to afford seven-membered rings in good yields. This transformation features wide substrate scope, the use of easily accessible materials and excellent functional group tolerance.

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Benzoxepines are ubiquitous oxygen-containing heterocycles widely found in a variety of natural alkaloids and biologically active compounds, such as pterulone, IV ptaeroxylin and bauhinoxepin A (Scheme 1a).¹ Therefore, the development of efficient and straightforward synthetic strategies to easily assemble benzoxepine scaffolds has gained substantial attention. In the past decade, various synthetic methodologies have been established for the construction of the benzoxepine skeleton.² Among the elegant strategies for the synthesis of benzoxepines are transition-metal-catalyzed intramolecular cyclization reactions³ and transition-metal-catalyzed intermolecular [m + n]-cyclization reactions.⁴ For example, Saá and co-workers reported an osmium-catalyzed regioselective 7-endo heterocyclization of aromatic alkynols to form benzoxepines in 2010.^3a Very recently, the Ruijter group employed a Pd-catalyzed intermolecular cascade [4 + 3] cyclocondensation of salicylaldehydes with vinylcyclopropanes for the synthesis of benzoxepines.^4b It should be noted that many organic chemists, for example Heo's,^5a Reddy's,^5b Jiang's,^5c and other groups,^5d,e have reported the synthesis of these seven-membered heterocyclic compounds under transition-metal-free conditions. In contrast, radical methodologies for the construction of benzoxepines have been much less well-developed. For instance, in 2018, Yang and co-workers developed a visible light induced cascade cyclization of (E)-1-(2-(allyloxy)phenyl)-3-(dimethylamino)-prop-2-en-1-ones with 2-bromo-2,2-difluoro-acetic acid ethyl ester (BrCF₂COOEt), generating functionalized CF₂-containing benzoxepine derivatives.⁶ Despite these significant advances, the exploration of an efficient, convenient and ecofriendly approach to diverse functionalized benzoxepines is still of importance.

Scheme 1 (a) Examples of biologically active compounds containing the benzoxepine scaffold; (b) Our work concept.

Organosulfone compounds have found extensive use in organic and medicinal chemistry.⁷ Indeed, heterocyclic compounds containing a sulfone group exhibit unique chemical activities and biological properties.⁸ Therefore, developing a new, sustainable and straightforward protocol for the construction of sulfone-containing compounds is considered highly desirable.⁹ Sulphonyl chlorides are commercially available reagents and relatively stable, and have been widely used as sulfonylation agents to synthesize organosulfones.¹⁰ To date, there has been a lack of methods for the construction of sulfonated benzoxepines via sulfonylation and cyclization of 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl radicals, which are formed from sulphonyl chlorides.

Visible light induced photoredox catalysis has been a powerful tool for synthetic conversions in organic synthesis due to its safety, high efficiency, operational simplicity and environmental friendliness.¹¹ Very recently, our group reported a visible-light-induced synthesis of 4-sulfonated cyclopenta[gh]phenanthridines via radical cyclization of 3-(arylethynyl)-[1,1′-biphenyl]-2-carbonitriles.^11k In continuation of our interest in the construction of sulfone-containing compounds,¹² we reported herein the visible-light-induced cascade sulfonylation/cyclization of 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl chlorides for the construction of sulfonated benzoxepines (Scheme 1b).

Initially, we chose 1-(allyloxy)-2-(1-phenylvinyl)benzene 1a and p-tosyl chloride 2a as model substrates to identify the optimal reaction conditions. As shown in Table 1, the reaction employing fac-Ir(ppy)₃ as a photocatalyst, K₂CO₃ as a base, MeCN as a solvent and irradiation with a 12 W blue LED light source at 100 °C under an argon atmosphere for 18 h can afford the product 3aa in 58% yield. Several photoredox catalysts such as Ru(bpy)₃(PF₆)₂ and Eosin Y were evaluated, and Eosin Y could afford a higher yield than fac-Ir(ppy)₃ (entries 2 and 3). Then, a series of bases such as NaHCO₃, NaOAc, Na₂HPO₄, Na₂CO₃, Et₃N and 2,6-lutidine were examined (entries 4–9), and Na₂CO₃ was determined to be the best choice (entry 7). Several other solvents such as DCM, THF, DMF and DMSO were screened, and none of them could afford higher yields than MeCN (entries 10–13). When the reaction was conducted at room temperature, product 3aa was obtained (entry 14). Raising the reaction temperature or decreasing the reaction temperature did not give a better result (entries 15 and 16). When a 33 W fluorescent light bulb was used as a light source, 3aa was obtained in 83% yield (entry 17). Using 5 W blue LED light instead of 12 W blue LED light also could not improve the reaction yield (entry 18). Finally, the desired product 3aa was not obtained when the reaction was conducted in the absence of Eosin Y or visible light (entries 19 and 20), and only 16% yield of 3aa was detected without a base (entry 21).

Table 1 Optimization of reaction conditions^a

Entry	Photocatalyst	Base	Solvent	Yield^b (%)
a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), base (0.25 mmol), photocatalyst (2 mol%), solvent (1 mL), a 12 W blue LED under Ar at 100 °C for 18 h. b Isolated yield. c Reaction temperature: 25 °C. d Reaction temperature: 80 °C. e Reaction temperature: 120 °C. f Using a 33 W fluorescent light bulb. g Using a 5 W blue LED light. h In the dark.
1	fac-Ir(ppy)3	K2CO3	MeCN	58
2	Ru(bpy)3(PF6)2	K2CO3	MeCN	10
3	Eosin Y	K2CO3	MeCN	67
4	Eosin Y	NaHCO3	MeCN	66
5	Eosin Y	NaOAc	MeCN	57
6	Eosin Y	Na2HPO4	MeCN	70
7	Eosin Y	Na2CO3	MeCN	88
8	Eosin Y	Et3N	MeCN	35
9	Eosin Y	2,6-Lutidine	MeCN	58
10	Eosin Y	Na2CO3	DCM	53
11	Eosin Y	Na2CO3	THF	41
12	Eosin Y	Na2CO3	DMF	12
13	Eosin Y	Na2CO3	DMSO	15
14^c	Eosin Y	Na2CO3	MeCN	0
15^d	Eosin Y	Na2CO3	MeCN	75
16^e	Eosin Y	Na2CO3	MeCN	84
17^f	Eosin Y	Na2CO3	MeCN	83
18^g	Eosin Y	Na2CO3	MeCN	78
19^h	Eosin Y	Na2CO3	MeCN	0
20	—	Na2CO3	MeCN	0
21	Eosin Y	—	MeCN	16

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With the optimized conditions in hand, we then investigated the scope of sulfonyl chlorides 2 with 1-(allyloxy)-2-(1-phenylvinyl)benzene 1a for the synthesis of sulfonated benzoxepines. As shown in Table 2, arylsulfonyl chlorides bearing both electron-donating (MeO and tBu) and electron-withdrawing (F, Cl and Br) groups at the para-position of the aromatic ring underwent this transformation smoothly, and the desired products 3ab–3af were obtained in 78–85% yields. The meta- and ortho-substituted arylsulfonyl chlorides (2g and 2h) afforded the corresponding products (3ag and 3ah) in 84% and 76% yield, respectively. 4-Nitrobenzenesulfonyl chloride (2i) was compatible with the standard reaction conditions, providing the corresponding product 3ai in 77% yield. In addition, benzenesulfonyl chloride (2j) was also suitable for this conversion to give the desired product 3aj in good yield. Moreover, naphthalene-1-sulfonyl chloride (2k), naphthalene-2-sulfonyl chloride (2l) and thiophene-2-sulfonyl chloride (2m) were good sulfonating agents, delivering the corresponding benzoxepines 3ak–3am in 63% to 75% yields. Furthermore, aliphatic sulfonyl chlorides, such as methanesulfonyl chloride and cyclopropanesulfonyl chloride, were also proved to be viable substrates for this conversion, providing the desired products 3an and 3ao in 36% and 70% yields, respectively.

Table 2 Scope of sulfonyl chlorides^a

a Reaction conditions: 1a (0.1 mmol), 2 (0.2 mmol), Na₂CO₃ (0.25 mmol), Eosin Y (2 mol%), MeCN (1 mL), 12 W blue LEDs under Ar at 100 °C for 18 h; isolated yields. b 5 mmol scale.

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Next, the scope of the reaction with various 1-(allyloxy)-2-(1-arylvinyl)benzenes 1 was examined, as shown in Table 3. First, we investigated the effect of the substitution pattern on a benzene ring tethered alkene. The substrates bearing both electron-donating (Me) and electron-withdrawing (Br and CF₃) groups at the 2 or 4 positions of the phenyl ring could be engaged in this reaction and gave the desired 3ba–3ea in 73–86% yields. Subsequently, we investigated the effect of the substitution pattern on the aromatic ring of the phenol moiety. This cascade reaction was tolerant of electron-donating (Me and MeO) and electron-withdrawing (F, Cl and Br) groups at the 4 or 5 positions of the phenol moiety, and the desired sulfonated benzoxepines 3fa–3ka were obtained in moderate to good yields, suggesting that the position of the substituent on the aryl ring has limited effects on the reaction outcome. The structure of 3ka was confirmed by single-crystal X-ray analysis (see the ESI† for details). Furthermore, substrates bearing disubstituted groups, such as 4,6-ditert-butyl and 4,6-dichloro groups, were also suitable for this cascade cyclization, producing 3la and 3ma in 68% and 65% yields, respectively. Gratifyingly, 2-(allyloxy)-1-(1-phenylvinyl)naphthalene (1n) was also compatible with the optimized conditions, and the desired product 3na was obtained in 56% yield. It should be noted that 1-((2-methylallyl)oxy)-2-(1-phenylvinyl)benzene (1o) was used to react with 2a, affording the product 3oa in good yield. Unfortunately, when R² is a methyl group, no desired product 3pa was detected probably due to the lower stability of the benzyl radical intermediate. To our delight, N-allyl-4-methyl-N-(2-(1-phenylvinyl)phenyl)benzenesulfonamide 1q and 1-(but-3-en-1-yl)-2-(1-phenylvinyl)benzene 1r were also amenable to this transformation, giving the products 3qa and 3ra in 89% and 65% yields, respectively.

Table 3 Scope of 1-(allyloxy)-2-(1-arylvinyl)benzenes^a

a Reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol), base (0.25 mmol), Eosin Y (2 mol%), MeCN (1 mL), 12 W blue LEDs under Ar at 100 °C for 18 h; isolated yields.

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To understand the reaction mechanism, several radical trapping experiments were conducted, as shown in Scheme 2; when a radical scavenger such as TEMPO (2.5 equiv.) was added to the reaction, the reaction was completely inhibited and the TEMPO-Ts adduct 4 was detected by HRMS. When 1,4-dinitrobenzene was added to this reaction system, the desired product 3aa was not detected. The experimental results suggested that a radical process might be involved in this transformation (see the ESI† for details).

Scheme 2 Control experiment.

According to the presented results and literature,¹⁰ a plausible reaction mechanism was proposed, as shown in Scheme 3. Initially, the excited state Eosin Y* was formed under visible-light irradiation, and it was further oxidized by p-tosyl chloride 2a to generate Eosin Y⁺˙ and radical anion I. The radical anion I undergoes decomposition to generate the corresponding sulfonyl radical II, which causes an addition to the C=C bond of 1a to afford the alkyl radical III. Then, an intramolecular addition of radical III to the other C=C bond leads to benzyl radical intermediate IV. Subsequently, the radical IV is oxidized by Eosin Y⁺˙ to form intermediate V with the concurrent regeneration of Eosin Y. Finally, a deprotonation step occurred to provide product 3aa.

Scheme 3 Proposed mechanism.

In summary, we have successfully developed a facile and efficient protocol for the synthesis of sulfonated benzoxepine derivatives via a visible light induced cascade sulfonylation/cyclization process involving the reaction of the easily prepared 1-(allyloxy)-2-(1-arylvinyl)benzenes with sulfonyl chlorides. This protocol featured wide substrate scope, excellent functional group tolerance and good yields of products, providing a general approach toward sulfonated benzoxepines. Further reaction mechanism studies and applications of 1-(allyloxy)-2-(1-arylvinyl)benzenes are underway in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (21901006) and the Anhui Provincial Natural Science Foundation (1908085QB80). We thank Dr Yun Wei (AHNU) for her help with the X-ray crystallographic analyses.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2074239. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1qo00611h

‡ These authors contributed equally to this work.

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