A metal-free sigmatropic rearrangement/cyclization/aromatization cascade reaction of hydroxy/aminophenyl propargyl alcohols with fluoroalkanesulfinyl chlorides: synthesis of 3-fluoroalkanesulfonyl benzofurans and indoles

Abstract

A metal-free protocol to synthesize 2-alkyl-3-fluoroalkanesulfonyl benzofurans and indoles from o-hydroxyphenyl/o-aminophenyl propargyl alcohols or 2-propynolphenols/2-propynolanilines and fluoroalkanesulfinyl chlorides (R_FSOCl) was disclosed. The mechanism is proposed to involve a sequential [1,2]/[2,3]-sigmatropic rearrangement/intramolecular oxy-addition/1,3-H migration cascade of propargyl triflinates generated in situ via O-fluoroalkanesulfinylation of propargyl alcohols. The procedure utilizes readily available propargyl alcohols and R_FSOCl as starting materials and offers a broad substrate scope with excellent performance (55 examples, up to 98% yield).

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Introduction

Aryl and heteroaryl fluoroalkyl sulfones are important structural motifs commonly found in bioactive compounds,¹ chiral catalysts,² and functional materials.³ A number of direct and indirect synthetic methods toward aryl fluoroalkanesulfonylated units have been reported in the literature;⁴ however, only a few synthetic studies on benzofuryl and indolyl fluoroalkyl sulfones have been revealed hitherto. For the synthesis of benzofuryl fluoroalkyl sulfones, two representative synthetic methods have been developed. In 2016, Zhang and coworkers achieved 3-(trifluoromethanesulfonyl)benzofuran from [(2-methoxyphenyl)ethynyl](phenyl)iodonium tosylate and CF₃SO₂Na, but this method does not apply to sulfonyl benzofurans bearing a substituent at C2 (Scheme 1a).⁵ Very recently, during the preparation of this manuscript, Xu and coworkers reported a tandem cyclization of o-hydroxyphenyl propargyl alcohols with sodium fluoroalkanesulfinates (R_FSO₂Na) in 35–84% yields (Scheme 1b).⁶ This protocol involves a sulfur-Michael addition/cyclization process and shows a relatively broad reaction scope; however, many examples in this work suffer from moderate yields. For the synthesis of indolyl fluoroalkyl sulfones, there are several direct protocols in addition to oxidation from the corresponding fluoroalkylthiolated indoles.⁷ Shibata and coworkers developed a direct Friedel–Crafts trifluoromethanesulfonylation system of indoles with Tf₂O/TTBP (2,4,6-tri-tert-butylpyridine) under mild conditions in 2011 (Scheme 1c).⁸ In 2012 and 2013, they reported anionic thia-Fries rearrangement strategies to synthesize hydroxy heteroaryl triflones from the corresponding triflates.⁹ In 2021, Xu and coworkers constructed 2-methyl-3-sulfonylindoles via decarboxylative propargylation/hydroamination of ethynyl benzoxazinanones using sodium sulfinates (Scheme 1d).¹⁰ Despite significant progress toward fluoroalkanesulfonyl benzofurans and indoles, a “universal” synthetic method for both fluoroalkanesulfonyl benzofurans and indoles from readily available building blocks is still highly desirable.

Scheme 1 Representative methods for the synthesis of 3-fluoroalkanesulfonyl benzofurans and indoles.

Trifluoromethanesulfinyl chloride (CF₃SOCl) is a cheap commercially available reagent, and it could also be easily synthesized by treatment of CF₃SO₂Na with SOCl₂. It has attracted increasing attention as a low cost trifluoromethanethiolation,¹¹ trifluoromethanesulfinylation,¹² and trifluoromethanesulfonylation¹³ reagent. Disproportionation of the CF₃SO– group to CF₃SO₂– and CF₃S– often occurs in situ when CF₃SOCl reacts with indoles, thiophenes, ketones, alkynes,¹⁴ β-diketones,¹⁵ 2,3-allenoates¹⁶ and other substrates, leading to trifluoromethanethiolated products. Recently, Liu and coworkers achieved trifluoromethanesulfinylation of activated arenes with CF₃SOCl in the presence of Lewis acid and trifluoromethanesulfinylation of indoles without an additive. The disproportionation was avoided by modulating the reaction conditions.¹² CF₃SOCl could serve as a trifluoromethanesulfonylating reagent via sigmatropic rearrangement. Gao and coworkers reported an O-trifluoromethanesulfinylation/[2,3]-sigmatropic rearrangement cascade reaction of arylhydroxylamine to afford ortho-trifluoromethanesulfonylated aniline derivatives.¹³ Based on our continuous research on CF₃SOCl¹⁷ and o-hydroxyphenyl or o-aminophenyl propargyl alcohols,¹⁸ we designed an O-fluoroalkanesulfinylation/sigmatropic rearrangement¹⁹/cyclization/aromatization cascade reaction to synthesize both benzofuryl and indolyl fluoroalkyl sulfones under metal-free conditions.

Results and discussion

To test the feasibility of the designed reaction, o-hydroxyphenyl propargyl alcohol 1a was initially treated with CF₃SOCl (1.50 equiv.) in the presence of 2-methylpyridine (2-Me-py, 1.50 equiv.) in tetrahydrofuran (THF) at 40 °C (Table 1, entry 1). Fortunately, a new compound was isolated as a yellow solid, the structure of which was identified as benzofuran triflone 3a by NMR analysis and further supported by single-crystal X-ray diffraction (CCDC 2405045†).²⁰ Screening of various solvents revealed that toluene was superior to others (Table 1, entries 2–4) and allowed the formation of 3a in 83% yield. Different bases were screened and DABCO (1,4-diazabicyclo[2.2.2]octane) was found to give the best yield (Table 1, entries 5–11). A diminished yield of 70% was observed at room temperature (Table 1, entry 12). Elevating the reaction temperature to 50 °C afforded an excellent yield of 96%, but a further increase of the reaction temperature led to a slight decrease in yield (Table 1, entries 13 and 14). A control experiment was conducted and the result showed that no desired product was formed without a base (Table 1, entry 15). After the successful synthesis of benzofuran triflone 3a, the optimal conditions were tested in the construction of indole triflone 5a. Gratifyingly, the desired product was obtained in 79% yield (Table 1, entry 16). Next, 2 equivalents of DABCO were used, allowing the obtention of 4a in 88% yield (Table 1, entry 17). We also tried to further optimize the yield by changing the reaction temperature, but the yield decreased to 80% (Table 1, entry 18). Further details of the reaction optimization are summarized in the ESI (Table S1†).

Table 1 Optimization of the reaction conditions^a

Entry	X	Base	Solvent	Temp. (°C)	t (h)	Yield^b (%)
a Reaction conditions: compounds 1a or 4a (0.2 mmol), 2a (0.3 mmol), and a base (0.3 mmol) were stirred in a solvent (3 mL) at an indicated temperature. b Yield of the isolated product. c 0.4 mmol of DABCO was used.
1	O	2-Me-py	THF	40	24	50
2	O	2-Me-py	DCM	40	10	60
3	O	2-Me-py	ACN	40	10	65
4	O	2-Me-py	Toluene	40	3	83
5	O	Pyridine	Toluene	40	10	35
6	O	2,6-Lutidine	Toluene	40	10	80
7	O	Morpholine	Toluene	40	3	41
8	O	Piperidine	Toluene	40	3	30
9	O	Imidazole	Toluene	40	3	7
10	O	DMAP	Toluene	40	3	65
11	O	DABCO	Toluene	40	3	85
12	O	DABCO	Toluene	rt	3	70
13	O	DABCO	Toluene	50	1	96
14	O	DABCO	Toluene	60	1	91
15	O	—	Toluene	40	3	0
16	NTs	DABCO	Toluene	50	0.5	79
17^c	NTs	DABCO	Toluene	50	0.5	88
18^c	NTs	DABCO	Toluene	70	0.5	80

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With the optimal reaction conditions in hand (Table 1, entries 13 and 17), we next evaluated the generality of this cascade reaction. First, we addressed different o-hydroxyphenyl propargyl alcohols (Scheme 2). Our method is compatible with a broad range of substitution patterns. Both electron-donating (such as Me and OMe) and electron-withdrawing (such as Cl and Br) groups on the phenol ring were well tolerated and afforded the desired products 3b–3f and 3h–3k in good yields. Notably, a strong electron-withdrawing nitro (NO₂) group was tolerable and gave the corresponding triflone 3g in 49% yield. Various o-hydroxyphenyl propargyl alcohols bearing para-, meta-, and ortho-substituted phenyls at R² delivered the corresponding benzofuran triflones 3l–3q in excellent yields (82–98%). Substrates with other aryls such as 2-naphthyl and 2-thiophenyl at R² also proceeded efficiently, giving the desired products 3r and 3s in 76% and 83% yields, respectively. Substrates bearing alkyl and hydrogen atoms at R² were smoothly transformed to the desired triflones 3t–3w in 55–84% yields. Next, the β-naphthol substrate was tested and it afforded the corresponding triflone 3x in 67% yield under the standard conditions. When propargyl alcohol 1a was treated with other fluoroalkanesulfinyl chlorides, the corresponding fluoroalkanesulfonyl benzofurans were obtained in moderate yields (3y and 3z). Non-fluorinated alkanesulfinyl chlorides, such as tert-butylsulfinyl chloride and phenylsulfinyl chloride, could give the corresponding sulfonyl benzofurans in moderate yields (3aa and 3ab) under the standard conditions. Notably, successful conversion was also achieved with benzenethiol substrate 6, providing 3-fluoroalkanesulfonyl benzothiophene 7 in 66% yield. Tertiary propargyl alcohols remained unchanged upon treatment with CF₃SOCl and DABCO (Scheme 2, unsuccessful example).

Scheme 2 Synthesis of 3-fluoroalkanesulfonyl benzofurans and benzothiophenes. Reaction conditions: compounds 1 (0.2 mmol) or 6 (0.2 mmol) and 2 (0.3 mmol), toluene (3 mL), DABCO (0.3 mmol), 50 °C; yields of the isolated products are given.

Various indole triflones could be synthesized in good yields when o-aminophenyl propargyl alcohols were treated with CF₃SOCl and DABCO. As shown in Scheme 3, a variety of functional groups with different electronic properties, such as methyl (5b, 5f), methoxyl (5c), chloro (5d, 5g), and bromo (5e) at different positions of the aniline moiety, were all found to be successfully accommodated in this cascade reaction, giving the corresponding indole triflones in 63–91% yields. Phenyls at R² decorated with either electron-donating or -withdrawing substituents at different positions were explored and all the substrates underwent the desired process smoothly, providing indole triflones 5h–5p in 71–87% yields. o-Aminophenyl propargyl alcohols with 2-naphthyl and alkyl at R² gave the corresponding products in 70% and 80% yields, respectively (5q, 5r). 2-Methyl-3-fluoroalkanesulfonyl indole triflones were prepared in greater than 80% yields from terminal alkynyl substrates (5s, 5t). Protecting groups on the N atom other than tosyl, such as p-nosyl and tert-butyloxycarbonyl groups, were also tested, producing the desired indole triflones 5u and 5v in good yields.

Scheme 3 Synthesis of 3-fluoroalkanesulfonyl indoles. Reaction conditions: compounds 4 (0.2 mmol) and 2a (0.3 mmol), toluene (3 mL), DABCO (0.4 mmol), 50 °C; yields of the isolated products are given.

Based on our experimental observations and literature surveys, a plausible mechanism accounting for the formation of 3a from 1a is illustrated in Scheme 4. The cascade reaction starts with O-trifluoromethanesulfinylation to yield propargyl triflinate A, which then undergoes a [1,2]-sigmatropic rearrangement²¹ to sulfone B. Next, propargyl–allenyl isomerization of sulfone B with the assistance of a base affords allene intermediate C. Finally, sequential intramolecular oxy-addition and 1,3-H migration leads to cyclization and aromatization, giving benzofuran triflone 3a.

Scheme 4 Proposed reaction mechanisms.

To further validate the reaction mechanism and expand the scope of this cascade reaction, 2-propynolphenols and 2-propynolanilines with the hydroxyl group and alkynyl moiety interchanged in propargylic alcohols were explored. According to the above-mentioned results and mechanisms, these propargyl alcohols would undergo an O-trifluoromethanesulfinylation/[1,2]-sigmatropic rearrangement/propargyl–allenyl isomerization/intramolecular oxy-addition cascade process to form 2-trifluoromethanesulfonylmethyl benzofuran G (Scheme 5, pathway a). Unexpectedly, only 3-trifluoromethanesulfonyl benzofuran 3a was isolated upon treatment of 8a with CF₃SOCl and DABCO. A revised reaction mechanism could account for the formation of 3a from 8a. As shown in Scheme 5, pathway b, O-trifluoromethanesulfinylation followed by [2,3]-sigmatropic rearrangement¹⁹ would directly give allenyl intermediate C, which then transforms to 3avia intramolecular oxy-addition and 1,3-H migration. Under slightly modified reaction conditions, four 2-propynolphenols bearing different substituents were transformed to the corresponding benzofuran triflones in 80–86% yields and two 2-propynolanilines were transformed to the corresponding indole triflones in 78–82% yields (Scheme 5, representative examples).

Scheme 5 Synthesis of 3-fluoroalkanesulfonyl benzofurans/indoles from 2-propynolphenols/2-propynolanilines. Reaction conditions: compounds 8 (0.2 mmol) and 2a (0.3 mmol), DCE (3 mL), DABCO (0.24 mmol), 50 °C; 1 h (for benzofurans) or 2 h (for indoles); yields of the isolated products are given. DCE: 1,2-dichloroethane.

Further utility of this methodology was demonstrated by the gram-scale synthesis of 3a and 5a, which retained very good yields under the standard conditions (Scheme 6, gram-scale experiments). Additionally, synthetic manipulation was then carried out. Considering the importance of the trifluoromethylthio group (CF₃S–) in pharmaceutical and agrochemical products,²² a reduction reaction of 3a was first tested. Delightfully, treatment of 3a with DIBAL-H provided trifluoromethanethiolated 2-benzyl benzofuran 9 in 62% yield, demonstrating the rich synthetic potential of the described approach in the synthesis of fluoroalkanethiolated benzoheterocyclic compounds. The triflones prepared in this work could serve as good coupling partners to achieve densely substituted benzofurans and indoles. For example, the Suzuki–Miyaura coupling of triflone 3a with 4-methoxyphenylboronic acid afforded 2-benzyl-3-(4-methoxyphenyl) benzofuran 10.

Scheme 6 Scaled-up preparation of 3a/5a and derivatization of 3a.

Conclusions

In conclusion, we have developed a metal-free protocol to synthesize 2-alkyl-3-fluoroalkanesulfonyl benzofurans and indoles from o-hydroxyphenyl/o-aminophenyl propargyl alcohols or 2-propynolphenols/2-propynolanilines. This protocol is also suitable for the synthesis of non-fluorinated sulfones and 2-alkyl-3-fluoroalkanesulfonyl benzothiophene. The procedure utilizes readily available propargyl alcohols and fluoroalkanesulfinyl chlorides as starting materials. The mechanism likely involves a sequential [1,2]/[2,3]-sigmatropic rearrangement/intramolecular oxy-addition/1,3-H migration cascade of propargyl triflinates generated in situ via O-fluoroalkanesulfinylation of propargyl alcohols. Over 50 examples are presented in 46–98% yields, good yields could be obtained in gram-scale experiments, and the fluoroalkanesulfonyl group could be easily manipulated, exhibiting wide application prospects of this methodology.

Author contributions

Zhen-Jiang Liu and Hui-Yu Chen conceived the project. Si-Jing Jiang, Zhao-Zhao Li, Li-Na Yang, and Xiao-Han Qiu performed the experiments and analysed and interpreted the experimental data. Zhen-Jiang Liu, Ming Bian, Yu-Ning Gao, and Hui-Yu Chen supervised the project and drafted the paper. Si-Jing Jiang and Zhao-Zhao Li contributed equally to this work. All the authors discussed the results and contributed to the preparation of the final manuscript.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 3a have been deposited at the CCDC under 2405045† and can be obtained from https://doi.org/10.1039/D5QO00107B.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (No. 21801032), the Science and Technology Commission of Shanghai Municipality (No. 20090503300), the Shanghai Municipal Education Commission, Shanghai Institute of Technology (No. BJPY2023-2, ZQ2022-13, ZQ2023-11 and ZQ2024-9), and the Leading Talent Program of Yancheng City is gratefully acknowledged.

References

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic data for new compounds, NMR spectra, CIF files of products 3a. CCDC 2405045. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qo00107b

‡ These authors have contributed equally.

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