Synthesis of trifluoromethylated 3,4-dihydroquinolin-2(1H)-ones via a photo-induced radical cyclization of benzene-tethered 1,7-enynes with Togni reagent

Abstract

A photoinduced radical cyclization of benzene-tethered 1,7-enynes with Togni reagent in the presence of sodium iodide is developed. Under ultraviolet irradiation, (Z)-4-(iodomethylene)-3-(2,2,2-trifluoroethyl)-3,4-dihydroquinolin-2(1H)-ones are generated in moderate to good yields. The transformation works well without any metals or photo-redox catalysts at room temperature. During the reaction process, the reaction proceeds through a trifluoromethyl radical-triggered α,β-conjugated addition/intramolecular 6-exo cyclization/iodination, with the formation of multiple bonds. Good functional group tolerance is observed under the reaction conditions.

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Introduction

Continuous efforts have been made to develop methodologies for the rapid generation of nitrogen-containing heterocycles.¹ Meanwhile, there is growing interest in the incorporation of fluorinated substituents into privileged scaffolds, with the expectation of improving their biological properties.² In particular, much attention has been paid to methods for the introduction of a trifluoromethyl group into privileged scaffolds,³ due to the unique properties of trifluoromethylated molecules. So far, many strategies have been developed and various trifluoromethyl reagents such as the Togni reagent, Umemoto's reagent, TMSCF₃, or CF₃SO₂Na are involved in this system.⁴ Among the protocols for fluorination, radical processes under photoredox conditions are attractive for access to fluoro-containing molecules. Usually, the reaction affords a trifluoromethyl radical with high reactivity in the process.^5,6 As part of a program for the generation of natural product-like compounds,⁷ we are interested in the synthesis of trifluoromethylated nitrogen-containing heterocycles and their applications in various biological evaluations. Very recently, we described an efficient synthesis of fluorinated 3,3-disubstituted 2-oxindoles through a photoinduced radical cyclization of N-arylacrylamides under ultraviolet irradiation.^5d During the reaction process, no oxidant or photoredox catalyst were needed, and the trifluoromethyl radical could be generated efficiently under the conditions.

Recently, 1,n-enynes have been used as convenient building blocks for the construction of diverse fused polycyclic skeletons via [2 + 2 + m] annulation reactions.^8,9 The unsaturated bonds of 1,n-enynes are reactive for functionalization. Different metal catalysts have been utilized in the cyclization reactions of 1,n-enynes. For example, Li and co-workers developed an efficient and selective route to fused [6.6.6] pyran derivatives via an iron-catalyzed radical [2 + 2 + 2] annulation of benzene-linked 1,7-enynes with aldehydes.^8b Additionally, 1,n-enyne cyclization reactions could also proceed well in a photocatalytic system. For instance, Li's group^9a and Xia's group^9b both disclosed a visible light initiated 1,5-hydride radical shift strategy starting from 1,n-enynes. Metal-free radical cyclization reactions of 1,n-enynes were also established and diverse fused polycyclic compounds could be assembled in efficient ways. Jiang and Tu reported a metal-free radical haloazidation of benzene-tethered 1,7-enynes using PhI(OAc)₂ as the oxidant in the presence of NBS (or NCS, NIS).^9e

Prompted by the advancement of 1,n-enyne chemistry and the photoinduced trifluoromethylation, we postulated that trifluoromethylated 3,4-dihydroquinolin-2(1H)-ones could also be generated through a photo-induced radical cyclization of benzene-tethered 1,7-enynes with the Togni reagent. We envisioned that under ultraviolet irradiation, this reaction might proceed in the absence of metal catalyst and oxidant. Therefore, we started to explore the feasibility of this proposed photoinduced trifluoromethylation of benzene-tethered 1,7-enynes.

Results and discussion

Initially, our studies focused on the model reaction of benzene-tethered 1,7-enyne 1a with Togni reagent 2 under ultraviolet irradiation at room temperature. The reaction was performed under ultraviolet irradiation (mercury lamp, 0.67 W cm⁻² using a standard photo-reactor) at room temperature (see ESI†). It was found that no reaction took place when the transformation occurred in MeCN or 1,4-dioxane (Table 1, entries 1 and 2). To our delight, the desired trifluoromethylated 3,4-dihydroquinolin-2(1H)-one 3a was isolated and obtained in 26% yield when the solvent was changed to 1,2-dichloroethane (Table 1, entry 3). The structure of compound 3a was confirmed by X-ray diffraction analysis (Fig. 1). A lower yield was observed when the solvent was replaced by toluene (Table 1, entry 4). From the structure of compound 3a, it was found that iodide anions would be involved in the reaction. Thus, tetrabutylammonium iodide (TBAI) was added to the reaction and the corresponding product 3a was generated in 35% yield (Table 1, entry 5). The yield reached up to 43% when the amount of Togni reagent was increased to 1.5 equivalents (Table 1, entry 6). We further examined other iodide sources. The desired product 3a was obtained in 45% yield when the reaction occurred in the presence of 1.0 equivalent of sodium iodide (Table 1, entry 7). We assumed that 2-iodobenzoic acid would be formed in situ, which might promote the reaction. We therefore tested the reaction with the addition of 2-Iodobenzoic acid (Table 1, entry 8). Gratifyingly, the expected product 3a was obtained in 54% yield. The yield was increased to 60% when benzoic acid was used instead (Table 1, entry 9). The conversion was retarded when the reaction was performed in the dark (Table 1, entry 10). These results were promising and attractive, since the reaction occurred in the absence of any metals or photo-redox catalysts under mild conditions at room temperature.

Fig. 1 ORTEP illustration of compound 3a (30% probability ellipsoids).

Table 1 Initial studies for the photoinduced trifluoromethylation of benzene-tethered 1,7-enynes^a

Entry	Solvent	Additive	Yield^b (%)
a N-Methyl-N-(2-(phenylethynyl)phenyl)methacrylamide 1a (0.2 mmol), Togni reagent 2 (1.2 equiv.), solvent (4.0 mL), irradiation supplied by 600 W Hg light, rt, 12 h. b Isolated yield based on N-methyl-N-(2-(phenylethynyl)phenyl)methacrylamide 1a. c In the presence of 1.5 equiv. of Togni reagent. d The reaction was performed in the dark.
1	MeCN	—	nr
2	1,4-Dioxane	—	nr
3	DCE	—	26
4	Toluene	—	22
5	DCE	TBAI	35
6^c	DCE	TBAI	43
7^c	DCE	NaI	46
8^c	DCE	2-Iodobenzoic acid/NaI	54
9^c	DCE	Benzoic acid/NaI	60
10^c^,^d	DCE	Benzoic acid/NaI	Trace

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Encouraged by the above results, we next explored the scope of the reaction of benzene-tethered 1,7-enynes 1 with Togni reagent 2 in the presence of sodium iodide. The results are summarized in Table 2. Various N-(2-(alkynyl)phenyl)methacrylamides 1 were treated with Togni reagent 2 under the optimized conditions, resulting in the corresponding trifluoromethylated 3,4-dihydroquinolin-2(1H)-ones 3 in moderate yields. Both electron-rich or electron-poor benzene-tethered 1,7-enynes 1 were compatible under the standard conditions. Different functional groups were tolerated, including methyl, methoxy, chloro, fluoro, acetoxyl, and trifluoromethyl groups. For instance, N-(4-fluoro-2-(phenylethynyl)phenyl)-N-methylmethacrylamide reacted with Togni reagent 2 gave rise to the product 3b in 64% yield. Additionally, this photoinduced CF₃-containing 3,4-dihydroquinolin-2(1H)-one formation was found feasible when thiophenyl-substituted alkyne 1m was utilized, providing the corresponding product 3m in 45% yield.

Table 2 Scope exploration of the photoinduced trifluoromethylation of benzene-tethered 1,7-enynes^a

a Reaction conditions: N-(2-(alkynyl)phenyl)methacrylamide 1 (0.2 mmol), Togni reagent 2 (0.3 mmol), NaI (0.2 mmol), PhCOOH (0.2 mmol), DCE (4.0 mL), N₂ atmosphere, irradiation supplied by 600 W Hg light, rt, 12 h. Isolated yield is based on benzene-tethered 1,7-enyne 1.

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On the basis of the present results and the previous reports,^5,6 we proposed a possible mechanism for the photoinduced radical trifluoromethyliodation of benzene-tethered 1,7-enynes (Scheme 1). We reasoned that the trifluoromethyl radical would be formed in the presence of the Togni reagent 2 under ultraviolet irradiation. Subsequently, the addition of the trifluoromethyl radical to the double bond of N-methyl-N-(2-(phenylethynyl)phenyl)methacrylamide 1 would afford intermediate A. Then an intramolecular radical cyclization would occur, resulting in the formation of the vinyl radical intermediate B. The following electron transfer would provide the cation intermediate C, which would combine with iodide to furnish the corresponding product 3. However, since the corresponging product 3a could also be obtained in the absence of sodium iodide (Table 1, entries 3 and 4), we postulated that another possible route could not be excluded. We reasoned that under ultraviolet irradiation, the Togni reagent 2 would provide a trifluoromethyl radical and 2-iodobenzoic acid concurrently. Recently, we and others reported that a halo radical could be generated from aryl halide via photo-induced Ar–X bond dissociation under metal-free conditions.¹⁰ Thus, an iodo radical would be formed subsequently from 2-iodobenzoic acid, which might react with intermediate B to furnish the final product 3.

Scheme 1 A plausible mechanism for the photoinduced radical trifluoromethyliodation of benzene-tethered 1,7-enyne 1.

Conclusions

In conclusion, we have described a photoinduced radical cyclization of benzene-tethered 1,7-enynes with Togni reagent in the presence of sodium iodide. Under ultraviolet irradiation, (Z)-4-(iodomethylene)-3-(2,2,2-trifluoroethyl)-3,4-dihydroquinolin-2(1H)-ones are generated in moderate yields. The transformation works well without any metals or photo-redox catalysts at room temperature. During the reaction process, the reaction proceeds through the trifluoromethyl radical-triggered α,β-conjugated addition/intramolecular 6-exo cyclization/iodination, with the formation of multiple bonds. Good functional group tolerance is observed under the reaction conditions. Further elaboration of (Z)-4-(iodomethylene)-3-(2,2,2-trifluoroethyl)-3,4-dihydroquinolin-2(1H)-ones could be expected via cross-coupling reactions. Further trifluoromethylation of other nitrogen-containing heterocycles under ultraviolet irradiation is under exploration in our laboratory.

Acknowledgements

Financial support from National Natural Science Foundation of China (no. 21372046 and 21532001) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, characterization data, ¹H & ¹³C NMR spectra of compound 3. CCDC 1477485. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00267f

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