Four-component acyloxy-trifluoromethylation of arylalkenes mediated by a photoredox catalyst

Abstract

A four-component intermolecular trifluoromethylation–acyloxylation of arylalkenes induced by visible light has been developed in the presence of the photoredox catalyst Ru(bpy)₃(PF₆)₂ under mild reaction conditions. A new Umemoto's reagent was used as a trifluoromethyl radical source, and this redox neutral reaction demonstrated good functional group tolerance for aryl alkenes with high yields up to 91%. The detailed reaction process was investigated based on control, deuterium and O¹⁸-labeling experiments to support that N,N-dimethylformamide (DMF)/H₂O acted as an acyloxyl source.

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Introduction

The trifluoromethyl (CF₃) group is a useful structural motif in many biologically active molecules because it influences their chemical and metabolic stabilities, lipophilicity, and binding selectivities.¹ Thus, the development of new methodologies for highly efficient introduction of the CF₃ group into organic molecules has attracted plenty of attention in organic synthesis and drug discovery.² Among the approaches, photocatalytic difunctionalization of unsaturated double bonds induced by visible light is considered as a useful protocol for the synthesis of diverse trifluoromethylated compounds.^3,4 Over the past decade, various trifluoromethylation reactions of alkenes,⁵ like oxytrifluoromethylation,⁶ halotrifluoromethylation,⁷ carbotrifluoromethylation,⁸ hydrotrifluoromethylation,⁹ aminotrifluoromethylation,¹⁰ and thiotrifluoromethylation,¹¹ have been reported using CF₃I,^7a,9c CF₃Br,^5k CF₃SO₂Cl,^7b,c CF₃SO₂Na,^9b,10c Togni's reagent,^8a–d or Umemoto's reagent^{7d,8e,9a,10a,b} as a CF₃ radical source (Scheme 1a). Despite these achievements, other methods to make structurally complex trifluoromethylated compounds still need to be developed. There have been some reports employing N,N-dimethylformamide (DMF) as the solvent and acetylation reagent in a visible light-induced photocatalytic reaction.¹² Recently, König's group reported an intermolecular tandem arylation/formyloxyarylation of alkenes by the photoredox Meerwein reaction (Scheme 1b).^12d However, the acyloxy-trifluoromethylation of alkenes with DMF has not been reported. Here, we report the first visible light-promoted tandem acyloxy-trifluoromethylation of alkenes (Scheme 1c).

Scheme 1 Photocatalytic difunctionalization of alkenes.

Results and discussion

Recently, a new Umemoto's reagent was developed which was thermally stable, air-stable, and one-pot-preparable,¹³ and has been used as a CF₃ reagent in several types of reactions by Umemoto and co-workers.^13,14 We initially examined the photocatalytic reaction of styrene and the air-stable Umemoto's reagent using eosin Y as the photocatalyst irradiated with 4 W white LEDs for 2 h, and only a trace amount of product was obtained (Table 1, entry 1). Rose Bengal did not work for the reaction (entry 2). However, 52% yield could be obtained in the presence of 1 mol% of Ru(bpy)₃(PF₆)₂ in the same reaction system (entry 3). Prolonging the reaction time to 6 h, Umemoto's reagent 2 could be consumed completely and 3a was obtained in 75% yield (entry 5). Moreover, both trifluoromethanesulfonate and tetrafluoroborate of 2 gave similar results (entries 4 and 5). The reaction yield could be further enhanced by increasing the amount of styrene (entries 7–9). However, the amount of water had little effect on the yields (entries 5, 10 and 11). Furthermore, the photocatalyst loading was also investigated, and 0.5 mol% of Ru(bpy)₃(PF₆)₂ could also give a satisfactory result with a 74% isolated yield (entries 7, 12–14). Control experiments showed that both the photocatalyst and visible light were essential for the success of the tandem acyloxy-trifluoromethylation reaction (entries 15 and 16). Other CF₃ reagents, like CF₃SO₂Cl, CF₃SO₂Na and Togni's reagent, were also investigated, and no product was detected (entries 17–19). These results demonstrated the advantage of the new Umemoto's reagent 2 as an electrophilic trifluoromethylating agent for this reaction using Ru(bpy)₃(PF₆)₂ as the photocatalyst. The old Umemoto's reagent can also be used as an efficient CF₃ radical source (entries 20 vs. 6). Additionally, different Ru(II) and Ir(III)-based photocatalysts as well as various additives were also investigated (see Table S1†).

Table 1 Optimization of reaction conditions^a

Entry	Catalyst	1a/eq.	H2O/eq.	Time/h	Yield^a/%
a The yields were determined by ^19F NMR spectroscopy of crude reaction mixtures with 1-chloro-4-(trifluoromethyl)benzene as an internal standard, and isolated yields are given in parentheses for some cases. b Tetrafluoroborate of 2 was used. c No light. d CF3SO2Cl was used as a CF3 reagent. e CF3SO2Na was used as a CF3 reagent. f Togni's reagent was used as a CF3 reagent. g Old Umemoto's reagent was used.
1	Eosin Y (1.0 mol%)	1.0	1.0	2	Trace
2	Rose Bengal (1.0 mol%)	1.0	1.0	2	NR
3	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	1.0	2	52
4	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	1.0	4	71
5	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	1.0	6	75
6^b	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	1.0	6	74
7	Ru(bpy)3(PF6)2 (1.0 mol%)	1.2	1.0	6	78 (76)
8	Ru(bpy)3(PF6)2 (1.0 mol%)	1.5	1.0	6	79 (79)
9	Ru(bpy)3(PF6)2 (1.0 mol%)	2.0	1.0	6	80
10	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	2.0	6	74
11	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	5.0	6	73
12	Ru(bpy)3(PF6)2 (1.5 mol%)	1.2	1.0	6	76
13	Ru(bpy) 3 (PF 6 ) 2 (0.5 mol%)	1.2	1.0	6	78 (74)
14	Ru(bpy)3(PF6)2 (0.1 mol%)	1.2	1.0	6	72
15	No photocatalyst	1.2	1.0	6	NR
16^c	Ru(bpy)3(PF6)2 (1.0 mol%)	1.2	1.0	6	NR
17^d	Ru(bpy)3(PF6)2 (1.0 mol%)	1.2	1.0	6	0
18^e	Ru(bpy)3(PF6)2 (1.0 mol%)	1.2	1.0	6	0
19^f	Ru(bpy)3(PF6)2 (1.0 mol%)	1.2	1.0	6	0
20^b^,^g	Ru(bpy)3(PF6)2 (1.0 mol%)	1.0	1.0	6	70

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By employing the optimized conditions, a series of aryl alkenes were used to investigate the generality of the above visible light-promoted formyloxy-trifluoromethylation reaction in DMF (Table 2). The aryl alkenes were compatible with various electron-donating groups (3b–3e) and halogen substituents (3f–3j) to afford the desired products in 49–91% yields. However, the chlorine substituent on the meta- and ortho-positions resulted in a lower yield than that on the para-position. The aryl alkenes bearing a strong electron-withdrawing group, like the cyano or trifluoromethyl group, afforded products 3g–3m in 52–55% yields. The aryl alkene substrates with the ester or steric hindrance group could also proceed smoothly to give products 3n and 3o in 76% and 62% yields, respectively. However, 2-vinylpyridine was incompatible with this reaction and only a trace amount of product 3l was observed, which may be attributed to the poor stability of the 2-pyridinylmethyl cation compared with the benzyl cation.¹⁵ Notably, N,N-dimethylacetamide (DME) could also be used as a solvent and acetoxylation reagent to generate the corresponding products 4b–4d.

Table 2 Substrate scope^a

a Isolated yields.

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For gaining insight into the mechanism of the reaction, control experiments were conducted as shown in Scheme 2. In the presence of 2 equiv. of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), the acyloxy-trifluoromethylation of the 1-(tert-butyl)-4-vinylbenzene 1c was quenched, and no 3c could be detected by in situ¹⁹F NMR spectroscopy. However, as expected, the signals of both products 5a and 5b were observed in the in suit ¹⁹F NMR spectrum, whose chemical shifts were consistent with those reported in the literature^16,17 (see the ESI†). 5b could also be detected through LC-MS (eqn (1)). These results indicated that a radical-mediated pathway is involved. Moreover, replacing H₂O with D₂O (99.9% D) just afforded no deuterated product 3c (eqn (2)); however, the deuterated product 3c-D was obtained in 83% yield with >99% D incorporation on using DMF-d₇ (99.5% D) as the solvent (eqn (3)). These results demonstrated that the hydrogen atom of the formyloxy group was from the DMF, rather than H₂O. What's more, O¹⁸-labeling experiments were also conducted using H₂O¹⁸ (97% O¹⁸) instead of H₂O. The O¹⁸-labeled product 3c-O¹⁸ was successfully isolated in 81% yield (eqn (4)), which was further hydrolyzed to give the no O¹⁸-labeled product benzyl alcohol 6 (eqn (5)). These results supported that the oxygen atom of the formyl group in 3c-O¹⁸ was from H₂O, and the other oxygen atom originated from DMF. All the products 3c, 3c-D, 3c-O¹⁸, 5 and 6 were further confirmed by HRMS analyses (see the ESI†).

Scheme 2 Control and labeling experiments.

The light–dark interval experiment was also performed to investigate the possibility of the radical propagation. From Scheme 3, it can be clearly seen that continuous irradiation with visible light is essential for the transformation. Moreover, the quantum yield of this photochemical reaction was found to be 39%. Although a radical propagation cannot be completely excluded, these results demonstrated that the radical propagation (Scheme 4) is not a dominant reaction pathway, which were also described in the previous reports.^6a,18–20

Scheme 3 Light–dark interval experiment.

Scheme 4 Proposed mechanistic pathway.

Based on the above results and previous literature reports,¹² a plausible mechanism is depicted in Scheme 4. Photoexcitation of Ru^II(bpy)₃(PF₆)₂ is initiated with a white LED, followed by single-electron reduction of 2 to generate Ru^III(bpy)₃(PF₆)₂ and the electrophilic trifluoromethyl radical through the single-electron transfer (SET) pathway. Then the trifluoromethyl radical rapidly combines with arylalkene 1 to furnish the stable benzyl radical 7, which was supported by the control experiment (Scheme 2, eqn (1)). Radical 7 can be oxidized by Ru^III(bpy)₃(PF₆)₂ through the other SET process to afford benzyl carbocation 8, which was subject to nucleophilic attack by DMF or DME to give imine intermediate 9. Hydrolysis of 9 yielded the desired product 3 or 4, which was also supported by the deuterated and O¹⁸-labeling experiments discussed above.

Conclusions

In conclusion, we have successfully developed a visible-light-induced four-component acyloxy-trifluoromethylation of arylalkenes using Ru(bpy)₃(PF₆)₂ as the photoredox catalyst. This difunctionalization approach is a redox neutral reaction with complete regioselectivity. Moreover, the detailed reaction process of this transformation was studied based on the radical quenching experiment, in suit ¹⁹F NMR spectrum, LC-MS study, deuterium and O¹⁸-labeling experiments. This approach will be useful in organic synthesis and drug discovery fields owing to the mild reaction conditions, high efficiencies, reasonably broad substrate scope and good functional group tolerance, and also the compatibility with various substrates of the aryl alkenes and amides.

Experimental

General procedure for the reaction of arylalkenes and Umemoto reagent

A 10 mL Schlenk tube equipped with a magnetic stir bar was charged with arylalkene 1 (1.2 equiv.), Umemoto reagent 2 (1.0 equiv.), DMF or DMA (5.0 mL mmol⁻¹2), H₂O (1.0 equiv.) and Ru(bpy)₃(PF₆)₂ (0.5 mol%). The mixture was evacuated and back-filled with nitrogen three times. Then the tube was irradiated for 6 h with a 4 W white LED lamp placed at a distance of 1–2 cm. After that, 20 mL of water was added to the reaction mixture, which was then extracted with ethyl acetate (50 mL × 3). The combined organic phases were washed with water, dried over MgSO₄ and concentrated in a vacuum. The residue was purified by silica gel column chromatography to give the corresponding product 3 or 4.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (21602198, 21878276, 21776259 and 21476270) and the “Qianjiang Talents Plan” (Grant No. QJD1602017) for financial support. We thank Dr Teruo Umemoto for his technical instructions on the reaction of Umemoto's reagents and for reviewing the manuscript.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Preparation and characterisation data as well as ¹H, ¹³C, ¹⁹F NMR and HRMS spectra of all compounds. See DOI: 10.1039/c8ob02239a

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