Visible-light-induced three-component 1,2-difluoroalkylarylation of styrenes with α-carbonyl difluoroalkyl bromides and indoles

Abstract

A novel visible light photoredox catalysis three-component 1,2-difluoroalkylarylation of styrenes was disclosed, and two new C–C bonds were generated in a single step through regioselective incorporation of a CF₂ group and a variety of indoles to C=C bonds. The well-designed photoredox system achieved the synthesis of a series of difluoro-containing indole derivatives with mild conditions and a broad substrate scope.

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Introduction

Organic compounds with the difluoromethene group (CF₂) usually possess unique physical and chemical advantages which lead to the broad exploitation of this motif in the field of agrochemicals and pharmaceuticals.¹ Additionally, it is well established that the CF₂ group can act as a bioisostere for an oxygen atom or a carbonyl group, making it an interesting group with respect to the design of bioactive molecules or CF₂-containing drugs.² Therefore, the selective introduction of the CF₂ moiety into diverse skeletons has become a hot topic in synthetic chemistry.^3,4

Over the past few years, visible-light photoredox catalysis has been regarded as a powerful tool for the preparation of fluorinated compounds.⁵ Among the available methods, versatile fluoroalkylative difunctionalization of alkenes has been well-developed quite recently.⁶ In 2011, seminal studies by the Stephenson group reported an elegant visible-light-catalyzed difunctionalization of alkenes with a haloalkane (BrCF₂CO₂Et), providing a mild way for the halodifluoroalkylation of alkenes.^7a,b Later, the groups of Dolbier, Akita and Zhu developed the amino-,^7d,g aryl-,^7e halo-,^7f and oxy-^7h difluoroalkylation of alkenes (Scheme 1) via trapping the corresponding oxidized electrophilic cation (or its corresponding radical intermediate). For the past two years, our group has also extensively developed a photoredox-catalyzed difluoroalkylative difunctionalization of alkenes to construct fluorinated ketones,⁷ⁱ oxindoles,^7j and quinolone^7j and polycyclic lactone^7k scaffolds. Despite the excellent studies in the aforementioned difluoroalkylated difunctionalization research, most of them are limited to construct tandem C–CF₂R and C–X (X=H, NR, OR, halogen) bonds or two-component intramolecular C(sp²)–H functionalization and cyclization. To the best of our knowledge, visible-light-induced three-component difluoroalkylation of styrenes allowing the simultaneous formation of two new C–C bonds in one step remains underdeveloped. Indole, the core structure in naturally and artificially available bioactive compounds, has received particular attention in the modification of this privileged structural motif.⁸ Thus, given our group established interest in the photochemistry of fluorinated radicals,^7h–j,9 we envisaged that the appropriate choice of indole as a nucleophile could realize a novel photoredox-catalyzed difluoroalkylation of styrenes. Herein, we firstly reported the visible-light-promoted three-component 1,2-difluoroalkylarylation of styrene with α-carbonyl difluoroalkyl bromides as the CF₂ radical precursor and indoles as nucleophiles. This method enables the regioselective incorporation of a CF₂ group and a variety of indoles to C=C bonds by photoredox catalysis under mild conditions (Scheme 1).

Scheme 1 Photoredox-catalyzed difluoroalkylative difunctionalization of alkenes.

Results and discussion

In order to verify our hypothesis, we firstly commenced to use 1-methyl-1H-indole 1a with 1-methoxy-4-vinylbenzene 2a and ethyl 2-bromo-2,2-difluoroacetate 3a for model reaction to optimize the conditions (Table 1). Visible light irradiation (36 W household white LED lamp) of the reaction for 24 h led to the formation of the expected adduct 4a in 20% yield in the presence of 2 mol% of fac-Ir(ppy)₃, and 2.0 equiv. of Ag₂CO₃ (Table 1, entry 1). To improve the yield, bases were first screened and AgOAc proved to be a more competent base (entries 2–5). Next, an evaluation of solvents revealed that DCM provided a significant improvement in yield (entry 7, 70%). The catalyst Ru(bpy)₃Cl₂ or Ir(ppy)₂(dtbbpy)PF₆ failed to improve the reaction yield (entries 11–12). Finally, the reaction could not proceed either in the dark or in the absence of fac-Ir(ppy)₃ or base (entries 13–15), indicating that illumination, the photocatalyst and the base played key roles in this reaction system.

Table 1 Optimization of the reaction conditions^a

Entry	Photocatalyst	Base	Solvent	Yield^b [%]
a The reactions were carried out with 1a (0.2 mmol), 2a (0.4 mmol), 3a (0.4 mmol), base (0.4 mmol, 2 equiv.), photocatalyst (0.004 mmol, 2 mol%), solvent (2 mL), at room temperature, 36 W fluorescent light bulb, 36 h. b Yield of the isolated product based on the amount of 1a. c The reactions were carried out with 1a (0.2 mmol), 2a (0.3 mmol), and 3a (0.3 mmol). d The reactions were carried out with 1a (0.2 mmol), 2a (0.5 mmol), and 3a (0.5 mmol). e Without base.^10 f In the dark. bpy = bipyridine, ppy = phenylpyridine, dtbpy = 4,4′-di-tert-butyl-2,2′-bipyridine. DMF = N,N-dimethylformamide, DMSO = dimethylsulfoxide, DCM = dichloromethane.
1	fac-Ir(ppy)3	Ag2CO3	CH3CN	20
2	fac-Ir(ppy)3	K2CO3	CH3CN	53
3	fac-Ir(ppy)3	K2HPO4	CH3CN	51
4	fac-Ir(ppy)3	KOAc	CH3CN	60
5	fac-Ir(ppy)3	AgOAc	CH3CN	66
6	fac-Ir(ppy)3	AgOAc	DMF	68
7	fac-Ir(ppy)3	AgOAc	DCM	70
8	fac-Ir(ppy)3	AgOAc	DMSO	40
9^c	fac-Ir(ppy)3	AgOAc	DCM	60
10^d	fac-Ir(ppy)3	AgOAc	DCM	65
11	Ru(bpy)3Cl2	AgOAc	DCM	Trace
12	Ir(ppy)2(dtbbpy)PF6	AgOAc	DCM	56
13^e	fac-Ir(ppy)3	—	DCM	nd
14	—	AgOAc	DCM	nd
15^f	fac-Ir(ppy)3	AgOAc	DCM	nd

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With the optimal reaction conditions in hand, the substrate scope of indole, styrene, and difluoro-reagent was investigated, and the representative examples are shown in Scheme 2. When testing the effect of the N-substituent indoles, we found that the electronic properties of the N-substituent group were crucial for this successful transformation. The indole, protected by an electron-rich group such as the benzyl, could convert to the difluoroalkylated product (4b) in moderate yield (68%), whereas N-Boc, N-Ts indoles failed to undergo the expected transformation under the standard conditions (4c–4d). It was worth mentioning that 5-methoxy-1H-indole, having a free NH group, remained viable in this protocol with 46% yield (4e). Thus the process could be applicable to the modifications of the N–H bond in the synthesis of complicated molecules. Next, a wide range of substituent indoles were amenable to this reaction. Various substituents in the indole scaffold, such as methoxy, methyl, halogen, or cyan groups, furnished the CF₂-containing indoles (4f–4l) in good yields (52%–90%). Indole with a substituent at the 4-, 6- or 7-position smoothly produced the corresponding coupling products (4m–4p). Intriguingly, 2-substituted indoles also proceeded well to give the desired products (4q–4s) in 51%–68% yields, even for the bulky phenyl substituted substrate (2r). Nevertheless, no reaction occurred when a 3-substituted indole was used as the substrate, indicating that the tandem three-component reaction has excellent chemoselectivity at the C₃ position of indoles. Moreover, we found the N-hetero indole 1-methyl-1H-pyrrolo[2,3-b]pyridine (1u) was compatible with this process.

Scheme 2 Scope of different substituent indoles. Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol, 2.0 equiv.), 3a (0.4 mmol, 2.0 equiv.), AgOAc (0.4 mmol, 2.0 equiv.), fac-[Ir(ppy)₃] (0.004 mmol, 2 mol%), DCM (2 mL), at room temperature, 36 W LEDs, 48 h.

Next, we evaluated the scope of styrene, and the difluoro-reagent under the optimized reaction conditions. As illustrated in Scheme 3, different aryl alkenes were investigated. We found that the electronic nature of the aryl group had a fundamental influence on the reactivity. The electron-rich aryl alkene, bearing the o-CH₃OC₆H₄ and p-MeC₆H₄ group, afforded the expected products (4v, 4w) with 75% and 58% yield, respectively. However, the alkene with an electron-withdrawing substituent group was not a suitable substrate (see the ESI†). To our delight, the internal alkenes 1,2-dihydronaphthalene provided the CF₂-substituted products 4x in 65% yield. Subsequently, we set out to study difluorinated substrates. Delightfully, bromodifluoroketones and bromodifluoroacetamides as difluorinating reagents were well-tolerated, thus providing the corresponding products (4aa–4ac) with moderate to good yields (30%–80%).

Scheme 3 Variation of the scope of styrene 2 and difluoro-reagent 3. ^a See Table 1 and Scheme 2.

To gain an insight into the mechanism of this transformation, a radical-trapping experiment was carried out. Notably, the reaction was completely suppressed by the radical inhibitor TEMPO (2,2,6,6-tetramethylpiperidinooxy), thus implying that this current reaction involves a free radical process. Based on this result and previous reports,¹¹ a plausible reaction mechanism via SET processes is shown in Scheme 4. Firstly, irradiation of iridium photocatalyst 9 with visible light will produce the long-lived excited state 10 (−1.73 V vs. SCE in CH₃CN), which can undergo the SET process in the presence of 3a to generate the CF₂ radical 5 and Ir⁴⁺11 (+0.77 V vs. SCE in CH₃CN).¹² Then, addition of ˙CF₂5 to alkene 2a gives the more stable benzyl radical intermediate 6, which is subsequently oxidized into the cation intermediate 7 by Ir⁴⁺11. Finally, the β-difluoroalkylated carbocation intermediate 7 is attacked by nucleophilic indoles 1a to afford cation intermediate 8, followed by base-mediated deprotonation to give the final selective difluoroalkylated product 4a.

Scheme 4 Plausible reaction mechanism.

Conclusions

In conclusion, we have successfully developed the visible-light-catalyzed three-component 1,2-difluoroalkylarylation of styrenes for the synthesis of a series of CF₂-containing indole derivatives. This reaction allows a one-step regioselective construction of two C–C bonds with mild reaction conditions and a broad substrate scope. Given the prevalence of organofluorine compounds in medicinal agents, and life and materials sciences, we expect that this reaction will find immediate application in the synthesis of biologically active compounds.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (21474048 and 21372114) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6qo00393a

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