Merging Visible-Light Photocatalysis and Transition-Metal Catalysis in

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Merging Visible-Light Photocatalysis and Transition-Metal Catalysis in Three-Component Alkyl-Fluorination of Olefins with a Fluoride Ion Weili Deng,† Weiwei Feng,† Yajun Li,*,† and Hongli Bao*,†,‡ †

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Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis, Fujian Institute of Research on the Structure of Matter, University of Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China S Supporting Information *

ABSTRACT: The light-induced, Ru-catalyzed three-component alkyl-fluorination of olefins under mild reaction conditions is reported. This carbofluorination reaction features a wide substrate scope and good functional group tolerance. A key advantage of this photoredox reaction is the use of nucleophilic fluoride and generic alkyl groups. Late-stage functionalizations are achieved, and a copper-assisted oxidative quenching mechanism is proposed based on the mechanistic studies.

he catalytic site-specific incorporation of fluorine into organic molecules is a challenging and promising strategy for the construction of various fluoro-organic compounds,1 which are of great interest in pharmaceuticals, agrochemicals, and other functional materials.2 Selective vicinal carbofluorination of unsaturated carbon−carbon double bonds is an efficient method for conversion of olefins into valuable fluorochemicals with more complex structures.3,4 As exemplified in Scheme 1a, describing the carbofluorination of olefins, only a few catalytic reactions have been documented and there remains a significant demand for achievement of a challenging and sought-after reaction diversity of olefin carbofluorination. In most of the recent advances on vicinal carbofluorination of olefins, developments have been mainly achieved on (Meerwein-type) aryl-fluorination by Toste,4a Gouverneur,4b Tu,4c Heinrich,4d and Tang4e et al., electrophilic cyanofluorination by Dilman,4g carbonylative oxycarbonyl-fluorination by Liu,4h decarboxylative acyl-fluorination by Duan,4i oxidative trifluoromethyl-fluorination by Qing,4j oxycarbonylmethyl-fluorination by Li4k,m and Szabó,4l and intramolecular alkyl migration assisted alkyl-fluorination by Alexakis4n and Zhu4o et al. Most of the reported olefin carbofluorinations are exclusively compatible with highly reactive, oxidizing, and electrophilic “F+ ” sources, such as Selectfluor, NFSI, fluoroiodine reagents, etc. because the high-valent “F+” ion can act as an oxidant and enable the smooth turnover of the reaction.5,1f To the best of our knowledge, there is still a lack of methods for olefin carbofluorination with the nucleophilic fluoride.6 Furthermore, methodologies for alkyl-fluorination of olefins with a generic alkyl group are also extremely rare and highly sought after.7,8 It is worth mentioning that Li’s group quite recently reported a copper mediated/catalyzed trifluor-

T

© XXXX American Chemical Society

Scheme 1. Olefin Carbofluorination with Various Carbon Source and Photocatalysis

omethyl- and trichloromethyl-fluorination of unactivated alkenes with fluoride ions.7d Compared to the existing methods for olefin carbofluorination using the electrophilic “F+” ion, if the nucleophilic fluoride is used as the fluorine source, an oxidative reagent or reactant Received: May 25, 2018

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DOI: 10.1021/acs.orglett.8b01658 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

product either (Table 1, entry 6). While a reduced amount of Et3N·3HF leads to a lower yield (Table 1, entries 7 and 8 vs 2), an increased amount of Et3N·3HF enhances the performance of this reaction (Table 1, entry 9). The reaction fails to proceed in the absence of either ruthenium or light (Table 1, entry 10). This result implies that the copper additive cannot trigger the reaction, but plays a very important role in the catalytic cycle. Finally, it is found that 2 mol % of Cu(OAc)2 can afford an identical performance and offer an isolated yield as high as 76% (Table 1, entry 11). With these optimized reaction conditions in hand, the substrate scope of the olefins was studied, and the results are shown in Table 2. Styrenes containing a variety of electron-

might be necessary to complete the reaction, since atom transfer radical addition (ATRA) seems unlikely to happen with a fluorine atom.9 In 2014, Nicewicz et al. reported a visible-light-driven, anti-Markovnikov hydrofluorination of olefins via photoredox catalysis in a reductive quenching system (Scheme 1b-i).10 In addition, there are rare examples of photoinduced olefin carbofluorination.11 We postulate that it should be possible to use oxidative alkyl reagents12 and photocatalysis13 to achieve the desired olefin carbofluorination with generic alkyl groups and a fluoride (Scheme 1b-ii). In this paper, we report a photoredox-catalyzed threecomponent alkyl-fluorination of olefins with a fluoride ion and oxidative alkyl reagents (Scheme 1c). This method has the following features: first, use of the nucleophilic fluoride instead of an electrophilic “F+” ion as the fluorine source for the olefin carbofluorination; second, use of generic alkyl groups from an oxidative alkyl source for the olefin carbofluorination; and third, use of photocatalysis in an oxidative quenching system14 for the olefin carbofluorination. Our hypothesis concerning the photoinduced carbofluorination of olefins was examined with a model reaction involving styrene (1a), lauroyl peroxide (LPO, 2a), and triethylamine trihydrofluoride (Et3N·3HF) irradiated with 18 W blue LEDs in the presence of a ruthenium-based catalyst (Table 1).15 The

Table 2. Substrate Scope of Olefinsa,b

Table 1. Olefin Alkyl-Fluorination Reaction Optimizationa

entry

additive (10 mol %)

yield (%)b

1 2 3 4 5c 6d 7e 8f 9g 10h 11g,i

− Cu(OAc)2 CuF2 CuTC Cu(OAc)2 CuF2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2 Cu(OAc)2

40 72 69 57 0 0 49 61 79 trace 78(76)

a

1 (0.5 mmol, 1.0 equiv), 2a (2.0 equiv), Et3N·3HF (5 M), Ru(bpy)3Cl2·6H2O (2 mol %), and Cu(OAc)2 (2 mol %) in 2 mL of DCE at rt under a nitrogen atmosphere upon blue LED irradiation. b Isolated yield. cYield in the parentheses is for a 2 mmol reaction. d CuF2 was used in place of Cu(OAc)2.

a

1a (0.5 mmol, 1.0 equiv), 2a (2.0 equiv), Et3N·3HF (2.5 M), Ru(bpy)3Cl2·6H2O (2 mol %), and additive (10 mol %) in DCE (2 mL) at rt under a nitrogen atmosphere upon 18 W blue light irradiation. b1H NMR yield of the product; isolated yield in parentheses. cAnother fluoride source, such as CsF (5 equiv), AgF2 (5 equiv), Py·HF (5 equiv), NH4F (5 equiv), or Bu4NF (5 equiv), was used in place of Et3N·3HF. dCuF2 (5 equiv) was used without Et3N·3HF. e0.5 M Et3N·3HF was used. f1.25 M Et3N·3HF was used. g 5 M Et3N·3HF was used. hWithout Ru(bpy)3Cl2·6H2O or light. i Cu(OAc)2 (2 mol %) were used.

withdrawing or -donating groups on the phenyl ring produce the corresponding carbofluorinated products (3aa−3za) in moderate to good yields. Substituents such as alkyl groups (3ba−3ga), methoxyl (3ha), esters (3ka and 3la), halides (3ma−3ra), carboxyl (3sa), and chloromethyl (3ta) are all tolerated under the carbofluorination reaction conditions. Notably, substrates with a substituent at the ortho-position of the phenyl ring give the corresponding products (3ea, 3fa, and 3qa) in moderate yields. The 1,1-disubstitued olefin (1u) can be converted into the fluorinated product (3ua) in moderate yield. Furthermore, the reaction scope can be extended to 1,2disubstituted olefins. Although the substrate 1v delivers the corresponding product (3va) in a low yield, methyl cinnamate affords the desired product (3wa) in moderate yield. 5Vinylthiazole (1x), with its heteroaromatic ring, delivers the carbofluorinated product (3xa) in moderate yield. Conjugated enynes are also suitable substrates for this reaction, affording

desired fluorinated product (3aa) is obtained in this case in 40% yield, as determined by 1H NMR (Table 1, entry 1). The yield is improved to 72% yield when Cu(OAc)2 is added to the reaction mixture (Table 1, entry 2).16 Other copper salts, such as CuF2 and CuTC, can be used as the additive with which to promote the reaction (Table 1, entries 3 and 4). When another fluoride source, such as CsF, AgF2, Py·HF, NH4F, or Bu4NF, is used in place of Et3N·3HF, no desired product is detected (Table 1, entry 5). With CuF2 as both the additive and fluoride sources, the reaction does not deliver the corresponding B


Letter

Organic Letters the alkyl-fluorinated products with an internal C−C triple bond (3ya and 3za). The substrate scope of alkyl diacyl peroxides in the olefin carbofluorination was studied (Table 3). As reported

Scheme 3. Preliminary Mechanistic Studies

Table 3. Substrate Scope with Alkyl Diacyl Peroxidesa,b

product 8 in a 54% isolated yield through a ring-opening process of the cyclopropane ring (Scheme 3b). These results demonstrate that the reaction might involve a free radical process. To gain further understanding of this free radical process, methanol was added to the reaction mixture. The desired product (3aa) is formed in 35% yield (Scheme 3c), but in addition, the etherification product (9) is isolated in 15% yield, suggesting that the reaction might involve a benzyl carbocation intermediate.20 The results of a light ON/OFF experiment (see the Supporting Information (SI) for details) suggests that the radicals are generated via a photolytic pathway.21 Further, to identify which species can quench the excited ruthenium catalyst, a series of controlled experiments were conducted (see SI).22 The emission intensity of the excited ruthenium catalyst decreases slightly with increasing concentration of LPO or styrene, showing a weak quenching effect. Interesting, the addition of Cu(OAc)2 to the excited ruthenium catalyst resulted in a strongly enhanced quenching effect, indicating that a quenching of the excited ruthenium catalyst by Cu(OAc)2 is involved in the mechanism. Furthermore, mechanistic studies reveal that LPO can be fully decomposed by a stoichiometric amount of Cu(I) species in the dark, but most of LPO still remains when a stoichiometric amount of Cu(II) species is employed even under blue light irradiation.23 The experimental results imply that a Cu(I) species might be related to the LPO cleavage in the catalytic cycle.24 Because the Cu(II) additive plays an important role in this olefin carbofluorination, a copper-involved, plausible mechanism for the visible-light induced alkyl-fluorination of olefin via an oxidative quenching process is proposed and is depicted in Scheme 4. Visible light irradiation of [Ru(bpy)3]2+ will generate the excited state complex *[Ru(bpy)3]2+, which then engages in a single-electron transfer with a Cu(II) species

a

1a (0.5 mmol, 1.0 equiv), 2 (2.0 equiv), Et3N·3HF (5 M), Ru(bpy)3Cl2·6H2O (2 mol %), and Cu(OAc)2 (2 mol %) in 2 mL of DCE at rt under a nitrogen atmosphere upon blue LED irradiation. b Isolated yield.

previously, the alkyl diacyl peroxides can be easily synthesized from alkyl carboxylic acids.17 Simple primary alkyl substituted diacyl peroxides afford the corresponding alkyl-fluorinated products with good yields (3ab−3ah). Primary alkyl groups with additional functional groups such as phenyl (3ai and 3aj), chloro (3ak), bromo (3al), alkenyl (3am), or ester (3an) are well tolerated under the reaction conditions, affording the corresponding alkyl-fluorinated products in moderate to high yields. The reactions with secondary alkyl diacyl peroxides afford low yields, because primary alkyl diacyl peroxides are relatively more stable and accessible than secondary alkyl diacyl peroxides and tertiary alkyl diacyl peroxides. This methodology can be utilized in late-stage functionalization of natural product derivatives (Scheme 2). For instance, Scheme 2. Synthetic Applications of the Olefin Carbofluorination Method

Scheme 4. Proposed Reaction Mechanism for the Photoredox Catalyzed Olefin Carbofluorination

18

the vinyl compounds 4a and 4b, synthesized from estrone, were applied in the reaction with LPO and Et3N·3HF under the standard reaction conditions. The desired alkyl-fluorinated products (5a and 5b) are synthesized with acceptable efficiency. To gain an understanding of the mechanism of the reaction, a radical trapping experiment was conducted with 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO) as a radical-trapping reagent.19 In this case, GC-MS analysis fails to detect the desired product (3aa), but the alkylated-TEMPO compound (6) can be isolated in 65% yield (Scheme 3a). The photocatalytic reaction of compound 7 produces the linear C


Letter

Organic Letters to generate a Cu(I) species and an oxidized [Ru(bpy)3]3+ complex.25 Based on the above-mentioned copper(I)-triggered LPO cleavage, the Cu(I) species engages in a single-electron transfer with alkyl diacyl peroxide to afford the alkyl free radical (A), the alkyl carboxylic acid anion, carbon dioxide and the regenerated Cu(II) species. The alkyl radical A reacts with an olefin to deliver the benzyl free radical (B) [E1/2ox = +0.37 V vs SCE for Ph(CH3)HC•],26 which can be oxidized by the Ru(III) species [E1/2Ru(III)/Ru(II) = +1.29 V vs SCE]27 to form the benzyl carbocation (C) and the regenerated Ru(II) species. Trapped by the nucleophilic fluoride ion, intermediate C produces the carbofluorinated product. In conclusion, we have developed a photoredox-catalyzed alkyl-fluorination of olefins under mild reaction conditions. This highly desirable carbofluorination reaction has a wide substrate scope and good functional group tolerance. The nucleophilic fluoride is used instead of the highly valent electrophilic “F+” ion as the fluorine source in the reaction. Generic alkyl groups, originating from alkyl carboxylic acids, are applied to the olefin carbofluorination for the first time as the alkyl source. Based on these results, a copper-assisted oxidative quenching mechanism has been proposed for this Ru-catalyzed olefin carbofluorination.

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2014, 47, 1513−1522. (f) Liang, T.; Neumann, C. N.; Ritter, T. Angew. Chem., Int. Ed. 2013, 52, 8214−8264. (2) For selected reviews and books on the application of fluorine chemistry: (a) Müller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881−1886. (b) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320−330. (c) Wang, J.; Sánchez-Roselló, M.; Aceña, J. L.; Pozo, C.; Sorochinsky, A. E.; Fustero, S.; Soloshonok, V. A.; Liu, H. Chem. Rev. 2014, 114, 2432−2506. (d) Richardson, P. Expert Opin. Drug Discovery 2016, 11, 983−999. (e) Bégué, J.-P.; Bonnet-Delpon, D. Bioorganic and Medicinal Chemistry of Fluorine; John Wiley & Sons: Hoboken, NJ, 2008. (f) Ojima, I. Fluorine in Medicinal Chemistry and Chemical Biology; Wiley-Blackwell: Oxford, 2009. (g) Gouverneur, V.; Müller, K. Fluorine in Pharmaceutical and Medicinal Chemistry: From Biophysical Aspects to Clinical Applications; Imperial College Press: London, 2012. (3) For a review on olefin fluorofunctionalization: Xu, X.-H.; Qing, F.-L. Curr. Org. Chem. 2015, 19, 1566−1578. (4) For selected recent papers on olefin carbofluorination. For olefin aryl-fluorination: (a) Talbot, E. P. A.; Fernandes, T.; de, A.; McKenna, J. M.; Toste, F. D. J. Am. Chem. Soc. 2014, 136, 4101− 4104. (b) Wolstenhulme, J. R.; Rosenqvist, J.; Lozano, O.; Ilupeju, J.; Wurz, N.; Engle, K. M.; Pidgeon, G. W.; Moore, P. R.; Sandford, G.; Gouverneur, V. Angew. Chem., Int. Ed. 2013, 52, 9796−9800. (c) Chen, Z.-M.; Yang, B.-M.; Chen, Z.-H.; Zhang, Q.-W.; Wang, M.; Tu, Y.-Q. Chem. - Eur. J. 2012, 18, 12950−12954. (d) Kindt, S.; Heinrich, M. R. Chem. - Eur. J. 2014, 20, 15344−15348. (e) Guo, R.; Yang, H.; Tang, P. Chem. Commun. 2015, 51, 8829−8832. (f) Yang, B.; Chansaenpak, K.; Wu, H.; Zhu, L.; Wang, M.; Li, Z.; Lu, H. Chem. Commun. 2017, 53, 3497−3500. For olefin cyano-fluorination: (g) Dilman, A. D.; Belyakov, P. A.; Struchkova, M. I.; Arkhipov, D. E.; Korlyukov, A. A.; Tartakovsky, V. A. J. Org. Chem. 2010, 75, 5367− 5370. For olefin oxycarbonyl-fluorination: (h) Qi, X.; Yu, F.; Chen, P.; Liu, G. Angew. Chem., Int. Ed. 2017, 56, 12692−12696. For olefin acyl-fluorination: (i) Wang, H.; Guo, L.-N.; Duan, X.-H. Chem. Commun. 2014, 50, 7382−7384. For olefin trifluoromethylfluorination: (j) Yu, W.; Xu, X.-H.; Qing, F.-L. Adv. Synth. Catal. 2015, 357, 2039−2044. For olefin oxycarbonylmethyl-fluorination: (k) Zhu, L.; Chen, H.; Wang, Z.; Li, C. Org. Chem. Front. 2014, 1, 1299−1305. (l) Yuan, W.; Szabó, K. J. Angew. Chem., Int. Ed. 2015, 54, 8533−8537. (m) Chen, H.; Zhu, L.; Li, C. Org. Chem. Front. 2017, 4, 565−568. For intramolecular alkyl migration assisted olefin alkyl-fluorination: (n) Romanov-Michailidis, F.; Guénée, L.; Alexakis, A. Angew. Chem., Int. Ed. 2013, 52, 9266−9270. (o) Zhao, H.; Fan, X.; Yu, J.; Zhu, C. J. Am. Chem. Soc. 2015, 137, 3490−3493. (5) For selected reviews: (a) Nyffeler, P. T.; Durón, S. G.; Burkart, M. D.; Vincent, S. P.; Wong, C.-H. Angew. Chem., Int. Ed. 2005, 44, 192−212. (b) Badoux, J.; Cahard, D. Org. React. 2007, 69, 347. (6) For selected papers on nucleophilic fluoride ion: (a) Sun, H.; DiMagno, S. G. J. Am. Chem. Soc. 2005, 127, 2050−2051. (b) Grushin, V. V. Acc. Chem. Res. 2010, 43, 160−171. (c) Watson, D. A.; Su, M.; Teverovskiy, G.; Zhang, Y.; García-Fortanet, J.; Kinzel, T.; Buchwald, S. L. Science 2009, 325, 1661−1664. (d) Lu, D.-F.; Zhu, C.-L.; Sears, J. D.; Xu, H. J. Am. Chem. Soc. 2016, 138, 11360−11367. (7) There are few examples on alkyl-fluorinations of conjugate olefins with electrophilic “F+” ion: (a) Huang, Y.; Walji, A. M.; Larsen, C. H.; MacMillan, D. W. C. J. Am. Chem. Soc. 2005, 127, 15051− 15053. (b) Quintard, A.; Alexakis, A. Adv. Synth. Catal. 2010, 352, 1856−1860. (c) Wang, L.; Meng, W.; Zhu, C.-L.; Zheng, Y.; Nie, J.; Ma, J.-A. Angew. Chem., Int. Ed. 2011, 50, 9442−9446. During the preparation of this manuscript, a paper on trifluoromethyl- and trichloromethyl-fluorination of unactivated alkenes with fluoride ions was reported: (d) Liu, Z.; Chen, H.; Lv, Y.; Tan, X.; Shen, H.; Yu, H.Z.; Li, C. J. Am. Chem. Soc. 2018, 140, 6169−6175. (8) Other selected methods for the synthesis of benzylic fluoride. Benzylic C−H fluorination: (a) Hua, A. M.; Mai, D. N.; Martinez, R.; Baxter, R. D. Org. Lett. 2017, 19, 2949−2952. (b) Zhang, Q.; Yin, X.S.; Chen, K.; Zhang, S.-Q.; Shi, B.-F. J. Am. Chem. Soc. 2015, 137, 8219−8226. (c) Nodwell, M. B.; Bagai, A.; Halperin, S. D.; Martin, R. E.; Knust, H.; Britton, R. Chem. Commun. 2015, 51, 11783−11786.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01658.

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Experimental procedures, mechanistic experiments, and spectroscopic data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yajun Li: 0000-0001-6690-2662 Hongli Bao: 0000-0003-1030-5089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Key R&D Program of China (Grant No. 2017YFA0700100), the NSFC (Grant No. 21502191, 21672213), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), the Haixi Institute of CAS (Grant No. CXZX-2017-P01), “the 1000 Youth Talents Program”, and the Natural Science Foundation of Fujian Province (Grant No. 2018J05037) for financial support.

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REFERENCES

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Merging Visible-Light Photocatalysis and Transition-Metal Catalysis in

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