Electrochemical Oxidation with Lewis-Acid Catalysis Leads to

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

pubs.acs.org/OrgLett

Electrochemical Oxidation with Lewis-Acid Catalysis Leads to Trifluoromethylative Difunctionalization of Alkenes Using CF3SO2Na Lingling Zhang,† Guoting Zhang,† Pan Wang,† Yongli Li,† and Aiwen Lei*,† †

The Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, People’s Republic of China

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S Supporting Information *

ABSTRACT: The directly external oxidant-free oxytrifluoromethylation and aminotrifluoromethylation of alkenes have been developed through the constant current electrolysis synergizing with a Lewis-acid catalysis protocol. By using sodium trifluoromethanesulfinate as the trifluoromethyl source, the method enabled difunctionalization of C−C double bonds of a wide range of styrene derivatives.

T

oxidative conditions can be easily oxidized into CF3SO3Na, which is a stable compound; therefore, using CF3SO2Na as the trifluoromethyl radical (•CF3) sources is still a problem. (see Scheme 1.)

he trifluoromethyl group has a significant effect on properties such as lipophilicity, permeability, and metabolic stability of compounds.1 Therefore, the efficient introduction of trifluoromethyl groups into pharmaceutical and agrochemical compounds, as well as functional organic materials, has become an important research field of chemistry.1 Undoubtedly, new methodologies for the efficient and highly selective incorporation of this substituent into diverse molecular structures have received increasing attention.2,3 Because of the prevalence of alkenyl in biologically active compounds and synthetic intermediates, the trifluoromethylation of alkenes with the simultaneous formation of C−C or C− heteroatom bonds is an especially practical and powerful strategy for preparing trifluoromethylated building blocks for bioactive compounds.4 Great advances have been made in the trifluoromethylation of aromatic compounds.2,3 However, compared with the exhaustive studies in the trifluoromethylation of aromatic compounds, the trifluoromethylation of alkenes remains to be researched.5−7 Among the recent reports, most of the approaches used electrophilic trifluoromethylating reagents as CF3 source such as Umemoto’s reagent or Togni’s reagents.5,8 In these reactions, the trifluoromethyl radical was generated from SET reduction processes in the presence of the appropriate photoredox catalysts or copper catalysts. Addition of the CF3 radical to alkene provided the β-CF3-substituted radical intermediate, which occurred during the second SET oxidation to give the βCF3-substituted carbocation intermediate. The β-CF3-substituted carbocation intermediate then underwent a subsequent nucleophilic attack to produce difunctionalized products. In this area, Akita’s group have made many breakthroughs for the difunctionalization of alkene using the photoredox catalyst.8a,d,f However, the high cost of these trifluoromethylating reagents and the scarce availability greatly limit their practical usage. As an inexpensive, stable, and easily handled trifluoromethyl source, CF3SO2Na (Langlois reagent) has been extensively studied since 1991.9 However, the Langlois reagent under © XXXX American Chemical Society

Scheme 1. Previous Work on Difunctionalization of Alkenes Involving Formation of the C−CF3 Bond

During the past few years, electrosynthesis has been considered to be a practical and environmentally friendly synthetic tool. The application of electrochemical anodic oxidation in synthetic organic chemistry has drawn increasing attention.10,11 Electrochemical alkene difunctionalization has been demonstrated in several seminal examples for the formation of C−C, C−N, C−Cl, and C−O bonds.12 Nevertheless, the scope and generality of this electrochemical approach remains limited. Only limited examples of environmentally friendly electrocatalytic protocol for alkene trifluoromethylation have been developed.13 Rare-earth metal triflates are a new type of Lewis acid catalysts which are extremely stable under redox conditions, and they have been widely applied to various organic transformations.14 Of them, Y(OTf)3 poses very specific Received: September 26, 2018

A

DOI: 10.1021/acs.orglett.8b03081 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters coordination property and has become a well-established catalyst in many useful organic synthesis reactions.15 Despite its tremendous use in organic synthesis, there exists few examples of Y(OTf)3 to combine with an electrochemical method to catalyze organic transformations. Herein, we report novel intermolecular oxytrifluoromethylation and aminotrifluoromethylation of alkenes in the presence of yttrium triflate under constant-current conditions, using NaSO2CF3 as an inexpensive, stable, easily handled, and stored trifluoromethylation reagent. Initially, 4-methylstyrene (1a) was chosen as the model substrate to react with sodium trifluoromethanesulfinate 2a in methanol under constant current electrolysis conditions (see Table 1). After a series of optimizations, the product can be

Scheme 2. Substrate Scope of the Oxytrifluoromethylation of Styrene Derivativesa,b

Table 1. Effect of the Reaction Parametersa

a

entry

variation from standard conditions

yield (%)

1 2 3 4 5 6 7

none no Y(OTf)3 10 mA instead of 15 mA 20 mA instead of 15 mA LiClO4 instead of nBu4NBF4 Pt (+)|Pt(−) no electricity

85 60 25 76 83 54 n.d.b

Reaction conditions: C anode, Pt cathode, constant current = 15 mA, 1 (0.50 mmol), 2 (1.0 mmol), nBu4NBF4 (0.25 equiv), Y(OTf)3 (0.5 equiv), MeOH (10 mL), room temperature, 3 h. Yield determined by 19F NMR spectroscopy using trifluoroacetophenone as an internal standard. bIsolated yield.

Disubstituted substrate 1m was also compatible with this transformation, providing the desired β-CF3 tertiary alkyl ether 3m in 77% yield. Remarkably, internal alkenes 1n were found to exhibit a similar reactivity compared to the terminal alkenes. Then, the other O nucleophiles (ROH; 4) such as alcohols and carboxylic acids were further investigated (Scheme 3). As a

a

Reaction conditions: C anode, Pt cathode, constant current = 15 mA, 1a (0.50 mmol), 2 (1.0 mmol), nBu4NBF4 (0.25 equiv), Y(OTf)3 (0.5 equiv), MeOH (10 mL), room temperature, 3 h. Yield determined by 19F NMR spectroscopy using trifluoroacetophenone as an internal standard.; n.d. = not detected. bn.d. = not detected.

Scheme 3. Substrate Scope of the Oxytrifluoromethylation of O Nucleophilesa

obtained under a constant current electrolysis at 15 mA in an undivided cell using nBu4NBF4 as electrolyte and Y(OTf)3 as additive in MeOH at room temperature (Table 1, entry 1). A decreased reaction yield was obtained without the addition of Y(OTf)3 (Table 1, entry 2). Increasing or decreasing the constant current would also lead to decreased reaction yields (Table 1, entries 3 and 4). As for the choice of electrolyte, lithium perchlorate obtained the same reaction efficiency compared with ammonium salts (Table 1, entry 5). Moreover, the effect of the electrode material was also explored. Lower reaction yield was obtained when the carbon anode was replaced by platinum (Table 1, entry 6). Control experiments also showed that no desired product was generated without electricity (Table 1, entry 7). With the optimized reaction conditions in hand, the scope of this electrochemical three-component oxytrifluoromethylation was studied (Scheme 2). Styrene derivatives 1a−l bearing various substituents on the aromatic ring reacted smoothly and led to the corresponding trifluoromethylated ethers 3a−l in good efficiencies. The reaction efficiency was found not to be sensitive to styrene with electron-donating groups at ortho, meta, and para positions (3i, 3j, and 3a). In addition, this reaction could be applied to styrenes bearing halogen atoms such as F (1c), Cl (1d, 1h, 1l), and Br (1e), an ester group such as AcO (1g). The corresponding ether products 3c−e,3h, 3l and 3g were obtained in good yields without any loss of the functional groups. In particular, the Br functionalities can serve as potential handles for additional transformations. 1,1-

a

Reaction conditions: C anode, Pt cathode, constant current = 15 mA, 1a (0.50 mmol), 2 (1.0 mmol), nBu4NBF4 (0.25 equiv), Y(OTf)3 (0.5 equiv), ROH (6 mL), DMF(4 mL) room temperature, 3 h. Yield determined by 19F NMR spectroscopy using trifluoroacetophenone as an internal standard.

result, alkoxytrifluoromethylation and carboxytrifluoromethylation smoothly proceeded not only to introduce a CF3 group to an alkene but also to construct ether and ester functionalities. Reactions of 1a with primary (4a−4b and 4d−4e) and secondary (4c) alcohols produced the expected CF3-substituted ethers 3aa−3ae in good yields. Formic acid (4f) and acetic acid (4g) afforded the corresponding CF3containing esters 3af and 3ag in 59% and 60% yields, respectively. These results indicate that the present electrochemical three-component oxytrifluoromethylation leads to B

DOI: 10.1021/acs.orglett.8b03081 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

of 1a in MeOH was observed at 1.93 V (for details, see the black line in Figure S3). At the same time, oxidation peaks of 2 could be observed above 1.51 V (for details, see the red line in Figure S3). These results indicate that 2 was likely to be first oxidized under electrolytic conditions. Based on the aforementioned studies and the mechanistic insights of the previous cases, a plausible pathway for this oxytrifluoromethylation process is outlined in Scheme 6.

the efficient and regioselective reactions, regardless of the O nucleophiles. In further demonstration of the utility and applicability of this method, a gram-scale synthesis of 3a was performed. As shown in Figure S1 in the Supporting Information, the gramscale reaction proceeded well to form the corresponding product (3a) in 55% yield, demonstrating the capacity to apply of the protocol. Next, we investigated N nucleophiles. As a result, aminotrifluoromethylation smoothly proceeded not only to introduce a CF3 group to an alkene but also to construct many functional groups. Various pyrazoles could be tolerated, providing a green route for β-trifluoromethylated N-substituted azoles (Scheme 4, 3ah−3aj). When we use acetonitrile (MeCN) as an Nnucleophile, aminotrifluoromethylated products were obtained in good yields (Scheme 5, 3ba and 3oa).

Scheme 6. Proposed Mechanism

Scheme 4. Substrate Scope of the Aminotrifluoromethylation of N Nucleophilesa

Initially, CF3SO2Na can be oxidized at the anode via a single-electron-transfer (SET) process to afford CF3 radical, and alkene converts to intermediate A in the presence of Y(OTf)3 and nucleophiles. The attack of CF3 radical onto intermediate A generates radical intermediate B, which next can be further oxidized to intermediate C. Subsequent nucleophilic attack on the carbocation intermediate C produces the three-component coupled product 3. In conclusion, an environmentally friendly electrochemical reaction protocol was developed to achieve the multicomponent intermolecular oxytrifluoromethylation and aminotrifluoromethylation of alkenes. No chemical oxidants were needed in this transformation. Under undivided electrolysis conditions, a comprehensive scope of the developed process and the tolerance of a variety of functional groups were successfully demonstrated. Importantly, this reaction could be scaled up with good reaction efficiency, which is beneficial for practical applications.

a

Reaction conditions: C anode, Pt cathode, constant current = 15 mA, 1a (0.50 mmol), 2 (1.0 mmol), nBu4NBF4 (0.25 equiv), Y(OTf)3 (0.5 equiv), 5 (3.0 equiv) CH3CN (6 mL), CH2Cl2 (4 mL) room temperature, 3 h. Yield determined by 19F NMR spectroscopy using trifluoroacetophenone as an internal standard.

Scheme 5. Substrate Scope of the Aminotrifluoromethylation of N Nucleophiles

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Preliminary mechanistic studies were conducted to gain insights into this transformation using electron paramagnetic resonance (EPR) spectroscopy. As shown in Figure S2 in the Supporting Information, when radical spin trapping agent DMPO (5,5-dimethyl-1-pyrroline N-oxide) was added under standard reaction condition, no EPR signal was observed under standard reaction condition (for details, see blue line in Figure S2). While a strong radical signal was observed (for details, see red line in Figure S2) in the absence of styrene. By compared with the simulated EPR spectrum (for details, see black line in Figure S2), the radical signal belongs to the CF3 radical (g = 2.0069, AN = 14.6, AH = 13.5). Thus, we proposed that radical process was possibly involved under the electrochemical conditions. In the next step, we performed cyclic voltammetry (CV) experiments to study the redox potential of the substrates (see Figure S3 in the Supporting Information). An oxidation peak

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03081. The experimental procedure, characterization data, and copies of 1H and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aiwen Lei: 0000-0001-8417-3061 Notes

The authors declare no competing financial interest. C

DOI: 10.1021/acs.orglett.8b03081 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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(g) Wilger, D. J.; Gesmundo, N. J.; Nicewicz, D. A. Chemical Science 2013, 4, 3160−3165. (7) (a) Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131, 10875−10877. (b) Iqbal, N.; Choi, S.; Ko, E.; Cho, E. J. Tetrahedron Lett. 2012, 53, 2005−2008. (c) Ye, Y.; Sanford, M. S. J. Am. Chem. Soc. 2012, 134, 9034−9037. (d) Nguyen, J. D.; Tucker, J. W.; Konieczynska, M. D.; Stephenson, C. R. J. J. Am. Chem. Soc. 2011, 133, 4160−4163. (e) Wallentin, C.-J.; Nguyen, J. D.; Finkbeiner, P.; Stephenson, C. R. J. J. Am. Chem. Soc. 2012, 134, 8875−8884. (8) (a) Yasu, Y.; Koike, T.; Akita, M. Angew. Chem., Int. Ed. 2012, 51, 9567−9571. (b) Koike, T.; Akita, M. Acc. Chem. Res. 2016, 49, 1937−1945. (c) Janson, P. G.; Ghoneim, I.; Ilchenko, N. O.; Szabó, K. J. Org. Lett. 2012, 14, 2882−2885. (d) Yasu, Y.; Koike, T.; Akita, M. Org. Lett. 2013, 15, 2136−2139. (e) Dagousset, G.; Carboni, A.; Magnier, E.; Masson, G. Org. Lett. 2014, 16, 4340−4343. (f) Tomita, R.; Yasu, Y.; Koike, T.; Akita, M. Angew. Chem., Int. Ed. 2014, 53, 7144−7148. (9) (a) Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron Lett. 1991, 32, 7525−7528. (b) Clavel, J.-L.; Langlois, B.; Laurent, E.; Roidot, N. Phosphorus, Sulfur Silicon Relat. Elem. 1991, 59, 169−172. (c) Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron Lett. 1992, 33, 1291−1294. (d) Tommasino, J.-B.; Brondex, A.; Médebielle, M.; Thomalla, M.; Langlois, B. R.; Billard, T. Synlett 2002, 2002, 1697− 1699. (e) O’Brien, A. G.; Maruyama, A.; Inokuma, Y.; Fujita, M.; Baran, P. S.; Blackmond, D. G. Angew. Chem., Int. Ed. 2014, 53, 11868−11871. (10) (a) Moeller, K. D. Tetrahedron 2000, 56, 9527−9554. (b) Sperry, J. B.; Wright, D. L. Chem. Soc. Rev. 2006, 35, 605−621. (c) Yoshida, J.-i.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev. 2008, 108, 2265−2299. (d) Francke, R.; Little, R. D. Chem. Soc. Rev. 2014, 43, 2492−2521. (e) Yan, M.; Kawamata, Y.; Baran, P. S. Chem. Rev. 2017, 117, 13230−13319. (11) (a) Sauer, G. S.; Lin, S. ACS Catal. 2018, 8, 5175−5187. (b) Kurihara, H.; Tajima, T.; Fuchigami, T. Electrochemistry 2006, 74, 615−617. (c) Horn, E. J.; Rosen, B. R.; Chen, Y.; Tang, J.; Chen, K.; Eastgate, M. D.; Baran, P. S. Nature 2016, 533, 77−81. (d) Wang, P.; Tang, S.; Huang, P.; Lei, A. Angew. Chem., Int. Ed. 2017, 56, 3009− 3013. (e) Rafiee, M.; Wang, F.; Hruszkewycz, D. P.; Stahl, S. S. J. Am. Chem. Soc. 2018, 140, 22−25. (f) Li, Y.; Gao, H.; Zhang, Z.; Qian, P.; Bi, M.; Zha, Z.; Wang, Z. Chem. Commun. 2016, 52, 8600−8603. (g) Qian, P.; Su, J.-H.; Wang, Y.; Bi, M.; Zha, Z.; Wang, Z. J. Org. Chem. 2017, 82, 6434−6440. (12) (a) Fu, N.; Sauer, G. S.; Lin, S. J. Am. Chem. Soc. 2017, 139, 15548−15553. (b) Fu, N.; Sauer, G. S.; Saha, A.; Loo, A.; Lin, S. Science 2017, 357, 575−579. (c) Parry, J. B.; Fu, N.; Lin, S. Synlett 2018, 29, 257−265. (d) Senboku, H.; Komatsu, H.; Fujimura, Y.; Tokuda, M. Synlett 2001, 2001, 0418−0420. (e) Arai, K.; Watts, K.; Wirth, T. ChemistryOpen 2014, 3, 23−28. (f) Yan, W.-q.; Lin, M.-y.; Little, R. D.; Zeng, C.-C. Tetrahedron 2017, 73, 764−770. (g) Torii, S.; Liu, P.; Tanaka, H. Chem. Lett. 1995, 24, 319−320. (h) Torii, S.; Liu, P.; Bhuvaneswari, N.; Amatore, C.; Jutand, A. J. Org. Chem. 1996, 61, 3055−3060. (i) Wiese, U.; Schafer, H.; v. Schnering, H. G.; Brendel, C.; Rinke, K. Angew. Chem., Int. Ed. Engl. 1970, 9, 158−158. (13) Ye, K.-Y.; Pombar, G.; Fu, N.; Sauer, G. S.; Keresztes, I.; Lin, S. J. Am. Chem. Soc. 2018, 140, 2438−2441. (14) (a) Zhou, B.; Li, L.; Zhu, X.-Q.; Yan, J.-Z.; Guo, Y.-L.; Ye, L.W. Angew. Chem., Int. Ed. 2017, 56, 4015−4019. (b) Lee, S. G.; Kim, S.-G. Tetrahedron 2018, 74, 3671−3678. (c) Li, C.; Zhan, W.; Zhang, F. Catal. Commun. 2018, 103, 65−68. (d) Zhou, B.; Li, L.; Liu, X.; Tan, T.-D.; Liu, J.; Ye, L.-W. J. Org. Chem. 2017, 82, 10149−10157. (e) Han, Y.-P.; Song, X.-R.; Qiu, Y.-F.; Zhang, H.-R.; Li, L.-H.; Jin, D.-P.; Sun, X.-Q.; Liu, X.-Y.; Liang, Y.-M. Org. Lett. 2016, 18, 940− 943. (15) (a) Biswas, C. S.; Patel, V. K.; Vishwakarma, N. K.; Mishra, A. K.; Saha, S.; Ray, B. Langmuir 2010, 26, 6775−6782. (b) Biswas, C. S.; Vishwakarma, N. K.; Patel, V. K.; Mishra, A. K.; Saha, S.; Ray, B. Langmuir 2012, 28, 7014−7022 and references therein .

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21390400, 21520102003, 21272180, 21302148), the Hubei Province Natural Science Foundation of China (No. 2013CFA081), the Research Fund for the Doctoral Program of Higher Education of China (No. 20120141130002), and the Ministry of Science and Technology of China (No. 2012YQ120060). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.

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REFERENCES

(1) (a) Hiyama, T. Organofluorine Compounds: Chemistry and Applications; Springer: Berlin, 2000. (b) Kirsch, P. Modern Fluoroorganic Chemistry: Synthesis Reactivity Applications; Wiley− VCH: Weinheim, Germany, 2004. (c) Yamazaki, T.; Taguchi, T.; Ojima, I. Fluorine in Medicinal Chemistry and Chemical Biology; Wiley−Blackwell, Chichester, Great Britain, 2009. (d) Jeschke, P. ChemBioChem 2004, 5, 570−589. (e) Müller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881−1886. (2) For recent reviews on the synthesis of CF3-containing molecules, see: (a) Ma, J.-A.; Cahard, D. Chem. Rev. 2004, 104, 6119−6146. (b) Shimizu, M.; Hiyama, T. Angew. Chem. 2005, 117, 218−234; Angew. Chem., Int. Ed. 2005, 44, 214−231. (c) Ma, J.-A.; Cahard, D. J. Fluorine Chem. 2007, 128, 975−996. (d) Shibata, N.; Mizuta, S.; Toru, T. Yuki Gosei Kagaku Kyokaishi 2008, 66, 215−228. (e) Furuya, T.; Kamlet, A. S.; Ritter, T. Nature 2011, 473, 470−477. (f) Studer, A. Angew. Chem., Int. Ed. 2012, 51, 8950−8958; Angew. Chem. 2012, 124, 9082−9090. (g) O’Brien, A. G.; Maruyama, A.; Inokuma, Y.; Fujita, M.; Baran, P. S.; Blackmond, D. G. Angew. Chem., Int. Ed. 2014, 53, 11868−11871. (h) Liu, H.; Gu, Z.; Jiang, X. Adv. Synth. Catal. 2013, 355, 617−626. (i) Katsuyama, I. Yuki Gosei Kagaku Kyokaishi 2009, 67, 992−1000. (j) Amii, H. Yuki Gosei Kagaku Kyokaishi 2011, 69, 752−762. (k) Besset, T.; Schneider, C.; Cahard, D. Angew. Chem. 2012, 124, 5134−5136. (3) (a) Charpentier, J.; Früh, N.; Togni, A. Chem. Rev. 2015, 115, 650−682. (b) Yang, X.; Wu, T.; Phipps, R. J.; Toste, F. D. Chem. Rev. 2015, 115, 826−870. (c) Liu, X.; Xu, C.; Wang, M.; Liu, Q. Chem. Rev. 2015, 115, 683−730. (d) Chu, L.; Qing, F.-L. Acc. Chem. Res. 2014, 47, 1513−1522. (e) Besset, T.; Poisson, T.; Pannecoucke, X. Chem. - Eur. J. 2014, 20, 16830−16845. (f) Liang, T.; Neumann, C. N.; Ritter, T. Angew. Chem., Int. Ed. 2013, 52, 8214−8264. (g) Tomashenko, O. A.; Grushin, V. V. Chem. Rev. 2011, 111, 4475−4521. (h) Besset, T.; Schneider, C.; Cahard, D. Angew. Chem., Int. Ed. 2012, 51, 5048−5050. (i) Alonso, C.; Martínez de Marigorta, E.; Rubiales, G.; Palacios, F. Chem. Rev. 2015, 115, 1847−1935. (j) Chen, P.; Liu, G. Synthesis 2013, 45, 2919−2939. (4) (a) Egami, H.; Sodeoka, M. Angew. Chem., Int. Ed. 2014, 53, 8294−8308. (b) Merino, E.; Nevado, C. Chem. Soc. Rev. 2014, 43, 6598−6608. (c) Gao, P.; Song, X.-R.; Liu, X.-Y.; Liang, Y.-M. Chem. Eur. J. 2015, 21, 7648−7661. (5) (a) Parsons, A. T.; Buchwald, S. L. Angew. Chem. 2011, 123, 9286−9289; Angew. Chem., Int. Ed. 2011, 50, 9120−9123. (b) Xu, J.; Fu, Y.; Luo, D.-F.; Jiang, Y.-Y.; Xiao, B.; Liu, Z.-J.; Gong, T.-J.; Liu, L. J. Am. Chem. Soc. 2011, 133, 15300−15303. (c) Wang, X.; Ye, Y.; Zhang, S.; Feng, J.; Xu, Y.; Zhang, Y.; Wang, J. J. Am. Chem. Soc. 2011, 133, 16410−16413. (d) Yang, B.; Xu, X.-H.; Qing, F.-L. Org. Lett. 2015, 17, 1906−1909. (6) (a) Kamigata, N.; Fukushima, T.; Yoshida, M. J. Chem. Soc., Chem. Commun. 1989, 0, 1559−1560. (b) Kamigata, N.; Fukushima, T.; Terakawa, Y.; Yoshida, M.; Sawada, H. J. Chem. Soc., Perkin Trans. 1 1991, 0, 627−633. (c) Ignatowska, J.; Dmowski, W. J. Fluorine Chem. 2007, 128, 997−1006. (d) Wallentin, C.-J.; Nguyen, J. D.; Finkbeiner, P.; Stephenson, C. R. J. J. Am. Chem. Soc. 2012, 134, 8875−8884. (e) Mu, X.; Wu, T.; Wang, H.-y.; Guo, Y.-l.; Liu, G. J. Am. Chem. Soc. 2012, 134, 878−881. (f) Janson, P. G.; Ghoneim, I.; Ilchenko, N. O.; Szabó, K. J. Org. Lett. 2012, 14, 2882−2885. D

DOI: 10.1021/acs.orglett.8b03081 Org. Lett. XXXX, XXX, XXX−XXX