Generation of benzosultams via trifluoromethylation of 2-ethynylbenzenesulfonamide under visible light

Abstract

Under visible light irradiation, 2-ethynylbenzenesulfonamides react with Togni's reagent in the presence of a photocatalyst leading to 3-(2,2,2-trifluoroethylidene)-2,3-dihydrobenzo[d]isothiazole 1,1-dioxides in good yields. This transformation proceeds efficiently at room temperature through a photo-initiated trifluoromethylation. The subsequent C–N bond formation produces the corresponding benzosultams.

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Introduction

Currently, continuous efforts are being made towards the construction of nitrogen-containing heterocycles in the drug discovery process and for the studies of chemical genetics.¹ Among the scaffolds of nitrogen-containing heterocycles, benzosultams have attracted our attention due to their importance in natural products and pharmaceuticals with diverse biological activities.² Over the past decade, we have been interested in the generation of natural product-like compounds and their subsequent biological evaluations.³ During the studies, we identified that some hits with the core of benzosultam showed promising activities for inhibition of cervical carcinoma. It is known that the medicinal properties of drugs or leading compounds could often be significantly improved if the fluorine atom is incorporated into the molecules.⁴ Therefore, we initiated a program for the preparation of fluoro-substituted benzosultams, with an expectation to find more active compounds.

The photo-induced organic transformations attracted our interest due to the efficiency of these methods and mild reaction conditions.^5,6 Usually, a single electron transfer (SET) process is involved in photocatalysis, which promotes the completion of the reaction. The advantages and efficiency of photochemical reactions make them attractive in organic synthesis. Among the photo-induced transformations, trifluoromethylation under photoredox catalysis has made rapid progress.^7–10 Usually, trifluoromethylation occurs in the presence of visible light and a photocatalyst at room temperature. In most cases, transition metals and organic molecules were employed as catalysts in trifluoromethylation. These photocatalysts were crucial to fulfill the photocatalytic cycles via SET processes. In general, the photocatalyst was stimulated to the excited state with visible light, which then provided an electron to the fluorinating reagent with the release of a CF₃ radical. Prompted by the photoinduced trifluoromethylation,^7–10 we envisioned that trifluoromethyl-substituted benzosultams would be accessed under suitable conditions. Recently, we reported an efficient synthesis of 4-((trifluoromethyl)thio)-2H-benzo[e][1,2]thiazine 1,1-dioxides through a BiCl₃-promoted reaction of 2-(2-alkynyl)benzenesulfonamide with trifluoromethanesulfanylamide.^11a We conceived that 2-(2-alkynyl)benzenesulfonamides could be utilized as the starting materials as well for the photoinduced trifluoromethylation. Thus, we started to explore the feasibility for the formation of trifluoromethyl-substituted benzosultams via trifluoromethylation of 2-(2-alkynyl)benzenesulfonamides under photocatalysis.

Results and discussion

Due to the easy availability, Togni's reagent⁷ was utilized as the source of trifluoromethylation. At the outset, the reaction of N-methyl-2-(phenylethynyl)benzenesulfonamide¹¹ and Togni's reagent was studied by using Ir(ppy)₃ as the photocatalyst under visible light. However, no reaction took place although different solvents and bases were screened. Since the addition of a trifluoromethyl radical to the triple bond would be the key step during the reaction process, we postulated that the steric hinderance of the alkyne in N-methyl-2-(phenylethynyl)benzenesulfonamide would retard the reaction. Thus, 2-ethynyl-N-methylbenzenesulfonamide 1a was used as a replacement for N-methyl-2-(phenylethynyl)benzenesulfonamide with Togni's reagent, and the reaction was re-explored. Initially, the reaction was catalyzed with Ir(ppy)₃ (1 mol%) at 25 °C in MeCN under visible light irradiation in the presence of potassium carbonate as a base (Table 1, entry 1). To our delight, we observed the formation of desired products. The corresponding (Z)/(E)-3-(2,2,2-trifluoroethylidene)-2,3-dihydrobenzo[d]isothiazole 1,1-dioxides 2a were generated. These two compounds could be isolated, and the structures of (E)-2a and (Z)-2a were unambiguously identified via X-ray crystallography analysis (see the ESI†). Interestingly, only Ir(ppy)₃ was effective as the photocatalyst, and the reaction failed to provide the expected products when Ru(bpy)₃(PF₆)₂, eosin Y, or fluorescein was employed in the transformation (Table 1, entries 2–4). The presence of Ir(ppy)₃ was essential, since no reaction occurred in a controlled experiment without the addition of a photocatalyst (data not shown in Table 1). Subsequently, different solvents were evaluated (Table 1, entries 5–12). When 1,4-dioxane was used as the solvent, the desired (E)-2a and (Z)-2a were produced in 20% and 12% yields, respectively (Table 1, entry 5). Only a trace amount of products was detected when the reaction was performed in DMF, MeOH, or Et₂O. Compared with the results obtained in CH₂Cl₂, DCE, acetone, and EtOAc, acetonitrile was demonstrated as the best solvent of choice. Further survey of bases revealed that the reaction worked efficiently when NaHCO₃ was used as the base, which provided the corresponding (E)-2a and (Z)-2a in 48% and 30% yields, respectively (Table 1, entries 13–21). The presence of other bases would result in inferior yields.

Table 1 Initial studies on the photo-induced reaction of 2-ethynyl-N-methylbenzenesulfonamide with Togni's reagent^a

Entry	Cat.	Base	Solvent	Yield^b (%)
				(E)-2a	(Z)-2a
a Reaction conditions: 2-ethynyl-N-methylbenzenesulfonamide 1a (0.2 mmol), Togni's reagent (1.2 equiv., 0.24 mmol), base (1.5 equiv., 0.3 mmol), photocatalyst (1 mol%), solvent (4.0 mL), N2, room temperature, under visible light irradiation (8W) for 14 h. b Determined by ^19F NMR with the use of trifluoromethyl benzene as an internal standard.
1	Ir(ppy)3	K2CO3	MeCN	34	21
2	Ru(bpy)3(PF6)2	K2CO3	MeCN	Trace	Trace
3	Eosin Y	K2CO3	MeCN	Trace	Trace
4	Fluorescein	K2CO3	MeCN	Trace	Trace
5	Ir(ppy)3	K2CO3	Dioxane	20	12
6	Ir(ppy)3	K2CO3	DMF	Trace	Trace
7	Ir(ppy)3	K2CO3	CH2Cl2	18	13
8	Ir(ppy)3	K2CO3	MeOH	Trace	Trace
9	Ir(ppy)3	K2CO3	DCE	20	12
10	Ir(ppy)3	K2CO3	Acetone	26	14
11	Ir(ppy)3	K2CO3	Et2O	Trace	Trace
12	Ir(ppy)3	K2CO3	EtOAc	28	9
13	Ir(ppy)3	KHCO3	MeCN	36	24
14	Ir(ppy)3	K3PO4	MeCN	37	22
15	Ir(ppy)3	Na2CO3	MeCN	27	15
16	Ir(ppy)3	NaHCO3	MeCN	48	30
17	Ir(ppy)3	DABCO	MeCN	33	20
18	Ir(ppy)3	NaOAc	MeCN	22	13
19	Ir(ppy)3	KOAc	MeCN	20	11
20	Ir(ppy)3	Cs2CO3	MeCN	21	11
21	Ir(ppy)3	NaOH	MeCN	12	7

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With the above optimized conditions in hand, we next explored the scope of the reaction of 2-ethynylbenzenesulfonamide 1 with Togni's reagent in the presence of a photocatalyst under visible light irradiation. The result is shown in Table 2. It was found that all reactions worked well, affording (E)-2 and (Z)-2 in totally good yields. All compounds of (E)-2 and (Z)-2 could be isolated easily via column chromatography. The generality of this approach was demonstrated, since different functional groups could be tolerated under the conditions. For example, the amino group could be compatible during the reaction process (product 2m).

Table 2 Scope investigation of the photo-induced trifluoromethylation of 2-ethynylbenzenesulfonamide 1 with Togni's reagent^a

a Isolated yield based on 2-ethynylbenzenesulfonamide 1.

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It seemed that no big difference was displayed for the reactions of 2-ethynylbenzenesulfonamides 1 with electron-donating groups or electron-withdrawing groups attached to the aromatic ring. It is noteworthy that the halo group (F and Cl) could be retained during the reaction process. The ratio of products was influenced when the methyl group attached to the nitrogen atom was replaced by other groups. For instance, when 2-ethynyl-N-cyclohexylbenzenesulfonamide was used as the substrate, the corresponding products (E)-2f and (Z)-2f were obtained in 77% and 9% yields, respectively. Similar results were observed when the group attached to the nitrogen atom was changed to tert-butyl and sec-butyl groups.

Based on the photoinduced trifluoromethylation as reported previously,⁷ a plausible mechanism was proposed (Scheme 1). We postulated that the reaction proceeded initially through electron transfer from the photocatalyst, which was stimulated to the excited state by visible light. The fluorination reagent would accept an electron, thus resulting in the release of a CF₃ radical. The addition of the CF₃ radical to 2-ethynylbenzenesulfonamide 1 would generate the alkenyl radical C, which then underwent electron transfer to produce the alkenyl cation intermediate D. Then the subsequent intramolecular nucleophilic addition would provide 3-(2,2,2-trifluoroethylidene)-2,3-dihydrobenzo[d]isothiazole 1,1-dioxide 2.

Scheme 1 A proposed mechanism for the photo-induced trifluoromethylation of 2-ethynylbenzenesulfonamide 1 with Togni's reagent.

Conclusions

In conclusion, we have described a photo-induced reaction of 2-ethynylbenzenesulfonamide with Togni's reagent in the presence of a photocatalyst. Under visible light irradiation, 3-(2,2,2-trifluoroethylidene)-2,3-dihydrobenzo[d]isothiazole 1,1-dioxides are generated in good yields. The (E)- and (Z)-isomers can be isolated easily. This transformation works efficiently at room temperature through photo-initiated electron transfer from the photocatalyst. The trifluoromethyl radical generated in situ is the key intermediate, which undergoes trifluoromethylation and the subsequent C–N bond formation leading to the corresponding benzosultams.

Acknowledgements

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

Notes and references

1. Selected reviews on the synthesis of N-heterocycles: (a) A. Armstrong and J. C. Collins, Angew. Chem., Int. Ed., 2010, 49, 2282; (b) A. R. Katritzky and S. Rachwal, Chem. Rev., 2010, 110, 1564; (c) M. A. P. Martins, C. P. Frizzo, D. N. Moreira, L. Buriol and P. Machado, Chem. Rev., 2009, 109, 4140; (d) M. Alvarez-Corral, M. Munoz-Dorado and I. Rodrıguez-Garcıa, Chem. Rev., 2008, 108, 3174; (e) A. Minatti and K. Muniz, Chem. Soc. Rev., 2007, 36, 1142; (f) G. Zeni and R. C. Larock, Chem. Rev., 2006, 106, 4644; (g) G. Qiu and J. Wu, Chem. Rec., 2016, 16, 19; (h) G. Qiu, Y. Kuang and J. Wu, Adv. Synth. Catal., 2014, 356, 3483.
2. For selected examples, see: (a) L. Zhuang, J. S. Wai, M. W. Embrey, T. E. Fisher, M. S. Egbertson, L. S. Payne, L. P. Guare, J. P. Vacca, D. J. Hazuda, P. J. Felock, A. L. Wolfe, K. A. Stillmock, M. V. Witmer, G. Moyer, W. A. Schleif, L. J. Gabryelski, Y. M. Leonard, J. J. Lynch, S. R. Michelson and S. D. Young, J. Med. Chem., 2003, 46, 453; (b) C. Valente, R. C. Guedes, R. Moreira, J. Iley, J. Gut and P. J. Rosenthal, Bioorg. Med. Chem. Lett., 2006, 16, 4115; (c) M. Inagaki, T. Tsuri, H. Jyoyama, T. Ono, K. Yamada, M. Kobayashi, Y. Hori, A. Arimura, K. Yasui, K. Ohno, S. Kakudo, K. Koizumi, R. Suzuki, S. Kawai, M. Kato and S. Matsumoto, J. Med. Chem., 2000, 43, 2040; (d) F. Brzozowski, F. Saczewski and N. Neamati, Bioorg. Med. Chem. Lett., 2006, 16, 5298; (e) G. J. Wells, M. Tao, K. A. Josef and R. Bihovsky, J. Med. Chem., 2001, 44, 3488; (f) R. J. Cherney, R. Mo, D. T. Meyer, K. D. Hardman, R.-Q. Liu, M. B. Covington, M. Qian, Z. R. Wasserman, D. D. Christ, J. M. Trzaskos, R. C. Newton and C. P. Decicco, J. Med. Chem., 2004, 47, 2981; (g) M. Bartholow, Top 200 Drugs of 2011. Pharmacy Times. http://www.pharmacytimes.com/publications/issue/2012/July2012/Top-200-Drugs-of-2011, accessed on Jan 9, 2013; (h) For a list of top drugs by year, see: http://cbc.arizona.edu/njardarson/group/top-pharmaceuticals-poster, accessed on Jan 9, 2013; (i) J. Drews, Science, 2000, 287, 1960.
3. For selected reviews, see: (a) H. Wang, Y. Kuang and J. Wu, Asian J. Org. Chem., 2012, 1, 302; (b) L. He, H. Nie, G. Qiu, Y. Gao and J. Wu, Org. Biomol. Chem., 2014, 12, 9045; (c) G. Qiu, Q. Ding and J. Wu, Chem. Soc. Rev., 2013, 42, 5257; (d) Y. Luo, X. Pan, X. Yu and J. Wu, Chem. Soc. Rev., 2014, 43, 834; (e) G. Qiu and J. Wu, Synlett, 2014, 2703.
4. (a) R. Filler and Y. Kobayashi, Biomedicinal Aspects of Fluorine Chemistry, Elsevier, Amsterdam, 1982; (b) Fluorine in Bioorganic Chemistry, ed. J. T. Welch and S. Eswarakrishman, Wiley, New York, 1991.
5. For reviews, see: (a) T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2, 527; (b) D. M. Schultz and T. P. Yoon, Science, 2014, 343, 1239176; (c) J. M. R. Narayanam and C. R. J. Stephenson, Chem. Soc. Rev., 2011, 40, 102; (d) J. Xuan and W.-J. Xiao, Angew. Chem., Int. Ed., 2012, 51, 6828; (e) L. Shi and W. Xia, Chem. Soc. Rev., 2012, 41, 7687; (f) Y.-M. Xi, H. Yi and A.-W. Lei, Org. Biomol. Chem., 2013, 11, 2387; (g) C. K. Prier, D. A. Rankic and D. W. C. MacMillan, Chem. Rev., 2013, 113, 5322; (h) D. M. Schultz and T. P. Yoon, Science, 2014, 343, 985; (i) M. N. Hopkinson, B. Sahoo, J.-L. Li and F. Glorius, Chem. – Eur. J., 2014, 20, 3874; (j) J. Xuan, Z.-G. Zhang and W.-J. Xiao, Angew. Chem., Int. Ed., 2015, 54, 15632; (k) J.-R. Chen, X.-Q. Hu, L.-Q. Lu and W.-J. Xiao, Chem. Soc. Rev., 2016, 45, 2044; (l) S. Protti, M. Fagnoni and D. Ravelli, ChemCatChem, 2015, 7, 1516; (m) M. D. Karkas, B. S. Matsuura and C. R. J. Stephenson, Science, 2015, 349, 1285; (n) R. Brimioulle, D. Lenhart, M. M. Maturi and T. Bach, Angew. Chem., Int. Ed., 2015, 54, 3872; (o) D. Ravelli, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2013, 42, 97; (p) T. P. Yoon, ACS Catal., 2013, 3, 895; (q) M. A. Ischay and T. P. Yoon, Eur. J. Org. Chem., 2012, 3359; (r) N. Hoffmann, ChemSusChem, 2012, 5, 352; (s) J. W. Tucker and C. R. J. Stephenson, J. Org. Chem., 2012, 77, 1617.
6. For selected examples, see: (a) C. Uyeda, Y. Tan, G. C. Fu and J. C. Peters, J. Am. Chem. Soc., 2013, 135, 9548; (b) S. E. Creutz, K. J. Lotito, G. C. Fu and J. C. Peters, Science, 2012, 338, 647; (c) D. T. Ziegler, J. Choi, J. M. Muñoz-Molina, A. C. Bissember, J. C. Peters and G. C. Fu, J. Am. Chem. Soc., 2013, 135, 13107; (d) J. A. Terrett, J. D. Cuthbertson, V. W. Shurtleff and D. W. C. MacMillan, Nature, 2015, 524, 330; (e) C. C. Nawrat, C. R. Jamison, Y. Slutskyy, D. W. C. MacMillan and L. E. Overman, J. Am. Chem. Soc., 2015, 137, 11270; (f) J. Jin and D. W. C. MacMillan, Nature, 2015, 525, 87; (g) X.-Q. Hu, J.-R. Chen, Q. Wei, F.-L. Liu, Q.-H. Deng, A. M. Beauchemin and W.-J. Xiao, Angew. Chem., Int. Ed., 2014, 53, 12163; (h) Y. Masuda, N. Ishida and M. Murakami, J. Am. Chem. Soc., 2015, 137, 14063; (i) M. El Khatib, R. A. M. Serafim and G. A. Molander, Angew. Chem., Int. Ed., 2016, 55, 254; (j) J. C. Tellis, D. N. Primer and G. A. Molander, Science, 2014, 345, 433; (k) H.-Q. Do, S. Bachman, A. C. Bissember, J. C. Peters and G. C. Fu, J. Am. Chem. Soc., 2014, 136, 2162; (l) A. C. Bissember, R. J. Lundgren, S. E. Creutz, J. C. Peters and G. C. Fu, Angew. Chem., Int. Ed., 2013, 52, 5129; (m) W. Liu, L. Li and C.-J. Li, Nat. Commun., 2015, 6, 6526; (n) L. Li, W. Liu, H. Zeng, X. Mu, G. Cosa, Z. Mi and C.-J. Li, J. Am. Chem. Soc., 2015, 137, 8328; (o) Y. Masuda, N. Ishida and M. Murakami, J. Am. Chem. Soc., 2015, 137, 14063.
7. For selected examples, see: (a) S. P. Pitre, C. D. McTiernan, H. Ismaili and J. C. Scaiano, ACS Catal., 2014, 4, 2530; (b) J. Xie, X. Yuan, A. Abdukader, C. Zhu and J. Ma, Org. Lett., 2014, 16, 1768; (c) P. Xu, J. Xie, Q. Xue, C. Pan, Y. Cheng and C. Zhu, Chem. – Eur. J., 2013, 19, 14039; (d) F. Gao, C. Yang, G. Gao, L. Zheng and W. Xia, Org. Lett., 2015, 17, 3478; (e) R. Tomita, Y. Yasu, T. Koike and M. Akita, Angew. Chem., Int. Ed., 2014, 53, 7144; (f) Y. Yasu, T. Koike and M. Akita, Chem. Commun., 2013, 49, 2037; (g) P. Xu, A. Abdukader, K. Hu, Y. Cheng and C. Zhu, Chem. Commun., 2014, 50, 2308; (h) S. Mizuta, K. M. Engle, S. Verhoog, O. Galicia-López, M. O'Duill, M. Médebielle, K. Wheelhouse, G. Rassias, A. L. Thompson and V. Gouverneur, Org. Lett., 2013, 15, 1250; (i) R. Beniazza, F. Molton, C. Duboc, A. Tron, N. D. McClenaghan, D. Lastécouèresa and J.-M. Vincent, Chem. Commun., 2015, 51, 9571.
8. For selected examples, see: (a) S. Mizuta, S. Verhoog, K. M. Engle, T. Khotavivattana, M. O'Duill, K. Wheelhouse, G. Rassias, M. Medebielle and V. Gouverneur, J. Am. Chem. Soc., 2013, 135, 2505; (b) Y. Yasu, T. Koike and M. Akita, Org. Lett., 2013, 15, 2136; (c) A. Carboni, G. Dagousset, E. Magnier and G. Masson, Chem. Commun., 2014, 50, 14197; (d) Q. Deng, J. Chen, Q. Wei, Q. Zhao, L. Lu and W. Xiao, Chem. Commun., 2015, 51, 3537; (e) N. Noto, K. Miyazawa, T. Koike and M. Akita, Org. Lett., 2015, 17, 3710; (f) Y. Yasu, Y. Arai, R. Tomita, T. Koike and M. Akita, Org. Lett., 2014, 16, 780; (g) Q. Wei, J. Chen, X. Hu, X. Yang, B. Lu and W. Xiao, Org. Lett., 2015, 17, 4464; (h) Q. Lin, X. Xu and F. Qing, J. Org. Chem., 2014, 79, 10434; (i) A. Sahoo, J. Li and F. Glorius, Angew. Chem., Int. Ed., 2015, 54, 11577; (j) S. H. Oh, Y. R. Malpani, N. Ha, Y. Jung and S. B. Han, Org. Lett., 2014, 16, 1310; (k) X. Tang and W. R. Dolbier Jr., Angew. Chem., Int. Ed., 2015, 54, 4246; (l) D. B. Bagal, G. Kachkovskyi, M. Knorn, T. Rawner, B. M. Bhanage and O. Reiser, Angew. Chem., Int. Ed., 2015, 54, 6999.
9. For recent examples, see: (a) D. Ravelli, S. Protti and M. Fagnoni, Chem. Rev. DOI:10.1021/acs.chemrev.5b00662; (b) Q. Lefebvre, N. Hoffmann and M. Rueping, Chem. Commun., 2016, 52, 2493; (c) B. Wang, D. Xiong and X. Ye, Org. Lett., 2015, 17, 5698; (d) L. Zhu, L. Wang, B. Li, B. Fu, C. Zhang and W. Li, Chem. Commun., 2016, 52, 6371; (e) P. Gao, X. Song, X. Liu and Y. Liang, Chem. – Eur. J., 2015, 21, 7648; (f) T. Besset, T. Poisson and X. Pannecoucke, Eur. J. Org. Chem., 2015, 2765; (g) F. Wang, N. Zhu, P. Chen, J. Ye and G. Liu, Angew. Chem., Int. Ed., 2015, 54, 9356; (h) Y. He, Q. Wang, L. Li, X. Liu, P. Xu and Y. Liang, Org. Lett., 2015, 17, 5188; (i) R. Tomita, T. Koike and M. Akita, Angew. Chem., Int. Ed., 2015, 54, 12923; (j) N. Iqbal, J. Jung, S. Park and E. Cho, Angew. Chem., Int. Ed., 2014, 53, 539; (k) H. Hua, Y. He, Y. Qiu, Y. Li, B. Song and Y. Liang, Chem. – Eur. J., 2015, 21, 1468; (l) J. Jacquet, S. Blanchard, E. Derat, M. Murr and L. Fensterbank, Chem. Sci., 2016, 7, 2030.
10. For selected recent examples, see: (a) C. Ni, M. Hu and J. Hu, Chem. Rev., 2015, 115, 765; (b) X. Liu, C. Xu, M. Wang and Q. Liu, Chem. Rev., 2015, 115, 683; (c) C. Alonso, E. M. De Marigorta, G. Rubiales and F. Palacios, Chem. Rev., 2015, 115, 1847; (d) E. Merino and C. Nevado, Chem. Soc. Rev., 2014, 43, 6598; (e) P. Yu, J.-S. Lin, L. Li, S.-C. Zheng, Y.-P. Xiong, L.-J. Zhao, B. Tan and X.-Y. Liu, Angew. Chem., Int. Ed., 2014, 53, 11890; (f) C. Yu, N. Iqbal, S. Park and E. J. Cho, Chem. Commun., 2014, 50, 12884; (g) X.-L. Jiang, Z.-H. Chen, X.-H. Xu and F.-L. Qing, Org. Chem. Front., 2014, 1, 774; (h) C. Matheis, K. Jouvin and L. J. Goossen, Org. Lett., 2014, 16, 5984; (i) D. J. Wilger, N. J. Gesmundo and D. A. Nicewicz, Chem. Sci., 2013, 4, 3160; (j) G. Landelle, A. Panossian, S. Pazenok, J. Vors and F. R. Leroux, Beilstein J. Org. Chem., 2013, 9, 2476; (k) T. Xu and G. Liu, Org. Lett., 2012, 14, 5416; (l) X. Yang, T. Wu, R. J. Phipps and F. D. Toste, Chem. Rev., 2015, 115, 826; (m) X.-H. Xu, K. Matsuzaki and N. Shibata, Chem. Rev., 2015, 115, 731; (n) L. Chu and F.-L. Qing, Acc. Chem. Res., 2014, 47, 1513; (o) Y. A. Serguchev, M. V. Ponomarenko and N. V. Ignaťev, J. Fluorine Chem., 2016, 185, 1; (p) T. H. Rehm, Chem. Eng. Technol., 2016, 39, 66; (q) Y. Zeng, C. Ni and J. Hu, Chem. – Eur. J., 2016, 22, 3210; (r) X. Xu and F. Qing, Curr. Org. Chem., 2015, 19, 1566; (s) C. Ni, M. Hu and J. Hu, Chem. Rev., 2015, 115, 765; (t) Y. Lu, C. Liu and Q. Chen, Curr. Org. Chem., 2015, 19, 1638; (u) G. K. S. Prakash and J. Hu, Acc. Chem. Res., 2007, 40, 921; (v) C. Ni and J. Hu, Synthesis, 2014, 842; (w) X. Shen and J. Hu, Eur. J. Org. Chem., 2014, 4437.
11. (a) Q. Xiao, J. Sheng, Z. Chen and J. Wu, Chem. Commun., 2013, 49, 8647; (b) Q. Xiao, Y. Luo, Y. Yan, Q. Ding and J. Wu, RSC Adv., 2013, 3, 5779; (c) H. Wang, Y. Luo, X. Hou and J. Wu, Dalton Trans., 2013, 42, 4410.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure and related data. CCDC 1469995–1469996. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00120c

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