Ruthenium-catalyzed 1,2,3-triazole directed intermolecular C–H amidation of arenes with sulfonyl azides

Abstract

Synthesis of 2-(2H-1,2,3-triazole-2-yl)aniline derivatives from 2-aryl-1,2,3-triazoles and sulfonylazides through ruthenium-catalyzed intermolecular C–H amidation is achieved. This reaction provides an environmentally benign protocol for C–N bond formation, producing N₂ gas as the sole byproduct. Yet a number of functionalities are well tolerated, obtaining the desired products in moderate to excellent yields.

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Aryl amines are valuable synthetic intermediates in a variety of organic reactions and have various potential applications in pharmaceutical and material sciences.¹ As a result, a great deal of attention has been devoted to designing new protocols for construction of aryl amines. In the early twentieth century, Ullmann and Goldberg reported the preparation of aniline derivatives from haloarenes, and opened new avenues for the transition-metal-catalyzed region-selective amination of C(sp²)-halide compounds.² During the past years, protocols for synthesizing aryl amines have been developed, among which Buchwald–Hartwig amination³ and Chan–Evans–Lam coupling⁴ are reliable and powerful protocols (Scheme 1a). However, Ullmann-type reaction, Buchwald–Hartwig amination and Chan–Evans–Lam coupling all require prefunctionalized arenes, and stoichiometric amounts of byproducts would be generated during the transformation. To overcome these issues, transition-metal-catalyzed direct C–H bond amination of arenes has become a powerful and efficient protocol owing to its step- and atom-economy.⁵ Until now, significant progress have been made on direct C–H bond amination, especially on the directing-group-assisted ortho-C–H bond amination.^6,7 Basically there are two protocols for direct C–H amination. One employs electrophilic amines as coupling partners, such as N-substituted hydroxylamines,^6a,bN-carboxylates,^6c–6fN-tosylates,^6gN-halides,^6h,i NFSI^6k and (Scheme 1b). The other uses neutral amines as nitrogen sources to obtain aniline derivatives through cross dehydrogenative couplings (Scheme 1c).⁷ However, these protocols are associated with some drawbacks. For example, the former method cannot avoid generation of byproducts and/or requires tedious pre-synthesis of each nitrogen-containing coupling partner, while the latter requires external oxidants to trap two hydrogen atoms from the C–H amination.⁸ Recent studies have demonstrated that nitrosobenzenes could also serve as the coupling partners for C–H bond amination.⁹ Despite the significant advances made in C–H amination, new atom and step economical procedures are still highly desired.

Scheme 1 Methods for synthesis of aryl arenes.

To avoid the tedious pre-synthesis procedures and to minimize the usage of external oxidants, organic azides have attracted much attention. In the field of organic synthesis, azides are ubiquitous and versatile building blocks.¹⁰ In 2012, Chang's group disclosed rhodium-catalyzed direct C–H amidation of arenes with sulfonyl azides (Scheme 2a).¹¹ This reaction reveals that azide is an ideal coupling partner for C–N bond formation given the advantages like it is easy for preparation, has sole byproduct (N₂), and is an potential internal oxidant.¹² Therefore, continuous efforts had been thrown to explore direct C–H amidation with azides in the presence of Rh, Ru, and Ir complexes, and there have many efficient strategies been reported during the past years.^12a,13

Scheme 2 Strategies for direct C–H amidation.

On the other hand, 2-aryl-1,2,3-triazoles, the essential structural motifs in medicinal chemistry, exhibit an array of chemical and structural properties.¹⁴ They have been employed as excellent substrates in functionalizing the ortho-C–H bond, including ortho-acylation, acyloxylation, alkoxylation, halogenation and arylation.¹⁵ These strategies enrich the molecular diversity and structural complexity of 2-aryl-1,2,3-triazoles. However, direct C–H bond amidation of 2-aryl-1,2,3-triazoles with sulfonyl azides is rare in literature. This motivates us to explore the direct amidation of 2-aryl-1,2,3-triazoles to obtain 2-(2H-1,2,3-triazole-2-yl)aniline structures, which have potential values for biological and chemical research.¹⁶ While we were working on this project, we noticed that Wu et al. reported the iridium-catalyzed direct C–H sulfonamidation.¹⁷ In their work, they mainly employed 1,2,3-triazoles N-oxides as directing groups and only offered a few examples of ortho-C–H bond amidation using 1,2,3-triazole as a directing group. Herein, we report our results on the development of ruthenium-catalyzed amidation of 2-aryl-1,2,3-triazoles with sulfonylazides (Scheme 2b). Compared to expensive transition metals such as rhodium and iridium complexes, less expensive [RuCl₂(p-cymene)]₂ has proved its efficiency in this context, providing 2-(2H-1,2,3-triazole-2-yl)aniline derivatives with good compatibility of functional groups. Moreover, neither pre-synthesis of electrophilic amines nor additional usage of external oxidants is required when employing commercial azides as amino source. Yet this procedure releases N₂ and omits the generation of stoichio-metric amounts of byproducts when using other amino sources.

Our initial experiment was carried out with 2-(m-tolyl)-2H-1,2,3-triazole 1a and 4-methyl benzene sulfonyl azide (TsN₃) 2a, in 1,2-dichloroethane (DCE). Transition metals that were effective in other C–H bond functionalization showed no effect in this reaction when they are used alone (Table 1, entries 1–4). Gratifyingly, the desired product 3aa was obtained in 31% yield by employing [RuCl₂(p-cymene)]₂ as catalyst with AgSbF₆ (Table 1, entry 6). Encouraged by this result, we next screened various solvents in order to identify a better reaction media. Variation of the solvent revealed that hexafluoroisopropyl-alcohol (HFIP) is the most effective solvents among others (Table 1, entries 7–12). Thus, later reactions were carried out in HFIP. In a search for an even higher yield, we then focused on finding an additive for facilitating this transformation. Interestingly, it was found that NaOAc was more effective than other additives such as PivOH, Cu(OAc)₂, and KPF₆/Zn(OAc)₂. Furthermore, evaluations of other silver salts had been carried out, and the results suggested that AgSbF₆ was still the most suitable one. Accordingly, the reaction conditions are optimized as follows: 5 mol% [RuCl₂(p-cymene)]₂, 20 mol% AgSbF₆, 0.5 equiv. NaOAc in HFIP at 80 °C for 12 h.

Table 1 Optimization of the reaction conditions^a,b

Entry	Catalyst (mol%)	Additive (equiv.)	Solvent	Yield^c (%)
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (as indicated), solvent (1.0 ml) and additive (as indicated) were heated at 80 °C for 12 h. b Cp* = pentamethylcyclopentadienyl; p-cymene = 4-isopropyltoluene; PivOH = trimethylacetic acid; t-amyl-OH = tert-pentyl alcohol; NMP = N-methyl-2-pyrrolidone; HFIP = hexafluoroisopropylalcohol. c Isolated yield. d N.R. = No reaction.
1	Pd(OAc)2 (10)	—	DCE	N.R.^d
2	Cu(OAc)2 (10)	—	DCE	N.R.
3	[RhCp*Cl2]2 (5)	—	DCE	N.R.
4	[RuCl2(p-cymene)]2 (5)	—	DCE	N.R.
5	[RhCp*Cl2]2 (5)/AgSbF6 (20)	—	DCE	24
6	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	DCE	31
7	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	Toluene	Trace
8	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	MeCN	N.R.
9	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	t-Amyl-OH	N.R.
10	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	DMSO	N.R.
11	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	NMP	N.R.
12	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	—	HFIP	47
13	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	PivOH (0.5)	HFIP	60
14	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	Cu(OAc)2 (0.5)	HFIP	69
15	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	KPF6 (1.0)/Zn(OAc)2 (1.0)	HFIP	66
16	[RuCl2(p-cymene)]2 (5)/AgSbF6 (20)	NaOAc (0.5)	HFIP	82
17	[RuCl2(p-cymene)]2 (5)/AgOAc (20)	NaOAc (0.5)	HFIP	72
18	[RuCl2(p-cymene)]2 (5)/Ag2CO3 (20)	NaOAc (0.5)	HFIP	63
19	[RuCl2(p-cymene)]2 (5)/CF3COOAg (20)	NaOAc (0.5)	HFIP	75

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Once the efficient conditions had been established, we then focused on the synthesis of 2-aryl-1,2,3-triazole derivatives. The results were summarized in Table 2. Initially, ortho-substituted arenes were probed. The halo-substituted arenes underwent the C–H amidation smoothly and obtained the corresponding products in moderate to good yield (Table 2, compound 3ba–3da). Notably, substrates with ortho-amido groups gave higher yields than those with electron-withdrawing groups (EWGs) such as CN and NO₂ (Table 2, compound 3ea–3ja). However, substrates bearing COOEt and COOMe were in contrast to the above EWG-containing arenes. The corresponding products 3ka and 3la were obtained in yields of 81% and 73%, respectively (Table 2, compound 3ka–3la). Then the meta-substituted substrates were screened. Not surprisingly, the less hindered ortho-C–H bond of aryl ring had been exclusively functionalized for all the substrate except meta-chloro substituted substrate, indicating regioselective C–H amidation of this reaction (Table 2, compound 3aa, 3ma–3oa). Reactions with meta-chloro substituted substrate resulted three separable regioisomeric products.¹⁸ Continued exploration on substrate scope showed that the reaction of para-substituted 2-aryl-1,2,3-triazole with TsN₃ provided mono- and di-amidation products. For example, the mono- and di-amidation products 3pa and 3pa′ were obtained in 32% and 20% yields under the optimal conditions, respectively. Interestingly, when the amount of TsN₃ was increased to 3.0 equiv., only di-amination product was obtained (Table 2, compound 3pa′).¹⁹ Thus, the modified conditions were then applied to other para-substituted compounds (Table 2, compound 3qa′–3ra′). On the other hand, different sulfonyl azides have also been examined for compatibility in this reaction. Benzenesulfonyl-azides with functional groups at 4^th position were well tolerated during the reaction. The corresponding ortho-amidated products were achieved in good to excellent yields (Table 2, compound 3ab–3ae). The results revealed that electron-donating methoxy group could afford a higher yield than the electron-withdrawing groups. In addition, methane sulfonylazide was a suitable partner for amidation, affording the 3af in 72% yield (Table 2, compound 3af).

Table 2 Substrate scope^a,b

a Unless noted otherwise, reactions were carried out with 1 (0.2 mmol), 2 (0.3 mmol), [RuCl₂(p-cymene)]₂ (5 mol%), AgSbF₆ (20 mol%) and NaOAc (0.5 equiv.) in HFIP at 80 °C for 12 h. b Isolated yield. c Yields based on the recovered starting material are list in the square brackets. d Major product is shown. See ref. 18 for more details. e 0.6 mmol TsN₃ is used.

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On the basis of previous reports, a plausible mechanistic pathway is depicted in Scheme 3 with 1a and 2a as model substrates.^{11,12a,13,15,20} Firstly, treatment of [RuCl₂(p-cymene)]₂ with AgSbF₆ and OAc⁻ is believed to convert the neutral ruthenium precursor to a cationic Ru(II) species, facilitating the key C–H bond activation to afford the five-membered ruthenium intermediate 4. Subsequently, the coordination of azide to intermediate 4, followed by the insertion of sulfonamide moiety, releases N₂ molecule, resulting the six-membered ruthenium complex 5. Finally, intermediate 5 undergoes protonation to afford the final product 3aa to fulfill the catalytic cycle.

Scheme 3 Possible mechanism.

Conclusions

In summary, we have described a ruthenium-catalyzed, 1,2,3-triazole directed ortho-amidation of C–H bond with sulfonyl azides. A number of arene substrates were selectively amidated in moderate to excellent yields, exhibiting good functional group tolerance. Moreover, the reaction offers an environmentally benign protocol for C–N bond formation without the requirement of external oxidants and/or the tedious pre-synthetic procedures. This atom- and step-economical method will have a promising application in the synthesis for the 2-(2H-1,2,3-triazole-2-yl)aniline drugs and intermediates.

Acknowledgements

We are grateful for support from the National Natural Science Foundation of China (NSFC) (grant number 81373259 & 81573286).

Notes and references

1. Amino Group Chemistry: From Synthesis to the Life Sciences, ed. A. Ricci, Wiley-VCH, Weinheim, 2007.
2. (a) F. Ullmann, Ber. Dtsch. Chem. Ges., 1903, 36, 2211; (b) I. Goldberg, Ber. Dtsch. Chem. Ges., 1906, 39, 1691; (c) G. Evano, N. Blanchard and M. Toumi, Chem. Rev., 2008, 108, 3054.
3. (a) D. S. Surry and S. L. Buchwald, Angew. Chem., Int. Ed., 2008, 47, 6338; (b) J. F. Hartwig, Acc. Chem. Res., 2008, 41, 1534.
4. (a) P. Y. S Lam, G. Vincent, D. Bonne and C. G. Clark, Tetrahedron Lett., 2003, 44, 4927; (b) D. M. T. Chan, K. L. Monaco, R. Li, D. Bonne, C. G. Clark and P. Y. S. Lam, Tetrahedron Lett., 2003, 44, 3863; (c) D. A. Evans, J. L. Katz and T. R. West, Tetrahedron Lett., 1998, 39, 2937; (d) D. M. T. Chan, K. L. Monaco, R. Wang and M. P. Winters, Tetrahedron Lett., 1998, 39, 2933; (e) P. Y. S. Lam, C. G. Clark, S. Saubern, J. Adams, M. P. Winters, D. M. T. Chan and A. Combs, Tetrahedron Lett., 1998, 39, 2941.
5. For selected examples, see: (a) J. Jiao, K. Murakami and K. Itami, ACS Catal., 2016, 6, 610; (b) M. R. Yadav, M. Shankar, E. Ramesh, K. Ghosh and A. K. Sahoo, Org. Lett., 2015, 17, 1886; (c) P. Starkov, T. F. Jamison and I. Marek, Chem.–Eur. J., 2015, 21, 5278; (d) M. Louillat and F. W. Patureau, Chem. Soc. Rev., 2014, 43, 901; (e) G. Dequirez, V. Pons and P. Dauban, Angew. Chem., Int. Ed., 2012, 51, 7384; (f) F. Collet, C. Lescot and P. Dauban, Chem. Soc. Rev., 2011, 40, 1926; (g) F. Collet, R. H. Dodd and P. Dauban, Chem. Commun., 2009, 40, 5061.
6. For selected examples, see: (a) W. Yang, J. Sun, X. Xu, Q. Zhang and Q. Liu, Chem. Commun., 2014, 50, 4420; (b) B. Zhou, J. Du, Y. Yang, H. Feng and Y. Li, Org. Lett., 2014, 16, 592; (c) P. Patel and S. Chang, Org. Lett., 2014, 16, 3328; (d) M. Shang, S. Zeng, S. Sun, H. Dai and J. Yu, Org. Lett., 2013, 15, 5286; (e) C. Grohmann, H. Wang and F. Glorius, Org. Lett., 2013, 15, 3014; (f) E. J. Yoo, S. Ma, T. Mei, K. S. Chan and J. Q. Yu, J. Am. Chem. Soc., 2011, 133, 7652; (g) K. Ng, A. S. C. Chan and W. Yu, J. Am. Chem. Soc., 2010, 132, 12862; (h) T. Matsubara, S. Asako, L. Ilies and E. Nakamura, J. Am. Chem. Soc., 2014, 136, 646; (i) F. Ng, Z. Zhou and W. Yu, Chem.–Eur. J., 2014, 20, 4474; (j) C. Grohmann, H. Wang and F. Glorius, Org. Lett., 2012, 14, 656; (k) R. Tang, C. Luo, L. Yang and C. Li, Adv. Synth. Catal., 2013, 355, 869.
7. For selected examples, see: (a) G. Li, C. Jia, Q. Chen, K. Sun, F. Zhao, H. Wu, Z. Wang, Y. Lv and X. Chen, Adv. Synth. Catal., 2015, 357, 1311; (b) H. Xu, X. Qiao, S. Yang and Z. Shen, J. Org. Chem., 2014, 79, 4414; (c) H. Zhao, Y. Shang and W. Su, Org. Lett., 2013, 15, 5106; (d) L. D. Tran, J. Roaneand and O. Daugulis, Angew. Chem., Int. Ed., 2013, 52, 6043; (e) A. John and K. M. Nicholas, J. Org. Chem., 2011, 76, 4158; (f) Q. Shuai, G. Deng, Z. Chua, D. S. Bohleand and C. Li, Adv. Synth. Catal., 2010, 352, 632; (g) H. Thu, W. Yu and C. Che, J. Am. Chem. Soc., 2006, 128, 9048.
8. (a) H. Kim and S. Chang, ACS Catal., 2016, 6, 2341; (b) Q. Meng, J. Zhong, Q. Liu, X. Gao, H. Zhang, T. Lei, Z. Li, K. Feng, B. Chen, C. Tung and L. Wu, J. Am. Chem. Soc., 2013, 135, 19052.
9. (a) J. Du, Y. Yang, H. Feng, Y. Li and B. Zhou, Chem.–Eur. J., 2014, 20, 5727; (b) B. Zhou, J. Du, Y. Yang, H. Feng and Y. Li, Org. Lett., 2013, 15, 6302.
10. S. Bräse, C. Gil, K. Knepper and V. Zimmermann, Angew. Chem., Int. Ed., 2005, 44, 5188.
11. J. Y. Kim, S. H. Park, J. Ryu, S. H. Cho, S. H. Kim and S. Chang, J. Am. Chem. Soc., 2012, 134, 9110.
12. (a) K. Shin, H. Kim and S. Chang, Acc. Chem. Res., 2015, 48, 1040; (b) F. W. Patureau and F. Glorius, Angew. Chem., Int. Ed., 2011, 50, 1977.
13. For selected examples, see: (a) W. Hou, Y. Yang, W. Ai, Y. Wu, X. Wang, B. Zhou and Y. Li, Eur. J. Org. Chem., 2015, 2015, 395; (b) K. Shin, J. Ryu and S. Chang, Org. Lett., 2014, 16, 2022; (c) Q. Zheng, Y. Liang, C. Qin and N. Jiao, Chem. Commun., 2013, 49, 5654; (d) M. Bhanuchandra, M. R. Yadav, R. K. Rit, M. RaoKuram and A. K. Sahoo, Chem. Commun., 2013, 49, 5225; (e) V. S. Thirunavukkarasu, K. Raghuvanshi and L. Ackermann, Org. Lett., 2013, 15, 3286; (f) B. Zhou, Y. Yang, J. Shi, H. Feng and Y. Li, Chem.–Eur. J., 2013, 19, 10511; (g) M. R. Yadav, R. K. Rit and A. K. Sahoo, Org. Lett., 2013, 15, 1638; (h) J. Kim, J. Kim and S. Chang, Chem.–Eur. J., 2013, 19, 7328; (i) K. Shin, Y. Baek and S. Chang, Angew. Chem., Int. Ed., 2013, 52, 8031; (j) J. Ryu, K. Shin, S. H. Park, J. Y. Kim and S. Chang, Angew. Chem., Int. Ed., 2012, 51, 9904; (k) J. Shi, B. Zhou, Y. Yang and Y. Li, Org. Biomol. Chem., 2012, 10, 8953.
14. (a) J. I. Montgomery, M. F. Brown, U. Reilly, L. M. Price, J. A. Abramite, J. Arcari, R. Barham, Y. Che, J. M. Chen, S. W. Chung, E. M. Collantes, C. Desbonnet, M. Doroski, J. Doty, J. J. Engtrakul, T. M. Harris, M. Huband, J. D. Knafels, K. L. Leach, S. Liu, A. Marfat, L. McAllister, E. McElroy, C. A. Menard, M. Mitton-Fry, L. Mullins, M. C. Noe, J. O'Donnell, R. Oliver, J. Penzien, M. Plummer, V. Shanmugasundaram, C. Thoma, A. P. Tomaras, D. P. Uccello, A. Vaz and D. G. Wishka, J. Med. Chem., 2012, 55, 1662; (b) R. Suzuki, D. Nozawa, A. Futamura, R. Nishikawa-Shimono, M. Abe, N. Hattori, H. Ohta, Y. Araki, D. Kambe, M. Ohmichi, S. Tokura, T. Aoki, N. Ohtake and H. Kawamoto, Bioorg. Med. Chem., 2015, 23, 1260; (c) K. F. McClure, M. Jackson, K. O. Cameron, D. W. Kung, D. A. Perry, S. T. Orr, Y. Zhang, J. Kohrt, M. Tu, H. Gao, D. Fernando, R. Jones, N. Erasga, G. Wang, J. Polivkova, W. Jiao, R. Swartz, H. Ueno, S. K. Bhattacharya, I. A. Stock, S. Varma, V. Bagdasarian, S. Perez, D. Kelly-Sullivan, R. Wang, J. Kong, P. Cornelius, L. Michael, E. Lee, A. Janssen, S. J. Steyn, K. Lapham and T. Goosen, Bioorg. Med. Chem. Lett., 2013, 23, 5410.
15. For selected examples, see: (a) P. He, Q. Tian and C. Kuang, Synthesis, 2015, 47, 1309; (b) S. Shi, W. Liu, P. He and C. Kuang, Org. Biomol. Chem., 2014, 12, 3576; (c) S. Ping and C. Kuang, J. Org. Chem., 2014, 79, 6105; (d) Z. Wang and C. Kuang, Adv. Synth. Catal., 2014, 356, 1549; (e) Z. Wang, Q. Tian, X. Yu and C. Kuang, Adv. Synth. Catal., 2014, 356, 961; (f) Q. Tian, X. Chen, W. Liu, Z. Wang, S. Shi and C. Kuang, Org. Biomol. Chem., 2013, 11, 7830; (g) L. Ackermann, R. Born and R. Vicente, ChemSusChem, 2009, 2, 546; (h) L. Ackermann, R. Vicente and A. Althammer, Org. Lett., 2008, 10, 2299.
16. (a) L. A. Gharat, L. Narayana, N. Khairatkar-Joshi and V. G. Kattige, WO 2009090548, 2009; (b) T. Fujimoto and Y. Tomata, JP 2010155827, 2010; (c) A. Jonczyk and W. Rautenberg, WO 2007115670, 2007; (d) T. Morofuji, A. Shimizu and J. Yoshida, J. Am. Chem. Soc., 2015, 137, 9816; (e) A. Oguri, T. Shiozawa, Y. Terao, H. Nukaya, J. Yamashita, T. Ohe, H. Sawanishi, T. Katsuhara, T. Sugimura and K. Wakabayashi, Chem. Res. Toxicol., 1998, 11, 1195; (f) R. J. Lu, J. A. Tucker, T. Zinevitch, O. Kirichenko, V. Konoplev, S. Kuznetsova, S. Sviridov, J. Pickens, S. Tandel, E. Brahmachary, Y. Yang, J. Wang, S. Freel, S. Fisher, A. Sullivan, J. Zhou, S. Stanfield-Oakley, M. Greenberg, D. Bolognesi, B. Bray, B. Koszalka, P. Jeffs, A. Khasanov, Y. A. Ma, C. Jeffries, C. Liu, T. Proskurina, T. Zhu, A. Chucholowski, R. Li and C. Sexton, J. Med. Chem., 2007, 50, 6535.
17. B. Zhu, X. Cui, C. Pi, D. Chen and Y. Wu, Adv. Synth. Catal., 2016, 358, 326.
18. Three compounds named 3ka, 3ka′, 3ka′′ are identified. In Table 2, only major product 3ka is shown. For more details, please see ESI Table S1.†.
19. See ESI† for more details about the yields of mono- and diaminated products from para-substituted 2-aryl-1,2,3-triazoles (Table S1†).
20. (a) D. L. Davies, O. Al-Duaij, J. Fawcett, M. Giardiello, S. T. Hilton and D. R. Russell, Dalton Trans., 2003, 21, 4132; (b) L. Ackermann, Chem. Rev., 2011, 111, 1315; (c) K. Ghosh, R. K. Rit, E. Ramesh and A. K. Sahoo, Angew. Chem., Int. Ed., 2016, 55, 7821; (d) M. R. Yadav, R. K. Rit, M. Shankar and A. K. Sahoo, J. Org. Chem., 2014, 79, 6123.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra. See DOI: 10.1039/c6ra14020c

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