Generation of 1-amino-isoquinoline-N-oxides via a tandem reaction of 2-alkynylbenzaldoxime with secondary amines in the presence of silver(I) and copper(I)

Abstract

1-Amino-isoquinoline-N-oxides are generated under mild conditions through a tandem reaction of 2-alkynylbenzaldoxime with secondary amines in the presence of silver(I) and copper(I). The reaction proceeds smoothly at room temperature under air, leading to the corresponding products in good yields. During the reaction process, a silver-catalyzed 6-endo cyclization and a copper(I)-catalyzed C–H bond activation are involved.

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1. Introduction

In the past decade, the achievement of the diversity-oriented synthesis of natural product-like compounds with privileged scaffolds has been witnessed.¹ A range of N-heterocycles have been applied broadly in the drug discovery process. We have also been involved in the construction of libraries of small molecules with privileged scaffolds.² Some hits have been identified during the subsequent biological assays. For instance, 2-alkynylbenzaldoxime as a versatile building block has been applied widely for the construction of N-heterocycles.³ Since it can be easily transferred to isoquinoline N-oxide via intramolecular 6-endo cyclization in the presence of metal catalysts or electrophiles,⁴ the subsequent [3 + 2] cycloaddition/nucleophilic addition and rearrangement could be expected. On the other hand, a Beckmann rearrangement could happen first since an oxime moiety is present in the molecule, which would then undergo an intramolecular cyclization to furnish nitrogen-containing heterocycles.⁵ Therefore, 2-alkynylbenzaldoxime is recognized as a good candidate for the generation of libraries of small molecules. In most cases, the chemistry is related to isoquinoline N-oxide, which can be isolated and obtained from 2-alkynylbenzaldoxime via 6-endo cyclization. A large collection of small molecules would be produced if diverse isoquinoline N-oxides could be easily accessible.

Recently, the transition metal catalyzed direct C–H oxidative amination of arenes has attracted much attention.⁶ For instance, Wu and Cui reported a copper(I)-catalyzed reaction of quinoline-N-oxide with amines.^6a During the process, the C–H bond at the ortho-position of the N-oxide was activated, thus undergoing subsequent amination. Encouraged by this result, we envisioned that the C–H bond at the ortho-position of isoquinoline N-oxide could be activated as well, which would react with amines to provide diverse 1-amino-isoquinoline-N-oxides. The proposed synthetic route is present in Scheme 1. We conceived that in the presence of silver(I) and copper(I) salts as co-catalysts, the reaction of 2-alkynylbenzaldoxime 1 with amines 2 would produce the corresponding 1-amino-isoquinoline-N-oxides. During the transformation, a silver(I)-catalyzed 6-endo cyclization would occur first, to afford isoquinoline N-oxide A. Subsequently, a C–H bond activation would take place in the presence of copper(I) salt to form intermediate B. If the reaction was performed under air atmosphere, the oxygen would act as an oxidant to involve amines in the transformation, leading to intermediate C with the release of a molecule of water. The following reductive elimination will deliver 1-amino-isoquinoline-N-oxides 3. With this consideration in mind, we thus initiated a program for the generation of 1-amino-isoquinoline-N-oxides.

Scheme 1 A proposed synthetic route to 1-amino-isoquinoline-N-oxides.

2. Results and discussion

At the outset, the studies were initially performed for a model reaction of 2-alkynylbenzaldoxime 1a with piperidine 2a at 50 °C in toluene under air atmosphere (Table 1). A control experiment catalysed by 10 mol% of silver triflate without the addition of copper(I) salt indicated that no desired product 3a was formed (Table 1, entry 1). Only isoquinoline-N-oxide A1 was isolated. This result demonstrated the important role of the copper(I) catalyst in the reaction process for the activation of the C–H bond at the ortho-position of isoquinoline N-oxide. As expected, 1-amino-isoquinoline-N-oxide 3a was obtained in 87% yield when Cu₂O (5 mol%) was added to the reaction system (Table 1, entry 2). The structure of 3a was confirmed by X-ray crystallographic analysis (Fig. 1). A similar result was observed when the copper salt was changed to CuCl or CuBr (Table 1, entries 3 and 4). The reaction co-catalyzed by silver triflate and palladium acetate gave rise to the expected product 3a in 17% yield (data not shown in Table 1). Gratifyingly, the corresponding product 3a was furnished in 95% yield when copper(I) iodide was used as the catalyst (Table 1, entry 5). No better yields were isolated when other solvents were utilized (Table 1, entries 6–10). The efficiency was not affected when the reaction occurred at room temperature (Table 1, entry 11). The same result was observed when the reaction was performed at 80 °C (Table 1, entry 12).

Fig. 1 X-ray ORTEP illustration of compound 3a.

Table 1 Initial studies for the reaction of 2-alkynylbenzaldoxime 1a with piperidine 2a

Entry	[Cu]	Temp (°C)	Solvent	Yield^a (%)
a Isolated yield based on 2-alkynylbenzaldoxime 1a. b In the absence of AgOTf.
1	—	50	Toluene	nr
2	Cu2O	50	Toluene	87
3	CuCl	50	Toluene	85
4	CuBr	50	Toluene	88
5	CuI	50	Toluene	95
6	CuI	50	DCE	82
7	CuI	50	MeCN	79
8	CuI	50	1,4-Dioxane	83
9	CuI	50	THF	80
10	CuI	50	DMA	76
11	CuI	25	Toluene	96
12	CuI	80	Toluene	95
13^b	CuI	25	Toluene	21

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The above mild optimized conditions prompted us to explore the scope of this silver(I) and copper(I) co-catalyzed reaction of 2-alkynylbenzaldoximes 1 with amines 2. The results are summarized in Tables 2 and 3. At the beginning, reactions of 2-alkynylbenzaldoximes 1 with piperidine 2a were examined (Table 2). It was found that in most cases, the reactions worked well to afford the expected products 3 in good to excellent yields. 2-Alkynylbenzaldoximes 1 with different groups attached on the aromatic ring were all good reactants under the standard conditions. Reactions of 2-alkynylbenzaldoximes 1 with aryl groups attached on the triple bond (R²) proceeded smoothly, leading to the corresponding products in good yields. Inferior results were obtained for the reaction of 2-alkynylbenzaldoximes 1 with alkyl groups attached on the triple bond (compounds 3e and 3f).

Table 2 Synthesis of 1-amino-isoquinoline-N-oxides 3 through a silver(I) and copper(I) co-catalyzed reaction of 2-alkynylbenzaldoxime 1 with piperidine 2a^a

a Isolated yield based on 2-alkynylbenzaldoxime 1.

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Table 3 Synthesis of 1-amino-isoquinoline-N-oxides 3 through a silver(I) and copper(I) co-catalyzed reaction of 2-alkynylbenzaldoxime 1a with secondary amines 2^a

a Isolated yield based on 2-alkynylbenzaldoxime 1a.

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We next investigated the silver(I) and copper(I) co-catalyzed reaction of 2-alkynylbenzaldoxime 1a with various amines (Table 3). Not only pyrrolidine but also azepane was a good partner in this transformation. The reaction of 2-alkynylbenzaldoxime 1a with morpholine gave rise to the expected product 3n in 93% yield. However, reaction of 2-alkynylbenzaldoxime 1a with 2,2,6,6-tetramethylpiperidine did not afford the desired product 3o. From these results, it was shown that the outcome was affected by the steric hindrance of the substrates. Other acyclic amines such as diethyl amine and dihexyl amine were employed in the reaction, which afforded a trace amount of the desired products. Moreover, primary amines are also not effective under these conditions (data not shown in Table 3), and no expected products 3 were detected when primary amines were used as reaction partners under these conditions.

3. Conclusion

In conclusion, we have reported the synthesis of 1-amino-isoquinoline-N-oxides through a silver(I) and copper(I) co-catalyzed reaction of 2-alkynylbenzaldoxime with secondary amines under mild conditions. The transformation proceeds smoothly at room temperature under air, leading to the corresponding products in good yields. During the reaction process, a silver-catalyzed 6-endo cyclization and a copper(I)-catalyzed C–H bond activation are involved. The reaction scope has been demonstrated and a range of 1-amino-isoquinoline-N-oxides is produced efficiently. Currently, application of the 1-amino-isoquinoline-N-oxides for the construction of diverse N-heterocycles is ongoing in our laboratory.

4. Experimental section

General experimental procedure for the synthesis of 1-amino-isoquinoline-N-oxides 3 through a silver(I) and copper(I) co-catalyzed reaction of 2-alkynylbenzaldoximes 1 with amines 2: a mixture of silver triflate (0.02 mmol, 5.1 mg) and 2-alkynylbenzaldoxime 1 (0.2 mmol) in toluene (1.0 mL) was stirred at 70 °C for 1 hour. Then secondary amine 2 (1.6 mmol), CuI (0.01 mmol, 1.9 mg), and toluene (1.0 mL) were added subsequently. The reaction was stirred at 25 °C until completion of the reaction as indicated by TLC. The solvent was evaporated and the residue was purified by column chromatography on silica gel to provide the product 3.

3-Phenyl-1-(trifluoromethyl)isoquinoline 3a

¹H NMR (400 MHz, CDCl₃) δ 8.19 (d, J = 8.2 Hz, 1H), 7.75–7.70 (m, 3H), 7.61–7.49 (m, 3H), 7.49–7.38 (m, 3H), 3.79 (s, 2H), 3.17 (s, 2H), 1.83–1.75 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 151.1, 147.5, 133.8, 130.2, 129.9, 128.9, 128.2, 128.0, 127.5, 127.3, 126.9, 124.2, 120.8, 49.7, 26.6, 24.4; HRMS (ESI) calcd for C₂₀H₂₀N₂O: 305.1648 (M + H⁺), found: 305.166.

5-Methoxy-3-phenyl-1-(piperidin-1-yl)isoquinoline 2-oxide 3b

¹H NMR (400 MHz, CDCl₃) δ 7.94 (s, 1H), 7.77–7.74 (m, 3H), 7.48–7.38 (m, 4H), 6.86 (d, J = 7.7 Hz, 1H), 3.97 (s, 3H), 3.72 (s, 2H), 3.16 (s, 2H), 1.79–1.75 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 155.2, 150.9, 146.6, 134.2, 130.9, 129.9, 128.7, 128.6, 127.9, 122.3, 116.2, 115.6, 106.4, 55.7, 49.6, 26.5, 24.4; HRMS (ESI) calcd for C₂₁H₂₂N₂O₂: 335.1754 (M + H⁺), found: 335.1758.

7-Chloro-3-phenyl-1-(piperidin-1-yl)isoquinoline 2-oxide 3c

¹H NMR (400 MHz, CDCl₃) δ 8.15 (s, 1H), 7.72 (dd, J = 8.0, 1.6 Hz, 2H), 7.65 (d, J = 8.4 Hz, 1H), 7.51 (s, 1H), 7.48–7.40 (m, 4H), 3.80 (s, 2H), 3.09 (s, 2H), 1.85–1.75 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 150.5, 147.9, 134.3, 133.4, 130.9, 129.7, 129.1, 128.9, 128.5, 128.2, 128.1, 123.0, 120.6, 49.3, 26.5, 24.3; HRMS (ESI) calcd for C₂₀H₁₉ClN₂O: 339.1259 (M + H⁺), found: 339.1256.

7-Fluoro-3-phenyl-1-(piperidin-1-yl)isoquinoline 2-oxide 3d

¹H NMR (400 MHz, CDCl₃) δ 7.81 (dd, J = 10.3, 2.0 Hz, 1H), 7.73–7.70 (m, 3H), 7.54 (s, 1H), 7.48–7.40 (m, 3H), 7.31–7.26 (m, 1H), 3.78 (s, 2H), 3.08 (s, 2H), 1.84–1.75 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 162.2 (d, ¹J_CF = 247.4 Hz), 150.7, 147.1, 133.5, 130.9, 129.8, 129.0, 128.8, 128.1, 127.0, 120.7, 118.2 (d, ²J_CF = 25.4 Hz), 108.1 (d, ²J_CF = 24.0 Hz), 49.2, 26.5, 24.3; HRMS (ESI) calcd for C₂₀H₁₉FN₂O: 323.1554 (M + H⁺), found: 323.1561.

3-Butyl-1-(piperidin-1-yl)isoquinoline 2-oxide 3e

¹H NMR (400 MHz, CDCl₃) δ 8.14–8.12 (m, 1H), 7.66–7.64 (m, 1H), 7.50–7.48 (m, 2H), 7.33 (s, 1H), 3.75 (s, 2H), 3.10 (s, 1H), 3.03–2.95 (m, 2H), 2.17 (s, 1H), 1.78–1.72 (m, 8H), 1.54–1.45 (m, 2H), 0.99 (t, J = 7.3 Hz, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 150.8, 149.8, 130.2, 128.0, 127.3, 126.2, 124.1, 117.9, 49.6, 30.4, 29.2, 26.6, 24.3, 22.7, 13.9; HRMS (ESI) calcd for C₁₈H₂₄N₂O: 285.1961 (M + H⁺), found: 285.1952.

3-(tert-Butyl)-1-(piperidin-1-yl)isoquinoline 2-oxide 3f

¹H NMR (400 MHz, CDCl₃) δ 8.11 (d, J = 8.1 Hz, 1H), 7.66 (d, J = 7.7 Hz, 1H), 7.51–7.43 (m, 3H), 3.63 (s, 2H), 3.11 (s, 2H), 1.84–1.75 (m, 6H), 1.59 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ 155.5, 151.9, 129.6, 127.5, 126.7, 123.5, 117.2, 49.1, 36.6, 28.3, 26.5, 24.3; HRMS (ESI) calcd for C₁₈H₂₄N₂O: 285.1961 (M + H⁺), found: 285.1941.

7-Methyl-3-phenyl-1-(piperidin-1-yl)isoquinoline 2-oxide 3g

¹H NMR (400 MHz, CDCl₃) δ 7.95 (s, 1H), 7.74 (d, J = 6.7 Hz, 2H), 7.62 (d, J = 7.9 Hz, 1H), 7.48–7.40 (m, 4H), 7.36 (d, J = 8.1 Hz, 1H), 3.74 (s, 2H), 3.14 (s, 2H), 2.56 (s, 3H), 1.81 (s, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 150.8, 146.6, 138.3, 133.9, 130.9, 130.4, 129.8, 128.7, 127.9, 127.4, 126.8, 123.1, 120.7, 49.4, 26.5, 24.3, 22.2; HRMS (ESI) calcd for C₂₁H₂₂N₂O: 319.1805 (M + H⁺), found: 319.1802.

6-Fluoro-3-phenyl-1-(piperidin-1-yl)isoquinoline 2-oxide 3h

¹H NMR (400 MHz, CDCl₃) δ 8.19 (m, 1H), 7.73 (d, J = 6.6 Hz, 2H), 7.48–7.41 (m, 4H), 7.33–7.29 (m, 2H), 3.72 (s, 2H), 3.13 (s, 2H), 1.71 (s, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 162.1 (d, ¹J_CF = 249.6 Hz), 151.2, 148.6, 133.5, 131.3, 129.7, 129.1, 128.0, 127.3 (d, ³J_CF = 8.9 Hz), 124.4, 120.1 (d, ⁴J_CF = 5.0 Hz), 118.3 (d, ²J_CF = 24.9 Hz), 110.9 (d, ²J_CF = 21.7 Hz), 49.5, 26.5, 24.3; HRMS (ESI) calcd for C₂₀H₁₉FN₂O: 323.1554 (M + H⁺), found: 323.1557.

3-(4-Chlorophenyl)-1-(piperidin-1-yl)isoquinoline 2-oxide 3i

¹H NMR (400 MHz, CDCl₃) δ 8.17 (d, J = 8.3 Hz, 1H), 7.72–7.69 (m, 3H), 7.60–7.50 (m, 3H), 7.42 (d, J = 8.5 Hz, 2H), 3.73 (s, 2H), 3.19 (s, 2H), 1.85–1.75 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 151.2, 146.2, 134.9, 132.2, 131.2, 130.2, 128.4, 128.3, 127.3, 126.9, 124.2, 120.7, 49.6, 26.5, 24.3; HRMS (ESI) calcd for C₂₀H₁₉ClN₂O: 339.1259 (M + H⁺), found: 339.1258.

3-(4-Fluorophenyl)-1-(piperidin-1-yl)isoquinoline 2-oxide 3j

¹H NMR (400 MHz, CDCl₃) δ 8.18 (d, J = 8.1 Hz, 1H), 7.76–7.70 (m, 3H), 7.59–7.50 (m, 3H), 7.13 (t, J = 8.4 Hz, 2H), 3.73 (s, 2H), 3.19 (s, 2H), 1.80 (s, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 163.1 (d, ¹J_CF = 247.3 Hz), 151.2, 146.4, 131.9 (d, ³J_CF = 8.2 Hz), 130.2, 129.8, 128.3 (d, ³J_CF = 7.7 Hz), 127.2, 126.9, 124.2, 120.7, 115.0 (d, ²J_CF = 21.6 Hz), 49.6, 26.5, 24.3; HRMS (ESI) calcd for C₂₀H₁₉FN₂O: 323.1554 (M + H⁺), found: 323.1543.

3-(4-Methoxyphenyl)-1-(piperidin-1-yl)isoquinoline 2-oxide 3k

¹H NMR (400 MHz, CDCl₃) δ 8.17 (d, J = 8.1 Hz, 1H), 7.73–7.68 (m, 3H), 7.59–7.48 (m, 3H), 6.98 (d, J = 8.8 Hz, 2H), 3.85 (s, 3H), 3.73 (s, 2H), 3.17 (s, 2H), 1.84–1.75 (m, 6H); ¹³C NMR (100 MHz, CDCl₃) δ 160.1, 151.1, 147.2, 131.3, 130.3, 128.1, 127.9, 127.0, 126.7, 126.1, 124.1, 120.4, 113.4, 55.4, 49.5, 26.6, 24.3; HRMS (ESI) calcd for C₂₁H₂₂N₂O₂: 335.1754 (M + H⁺), found: 335.1753.

3-Phenyl-1-(pyrrolidin-1-yl)isoquinoline 2-oxide 3l

¹H NMR (400 MHz, CDCl₃) δ 8.18 (d, J = 8.2 Hz, 1H), 7.75–7.71 (m, 3H), 7.57–7.52 (m, 3H), 7.50–7.41 (m, 3H), 3.64–3.52 (m, 4H), 2.15–2.05 (m, 4H); ¹³C NMR (100 MHz, CDCl₃) δ 149.7, 147.6, 133.9, 130.9, 129.9, 129.8, 128.9, 128.6, 128.2, 128.1, 126.7, 124.7, 120.9, 48.8, 26.5; HRMS (ESI) calcd for C₁₉H₁₈N₂O: 291.1492 (M + H⁺), found: 291.1492.

1-(Azepan-1-yl)-3-phenylisoquinoline 2-oxide 3m

¹H NMR (400 MHz, CDCl₃) δ 8.27 (d, J = 8.3 Hz, 1H), 7.76–7.72 (m, 3H), 7.60–7.57 (m, 2H), 7.54–7.50 (m, 1H), 7.48–7.40 (m, 3H), 3.43 (s, 4H), 1.83 (s, 8H); ¹³C NMR (100 MHz, CDCl₃) δ 152.5, 147.6, 133.7, 130.9, 130.1, 129.8, 128.9, 128.4, 128.2, 128.0, 126.8, 124.4, 121.4, 51.1, 30.5, 28.2; HRMS (ESI) calcd for C₂₁H₂₂N₂O: 319.1805 (M + H⁺), found: 319.1803.

1-Morpholino-3-phenylisoquinoline 2-oxide 3n

¹H NMR (400 MHz, CDCl₃) δ 8.19 (d, J = 8.3 Hz, 1H), 7.75–7.72 (m, 3H), 7.61–7.54 (m, 3H), 7.50–7.43 (m, 3H), 3.96 (s, 8H); ¹³C NMR (100 MHz, CDCl₃) δ 149.5, 147.5, 133.5, 130.9, 130.2, 129.7, 129.0, 128.5, 128.4, 128.1, 127.1, 123.8, 121.3, 67.9, 48.5; HRMS (ESI) calcd for C₁₉H₁₈N₂O₂: 307.1441 (M + H⁺), found: 307.1447.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21172038, 21372046, 21262015), the Science Funds of the Natural Science Foundation of Jiangxi Province (20122BAB203005), and the Project of the Science Funds of Jiangxi Education Office (KJLD13069).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental procedure, characterization data, ¹H and ¹³C NMR spectra of compounds 3, the CIF file of compound 3a. CCDC 1016318. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qo00209a

‡ These authors contributed equally.

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