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DOI: 10.1039/C5RA11536A (Communication) RSC Adv., 2015, 5, 70910-70914

Construction of spirocycles containing highly substituted pyrroli-dine and 1-indanone motifs with spiro quaternary stereogenic centers via 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone and azomethine ylides promoted by simple imidazolium salts†

Wei-Qiang Hua, Yan-Su Cui‡ a, Zhi-Jun Wub, Chuan-Bao Zhanga, Pei-Hao Doua, Song-Yang Niua, Ji-Ya Fu*a and Yong Liu*a
aInstitute of Fine Chemistry and Engineering, Henan Engineering Laboratory of Flame-Retardant and Functional Materials, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China. E-mail: liuyong79@126.com; fujiya@126.com
bChengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China

Received 16th June 2015 , Accepted 7th August 2015

First published on 10th August 2015

Abstract

The 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides has been developed successfully promoted by simple imidazolium salts to construct a variety of spirocycles containing highly substituted pyrrolidines efficiently, affording the corresponding compounds in excellent yields (up to 99%). The highly efficient catalytic system exhibited broad substrate scopes under mild conditions.

Highly substituted pyrrolidines are prevalent in many natural products which exhibit a broad range of biological and pharmaceutical activities.1 In particular, highly substituted pyrrolidine motifs containing a spiro quaternary stereogenic carbon center and fused with 1-indanone are versatile synthetic motifs for the biologically active natural products and pharmaceuticals.2 The significance of those molecules has stimulated intensive interests to develop numerous excellent manufacturing approaches.3,4 Among them, 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides is an extremely powerful and efficient strategy for the construction of spiro[pyrrolidine-1-indanone] derivatives and widely applied in organic synthesis.5 Wang6 reported the first example of 1,3-dipolar cycloaddition reaction of 2-alkylidene-ketones with azomethine ylide using Ag(I)/TF-BiphamPhos complex. To the best of our knowledge, no successful and efficient organocatalytic protocols for the 1,3-dipolar cycloaddition reaction of 2-alkylidene-1-indanone with azomethine ylides have been reported. And it is still highly challenging and desirable to develop new efficient catalytic systems for this transformation.4

Ionic liquids, which are powerful tools to activate donors or acceptors, can be designed for task-specific applications through smart choice of the respective cation and/or anion.7 Since the last century, ionic liquids (ILs) have been attracting organic chemists' attentions because of their advantages of friendly to environment, convenient to use, controlled properties and thermal stability.8 Especially, functionalized ILs, such as acidic, basic and other ionic liquids have been synthesized and used to promote various chemical reactions.9 Although several kinds of functionalized ILs have been used for excellent catalysts for a number of reactions,10 such as Michael addition,11 Diels–Alder addition12 etc., rarely functionalized ILs were applied to 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides.

Herein, as a part of our program to explore the potential of functional ILs in chemical transformations, we described the first 1,3-dipolar cycloaddition reaction of 2-alkylidene-1-indanone with various azomethine ylide promoted by imidazolium salts, one kind of simple and useful functionalized ILs, for the synthesis of spiro heterocyclic compounds containing highly substituted pyrrolidine and 1-indanone motifs with excellent results.

For investigation on 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides, we choose 2-alkylidene-1-indanone (1a) and azomethine ylide (2a) as model reactants to determine the optimal conditions and the results are summarized in Table 1. A variety of imidazolium salts (3a–3h) were synthesized and tested in the model reaction in acetonitrile, and moderate to excellent yields (52–99%) were obtained (Table 1, entries 4–11). Catalysts 1-butyl-3-methylimidazolium bromide ([bmim]Br) (3a) was first evaluated, and gave the desired product (4aa) in 66% yield (Table 1, entry 4). The anion of the imidazolium salts affected the results obviously (Table 1, entries 4–7). The catalyst with dicarbonate as anion (3b) was tested and 52% yield was obtained (Table 1, entry 5). With carboxylate as anion, catalysts 3e–3g provided 4aa in 90–99% yields (Table 1, entries 8–11). Especially, catalyst [bmim]CH3(CH2)2CO2 gave 4aa in 99% yield, and it turned out to be efficient to promote the reaction (Table 1, entry 10).

Table 1 Screening of catalysts and optimized the conditionsa

image file: c5ra11536a-u1.tif
Entry Cat. Solvent Time (h) Yieldb (%)
a Unless otherwise specified, all reaction carried out with 2-alkylidene-1-indanone (1a, 0.20 mmol, 1.0 equiv.), azomethine ylide (2a, 0.2 mmol, 1.0 equiv.), solvent (1 mL) and catalyst 3 (20 mmol%) at 40 °C.b Isolated yields.c Carried out at 25 °C.d Carried out with 10 mol% catalyst 3g.
1 K2CO3 CH3CN 48 Trace
2 DABCO CH3CN 48 Trace
3 NaOAc CH3CN 48 24
4 3a CH3CN 168 66
5 3b CH3CN 168 52
6 3c CH3CN 168 72
7 3d CH3CN 168 62
8 3e CH3CN 20 90
9 3f CH3CN 5 97
10 3g CH3CN 5 99
11 3h CH3CN 46 80
12 3g Toluene 48 66
13 3g CHCl3 48 70
14 3g ClCH2CH2Cl 48 21
15 3g CH3CO2Et 48 82
16 3g Et2O 2 80
17 3g THF 2 85
18 3g MeOH 24 93
19 3g DMF 3 92
20 3g DMSO 3 69
21c 3g CH3CN 46 81
22d 3g CH3CN 29 86

Then a series of solvents were evaluated with 3g as catalyst and the results were presented in Table 1 (entries 12–20). All solvents gave moderate to good yields (66–92%). In polar solvents such as Et2O, THF, DMF, DMSO, 3g delivered high yields (80–92%) (Table 1, entries 16, 17, 19 and 20). Protic solvents such as MeOH, afforded the cycloadduct 4aa in 93% yield (Table 1, entries 18). Among the screened solvents, acetonitrile, affording the desired products in 99% yields, was chosen for further optimization (Table 1, entry 10). Reaction temperature was studied with 3g as catalyst and acetonitrile as the solvent. Decreasing the temperature to 25 °C, the reaction proceeded smoothly and afforded the desired adduct 4aa in good yield (81%, Table 1, entry 21). Lowering catalyst loading to 10 mol%, the yields were decreased (Table 1, entries 22 vs. 10). Thus, the optimized reaction conditions were found to be reaction of 1.0 equiv. 2-alkylidene-1-indanone (1a) with 1.0 equiv. azomethine ylide (2a), in the presence of 20 mol% of 3g, in acetonitrile at 40 °C (Table 1, entry 10).

With the optimized reaction conditions in hand, the scopes of the substrates for this 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides promoted by BILs were evaluated and the results are summarized in Table 2. Generally, a series of the azomethine ylides were successfully employed in this transformation and proceeded smoothly affording the corresponding adducts 4 in moderate to excellent yields (61–99%), and the results strongly depended on the substituents of the aromatic ring. Substrates bearing various electron-withdrawing groups on the aromatic ring at 4-position, such as fluoro, chloro, trifluoromethyl, and nitro group, were well-tolerated, delivering the products 4ab–4af in 83–87% yields (Table 2, entries 2–6). The electron-donating substituents, such as methyl and methoxyl, obviously increased the reactivity of azomethine ylides in this reaction (Table 2, entries 7 and 8). Substrates with an electron-withdrawing or electron-donating groups on the aromatic ring at 2-position gave lower yields than those with substituents at 4-position, and the reaction rate was decreased (Table 2, entries 10, 11, 12 vs. 3, 4, 8). For azomethine ylide 2m, 90% yield was obtained (Table 2, entry 13). In addition, the 2-furyl substituted azomethine ylide gave 61% yield (Table 2, entry 14).

Table 2 Scope of substratesa

image file: c5ra11536a-u2.tif
Entry R1 R2 dr Yieldb (%)
a Unless otherwise specified, all reaction carried out with 2-alkylidene-1-indanone (1a, 0.20 mmol, 1.0 equiv.), azomethine ylide (2a, 0.2 mmol, 1.0 equiv.), solvent (1 mL) and catalyst 3g (20 mmol%) at 40 °C in 2–27 h.b Isolated yields.
1 H (1a) Ph (2a) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.63[thin space (1/6-em)]:[thin space (1/6-em)]0.21[thin space (1/6-em)]:[thin space (1/6-em)]0.78[thin space (1/6-em)]:[thin space (1/6-em)]0.73[thin space (1/6-em)]:[thin space (1/6-em)]0 99 (4aa)
2 H (1a) 4-FPh (2b) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.06[thin space (1/6-em)]:[thin space (1/6-em)]0.21[thin space (1/6-em)]:[thin space (1/6-em)]0.16[thin space (1/6-em)]:[thin space (1/6-em)]0.26[thin space (1/6-em)]:[thin space (1/6-em)]0.66 85 (4ab)
3 H (1a) 4-ClPh (2c) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.21[thin space (1/6-em)]:[thin space (1/6-em)]0.13[thin space (1/6-em)]:[thin space (1/6-em)]0.14[thin space (1/6-em)]:[thin space (1/6-em)]0.16[thin space (1/6-em)]:[thin space (1/6-em)]0.17 83 (4ac)
4 H (1a) 4-BrPh (2d) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.05[thin space (1/6-em)]:[thin space (1/6-em)]0.04[thin space (1/6-em)]:[thin space (1/6-em)]0.11[thin space (1/6-em)]:[thin space (1/6-em)]0.11[thin space (1/6-em)]:[thin space (1/6-em)]0.16 87 (4ad)
5 H (1a) 4-NO2Ph (2e) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.80[thin space (1/6-em)]:[thin space (1/6-em)]0.35[thin space (1/6-em)]:[thin space (1/6-em)]0.36[thin space (1/6-em)]:[thin space (1/6-em)]0.76[thin space (1/6-em)]:[thin space (1/6-em)]0.79 86 (4ae)
6 H (1a) 4-CF3Ph (2f) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.05[thin space (1/6-em)]:[thin space (1/6-em)]0.17[thin space (1/6-em)]:[thin space (1/6-em)]0.82[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]0 85 (4af)
7 H (1a) 4-CH3Ph (2g) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.21[thin space (1/6-em)]:[thin space (1/6-em)]0.44[thin space (1/6-em)]:[thin space (1/6-em)]0.14[thin space (1/6-em)]:[thin space (1/6-em)]0.58[thin space (1/6-em)]:[thin space (1/6-em)]0.51 93 (4ag)
8 H (1a) 4-CH3OPh (2h) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.10[thin space (1/6-em)]:[thin space (1/6-em)]0.55[thin space (1/6-em)]:[thin space (1/6-em)]0.96[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]0 98 (4ah)
9 H (1a) 4-N(Me)2Ph (2i) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.13[thin space (1/6-em)]:[thin space (1/6-em)]0.52[thin space (1/6-em)]:[thin space (1/6-em)]0.71[thin space (1/6-em)]:[thin space (1/6-em)]0.61[thin space (1/6-em)]:[thin space (1/6-em)]0.10 71 (4ai)
10 H (1a) 2-ClPh (2j) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.29[thin space (1/6-em)]:[thin space (1/6-em)]0.20[thin space (1/6-em)]:[thin space (1/6-em)]0.24[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]0 65 (4aj)
11 H (1a) 2-BrPh (2k) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.34[thin space (1/6-em)]:[thin space (1/6-em)]0.16[thin space (1/6-em)]:[thin space (1/6-em)]0.10[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]0 85 (4ak)
12 H (1a) 2-CH3OPh (2l) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.81[thin space (1/6-em)]:[thin space (1/6-em)]0.40[thin space (1/6-em)]:[thin space (1/6-em)]0.26[thin space (1/6-em)]:[thin space (1/6-em)]0.47[thin space (1/6-em)]:[thin space (1/6-em)]0.79 67 (4al)
13 H (1a) image file: c5ra11536a-u3.tif 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.27[thin space (1/6-em)]:[thin space (1/6-em)]0.29[thin space (1/6-em)]:[thin space (1/6-em)]0.59[thin space (1/6-em)]:[thin space (1/6-em)]0.71[thin space (1/6-em)]:[thin space (1/6-em)]0.35 90 (4am)
14 H (1a) image file: c5ra11536a-u4.tif 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.07[thin space (1/6-em)]:[thin space (1/6-em)]0.03[thin space (1/6-em)]:[thin space (1/6-em)]0.25[thin space (1/6-em)]:[thin space (1/6-em)]0.23[thin space (1/6-em)]:[thin space (1/6-em)]0.58 61 (4an)
15 H (1a) (CH3)2CH2CH2 (2p) n.d. Trace
16 4 F (1b) Ph (2a) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.30[thin space (1/6-em)]:[thin space (1/6-em)]0.25[thin space (1/6-em)]:[thin space (1/6-em)]0.40[thin space (1/6-em)]:[thin space (1/6-em)]0.40[thin space (1/6-em)]:[thin space (1/6-em)]0.19 80 (4ba)
17 4-Cl (1c) Ph (2a) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.08[thin space (1/6-em)]:[thin space (1/6-em)]0.22[thin space (1/6-em)]:[thin space (1/6-em)]0.47[thin space (1/6-em)]:[thin space (1/6-em)]0.16[thin space (1/6-em)]:[thin space (1/6-em)]0.85 76 (4ca)
18 4-Br (1d) Ph (2a) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.08[thin space (1/6-em)]:[thin space (1/6-em)]0.32[thin space (1/6-em)]:[thin space (1/6-em)]0.26[thin space (1/6-em)]:[thin space (1/6-em)]0.54[thin space (1/6-em)]:[thin space (1/6-em)]0.52 68 (4da)
19 4-CF3 (1e) Ph (2a) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.06[thin space (1/6-em)]:[thin space (1/6-em)]0.29[thin space (1/6-em)]:[thin space (1/6-em)]0.13[thin space (1/6-em)]:[thin space (1/6-em)]0.25[thin space (1/6-em)]:[thin space (1/6-em)]0.18 78 (4ea)
20 4-CH3O (1f) Ph (2a) 1.00[thin space (1/6-em)]:[thin space (1/6-em)]0.91[thin space (1/6-em)]:[thin space (1/6-em)]0.56[thin space (1/6-em)]:[thin space (1/6-em)]0.90[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]0 64 (4fa)

We next examined the scopes of this 1,3-dipolar cycloaddition by varying the 2-alkylidene-1-indanones under the optimized conditions (Table 2, entries 16–20). This transformation tolerated various 2-alkylidene-1-indanones with electron-donating or electron-withdrawing substituents on the aromatic ring at 4-position, affording the products (4ba–4fa) in moderate yields (Table 2, entries 16–20). The electron-withdrawing group, for example, fluoro, chloro, bromo and trifluoromethyl group, on the phenyl ring of the 2-alkylidene-1-indanones usually gave the slightly higher yield compared with the methoxyl-substituted one (Table 2, entries 20 vs. 16–19).

Encouraged by these results for the 2-alkylidene-ketones, we then investigated the 1,3-dipolar cycloaddition of 2-alkylidene-3-benzofuranone (1a′) or 3-alkylidene-2-benzofuranone (1a′′) with azomethine ylides (Scheme 1). To our delight, the reaction smoothly proceeded with ILs 3g as catalyst, and the highly substituted pyrrolidines were still successfully obtained to give the corresponding cycloadduct 4a′a and 4a′′a in 87% and 93% yields, respectively.


image file: c5ra11536a-s1.tif
Scheme 1 The results of 1,3-dipolar cycloaddition of 2-alkylidene-3-benzofuranone or 3-alkylidene-2-benzofuranone with azomethine ylides.

To test the catalyst reusability and be convenient for recycling, the reaction was carried out in the presence of a catalytic amount of imidazolium salts under the conditions with 1a (0.20 mmol), 2a (0.20 mmol), catalyst 3g (20 mol%), at 40 °C and the reaction was monitored by TLC. When the reaction was completed, the reaction mixture was concentrated in vacuo, and extracted with DCM, the ionic liquid left in the aqueous phase was concentrated in vacuo. Recycled 3g was directly used in the next run and the results indicated that the isolated yield of the product 4a was almost consistent after five runs and 3g could be reused at least five times (Fig. 1).


image file: c5ra11536a-f1.tif
Fig. 1 Recyclability of the catalyst. Reaction conditions: 1a (0.20 mmol, 1.0 equiv.), 2a (0.20 mmol, 1.0 equiv.), catalyst 3g (20 mol%), in CH3CN at 40 °C.

Based on the experimental results, we suggested a plausible bifunctional catalytic mechanism involving nucleophilic butyric anion as a base to capture a proton of azomethine ylides and an electrophilic imidazolium cation to accept the transferred electron to complete the reaction cycle as shown in Scheme 2. Azomethine ylides was activated by the butyric anion and 2-alkylidene-1-indanone might be activated by the imidazolium cation (Scheme 2).


image file: c5ra11536a-s2.tif
Scheme 2 Proposed mechanism.

Conclusions

In summary, we have developed an efficient catalyst to promote 1,3-dipolar cycloaddition of 2-alkylidene-1-indanone with azomethine ylides leading to the formation of useful spirocycles containing highly substituted pyrrolidines and 1-indanone motifs with spiro quaternary stereogenic centers. This is the first example of 1,3-dipolar cycloadditions of 2-alkylidene-1-indanone with azomethine ylides. Moreover, this transformation provides an efficient and practical method for the synthesis of structurally diverse 1-indanone derivatives under mild conditions.

Acknowledgements

This work was supported by the National Natural Science Foundations of China (No. 21206031), the fundamental research foundations for provincial colleges and universities (No. zzjj20140006) and the key project of science and technology of Henan Province (No. 15A150034).

Notes and references

  1. (a) S. Yu, D. Qin, S. Shangary, J. Chen, G. Wang, K. Ding, D. McEachern, S. Qiu, Z. Nikolovska-Coleska, R. Miller, S. Kang, D. Yang and S. Wang, J. Med. Chem., 2009, 52, 7970 CrossRef CAS PubMed ; (b) G. Gilles and S. L. Claudine, Stress, 2003, 6, 199 CrossRef PubMed ; (c) K. Bernard, S. Bogliolo and J. Ehrenfeld, Br. J. Pharmacol., 2005, 144, 1037 CrossRef CAS PubMed ; (d) S. G. Pyne, A. S. Davis, N. J. Gates, J. Nicole, J. P. Hartley, K. B. Lindsay, T. Machan and M. Tang, Synlett, 2004, 2670 CrossRef CAS ; (e) L. M. Harwood and R. J. Vickers, in Synthetic Applications of 1,3-Dipolar Cycloaddition Chemistry Toward Heterocycles and Natural Products, ed. A. Padwa and W. Pearson, Wiley & Sons, New York, 2002 Search PubMed .
  2. Selected example: A. C. Wei, M. A. Ali, Y. K. Yoon, R. Ismail, T. S. Choon, R. S. Kumar, N. Arumugam, A. I. Almansour and H. Osman, Bioorg. Med. Chem. Lett., 2012, 22, 4930 CrossRef CAS PubMed .
  3. Selected examples: (a) S. Sternativo, B. Battistelli, L. Bagnoli, C. Santi, L. Testaferri and F. Marini, Tetrahedron Lett., 2013, 54, 6755 CrossRef CAS PubMed ; (b) W. Fang, M. Presset, A. Guerinot, C. Bour, S. Bezzenine-Lafollee and V. Gandon, Chem.–Eur. J., 2014, 20, 5439 CrossRef CAS PubMed ; (c) G. M. Schwarzwalder, S. E. Steinhardt, H. V. Pham, K. N. Houk and C. D. Vanderwal, Org. Lett., 2013, 15, 6014 CrossRef CAS PubMed ; (d) F. Behler, F. Habecker, W. Saak, T. Kluener and J. Christoffers, Eur. J. Org. Chem., 2011, 4231 CrossRef CAS PubMed ; (e) A. C. Wei, M. A. Ali, Y. K. Yoon, R. Ismail, T. S. Choon and R. S. Kumar, Bioorg. Med. Chem. Lett., 2013, 23, 1383 CrossRef CAS PubMed ; (f) M. T. Ardolino and P. Morken, J. Am. Chem. Soc., 2014, 136, 7092 CrossRef CAS PubMed ; (g) Z. Y. Cao, X. M. Wang, C. Tan, X. L. Zhao, J. Zhou and K. L. Ding, J. Am. Chem. Soc., 2013, 135, 8197 CrossRef CAS PubMed .
  4. H. Huang, W. Wu, K. Zhu, J. Hu and J. Ye, Chem.–Eur. J., 2013, 19, 3838 CrossRef CAS PubMed .
  5. Selected examples: (a) J. Casas, R. Grigg, C. Nájera and J. M. Sansano, Eur. J. Org. Chem., 2001, 1971 CrossRef CAS ; (b) A. S. Gothelf, K. V. Gothelf, R. G. Hazell and K. A. JΦrgensen, Angew. Chem., Int. Ed., 2002, 41, 4236 CrossRef CAS ; (c) R. Narayan, M. Potowski, Z.-J. Jia, A. P. Antonchick and H. Waldmann, Acc. Chem. Res., 2014, 47, 1296 CrossRef CAS PubMed ; (d) T. Liu, Q. Li, Z. He, J. Zhang and C. Wang, Chin. J. Catal., 2015, 36, 68 CrossRef CAS ; (e) X. H. Chen, Q. Wei, S. W. Luo, H. Xiao and L. Z. Gong, J. Am. Chem. Soc., 2009, 131, 13819 CrossRef CAS PubMed ; (f) F. Shi, Z.-L. Tao, S.-W. Luo, S.-J. Tu and L.-Z. Gong, Chem.–Eur. J., 2012, 18, 6885 CrossRef CAS PubMed ; (g) F. Shi, R. Y. Zhu, X. Liang and S. J. Tu, Adv. Synth. Catal., 2013, 355, 2447 CrossRef CAS PubMed ; (h) C. S. Wang, R. Y. Zhu, J. Zheng, F. Shi and S. J. Tu, J. Org. Chem., 2015, 80, 512 CrossRef CAS PubMed ; (i) W. Dai, X. L. Jiang, Q. Wu, F. Shi and S. J. Tu, J. Org. Chem., 2015, 80, 5737 CrossRef CAS PubMed .
  6. T.-L. Liu, Z.-L. He, Q.-H. Li, H.-Y. Tao and C.-J. Wang, Adv. Synth. Catal., 2011, 353, 1713 CrossRef CAS PubMed .
  7. Selected examples: (a) P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772 CrossRef CAS ; (b) T. Welton, Chem. Rev., 1999, 99, 2071 CrossRef CAS PubMed ; (c) J. S. Wilkes, Green Chem., 2002, 4, 73 RSC ; (d) T. Welton, Chem. Rev., 1999, 99, 2071 CrossRef CAS PubMed ; (e) E. Tina, V. Jürgen, W. Frank and S. S. Ulrich, J. Mater. Chem., 2008, 18, 5267 RSC ; (f) W. Wassercheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772 CrossRef .
  8. Selected examples: (a) J. N. Rosa, C. A. M. Afonso and A. G. Santos, Tetrahedron, 2001, 57, 4189 CrossRef CAS ; (b) J.-C. Hsu, Y.-H. Yen and Y.-H. Chu, Tetrahedron Lett., 2004, 45, 4673 CrossRef CAS PubMed ; (c) E. J. Kim and S. Y. Ko, Helv. Chim. Acta, 2003, 86, 894 CrossRef CAS PubMed .
  9. Selected examples: (a) X. Mi, S. Luo and J.-P. Cheng, J. Org. Chem., 2005, 70, 2338 CrossRef CAS PubMed ; (b) P. Langer, Angew. Chem. Int., Ed., 2000, 39, 3049 CrossRef CAS ; (c) D. Basavaiah, P. D. Rao and R. S. Hyma, Tetrahedron, 1996, 52, 8001 CrossRef CAS ; (d) I. E. Marko, P. G. Giles and N. J. Hindley, Tetrahedron, 1997, 53, 1015 CrossRef CAS ; (e) S. E. Drewes and G. H. P. Roos, Tetrahedron, 1988, 44, 4653 CrossRef CAS ; (f) X. L. Mi, S. Z. Luo, J. Q. He and J.-P. Cheng, Tetrahedron Lett., 2004, 45, 4567 CrossRef CAS PubMed ; (g) R. Sheldon, Chem. Commun., 2001, 2399 RSC .
  10. X. L. Fu, Z. Zhang, C. M. Li, L. B. Wang, H. Y. Ji, Y. Yang, T. Zou and G. H. Gao, Catal. Commun., 2009, 10, 665 CrossRef CAS PubMed ; L. F. Zhang, Y. Yang, Y. R. Xue, X. L. Fu, Y. An and G. H. Gao, Catal. Today, 2010, 158, 279 CrossRef PubMed ; M. Selva, A. Perosa and M. Fabris, Green Chem., 2008, 10, 1068 RSC ; M. Selva, M. Fabris, V. Lucchini, A. Perosa and M. Noe, Org. Biomol. Chem., 2010, 8, 5187 Search PubMed .
  11. Selected examples: (a) K. Nzaira, X. H. Liu, L. Petra, P. Dimitrios and S. Grace, Tetrahedron, 2006, 62, 11039 CrossRef PubMed ; (b) C. J. Yu and C. J. Liu, Molecules, 2009, 14, 3222 CrossRef CAS PubMed ; (c) H. Guo, X. Li, J. L. Wang, X. H. Jin and X. F. Lin, Tetrahedron, 2010, 66, 8300 CrossRef CAS PubMed ; (d) B. C. Ranu and S. Banerjee, Org. Lett., 2005, 7, 3049 CrossRef CAS PubMed ; (e) L. Yang, L. W. Xu, W. Zhou, L. Li and C. G. Xia, Tetrahedron Lett., 2006, 47, 7723 CrossRef CAS PubMed ; (f) J. M. Xu, Q. Wu, Q. Y. Zhang, F. Zhang and X. F. Lin, Eur. J. Org. Chem., 2007, 1798–1802 CrossRef CAS PubMed ; (g) X. Xin, Y. Wang, W. Xu, Y. J. Lin, H. F. Duan and D. W. Dong, Green Chem., 2010, 12, 893 RSC .
  12. Selected examples: (a) D. A. Jaeger and C. E. Tucker, Tetrahedron Lett., 1989, 30, 178 CrossRef ; (b) P. Ludley and N. Karodia, Tetrahedron Lett., 2001, 42, 2011 CrossRef CAS ; (c) C. M. Gordon, J. D. Holbrey and K. R. Seddon, J. Mater. Chem., 1998, 8, 2627 RSC ; (d) J. D. Holbrey and K. R. Seddon, J. Chem. Soc., Dalton Trans., 1999, 2133 RSC ; (e) C. E. Song, E. J. Roh and J. H. Choi, Chem. Commun., 2000, 1695 RSC ; (f) J. A. Boon, J. A. Levisky, J. L. Pflug and J. S. Wilkes, J. Org. Chem., 1986, 51, 480 CrossRef CAS ; (g) J. S. Wilkes, Green Chem., 2002, 4, 73 RSC .

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11536a
‡ This author contributed equally to this work.
This journal is © The Royal Society of Chemistry 2015
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