A new green catalyst: 1,3,5-triazine-functionalized bisimidazolium dichloride tethered SPION catalyzed Betti synthesis

Abstract

A unique dicationic ionic liquid tethered to superparamagnetic iron oxide nanoparticles as a green and powerful catalyst for the efficient synthesis of Betti bases in high to excellent yields has been evaluated. Due to the high magnetization of the catalyst, it can be satisfactorily recovered by a simple external magnet. The catalyst could then be recycled and reused at least six times without any loss of activity.

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Introduction

Since the first reports on task specific ionic liquids (TSILs) have appeared in the past decade,¹ a wide variety of these materials with tailored compositions which possess different properties have been synthesized.^2–5 The chemical properties of TSILs can be modified with different chemical groups, by which their functionality and textural properties could be changed.⁶ Within this context, these compounds have attracted increasing amounts of attention in both academic and industrial fields owing to their unique properties, such as catalytic activity.⁷

The advantage of utilizing TSILs as environmentally friendly catalysts can be higher if efficient recovery and reuse of the catalyst can be performed, so improving the greenness of the process. In this context, one possible route is the immobilization of catalytic TSILs onto inorganic or polymer supports, which produces a new class of catalyst systems.⁸ In these systems, a thin film of TSIL is dispersed over the large internal surface of the support. Utilizing nano-supports has attracted considerable interest in recent years due to their high activity and environmental acceptability.⁹ Although these materials offer a high specific surface area for the active component, which enhances the contact between the reactants and supported catalyst, they are easily agglomerated and are difficult to separate. Therefore, to overcome this problem, it is essential to design an easily recoverable catalyst.

Currently, various nano-catalysts which employ magnetic separation properties have been developed in a number of organic transformations.¹⁰ These magnetic nanoparticles (MNPs) show excellent catalytic activities. The magnetic separation method presents many advantages over conventional ones. It can be considered environmentally benign as the filtration steps are omitted in the reaction. For instance, the catalyst can be recovered with an external magnetic field.

To date, although TSILs moieties are widely used in organic synthesis, there are only a few examples of ionic liquids anchored to MNPs which are used as catalysts in organic transformations.¹¹ In 2009, Zhao and co-workers reported the first example of an imidazole-based ionic liquid grafted onto SPION as a catalyst for the Knoevenagel condensation reaction.¹² SPION-decorated ionic liquids have also been used as catalysts for the cycloaddition of CO₂ and epoxides to produce cyclic carbonates.¹³ Very recently, Lue et al. reported a magnetic nanoparticle-supported acidic IL as a catalyst for the synthesis of benzoxanthenes.¹⁴ As a result, the ionic liquid based-catalysts bearing imidazolium groups are apparently more effective than catalysts with other groups.¹⁵ However, to the best of our knowledge, multicomponent reactions promoted by dicationic ionic liquids based on imidazolium cations anchored to MNPs as a new class of eco-friendly catalysts has not been reported.

A one-pot multicomponent reaction is a useful method for the synthesis of complex chemotypes which saves time, allows instantaneous access to large compound libraries with diverse functionalities and avoids costly purification processes by the systematic variation of the starting material, which could be either commercially available or easily prepared.¹⁶

One of the classic multicomponent reactions is the Betti bases synthesis.¹⁷ The classical Betti reaction is a three-component reaction between an aldehyde, ammonia and β-naphthol.¹⁸ This reaction was a milestone in organic synthesis and was the method used to obtain amidoalkyl naphthols¹⁷ which, in turn, are very important precursors for the synthesis of important bioactive 1-aminomethyl-2-naphthols, their bradycardia and hypotensive effects in humans having been evaluated.¹⁹ These are attractive compounds as chiral ligands in enantioselective reactions.²⁰ Moreover, they can be used as chiral shift reagents for carboxylic acids or chiral auxiliaries for the synthesis of α-aminophosphonic acids.²¹ Several studies have concentrated on the catalysis of the reaction, using different catalytic systems.²² Nevertheless, in the most recent report, separation of the catalyst from the reaction mixture was carried out via conventional methods, which lead to leaching of the utilized catalysts into the environment and the use of toxic organic solvents, which are critical disadvantages of these protocols.

Now, encouraged by previous results and our interest in developing efficient, sustainable and greener pathways for organic transformations,²³ we report herein a simple synthesis for a new dicationic ionic liquid with a 1,3,5-triazine core anchored to nano-superferromagnetic particles and its application in the Betti reaction to synthesize β-amidoalkylnaphthols via a one-pot multicomponent reaction.

The synthetic path for the catalyst is depicted in Scheme 1.

Scheme 1 The synthesis path of 1,3,5-triazine-functionalized bisimidazolium dichloride tethered to SPIONs.

The free-dicationic ionic liquid (Cl-ACl₂) was first prepared via the reaction of 1,3,5-trichlorotriazine (TCT) with two equimolar N-methylimidazole in dry dioxane. The key factor in the synthesis was a nucleophilic substitution reaction between TCT and N-methylimidazole to generate the desired bisimidazolium salt. Finally, the ionic liquid was anchored to the SPIONs by an ipso-substitution reaction with the OH group of silica-encapsulated Fe₃O₄ nanoparticles.

SPION-ACl₂ was characterized by means of fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TG), vibrating sample magnetometry (VSM) and transmission electron microscopy (TEM).

Fig. S1 (see ESI†) illustrates the FTIR spectra of Fe₃O₄ (a), silica-encapsulated Fe₃O₄ (b), the free-dicationic ionic liquid (Cl-ACl₂) (c) and the SPION-ACl₂ nanocatalyst (d), respectively.

The spectral features of ACl₂ are observed for SPION-ACl₂ by the presence of bands at 3141, 2953, 1720, 1440, 805 and 620 cm⁻¹. These bands are absent in the spectrum of the silica-encapsulated Fe₃O₄ nanoparticles and confirm the presence of ACl₂ on SPION.

The thermal stability of SPION-ACl₂ was also evaluated by TGA-DTG and the thermograms are given in Fig. S2 (see ESI†). These curves show about 5.04% weight loss below 120 °C, suggesting that 1 mg of SPION-ACl₂ still contained about 0.0028 mmol of H₂O, which it is due to the removal of adsorbed water. The decomposition temperature (T_d) was also calculated from the TGA-DTG curves. As exhibited in Fig. S2 (ESI†), the T_d value was in the range of ∼200–460 °C with about 33 wt.% loss for ACl₂. The remaining ca. 60% weight is revealed to be the silica-encapsulated Fe₃O₄ nanoparticles, which illustrates that the ratio between the organic moiety (ACl₂) and the inorganic phase is 33 : 60% approximately.

The dc magnetic characterization of SPION-ACl₂ and the neat silica-encapsulated Fe₃O₄ nanoparticles were examined at room temperature in an external field range of ±10 kOe (Fig. S3, ESI†).

In this investigation, due to the functionalization of the silica-encapsulated Fe₃O₄ by ACl₂, Ms (the saturation magnetization) is found to be 20.2 emu g⁻¹, which is considerably lower than that of the bulk magnetite (40.6 emu g⁻¹, Fig. S3, ESI†).

To study the morphology characteristics of SPION-ACl₂, TEM images were also investigated (Fig. 1).

Fig. 1 (a) TEM image of SPION-ACl₂ and (b) SPION-ACl₂ particle size distribution histogram.

The TEM images of SPION-ACl₂ revealed that it appears to have an almost spherical structure with a narrow size distribution (the average particle size is 10–35 nm, Fig. 1b), thereby maintaining a nanocrystalline appearance. Thus, the enormous active sites of this nanoparticle may allow for excellent activity in organic transformations, even with a low catalyst loading.

In order to show the catalytic activity of SPION-ACl₂ in a Betti reaction, a comparison between the type and texture of the catalyst was examined in a condensation reaction of 4-nitrobenzaldehyde (1 mmol), β-naphthol (1 mmol) and acetamide (1.2 mmol) as a template reaction under solvent-free conditions (Table 1, entries 1–9).

Table 1 The effect of the catalysts and temperature on the yield of 2h as a template

Entry	Catalyst	Temp. (°C)	Yield^a (%)
Reaction conditions: 4-nitrobenzaldehyde (1 mmol), β-naphthol (1 mmol), acetamide (1.2 mmol) and catalyst after 20 min.a Isolated yield.
1	SPION-ACl2 (50 mg, 6 mol%)	100 °C	94
2	SPION-ACl2 (40 mg, 4.8 mol%)	100 °C	81
3	—	100 °C	N.R.
4	Nano-SiO2	100 °C	2
5	Fe3O4	100 °C	7
6	SPION-A(PF6)2 (50 mg, 6 mol%)	100 °C	83
7	SPION-A(NTf2)2 (50 mg, 6 mol%)	100 °C	77
8	[bmim]Cl (6 mol%)	100 °C	<15
9	Cl-ACl2 (6 mol%)	100 °C	65
10	β-naphthol-ACl2 (6 mol%)	100 °C	37
11	CH3CONH-ACl2 (6 mol%)	100 °C	44
12	SPION-ACl2 (50 mg, 1.7 mol%)	100 °C	40
13	SPION-ACl2 (50 mg, 2.5 mol%)	100 °C	60
14	SPION-ACl2 (50 mg, 5 mol%)	100 °C	85
15	SPION-ACl2 (50 mg, 6 mol%)	110 °C	94
16	SPION-ACl2 (50 mg, 6 mol%)	120 °C	95
17	SPION-ACl2 (50 mg, 6 mol%)	90 °C	80

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Surprisingly, in the presence of SPION-ACl₂ (50 mg, 6 mol%), quantitative conversion was registered in 20 min and 94% of the Betti adduct, 2h, was isolated (Table 1, entry 1). Notably, lower yields were obtained when the same reaction was carried out while utilizing a lower catalyst loading (Table 1, entry 2).

No reaction occurred in the absence of a catalyst (Table 1, entry 3) and no valuable yield was obtained in the presence of nano-SiO₂ or Fe₃O₄ without ACl₂ (Table 1, entries 4 and 5). In contrast, using ionic liquids with different counter anions,²⁵ such as A(PF₆)₂ or A(NTf₂)₂ instead of ACl₂, evidently gave the product in lower yields (Table 1, entries 6 and 7). It is surprising that the desired product was obtained in small amounts in the presence of 1-butyl-3-methyl-imidazolium chloride (<15%), while the conversion of 2h while utilizing Cl-ACl₂ was 65% (Table 1, entries 8 and 9). It can be attributed to the fact that Cl-ACl₂ is not stable in the reaction and part of it is introduced into the reaction with the substrates (and was then consumed) and β-naphthol-ACl₂ and CH₃CONH-ACl₂, two of the byproducts, were detected. A closer examination illustrated that when the Cl-moiety of Cl-ACl₂ was replaced with functionalities such as β-naphthol or CH₃CONH and these were used as the catalyst instead of SPION-ACl₂, the yield was drastically reduced (Table 1, entries 10 and 11). On the other hand, the catalyst loading on the support was also investigated (Table 1, entries 12–14). As shown, the best result was obtained for 6 mol% loading. After further investigation, we also found that the reaction temperature can also be greatly affected in this transformation.

The obvious improvement in the conversion (94%) was achieved for the reaction under solvent-free conditions at 100 °C. A higher reaction temperature (110–120 °C) made no obvious difference to the yield of the products but using a lower reaction temperature (90 °C), sharply decreased the conversion to approximately 80%, even with longer reaction times (Table 1, entries 15–17). Accordingly, performing the reaction at 100 °C under solvent-free conditions is the optimal condition for the Betti reaction in the presence of SPION-ACl₂.

After the successful generation of 2h, we considered introducing more diversities into the β-amidoalkylnaphthol scaffold via the one-pot reaction. Therefore, with the optimized reaction conditions in hand, we examined several aryl aldehydes and found that SPION-ACl₂ is an efficient catalyst for the synthesis of a large spectrum of Betti bases through this reaction (Table 2).

Table 2 Synthesis of β-amidoalkylnaphthols catalyzed by SPION-ACl₂

Entry	Ar	R	Time (min)	Product	Yield^a (%)	TOF (h^−1)
TOF = (mmol of product/mmol of catalyst)/time (h).a Isolated yield.
1	C6H5	CH3	30	2a	85	28.4
2	2-ClC6H4	CH3	25	2b	87	34.8
3	4-ClC6H4	CH3	20	2c	95	47.5
4	2-BrC6H4	CH3	25	2d	90	36.0
5	3-BrC6H4	CH3	20	2e	94	47.0
6	4-BrC6H4	CH3	25	2f	89	35.6
7	4-FC6H4	CH3	15	2g	83	55.4
8	4-NO2C6H4	CH3	20	2h	94	48.5
9	3-NO2C6H4	CH3	20	2i	92	47.0
10	4-CNC6H4	CH3	20	2j	92	46.0
11	4-CH3C6H4	CH3	35	2k	85	24.3
12	4-CH3OC6H4	CH3	35	2l	82	23.4
13	C6H5	C6H5	30	2m	86	28.7
14	2-ClC6H4	C6H5	25	2n	89	35.6
15	4-ClC6H4	C6H5	20	2o	90	45.0
16	2-BrC6H4	C6H5	25	2p	88	35.2
17	3-BrC6H4	C6H5	25	2q	94	37.6
18	4-BrC6H4	C6H5	25	2r	97	38.8
19	4-FC6H4	C6H5	15	2s	92	61.4
20	4-NO2C6H4	C6H5	20	2t	95	47.5
21	3-NO2C6H4	C6H5	20	2u	92	46.0
22	4-CNC6H4	C6H5	20	2v	95	47.5
23	4-CH3C6H4	C6H5	30	2w	80	26.7
24	4-CH3OC6H4	C6H5	35	2x	81	23.1

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It was generally observed that high to excellent yields of the products were obtained in all cases almost irrespective of the type of substituent present in the aromatic ring of the aryl aldehyde, and the β-amidoalkylnaphthol derivatives, 2a–2x, were exclusively obtained with turnover frequencies (TOF) of 23.1–61.4 h⁻¹ (Table 2). It was found that aryl aldehydes carrying electron-donating groups could be smoothly converted to the desired products (Table 2, entries 11 and 12).

The coupling of aryl aldehydes containing electron-withdrawing groups also afforded the corresponding Betti bases in high to excellent isolated yields (85–98%, Table 2, 2b–2j). Moreover, this reaction works well with phenylacetamide (Table 2, entries 13–24). The successful production of the Betti base derivatives indicated that the one-pot reaction is a general route for this transformation. Thus, based on these considerations, we explored a new and facile one-pot procedure for the synthesis of β-amidoalkylnaphthols derivatives.

The SPION-ACl₂ catalyst prepared in this assay could be easily recovered by an applied magnetic field at the end of the reaction. The investigation of a solvent-free Betti reaction using SPION-ACl₂ was repeated six times to evaluate the catalyst's recyclability and stability (Fig. 2). After each run, the catalyst was separated from the reaction mixture by an applied magnetic field, washed with absolute ethanol and reused in the next cycle, which indicates the high stability of the catalyst under these reaction conditions (Fig. 2).

Fig. 2 Reuse of SPION-ACl₂ in the reactions: 4-nitrobenzaldehyde 1h (1 mmol), β-naphthol (1 mmol), acetamide (1.2 mmol) and SPION-ACl₂ (50 mg, 6 mol%) stirred at 100 °C for 20 min.

On the basis of these results, we propose a plausible pathway for the reaction (Scheme 2). The carbonyl group of aldehyde 1 may be activated by the coordination of the imidazolium cations of the catalyst, which may subsequently be attacked by the amide to afford the intermediate B. The reaction of Bvia β-naphthol affords the corresponding Betti adduct.

Scheme 2 Proposed mechanism.

In conclusion, we have demonstrated that SPION-ACl₂ acts as a unique and robust nano-catalyst for the efficient synthesis of Betti bases. This catalysis reaction furnishes a green, useful and practical procedure for the direct synthesis of β-amidoalkylnaphthols owing to the following profits: (a) the catalyst is very powerful, inexpensive and highly recyclable, (b) the highly efficient synthesis and separation processes of β-amidoalkylnaphthols, based on the present catalytic system, are promising for large-scale applications in an eco-friendly manner, (c) to the best of our knowledge, the synthesis of β-amidoalkylnaphthols in the presence of SPIONs as a modern nano-catalyst has not been achieved to date. This new bis-IL tethered SPION catalyst is a unique type of nano-superferromagnetic catalyst and may be applied to develop new catalytic systems for environmentally-friendly organic syntheses.

Experimental

All chemicals were of commercial reagent grade and were purified before use. All known organic products were identified by a comparison of their physical and spectral data with those of authentic samples. SPIONs (silica-encapsulated Fe₃O₄) were prepared according to a previously reported method.²⁴ Thin layer chromatography (TLC) was performed on UV-active aluminium-backed plates of silica gel (TLC Silica gel 60 F254). FT–IR spectra were obtained as potassium bromide pellets in the range of 400–4000 cm⁻¹ with a Nicolet Impact 400D instrument. ¹H, and ¹³C NMR spectra were measured on a Bruker DPX 400 MHz. The TEM images were taken with a Philips CM120 unit operating at 200 kV. The TGA curve was obtained with a heating rate of 10 °C per min on a TG 50 Mettler thermogravimetric analyzer. The magnetic measurements were performed with a vibrating sample magnetometer (VSM) at Meghnatis Daghigh Kavir Co.

Synthesis of Cl-ACl₂

A mixture of 1,3,5-trichlorotriazine (184 mg, 1 mmol) and N-methylimidazole (164 mg, 2 mmol) in 10 mL of dry dioxane was magnetically stirred for 5 h under reflux conditions. At the end of the reaction, the white precipitate was filtered, washed three times with THF (3 × 5 ml) and dried under reduced pressure at 60 °C. The ¹H and ¹³C NMR spectra are given below:

¹H-NMR (400 MHz, D₂O) δ_H (ppm) 3.98 (s, N-CH₃), 7.58 (t, 1H), 8.20 (t, 1H), 9.73 (s, 1H). ¹³C-NMR (100 MHz, D₂O) δ_C (ppm) 36.70, 119.00, 124.99, 136.47, 161.06, 170.59.

Synthesis of SPION-ACl₂

The SPIONs (silica-encapsulated Fe₃O₄), (60 mg) were first dispersed in 4 mL of dry THF by sonication in an ultrasonic bath for 15 min to form a brownish solution. The building block Cl-ACl₂ (40 mg, 0.12 mmol) was then added. After purging the mixture with N₂ for 5 min, the flask was sealed under a N₂ atmosphere with a rubber stopper. The mixture was sonicated for 1 h. The nanoparticles were then separated and collected by an external magnet, washed three times with THF (3 mL) and dried under reduced pressure at 80 °C. 50 mg of SPION-ACl₂ (3 mmol per ACl₂) was obtained.

General procedure for the synthesis of Betti bases catalyzed by SPION-ACl₂

Aryl aldehyde (1 mmol), amide (1.2 mmol) and β-naphthol (1 mmol) were added to a reaction tube charged with a magnetic stirrer bar and SPION-ACl₂ (50 mg, 6 mol%). The reaction mixture was stirred at 100 °C for 15–35 min (as indicated by the TLC). Most of the nanoparticles were adsorbed onto the magnetic stirrer bar when the stirring was stopped. After cooling to room temperature, the catalyst was collected by an external permanent magnet, washed two times with absolute ethanol (2 × 1 mL), air-dried and used directly for the next round of the reaction without further purification. After the separation of the catalyst, the collected solution was added to the residue of the reaction mixture and the volatiles were removed in vacuo. Recrystallisation of the solid product from ethanol–water afforded the pure corresponding product in 80–97% yields.

All the products are known in the literature and were identified by a comparison of their FT-IR, ¹H, and ¹³C NMR spectra with the literature data.²² As a sample, the characterization data for 2K is given below.

N-((2-Hydroxynaphthalen-1-yl)(p-tolyl)methyl)acetamide (2K). m.p. 222–223 °C; FT-IR (KBr) (ν_max/cm⁻¹): 3420, 3314, 3068, 1619, 1600, 935, 881. ¹H NMR (400 MHz, DMSO-d₆): δ = 1.95 (s, 3H), 2.20 (s, 3H), 7.09–7.04 (m, 5H), 7.20 (d, J = 8.8 Hz, 1H), 7.22 (t, J = 7.0 Hz, 1H), 7.32 (m, 1H), 7.72 (d, J = 8.8 Hz, 1H), 7.76 (d, J = 7.9 Hz, 1H), 7.80 (br, 1H), 8.35 (d, J = 8.1 Hz, 1H), 9.89 (s, 1H). ¹³C NMR (100 MHz, DMSO-d₆): 20.3, 22.5, 47.6, 118.2, 119.0, 122.1, 123.0, 126.1, 126.0, 128.6, 128.2, 129.2, 132.1, 134.9, 139.4, 143.3, 152.5, 169.0 ppm. EIMS (m/z): 305(M)⁺. Anal. Calcd. for C₂₀H₁₉NO₂: C: 78.65; H: 6.26; N: 4.50%. Found: C: 78.74; H: 6.20; N: 4.45%.

Acknowledgements

The support of this work by the Center of Excellence of Chemistry of the University of Isfahan (CECUI) is acknowledged.

Notes and references

1. (a) J. D. Holbrey and K. R. Seddon, Clean Prod. Processes, 1999, 1, 223; (b) J. H. Davis, Chem. Lett., 2004, 33, 1072; (c) T. Welton, Coord. Chem. Rev., 2004, 248, 2459.
2. N. V. Plechkovaa and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123.
3. (a) S. Yu, S. Lindeman and C. D. Tran, J. Org. Chem., 2008, 73, 2576; (b) R. Giernoth, Angew. Chem., Int. Ed., 2010, 49, 2.
4. Q. Zhang, S. Zhang and Y. Deng, Green Chem., 2011, 13, 2619.
5. (a) A. B. Pereiro, J. M. M. Araujo, J. M. S. S. Esperanca, I. M. Marrucho and L. P. N. Rebelo, J. Chem. Thermodyn., 2012, 46, 2; (b) T. Yu and R. G. Weiss, Green Chem., 2012, 14, 209.
6. (a) W. L. Hough, M. Smiglak, H. Rodńguez, R. P. Swatloski, S. K. Spear, D. T. Daly, J. Pernak, J. E. Grisel, R. D. Carliss, M. D. Soutullo, J. H. Davis and R. D. Rogers, New J. Chem., 2007, 31, 1429; (b) N. Liu, C. Liu and Z. Jin, Green Chem., 2012, 14, 592.
7. (a) T. L. Greaves and C. J. Drummond, Chem. Rev., 2008, 108, 206; (b) X. Ding, W. Tang, C. Zhu and Y. Cheng, Adv. Synth. Catal., 2010, 352, 108; (c) C. B. Yue, D. Fang, L. Liu and T. F. Yi, J. Mol. Liq., 2011, 163, 99.
8. (a) A. Riisager, R. Fehrmann, M. Haumann and P. Wasserscheid, Top. Catal., 2006, 40, 91; (b) Q. Gong, J. Klankermayer and B. Bluemich, Chem.–Eur. J., 2011, 17, 13795; (c) R. A. Watile, D. B. Bagal, K. M. Deshmukh, K. P. Dhake and B. M. Bhanage, J. Mol. Catal. A: Chem., 2011, 351, 196; (d) M. Haumann, M. Jakuttis, R. Franke, A. Schonweiz and P. Wasserscheid, ChemCatChem, 2011, 3, 1822; (e) H. Hagiwara, K. Sato, T. Hoshi and T. Suzuki, Synlett, 2011, 2545; (f) C. Fehér, E. Kriván, J. Hancsókb and R. Skoda-Földes, Green Chem., 2012, 14, 403.
9. (a) Y. Hu, H. M. Yang, Y. C. Zhang, Z. S. Hou, X. R. Wang, Y. X. Qiao, H. Li, B. Feng and Q. F. Huang, Catal. Commun., 2009, 10, 1903; (b) S. S. Moganty, N. Jayaprakash, J. L. Nugent, J. Shen and L. A. Archer, Angew. Chem., Int. Ed., 2010, 49, 9158; (c) M. E. Mahmoud and M. H. Al-Bishri, Chem.–Eng. J., 2011, 166, 157.
10. (a) S. Shylesh, V. Schünemann and W. R. Thiel, Angew. Chem., Int. Ed., 2010, 49, 3428; (b) E. Rafiee and S. Eavani, Green Chem., 2011, 13, 2116; (c) R. Arundhathi, D. Damodara, P. R. Likhar, M. L. Kantam, P. Saravanan, T. Magdaleno and S. H. Kwon, Adv. Synth. Catal., 2011, 353, 1591; (d) R. Hudson, C.-J. Li and A. Moores, Green Chem., 2012, 14, 622; (e) C. Lian, H. Liu, C. Xiao, W. Yang, K. Zhang, Y. Liu and Y. Wang, Chem. Commun., 2012, 48, 3124.
11. Q. Zhang, H. Su, J. Luo and Y. Wei, Green Chem., 2012, 14, 201.
12. Y. Zhang, Y. Zhao and C. Xia, J. Mol. Catal. A: Chem., 2009, 306, 107.
13. X. Zheng, S. Luo, l. Zhang and J.-P. Cheng, Green Chem., 2009, 11, 455.
14. Q. Zhang, H. Su, J. Luo and Y. Wei, Green Chem., 2012, 14, 201.
15. C. B. Yue, D. Fang, L. Liu and T. F. Yi, J. Mol. Liq., 2011, 163, 99.
16. (a) T. J. J. Mueller, Beilstein J. Org. Chem., 2011, 7, 960; (b) L. H. Choudhury and T. Parvin, Tetrahedron Lett., 2011, 67, 8213; (c) F. De Moliner, S. Crosignani, A. Galatini, R. Riva and A. Basso, ACS Comb. Sci., 2011, 13, 453; (d) N. Isambert, M. D. S. Duque, J. C. Plaquevent, Y. Genisson, J. Rodriguez and T. Constantieux, Chem. Soc. Rev., 2011, 40, 1347; (e) E. Ruijter, R. Scheffelaar and R. V. A. Orru, Angew. Chem., Int. Ed., 2011, 50, 6234.
17. C. Cardellicchio, M. A. M. Capozzi and F. Naso, Tetrahedron: Asymmetry, 2010, 21, 507.
18. M. Betti, Org. Synth. Collect., 1941, 1, 381.
19. A. Y. Shen, C. T. Tsai and C. L. Chen, Eur. J. Med. Chem., 1999, 34, 877.
20. (a) C. Cimarelli, A. Mazzanti, G. Palmieri and E. Volpini, J. Org. Chem., 2001, 66, 4759; (b) C. Cimarelli, G. Palmieri and E. G. Volpini, Tetrahedron: Asymmetry, 2002, 13, 2417; (c) C. Cimarelli, G. Palmieri and E. Volpini, J. Org. Chem., 2003, 68, 1200.
21. K. E. Metlushka, B. A. Kashemirov, V. F. Zheltukhin, D. N. Sadkova, B. Büchner, C. Hess, O. N. Kataeva, C. E. McKenna and V. A. Alfonsov, Chem.–Eur. J., 2009, 15, 6718.
22. (a) M. M. Khodaei, A. R. Khosropour and H. Moghanian, Synlett, 2006, 916; (b) H. R. Shaterian, H. Yarahmadi and M. Ghashang, Bioorg. Med. Chem. Lett., 2008, 18, 788; (c) P. Zhang and Z. H. Zhang, Monatsh. Chem., 2009, 140, 199; (d) M. Lei, L. Ma and L. Hu, Tetrahedron Lett., 2009, 50, 6393; (e) A. R. Hajipour, Y. Ghayeb, N. Sheikhan and A. E. Ruoho, Tetrahedron Lett., 2009, 50, 5649; (f) G. Rashinkar and R. Salunkhe, J. Mol. Catal. A: Chem., 2010, 316, 146; (g) Q. Zhang, J. Luo and Y. Wei, Green Chem., 2010, 12, 2246; (h) J. Luo and Q. Zhang, Monatsh. Chem., 2011, 142, 923; (i) B. Karmakar and J. Banerji, Tetrahedron Lett., 2011, 52, 4957; (j) C. Cimarelli, D. Fratoni, A. Mazzanti and G. Palmieri, Eur. J. Org. Chem., 2011, 2094.
23. (a) M. Rostami, A. R. Khosropour, V. Mirkhani, I. Mohammadpoor-Baltork, M. Moghadam and S. Tangestaninejad, Synlett, 2011, 1677; (b) M. Rostami, A. R. Khosropour, V. Mirkhani, I. Mohammadpoor-Baltork, M. Moghadam and S. Tangestaninejad, Monatsh. Chem., 2011, 142, 1175; (c) M. Rostami, A. R. Khosropour, V. Mirkhani, M. Moghadam, S. Tangestaninejad and I. Mohammadpoor-Baltork, Appl. Catal., A, 2011, 397, 27; (d) E. Ranjbakhsh, A. K. Bordbar, M. Abbasi, A. R. Khosropour and E. Shams, Chem.–Eng. J., 2012, 179, 272.
24. (a) W. Stöber, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, 26, 62; (b) X. Q. Liu, Z. Y. Ma, J. M. Xing and H. Z. Liu, J. Magn. Magn. Mater., 2004, 270, 1; (c) J. Park, K. J. An, Y. S. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891.
25. A(PF₆)₂ and A(NTf₂)₂ were synthesized via the “ionic liquids ion cross-metathesis” process, see: V. Zgonnik, C. Zedde, Y. Génisson, M.-R. Mazières and J.-C. Plaquevent, Chem. Commun., 2012, 48, 3185.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20187a

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