Novel Multiple-Acidic Ionic Liquids: Catalysts for Environmentally

Article pubs.acs.org/IECR

Novel Multiple-Acidic Ionic Liquids: Catalysts for Environmentally Friendly Benign Synthesis of trans-β-Nitrostyrenes under SolventFree Conditions Anguo Ying,*,† Songlin Xu,‡ Shuo Liu,‡ Yuxiang Ni,‡ Jianguo Yang,† and Chenglin Wu† †

School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 318000, People’s Republic of China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: A series of novel multiple-acidic ionic liquids, sulfated from the low-cost starting material 2-aminoethanol, were introduced as catalysts for the Henry reaction of versatile aldehydes and nitroalkane under solvent-free conditions. The ionic liquid with hydrogen sulfate as a counteranion showed the best catalytic efficiency and gave the corresponding trans-βnitrostyrenes in good to excellent yields. More importantly, the ionic liquid catalyst could be readily recovered and reused six times without considerable loss of its catalytic activity.

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INTRODUCTION

Compared with classical molecular solvents, ionic liquids (ILs) present some interesting properties including low vapor pressure, wide liquid range, high thermal stability, excellent solvation ability, and recyclability. Thus, ILs, especially taskspecific ionic liquids (TSILs), have been very frequently used as green solvents, as well as catalysts or promoters for various organic transformations. 27−36 2-Aminoethanol, which is commercially available and cost-effective, was utilized to prepare functionalized ionic liquids (IL A, IL B, Figure 1). These ILs have shown efficient catalytic activities for organic transformations, including Knoevenagel condensation,37,38

Because of the ready conversion of the nitro group into a variety of diverse functionalities, nitroalkenes are versatile synthetic blocks, for example, to produce phenylalkylamines,1−3 an important ingredients with some interesting bioactivities for medicinal usage. They are also powerful dienophiles in Diels− Alder reactions4 and can react with Grignard reagents,5,6 organoboranes,7 organozinc halides,8 organomanganese compounds,9 primary amines and dialkyl acetylenedicarboxylates,10 and 1,3-dicarbonyl compounds11 to generate structurally diverse products. Moreover, nitrostyrenes themselves exhibit medical properties.12,13 In view of their significant position in both synthetic chemistry and medicinal chemistry, two strategies for the preparation of nitrostyrenes have been developed. The first method is the direct nitration of aromatic olefins with nitric oxide (NO).14 The second and most versatile protocol for the synthesis of nitroolefins involves the Henry condensation of carbonyl compounds with a nitroalkane, followed by antiperiplanar elimination of water from the resulting 2-nitro alcohols. Henry reactions using heterogeneous catalysts such as polyamine-functionalized mesoporous zirconia,15 silica−alumina-supported amine,16 zeolite,17 primary amine immobilized on the silica gel KG-60,18 ammonium acetate under microwave irradiation,19,20 ceric ammonium nitrate (CAN),21 nickel hydroxyapatite,22 and acetic acid23 have been found to be efficient for the production of nitroalkenes. Very recently, several acid−base bifunctional catalysts grafted on mesoporous silica nanoparticles24−26 have also been utilized to obtain nitrostyrenes starting from benzaldehyde dimethyl acetal. However, the use of hazardous and carcinogenic solvents and the relatively lower catalytic activities compared to heterogeneous catalysts in the above methodologies are incompatible with “green chemistry”. In accordance with the principles of green chemistry, the design of catalysts for Henry condensation with ready separation, highly catalytic activity, and low cost is currently being pursued. © 2013 American Chemical Society

Figure 1. Task-specific ILs derived from 2-aminoethanol. Received: Revised: Accepted: Published: 547

October 8, 2013 December 11, 2013 December 11, 2013 December 11, 2013 dx.doi.org/10.1021/ie403372n | Ind. Eng. Chem. Res. 2014, 53, 547−552

Industrial & Engineering Chemistry Research

Article

Scheme 1

Michael addition,39 synthesis of arylalkylidene rhodanines,40 and preparation of dihydropyrano[3,2-c]chromene derivatives. 41 Encouraged by these exciting results and in continuation of our efforts to design novel functional ILs for application to organic synthesis,42−47 we report herein the development of a series of novel multiacidic ionic liquids (Figure 1) and their use as catalysts for the Henry reactions of various aromatic aldehydes and nitroalkanes to give βnitrostyrenes in high yields.

recrystallization in ethanol to give the desired product. The ionic liquid [SFHEA][HSO4] after extraction was dried in vacuo at 60 °C for 5 h. The recovered ionic liquid was then reused in subsequent reactions. The products were characterized by 1H NMR and mass spectroscopies and elemental analysis.

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RESULTS AND DISCUSSION Novel functionalized acidic ILs bearing two sulfonic acid groups were prepared by the reaction of 2-aminoethanol and chlorosulfonic acid, followed by treatment with different strong acids (Scheme 1). Considering the crucial role of the acidity of ILs in organic reactions, the Brønsted acidic scale of the four sulfonic functional ILs was measured on a Shimadzu model UV2401-PC spectrometer with 4-nitroaniline as the indicator.48,49 [IH]s and [I]s are the mole concentrations of the protonated and unprotonated forms, respectively, of the indicator 4-nitroaniline in dichloromethane (DCM). As shown in Figure 2, the maximum absorbance of the

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EXPERIMENTAL SECTION General. All chemicals were purchased from commercial sources and used without further purification. 1H and 13C NMR spectra were recorded on a Bruker Avance DPX 400 spectrometer at 400 and 100 MHz in CDCl3 and D2O, respectively. Chemical shifts are reported in parts per million (δ), relative to the internal standard of tetramethylsilane (TMS). UV−vis acidity evaluation was conducted on a Shimadzu UV2401-PC spectrometer at room temperature. Mass spectra were obtained with an automated Fininigan TSQ Advantage mass spectrometer. Elemental analysis was carried out on a Carlo Erba 1160 instrument. All reactions were monitored by thin layer chromatography (TLC) and were performed on 0.25-mm silica gel 60-F plates. A mixture of petroleum ether and ethyl acetate was used as the developing solvent. Flash chromatography separation was performed on silica gel (100−200 mesh). All condensation products were purified through column chromatography or recrystallization in ethanol and were characterized by NMR, mass, and elemental analyses. Synthesis of SO3H-Functionalized ILs. 2-Aminoethanol (0.1 mol) and dichloromethane (DCM; 30 mL) were charged into a 100 mL three-neck flask cooled in an ice bath. Chlorosulfonic acid (0.2 mol) was then added dropwise at a temperature of ≤5 °C with intensive mixing. After dropwise addition, the reaction mixture was removed from the ice bath and stirred at room temperature for 5 h. After filtration, the resulting zwitterion was dissolved in 50 mL of water, and different acids were added dropwise with vigorous stirring. The reaction mixture was heated at 60 °C for an additional 5 h. The reaction solution was subjected to vacuum distillation to remove water and was then dried in vacuo at 60 °C for another 8 h to afford the desired ILs as light yellow viscous liquids. General Procedure for Henry Reaction of Aldehydes and Nitroalkanes. To a mixture of aldehydes (1 mmol) and nitroalkane (1 mmol) in a 10 mL flask equipped with a magnetic stirrer was added the ionic liquid [SFHEA][HSO4] (0.2 mmol). The reaction mixture was stirred at 110 °C for the desired time until the disappearance of starting material as monitored by TLC. Upon completion of the reaction, the mixture was extracted with ethyl acetate several times. The combined organic phase was concentrated through vacuum evaporation, and the resulting crude product was purified by

Figure 2. Absorption spectra of 4-nitroaniline in four ionic liquids in DCM: (a) blank, (b) [SFHEA][NO3], (c) [SFHEA][CF3COO], (d) [SFHEA][CH3SO3], and (e) [SFHEA][HSO4].

unprotonated form of the indicator was observed at 348 nm in DCM, which decreased to some extent after addition of acidic ILs. Thus, the calculation of [I]s/[IH+]s could be realized by measuring the change in absorbance between the blank curve (no IL addition) and each curve of the acidic ILs. The acidity was determined through the calculation of the Hammett function, H0, using the equation H0 = pK (I)aq + log([I]s /[IH+]s )

(1)

where pK(I)aq is the pKa value of 4-nitroaniline, namely, 0.99 (Table 1). As is known, the lower the H0 value, the stronger the 548

dx.doi.org/10.1021/ie403372n | Ind. Eng. Chem. Res. 2014, 53, 547−552


Article

in a significant decrease in the reaction yield, whereas an increase in the temperature to 130 °C was not beneficial to the reaction efficiency (Table 2, entries 11−13). The reaction conditions of 20 mol % [SFHEA][HSO4] as the catalyst at 110 °C without any organic solvent were subjected to further examination. Notably, the reaction in the absence of any ionic liquid could not proceed at all, indicating the specific role of the multiacidic IL [SFHEA][HSO4] in the model (Table 2, entry 2). With the optimal conditions in hand, the different aldehydes were treated with nitromethane in the presence of a catalytic amount (20 mol %) of multisulfonic-functionalized [SFHEA][HSO4] for the Henry reaction, and the results are presented in Table 3. The reaction proceeded smoothly with functionality-

Table 1. Calculation of the Hammett Function for Various Acidic ILs in DCM at Room Temperature ionic liquid blank [SFHEA][HSO4] [SFHEA][NO3] [SFHEA] [CF3COO] [SFHEA] [CH3SO3]

absorbance

[I]s (%)

[IH+]s (%)

Hammett function, H0

1.751 0.596 0.881 0.816

100.0 34.0 50.3 46.6

0 66.0 49.7 53.4

− 0.70 1.00 0.93

0.753

43.0

57.0

0.87

acidity. Different anions lead to different absorbances, indicating the dependence of the acidity of the ILs on the nature of the counteranion. The acidity order was as follows: [SFHEA][HSO 4 ] > [SFHEA][CH 3 SO 3 ] > [SFHEA][CF3COO] > [SFHEA][NO3]. Initially, the reaction of benzaldehyde and nitromethane was selected as a model to optimize the reaction conditions. As shown in Table 2, two ionic liquids, [BmIm]BF4 and

Table 3. Comparison of the Present Catalytic System with Other Reported Protocols in the Model Reaction between 4Nitrobenzaldehyde and Ethyl Nitromethane entry

reaction conditions

1

Al−MCM−NH2−E catalyst, 90 °C, 30 min MSN−NNH2−SO3H catalyst, 90 °C, 6 h ETAM/SiO2, 85 °C, 24 h ultrasonic irradiation, 60 °C, 45 min [SFHEA][HSO4] catalyst, 110 °C, 1.5 h, neat

Table 2. Henry Condensation of Benzaldehyde (5 mmol) and Nitromethane (6 mmol) under Different Reaction Conditions

2 3 4 5 a

entry

IL (mol %)

temperature (°C)

1 2 3c 4 5 6 7 8 9 10 11 12 13

blank [BmIm]BF4 (50) [BmIM]PF6 (50) [MMIm][MSO4] (20) [SFHEA][NO3] (30) [SFHEA][CF3COO] (30) [SFHEA][CH3SO3] (30) [SFHEA][HSO4] (30) [SFHEA][HSO4] (5) [SFHEA][HSO4] (10) [SFHEA][HSO4] (20) [SFHEA][HSO4] (20) [SFHEA][HSO4] (20)

120 120 120 120 110 110 110 110 110 110 110 80 130

time (h)

yield (%)a

24 12 12 12 3 2 3 2 5 2 2 3 2

NRb NR NR 63 84 81 86 91 68 83 92 59 90

yield (%)

ref

58.2

26

38 (68:32)a 56 78 (43:35)b 89

24 51 19 this work

Nitroalcohol/nitroalkene ratio. bCis/trans ratio.

substituted aromatic aldehydes to afford the corresponding βnitrostyrenes in good to excellent yields (83−98%) within reaction times ranging from 1.5 to 5.0 h (Supporting Information, Table 1, entries 1−14). Among these cases, aromatic aldehydes bearing electron-withdrawing groups (NO2, F, CF3, Cl) reacted slightly more rapidly than those with electron-donating substituents (OMe, Me, OH, NMe2), which could be attributed to the increase in electrophilicity of the carbonyl carbon at the aromatic ring caused by withdrawing groups. Polycyclic aromatic aldehyde and heteroaryl aldehydes were also efficient substrates for the Henry reaction to give the desired products in the yields of 80−96% (Supporting Information, Table 1, entries 15−18). It was found that the reaction efficiency, in terms of reaction rate and yield, decreased when nitroethane instead of nitromethane was reacted with the aldehydes (Supporting Information, Table 1, entries 19−22). When the length of the alkyl chain of the nitroalkane was further extended as in 1-nitropropane, no product was formed at all, probably because of its much greater steric hindrance compared to that of nitromethane (Table 2, entry 11, and Supporting Information, Table 1, entries 19 and 23). It is noteworthy that all products obtained had an exclusively E geometry and no subsequent Michael adduct was detected even when excess nitromethane was added (Scheme 2). A plausible mechanism for the formation of trans-βnitrostyrenes catalyzed by triple acidic ionic liquid [SFHEA][HSO4] is depicted in Figure 3. We propose that [SFHEA][HSO4] catalyst might play a key role in promoting the Henry reaction of aromatic aldehydes and nitromethane through hydrogen-bonding interactions. The triple hydrogen-bonding interactions not only increase the electrophilicity of the aldehydes but also make the methyl group in nitromethane more active, which can facilitate the formation of 2-nitro-1-aryl alcohol A. Subsequently, a proton is transferred from catalyst to

a Isolated yield based on benzaldehyde. bNR: no reaction. c[BmIM]PF6: 1,3-dimethylimidazolium methyl sulfate.

[BmIM]PF6, usually utilized as reaction media, were found to be ineffective for the reaction. No desired product was formed even after the reaction time was prolonged to 12 h (Table 2, entries 2 and 3). The acidic ionic liquid [MMIm][MSO4]50 exhibited a slight catalytic activity to give the product in a moderate yield of 63% (Table 2, entry 4). To our pleasure, the model reaction proceeded smoothly in the presence of the novel acidic ILs [SFHEA][NO 3 ], [SFHEA][CH 3 SO 3 ], [SFHEA][CF3COO], and [SFHEA][HSO4], leading to 81− 91% yields of nitroolefin (Table 2, entries 5−8). Among the four ILs tested, [SFHEA][HSO4] was most effective, and an excellent yield of 91% was obtained, in accordance with the acidity order of these 2-aminoethanol-derived ILs. To optimize the amount of IL required, the reaction was carried out in the presence of 5, 10, 20, and 30 mol % [SFHEA][HSO4] under solvent-free conditions at 110 °C (Table 2, entries 8−11). The best catalyst loading was found to be 20 mol %. In addition, a further decrease of the reaction temperature to 80 °C resulted 549



Article

Scheme 2

Figure 3. Proposed mechanism for the Henry reaction catalyzed by [SFHEA][HSO4].

as the catalyst, and 56% yield of nitroalkene was obtained, even after the reaction time was prolonged to 24 h (Table 3, entry 3). The model reaction was also conducted under ultrasonic irradiation in the absence of catalyst. Although the reaction yield reached 78%, some cis byproduct appeared, which significantly limited the reactive selectivity (Table 3, entry 4). All of the results demonstrate that the present catalytic system is very efficient, green, and economical for the preparation of exclusively E-geometry nitroalkene (Table 3, entry 5). To demonstrate the industrial applicability of this methodology, the solvent-free Henry reaction of nitromethane and pmethoxybenzaldehyde was carried out on a larger scale (100 mmol). The reaction was completed in 4 h, affording the corresponding nitroolefin in excellent yield of 96%. On the same scale, the recyclability of the catalytic system was investigated using the same reaction as model reaction. Upon completion of the reaction, the product was isolated by several cycles of extraction with ethyl acetate, and the residue ionic liquid was dried to remove water at 60 °C under a vacuum. The combined solvent was evaporated by vacuum distillation to give the crude product, which was recrystallized in ethanol to give the nitrostyrenes in high purity. The recovered ionic liquid was

A to form the intermediate cation B, which tends to eliminate a molecule of water to give the final nitroalkene products. The mechanism can also be verified by experiment regarding the reaction of aldehyde and nitromethane conducted at 60 °C. A large amount of nitroalcohol A was observed from TLC plate, which directly confirms the rationality of the reaction pathway with 2-nitro-1-aryl alcohol A as a key intermediate. Reaction selectivity is an important parameter in evaluating a protocol. An excellent trans/cis selectivity (no cis byproduct detected) provided by the present methodology can be attributed to two factors as follows: The intermediate B tends to proceed with trans elimination in the presence of acidic catalyst.23 Second, much more thermodynamic and space stability of transnitrostyrene could significantly decrease the reaction rate of the cis byproduct. For the purpose of comparison with other methodologies in terms of catalytic efficiency, we carried out the reaction of the slugguish substrate 4-nitrobenzaldehyde with nitromethane. As shown in Table 3, mesoporous-material-supported catalysts with coexisting acidic and basic sites (Al−MCM−NH2−E, MSN−NNH2−SO3H) gave only moderate yields (Table 3, entries 1−2). Then, ethanolamine grafted on silica was selected 550



Article

(3) Fioravanti, S.; Pellacani, L.; Stabile, S.; Tardella, P. A.; Ballini, R. Solvent-free aziridination of α-nitroalkenes. Tetrahedron 1998, 54, 6169. (4) Fuji, K.; Node, M.; Nagasawa, H.; Nanima, Y.; Terada, S. Asymmetric Induction via Addition−Elimination Process: Nitroolefination of α-Substituted Lactones. J. Am. Chem. Soc. 1986, 108, 3855. (5) Yao, C.-F.; Chen, W.-W.; Lin, Y.-M. Reactions of β-nitrostyrenes with Grignard reagents. Tetrahedron Lett. 1996, 37, 6339. (6) Liu, J.-T.; Lin, W.-W.; Jang, J.-J.; Liu, J.-Y.; Yan, M.-C.; Hung, C.; Kao, K.-H.; Wang, Y.; Yao, C.-F. One-pot synthesis of five- or sixmembered carbocycles through intramolecular cycloadditions by the use of ethyl chloroformate. Tetrahedron 1999, 55, 7115. (7) Yao, C.-F.; Chu, C.-M.; Liu, J.-T. Free-Radical Reactions of Trialkylboranes with β-Nitrostyrenes To Generate Alkenes. J. Org. Chem. 1998, 63, 719. (8) Seebach, D.; Schafer, H.; Schmidt, B.; Schreiber, M. CC Coupling with NO2/Alkyl Substitution at the Vinylic Carbon by Reaction of 2-Aryl-l-nitro-1-alkenes with Dialkylzinc CompoundsA Novel Reaction. Angew. Chem., Int. Ed. Engl. 1992, 31, 1587. (9) Namboothiri, I. N. N.; Hassner, A. Additions of organomanganese reagents to conjugated nitroolefins. J. Organomet. Chem. 1996, 518, 69. (10) Ghabraie, E.; Balalaie, S.; Bararjanian, M.; Bijanzadeh, H. R.; Rominger, F. An efficient one-pot synthesis of tetra-substituted pyrroles. Tetrahedron 2011, 67, 5415. (11) Narayanaperumal, S.; da Silva, R. C.; Feu, K. S.; de la Torre, A. F.; Corrêa, A. G.; Paixão, W. Basic-functionalized recyclable ionic liquid catalyst: A solvent-free approach for Michael addition of 1,3dicarbonyl compounds to nitroalkenes under ultrasound irradiation. Ultrason. Sonochem. 2013, 20, 793. (12) Milhazes, N.; Calheiros, R.; Marques, M. P. M.; Garrido, J.; Cordeiro, M.; Rodrigues, C.; Quinteira, S.; Novais, C.; Peixe, L. βNitrostyrene derivatives as potential antibacterial agents: A structure− property−activity relationship study. Biorg. Med. Chem. 2006, 14, 4078. (13) Wang, W. Y.; Hsieh, P. W.; Wu, Y. C.; Wu, C. C. Synthesis and pharmacological evaluation of novel β-nitrostyrene derivatives as tyrosine kinase inhibitors with potent antiplatelet activity. Biochem. Pharmacol. 2007, 74, 601. (14) Jovel, I.; Prateeptongkum, S.; Jackstell, R.; Vogl, N.; Weckbecker, C.; Beller, M. A Selective and Practical Synthesis of Nitroolefins. Adv. Synth. Catal. 2008, 350, 2493. (15) Rana, S.; Mallick, S.; Parida, K. M. Facile Method for Synthesis of Polyamine-Functionalized Mesoporous Zirconia and Its Catalytic Evaluation toward Henry Reaction. Ind. Eng. Chem. Res. 2011, 50, 2055. (16) Motokura, K.; Teda, M.; Iwasawa, Y. Heterogeneous organic base-catalyzed reactions enhanced by acid supports. J. Am. Chem. Soc. 2007, 129, 9540. (17) Ballini, R.; Bigi, F.; Gogni, E.; Maggi, R.; Sartoli, G. Zeolite as Base Catalyst: Nitroaldolic Condensation. J. Catal. 2000, 191, 348. (18) Soldi, L.; Ferstl, W.; Loebbecke, S.; Maggi, R.; Malmassari, C.; Sartori, G.; Yada, S. Use of immobilized organic base catalysts for continuous-flow fine chemical synthesis. J. Catal. 2008, 258, 289. (19) Varma, R. S.; Dahiya, R.; Kumar, S. Microwave-assisted Henry reaction: Solventless synthesis of conjugated nitroalkenes. Tetrahedron Lett. 1997, 38, 5131. (20) Rodríguez, J. M.; Pujol, M. D. Straightforward synthesis of nitroolefins by microwave- or ultrasound-assisted Henry reaction. Tetrahedron Lett. 2011, 52, 2629. (21) Rao, A. S.; Srinivas, P. V.; Babu, S. K.; Rao, M. An efficient synthesis of conjugated nitro-olefins using ceric ammonium nitrate. Tetrahedron Lett. 2005, 46, 8141. (22) Neelakandeswari, N.; Sangami, G.; Emayavaramban, P.; Karvembu, R.; Dharmaraj, N.; Kim, H. Y. Mesoporous nickel hydroxyapatite nanocomposite for microwave-assisted Henry reaction. Tetrahedron Lett. 2012, 53, 2980.

reused in subsequent reactions. As shown in Figure 4, the acidic functional IL [SFHEA][HSO4] could be recycled six times

Figure 4. Recyclability investigation of [SFHEA][HSO4] used as a catalyst for the Henry reaction of nitromethane and p-methoxybenzaldehyde.

without significant loss of catalytic activity, and the reused ionic liuid remained intact (1H NMR spectroscopy).

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CONCLUSIONS In conclusion, we have developed a type of novel multipleacidic ILs, namely, [SFHEA][NO3], [SFHEA][CH3SO3], [SFHEA][CF3COO], and [SFHEA][HSO4]. Among these functional ILs, [SFHEA][HSO4] showed the best catalytic activity for the Henry reactions of various aldehydes and different nitroalkanes to afford the desired trans-β-nitrostyrenes in good to excellent yields. The good catalytic potency and excellent recyclability of [SFHEA][HSO4] make this protocol more useful for preparation of nitroolefin in large scale over reported methodologies. In view of the ready procedure for preparation and low cost of the 2-aminoethanol-based ILs, investigations of their further application for other organic transformations are underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Table of Henry reactions of various aldehydes and nitroalkanes, analysis data of four novel ILs, 1H and 13C NMR spectra of all four novel acidic ionic liquids, and 1H NMR spectra of all condensation products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: 86-576-88660359. E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support for this research from the National Natural Science Foundation of China (Grant 21106090), Foundation of Low Carbon Fatty Amine Engineering Research Center of Zhejiang Province (2012E10033), and Zhejiang Provincial Natural Science Foundation of China (No. LY12B02004).

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Novel Multiple-Acidic Ionic Liquids: Catalysts for Environmentally

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