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Catalysis Communications 9 (2008) 337–341 www.elsevier.com/locate/catcom
Brønsted acidic ionic liquids as novel catalysts for Prins reaction Wenjuan Wang, Lili Shao, Wenping Cheng, Jianguo Yang *, Mingyuan He Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, Shanghai 200062, China Received 16 May 2007; received in revised form 1 July 2007; accepted 2 July 2007 Available online 10 July 2007
Abstract Prins reaction, used to prepare dioxanes, has been limited by complex catalyst separation and reusability. In this article, six waterstable Brønsted acidic task-speciﬁc ionic liquids ([HMIM]BF4,[(CH2)4SO3HMIM][HSO4], [(Ac)2BIM]Br, [NMP][HSO4], [BMIM][HSO4] and [BMIM][H2PO4] were synthesized and used as environmentally benign catalysts for Prins reaction under mild reaction conditions for the ﬁrst time. The process is highly eﬀective and environmentally benign. Furthermore, [BMIM][HSO4] was conveniently separated with the products and easily recycled to catalyze Prins reaction again with excellent yields. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Brønsted acidic ionic liquid; Prins reaction; Catalyst
1. Introduction In recent years, ionic liquids (ILs) have attracted increasing interest and been successfully used in variety of catalytic reactions as environmentally benign solvents and catalysts due to their relatively low viscosities, low vapor pressure, and high thermal and chemical stability, etc. [1,2]. Much attention has currently been focused on the organic reactions with ILs as catalysts or solvents and many organic reactions were performed in ILs with high performances [3–5]. Recently, the synthesis of ‘‘task-speciﬁc’’ ILs with special functions according to the requirement of a speciﬁc reaction has become an attractive ﬁeld . All these studies oﬀered us the possibility of designing suitable catalysts for the appointed reaction. To the best of our knowledge, the synthesis of 1,3-dioxanes as one of the well-known method through Prins reaction using Brønsted acidic ILs as catalyst has not been reported. The Prins reaction involving the acid-catalyzed reaction of oleﬁns with aldehydes is an important carbon–carbon bond forming reaction and usually leads to the formation *
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1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.07.006
of dioxanes as common major products . It had received increasing attention as an important carbon–carbon bond forming reaction in organic synthesis since its ﬁrst discovery over one century age. It is one of the most eﬃcient method to make various oxaocyclic or carbocyclic moieties due to its major products of classical Prins reaction are normally 1,3-dioxanes, unsaturated alcohols and so on. The conventional approaches of Prins reaction using strong protonic acid catalysts, e.g. sulphuric acid, hydrochloric acid, or p-toluenesulphonic acid. However, the application of these homogeneous catalysts is limited due to their toxicity and corrosivity. Moreover, a major disadvantage is that the use of these homogeneous catalysts results in the tedious work-up procedure and the requirement for neutralization of the catalysts with strong acids produces undesired wastes. For economic and ecological reasons, there is a need to develop an environmentally benign method for the synthesis of the usable 1,3-dioxanes. In this article, Brønsted acidic ILs (Scheme 1) were synthesized and used to catalyze Prins reaction of styrene with formaldehyde under reﬂux condition (Scheme 2). The reaction proceeded smoothly and therefore the puriﬁcation of products was fairly simple. Furthermore, [BMIM][HSO4] was conveniently separated from the products and easily
W. Wang et al. / Catalysis Communications 9 (2008) 337–341 HOOC
N H 2 PO4
N + N
2 CH2O O O
recycled for another round of reaction. This will signiﬁcantly reduce the eﬀect of the reaction on environment and thus pave a way for the large scale applications. 2. Experimental 2.1. Preparation of ILs For the present study, we prepared a series of Brønsted acidic ionic liquids. The obtained ionic liquids were used as catalysts for synthesis of dioxanes. The ionic liquids [HMIM]BF4, [(CH2)4SO3HMIM][HSO4], [BMIM][HSO4] and [BMIM][H2PO4] were synthesized refer to the procedures reported in previous literature [8,9,11,12]. These previous studies oﬀered us the possibility of designing suitable catalysts for this reaction. (i) Preparation of [HMIM]BF4: 1-Methylimidazole (0.2 mol) was charged into a 250 mL three-neck ﬂask. Under vigorous magnetic stirring, an aqueous solution of 40 wt% HBF4 (0.2 mol) was added dropwise at 0 °C for 1 h. The mixture was continuously stirred at rt for 2 h. The mixture was distilled under vacuum for 2 h to remove redundant acetic acid, and then cooled to room temperature to get the white solid [HMIM]BF4 . (ii) Preparation of [(CH2)4SO3HMIM][HSO4]: A mixture of 1-Methylimidazole (0.2 mol) and 1, 4-butane sultone (0.2 mol) was charged into a 150 mL conical ﬂask. The mixture was stirred at room temperature for 4 days until it turned into solid. Thus, the white solid zwitterion formed was washed repeatedly with ether, ﬁltrated to remove non-ionic residues and dried in vacuum. Then, to a stoichiometric amount of con-
centrated sulfuric acid (98%, 10.9 mL) in a 150 mL conical ﬂask was added dropwise the white solid zwitterion slowly at 0 °C. The mixture was stirred at 80 °C for 6 h. Product was washed with diethyl ether and dried in vacuo at 50 °C for 2 h to get the viscous clear [(CH2)4SO3HMIM][HSO4]. (iii) Preparation of [(Ac)2BIM]Br: A mixture of imidazole (0.22 mol), acetone (40 mL) and dimethylmaleate (0.22 mol) was charged into a 250 mL three-necked ﬂask equipped with a condenser tube. The reaction was performed for 24 h under reﬂux condition. The mixture was distillated by a rotary evaporator at 80 °C for 2 h in vacuum to remove the volatile materials and obtain 1-(double-2-methyl butyrate) imidazole. Then, A mixture of 1-bromobutane (0.26 mol), certain amount of methanol and 1-(double-2-methyl butyrate) imidazole was charged into the threenecked ﬂask at room temperature and stirred at 80 °C for 12 h. Then distillated to remove methanol to obtain 1-(double-2-methyl butyrate)-3-butyl imidazole bromide . Moreover, suﬃcient amount of 36% HCl was added and the mixture stirred at 100 °C for 2 h. The hydrolyzed mixture was distillated by a rotary evaporator in vacuum over 2 h to remove the residues of HCl. The product was cooled to room temperature to get the viscous clear [(Ac)2BIM]Br . (iv) Preparation of [NMP][HSO4]: 1-Methyl-2-pyrodidone (0.2 mol) was charged into a 250 mL threenecked ﬂask with magnetic stirrer. Then equimolar concentrated sulphuric acid (98 wt %) was added dropwise slowly into the ﬂask at 80 °C for 12 h. The mixture was washed with ether three times to remove non-ionic residues and dried in vacuum by a rotary evaporator to obtain the viscous clear [NMP][HSO4]. (v) Preparation of [BMIM][HSO4] and [BMIM][H2PO4]: The novel [BMIM][HSO4] and [BMIM][H2PO4] derived from imidazole chloride salts (Scheme 1) were obtained by a dropwise addition of one equivalent of concentrated sulphuric acid (98%) or o-phosphoric acid (85%) to solution of the corresponding 1-butyl-
W. Wang et al. / Catalysis Communications 9 (2008) 337–341
3-methylimidazolium chloride in anhydrous methylene chloride . The reaction proceed at room temperature for 24 h with vigorous stirring under a stream of dry nitrogen. Then, the mixture was dried in vacuum by a rotary evaporator to remove the HCl and solvent to obtain the viscous clear [BMIM][HSO4] and [BMIM][H2PO4]. As the functional Brønsted acidic ILs, they are all stable under an inert atmosphere.
Qualitative and quantitative analysis was conducted with GC–MS (Agilent 6890 Series/5973N) and GC (Agilent 6850), respectively. The concentration of reactant and product was directly given by the system of chemstation according to the area of chromatograph peak. Conversion and selectivity were calculated according to the concentration of reactant and product. 3. Results and discussion 3.1. Eﬀect of diﬀerent ils
2.2. Selected spectral data [HMIM]BF4: 1H NMR (500 M Hz, DMSO-d6,TMS): d 3.898 (s, 3H), 7.688 (s, H), 7.712 (s, H), 9.050 (s, H). [(CH2)4SO3HMIM] [HSO4]: 1H NMR (500 M Hz, DMSO-d6, TMS): d 1.569 (t, 2H, J = 7.0Hz), 1.912 (t, 2H, J = 7.0 Hz), 2.538 (t, 2H, J = 1.5 Hz), 3.888 (s, 3H), 4.218 (t, 2H, J = 7.0 Hz), 7.742 (s, 1H), 7.810 (s, 1H), 9.166 (s, 1H). [(Ac)2BIM]Br: 1H NMR (500 MHz, DMSO-d6, TMS) : d 0.894 (t, 3H, J = 3.5 Hz), 1.241 (m, 2H, J = 7 Hz), 1.799 (t,2H, J = 7.5 Hz), 3.281 (d, 2H, J = 5.0 Hz), 4.238 (t, 2H, J = 7.0 Hz), 5.662 (d, 1H,J = 2.0 Hz), 7.708 (d, 1H, J = 2.0 Hz), 7.817 (t ,1H, J = 1.0 Hz), 9.364 (s, 1H), 12.854 (bs, 1H). [NMP][HSO4]: 1H NMR (500 M Hz, DMSO-d6, TMS): d 1.901 (t, 2H, J = 6.1 Hz), 2.331 (t, 2H, J = 7.4 Hz), 2.698 (s, 3H), 3.308 (t, 2H, J = 7.1 Hz), 8.263 (bs, 1H). [BMIM][HSO4]: 1H NMR (500 M Hz, DMSO-d6,TMS): d 0.941 (t, 3H,J = 7.5 Hz), 1.264 (m, 2H, J = 7.1 Hz), 1.766 (m, 2H, J = 7.4 Hz), 2.52 (s, H), 3.876 (s, 3H), 4.192 (t, 3H, J = 7.0 Hz), 7.754 (s, H), 7.824 (s, H), 9.304 (s,H). 1H NMR data of [BMIM][H2PO4] and [BMIM][HSO4] is similar. 2.3. General procedure of Prins reaction In a typical reaction, styrene, formaldehyde and Brønsted acidic IL as catalyst were added successively into a 100 mL round ﬂask equipped with a thermometer, a magnetic stirrer, and a N2-inlet valve without any additional organic solvents. Formaldehyde was added dropwise into the ﬂask for 2 h. The ratio of formaldehyde and styrene was 2.0–3.0 mol%. The concentration of IL in the reaction mixture was 5.0–20.0 mol%. The reaction proceeded under reﬂux (94–96 °C) for 2–10 h. After the reaction, the mixture was placed for a while for the formation of two phases, the upper organic phase and the lower ionic catalyst phase. Based on the basic properties of our catalysts and products, these acidic ILs are insoluble in the upper organic products, but highly soluble in water, the resulted product could be easily separated with catalysts and the catalysts could be easily reused after vacuum distillation at 80 °C for 1 h to remove redundant mixed solvent. IL could be recovered easily and directly reused in subsequent runs.
Our eﬀects were focused on investigating the Prins reaction of styrene and formaldehyde dioxanes using these Brønsted acidic ILs as the new catalyst in a batch-type process. The results are summarized in Table 1. As shown in Table 1, acidity and the structure of ILs, molar ratio of IL/substrate, reaction time all have signiﬁcant eﬀects on the reaction. It could be seen that with the increase of the molar ratio of IL/substrate from 5 % to 10 %, reaction yield was signiﬁcantly improved (Table 1). Experiments show that the catalytic performance of [(CH2)4SO3HMIM][HSO4], [(Ac)2BIM]Br, [NMP][HSO4], [BMIM][HSO4], [BMIM][H2PO4] could be much better than [HMIM]BF4 under the same reaction conditions. Poor results (entries 1,2, Table 1) were observed using [HMIM]BF4 as catalyst due to its weak acidity. It should be noted that the reaction rate is dependent upon the IL chosen and in this reaction, [BMIM][H2PO4] gives lower rate enhancement (entries 15–17, Table 1) than the [BMIM][HSO4] and [NMP][HSO4] as catalysts (entries 9–14, Table 1). Probably, this is due to the lower Brønsted
Table 1 Result of Prins reaction of styrene and formaldehyde in diﬀerent Brønsted acidic ILs as catalyst Entry Ionic liquid (IL)
Molar ratio of Conversion Selectivity IL/substrate (%) (%) (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
10 20 5 8 10 5 8 10 5 8 10 5 8 10 5 8 10
[HMIM]BF4 [HMIM]BF4 [(CH2)4SO3HMIM][HSO4] [(CH2)4SO3HMIM][HSO4] [(CH2)4SO3HMIM][HSO4] [(Ac)2BIM]Br [(Ac)2BIM]Br [(Ac)2BIM]Br [NMP][HSO4] [NMP][HSO4] [NMP][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][HSO4] [BMIM][H2PO4] [BMIM][H2PO4] [BMIM][H2PO4]
9.7 17.4 52.2 68.5 81.9 56.5 62.1 70.6 91.2 92.8 95.5 91.3 92.0 95.8 65.4 76.5 94.3
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
All reactions were run using 0.05 mol styrene, 0.1 mol formaldehyde. Reaction time: 10 h. Reaction proceeded under reﬂux condition.
W. Wang et al. / Catalysis Communications 9 (2008) 337–341
Table 2 Results of Prins reaction of styrene and formaldehyde [BMIM][HSO4] as catalyst under diﬀerent reaction time
Reaction time (h)
1 2 3 4 5
4 6 8 10 12
48.4 64.6 78.7 91.3 91.4
100 100 100 100 100
80 70 60
All reactions were run using 0.05 mol styrene, 0.1 mol formaldehyde. Molar ratio of [BMIM][HSO4]/substrate: 5%. Reaction proceeded under reﬂux condition.
acidity of dihydrogen phosphate counteranion. Furthermore, it can be seen that the conversion for reaction with [BMIM][HSO4] and [NMP][HSO4] as catalysts were better than [(CH2)4SO3HMIM] [HSO4] and [(Ac)2BIM]Br. These results suggest that the performance of these ILs is dependent upon the character of the side chain of the cation and the N-heterocyclic ring. These preliminary experimental results showed that the change of Brønsted acid counteranion would increase the reaction rate, and the change of acid group introduced into the imidazolium cation would inﬂuence the acidic activity, polarity and the solvent properties of the ILs. In this method, Brønsted acidic ILs play an important role in signiﬁcantly enhancing the reactivity as well as reducing formation of byproducts. The use of these new novel acidic ILs in this reaction can be considered as an interesting new alternation. 3.2. Eﬀect of diﬀerent reaction conditions using [BMIM][HSO4] as catalyst [BMIM][HSO4] exhibited the most excellent catalytic performance with high conversion under the same reaction conditions (entries 12–14, Table 1). Therefore, we chose [BMIM][HSO4] as the high eﬀective catalyst to investigate further the eﬀect of molar ratio of formaldehyde and styrene, reaction time and possibility of reusability on this reaction. Eﬀect of reaction time was ﬁrst examined. As shown in Table 2, increasing reaction time accelerated the reaction dramatically. However, along with prolong of the reaction
Table 3 Results of Prins reaction of diﬀerent molar ratio of formaldehyde/styrene using [BMIM][HSO4] as catalyst Entry
Molar ratio of formaldehyde/styrene
Reaction time (h)
1 2 3
2/1 2.5/1 3/1
10 8 8
91.3 93.6 93.6
100 100 100
All reactions were run using 0.05 mol styrene, 0.125 mol formaldehyde. Reaction proceeded under reﬂux condition. Molar ratio of [BMIM][HSO4]/substrate: 5%.
Run number Fig. 1. aAll reactions were run using 50 mol styrene, 125 mol formaldehyde; bMolar ratio of [BMIM][HSO4]/substrate: 5%; cReaction proceeded under reﬂux condition; dReaction time:10 h; eGC yields refer to the recovered crude reaction mixture without further puriﬁcation.
time, reaction rate become lower gradually and the whole reaction tend to equilibrium. This reaction nearly tend to equilibrium after 10 h. So, the conversion no longer has obvious increase when reaction time exceed 10 h (entries 4,5, Table 2). In this work, the eﬀect of diﬀerent molar ratio of formaldehyde/styrene on this reaction was examined due to formaldehyde is a volatile matter. Excess amount of formaldehyde was used. The results are shown in Table 3. It could be seen that with the increase of the molar ratio of formaldehyde/styrene from 2:1 to 2.5:1, the conversion has slightly improved (entries 1,2, Table 3). However, when the molar ratio of formaldehyde/styrene increased from 2.5:1 to 3:1, the conversion had no obvious increase (entries 2,3, Table 3). 3.3. Reusability of [BMIM][HSO4] as catalyst As one of the most eﬀective functional ILs, [BMIM] [HSO4] was selected to investigate the possibility of reusability (Fig. 1). After reaction, the mixture was cooled to room temperature for the formation of two phases. The upper organic phase was decanted and monitored by GC and the lower ionic catalyst phase was distillated by a rotary evaporator at 80 °C for 1 h under vacuum to remove redundant mixed solvent. [BMIM][HSO4] could be recovered easily and directly reused in subsequent runs. As it shown in Fig. 1, no obvious change was observed on the recovered catalytic and excellent yield was obtained. After ﬁve reusability, [BMIM][HSO4] still stable enough and has less impact on the catalytic activity. This indicated that [BMIM][HSO4] was high eﬃcient and recyclable catalyst for Prins reaction. 4. Conclusion In summary, six Brønsted acidic ILs were successfully prepared and utilized as eﬀective catalysts for Prins reaction under smooth conditions. The product was easily
W. Wang et al. / Catalysis Communications 9 (2008) 337–341
separated with high yields. Catalysts were readily recycled and reused to produce almost identical results. No organic solvent was used, resulting in eco-friendly process. The use of these new novel ILs in this reaction provides a better and practical alternative to the existing procedures and provides great promise toward further useful applications. This process will pave a way for large scale applications of Prins reaction. Further applications for other reaction systems are currently under investigation. Acknowledgements We gratefully appreciate the ﬁnancial support from the National Key Technology R&D Program (No. 2006BAE 03B06), and National Nature Science Foundation of China (No. 20590366).
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