A new approach to the synthesis of diacetals (diketals) pentaerythritol catalyzed by SO3H-functionalized ionic liquids

Article abstract of DOI:10.1016/j.cclet.2009.12.014

In this article, an efficient, simple and environmentally friendly approach to the synthesis of diacetals (diketals) pentaerythritol using SO₃H-functionalized ionic liquids (ILs) as catalysts was reported. The ILs show high catalytic activity and reusability with good to excellent yields of the desired products. Hammett method has been used to determine the acidity order of these ionic liquids and the results are consistent with the catalytic activities observed in acetalization reaction. Maximum product yield of 93% was observed on using [PSPy][OTf] as catalyst and it can be reused at least 8 times without obvious activity loss.

Full text of DOI:10.1016/j.cclet.2009.12.014

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 524–528
www.elsevier.com/locate/cclet
A new approach to the synthesis of diacetals (diketals)
pentaerythritol catalyzed by SO3H-functionalized ionic liquids
*
Yuan Yuan Wang, Yan Nan Xu, Zhi Zhong Wang, Li Yi Dai
Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry,
East China Normal University, Shanghai 200062, China
Received 30 September 2009
Abstract
In this article, an efficient, simple and environmentally friendly approach to the synthesis of diacetals (diketals) pentaerythritol
using SO3H-functionalized ionic liquids (ILs) as catalysts was reported. The ILs show high catalytic activity and reusability with
good to excellent yields of the desired products. Hammett method has been used to determine the acidity order of these ionic liquids
and the results are consistent with the catalytic activities observed in acetalization reaction. Maximum product yield of 93% was
observed on using [PSPy][OTf] as catalyst and it can be reused at least 8 times without obvious activity loss.
# 2009 Li Yi Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: SO3H-functionalized ionic liquids; Diacetals (diketals) pentaerythritol; Catalyze; Hammett method; Reusability
Acetals are biologically important due to their role in the chemistry of carbohydrates [1,2]. Also, in the fragrance
and pharmaceutical industries, acetals are used both as intermediates and as end products [3,4]. Several publications
have described the preparation of diacetals pentaerythritol under acidic conditions [5–7]. However, many of these
reported methods suffer from drawbacks such as harsh reaction conditions, unsatisfactory yields, cumbersome product
isolation procedures, large excess of aldehyde (ketones), volatile and hazardous organic solvents, which have to be
recovered, treated and disposed of. Therefore, the design and development of highly active catalysts aimed at
overcoming these limitations have proven to be a significant challenging task.
During the past decade, ionic liquids (ILs) have received considerable attention as environmentally friendly and
effective reaction media for a wide variety of organic reaction and other applications in chemistry [8–10]. The
introduction of specific functionality has led to the identification of functionalized ionic liquids (FILs, or task-specific
ionic liquids, TSILs) [11]. Among different kinds of FILs, SO3H-functionalized ILs have been intensively studied over
the past five years, because the existence of SO3H-functional groups can obviously enhance their acidities and water
solubilities [12,13]. And we also have reported that FILs with benzimidazolium cations could efficiently catalyze the
acetalization of aromatic aldehydes with diols [14,15]. As an extension to our previous work on FILs catalysis, we
envisaged that SO3H-functionalized ILs (Fig. 1.) would be able to promote the synthesis of diacetals (diketals)
* Corresponding author.
E-mail address: lydai@chem.ecnu.edu.cn (L.Y. Dai).
1001-8417/$ – see front matter # 2009 Li Yi Dai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.12.014

Y.Y. Wang et al. / Chinese Chemical Letters 21 (2010) 524–528
525
Fig. 1. Structures of SO3H-functionalized ILs used in this paper.
pentaerythritol, and meanwhile maintain the biphasic properties of ionic liquid, thereby ensuring good recyclability
and reusability. As far as we know, this is the first example of synthesis of diacetals (diketals) pentaerythritol in ILs.
1. Experimental
SO3H-Functionalized ILs based on trimethylamine, 1-methylimidazole and pyridine were prepared according to
the procedures reported by Cole et al. [16], Li et al. [17] and Xing et al. [18], respectively. The obtained ILs were
1
identified by H NMR and IR.
Typical procedure for the synthesis of diacetals (diketals) pentaerythritol: Weighted amounts of pentaerythritol,
aldehyde (ketone) and ILs were added into a 25 mL round-bottom flask fitted with a reflux condenser. The
acetalization was typically allowed to proceed for 1.5–4 h with vigorous stirring and maintained at the desired
temperature. After the reaction, the mixture was placed for a while for the formation of two phases. The crude product
was isolated by distillation and purified through recrystallization with ethanol as a solvent to give the title compound,
1
which were characterized by FT-IR and H NMR. The IL phase contained the produced water, could be easily
recovered and reused in the next run after heat treatment under vacuum at 70 8C for 2 h.
2. Results and discussion
The measurement of the acidic scales of these SO3H-functionalized ILs was conducted on an Agilent 8453 UV–vis
spectrophotometer with a basic indicator according to the literature reported previously [15,16]. The maximal
Fig. 2. Absorbance spectra of 4-nitroaniline for various SO3H-functionalized ILs in CH2Cl2.

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Y.Y. Wang et al. / Chinese Chemical Letters 21 (2010) 524–528
Table 1
Calculation and comparison of H0 values of different SO3H-functionalized ILs in CH2Cl2 (20 8C).
Entry
ILs
Amax
[I] (%)
100.0
71.8
84.8
50.3
59.2
81.3
49.8
65.3
86.6
37.5
[IH+] (%)
0
H0
1
2
–
2.353
1.689
1.996
1.184
1.393
1.912
1.173
1.537
2.038
0.882
–
[PSTma][HSO4]
[PSTma][H2PO4]
[PSTma][OTf]
[PSMim][HSO4]
[PSMim][H2PO4]
[PSMim][OTf]
[PSPy][HSO4]
[PSPy][H2PO4]
[PSPy][OTf]
28.2
15.2
49.7
40.8
18.7
50.2
34.7
13.4
62.5
1.40
1.74
1.00
1.15
1.63
0.99
1.26
1.80
0.77
3
4
5
6
7
8
9
10
Indicator: 4-nitroanline.
Table 2
Synthesis of dibenzalpentaerythritol catalyzed by various SO3H-functionalized ILs.
Entry
ILs
Yield (%)a
1
2
3
4
5
6
7
8
9
[PSTma][HSO4]
[PSTma][H2PO4]
[PSTma][OTf]
[PSMim][HSO4]
[PSMim][H2PO4]
[PSMim][OTf]
[PSPy][HSO4]
[PSPy][H2PO4]
[PSPy][OTf]
88
86
90
88
86
90
89
86
93
a
Yield refers to isolated pure products: pentaerythritol (10 mmol), benzaldehyde (20 mmol), SO3H-functionalized ILs (3 mmol), T = 100 8C,
t = 1.5 h.
absorbance of the unprotonated form of the indicator was observed at 350 nm in CH2Cl2, Fig. 2. When an IL was
added, the absorbance of the unprotonated form of the indicator decreased. Ultimately, we obtained the acidity order of
the nine ILs with the following H0 values (Table 1): [PSPy][OTf] (0.77) > [PSMim][OTf] (0.99) > [PSTma][OTf]
(1.00) > [PSMim][HSO4] (1.15) > [PSPy][HSO4] (1.26) > [PSTma][HSO4] (1.40) > [PSMim][H2PO4] (1.63) >
[PSTma][H2PO4] (1.74) > [PSPy][H2PO4] (1.80), suggesting that the Brønsted acidity of [PSPy][OTf] was relatively
stronger than the other eight ILs used in this work (Table 1, entries 2–10).
The reactants have good solubility in SO3H-functionalized ILs while the diacetals and diketals are almost
immiscible with ILs, which facilitate the shift of the equilibrium of acetalization reaction. The acetalization started as a
homogeneous process and ended as biphasic. Table 2 shows the results of experiments carried out under the same
Table 3
Synthesis of pentaerythritol diacetals (diketals) catalyzed by IL [PSPy][OTf].
Entry
Aldehydes (Acetones)
Time (/h)
Yield (%)a
Mp (8C)
Found
Reported
1
2
3
4
5
6
7
8
9
10
Butyraldehyde
1.5
1.5
1.5
1.5
1.5
2.5
2.0
1.5
3.0
4.0
91
89
93
94
94
90
92
95
86
87
43–45
158–159
155–156
141–142
201–202
183–184
212–213
226–227
147–148
114–115
43–45
159–160
155–156
141
2-Furaldehyde
Benzaldehyde
2-Chloro benzaldehyde
4-Chloro benzaldehyde
4-Methoxy benzaldehyde
4-Methyl benzaldehyde
4-Nitro benzaldehyde
Acetophenone
200–201
184–185
211–213
227–228
147–148
113–114
Cyclohexanone
a
Yield refers to isolated pure products: pentaerythritol (10 mmol), aldehyde (ketone) (20 mmol), [PSPy][OTf] (3 mmol), T = 100 8C.

Y.Y. Wang et al. / Chinese Chemical Letters 21 (2010) 524–528
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Table 4
Recycling of the IL [PSPy][OTf] in the synthesis of dibenzalpentaerythritol.
Recycling time
1
2
3
4
5
6
7
8
Yield (%)a
93
93
92
93
92
91
91
90
a
Yield refers to isolated pure products: pentaerythritol (10 mmol), aldehyde (ketone) (20 mmol), [PSPy][OTf] (3 mmol), T = 100 8C, t = 1.5 h.
reaction conditions for comparing the catalytic performances of these SO3H-functionalized ILs. We found that
[PSPy][Tf] might be best for the acetalization in all SO3H-functionalized ILs, leading to 93% yield of
dibenzalpentaerythritol. The sequence of catalytic activity observed in acetalization is in good agreement with the
acidity order determined by the Hammett method. The increasing of the anion’s Brønsted acidity of ILs improves the
catalytic activity of the acidic ILs.
The acetalization of various aldehydes (ketones) with pentaerythritol has been investigated in IL [PSPy][OTf]
(Table 3, entries 1–10). Ketones show less reactivity than aldehydes for this reaction, for example
dibenzalpentaerythritol (Table 3, entry 3) was obtained in quantitative yield (93%) for 1.5 h whereas acetophenone
(Table 3, entry 9) provided 86% yield of product for 3.0 h. As can be known from entries 3–8 in Table 3, a change in the
substitutional group of the reactants impacts the yield of products. When 4-methoxybenzaldehyde (Table 3, entry 6)
was treated with pentaerythritol in IL the reaction took longer (2.5 h) and the yield of di-4-methoxybenzal
pentaerythritol was lower (90%), for the strong donor methoxy group reduced the reactivity. The reactivity order of
aromatic aldehydes is 4-nitrobenzaldehyde > 4-chlorobenzaldehyde > benzaldehyde > 4-methylbenzaldehyde > 4-
methoxybenzaldehyde, which is consistent with the report [19] that showed electron-withdrawing substituents
enhanced the rate of acetal formation.
Compared with traditional acid catalysts, easy recycling is an attractive property of the acidic ILs. Consequently,
we investigated the catalytic activity of recycled [PSPy][OTf] in the synthesis of dibenzalpentaerythritol. After
1
acetalization the crude product could be conveniently isolated by distillation. The IL was assessed by H NMR
spectroscopy after treated under vacuum (0.01 T) at 70 8C for 2 h, and no traces of benzaldehyde and pentaerythritol
were detected. As shown in Table 4, [PSPy][OTf] could be reused at least 8 times and the yield of
dibenzalpentaerythritol was not less than 90%, which indicated that [PSPy][OTf] was highly efficient and recyclable
catalyst for the synthesis of diacetals (diketals) pentaerythritol.
3. Conclusions
In conclusion, a new approach to the synthesis of diacetals (diketals) pentaerythritol was developed using SO3H-
functionalized ILs as catalysts. This catalytic system is stable, easily separable, and reusable. Furthermore, this series
of SO3H-functionalized ILs have higher catalytic activities under mild reaction condition without addition of any
organic solvents or catalysts. Further investigations concerning the molecular geometries of these SO3H-
functionalized ILs and structure-catalytic activity relationship are currently underway.
Acknowledgments
This research is supported by National 863 High-Tech Research and Development Program of China (No.
2007AA05Z101).
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