SO3H-functionalized ionic liquids as efficient and recyclable catalysts for the synthesis of pentaerythritol diacetals and diketals

Article abstract of DOI:10.1016/j.molcata.2010.01.011

The efficient, simple and environmentally friendly synthesis of pentaerythritol diacetals and diketals using SO₃H-functionalized ionic liquids (ILs) as catalysts was reported. The ILs showed high catalytic activities and reusabilities with good to excellent yields of the desired products. H₀ (Hammett function) values and the minimum-energy geometries of SO₃H-functionalized ILs were determined and the results revealed that the acidities and catalytic activities of ILs in acetalization were related to their structures. The IL [PSPy][OTf] with the shortest H-O bond distance had the strongest acidity and the highest catalytic activity in the synthesis of pentaerythritol diacetals and diketals.

Full text of DOI:10.1016/j.molcata.2010.01.011

Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
SO3H-functionalized ionic liquids as efficient and recyclable catalysts for the
synthesis of pentaerythritol diacetals and diketals
Yuanyuan Wang, Xinxin Gong, Zhizhong Wang, Liyi Dai∗
Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University,
North Zhong Shan Road 3663, 200062 Shanghai, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The efficient, simple and environmentally friendly synthesis of pentaerythritol diacetals and diketals
using SO3H-functionalized ionic liquids (ILs) as catalysts was reported. The ILs showed high catalytic
activities and reusabilities with good to excellent yields of the desired products. H0 (Hammett function)
values and the minimum-energy geometries of SO3H-functionalized ILs were determined and the results
revealed that the acidities and catalytic activities of ILs in acetalization were related to their structures.
The IL [PSPy][OTf] with the shortest H–O bond distance had the strongest acidity and the highest catalytic
activity in the synthesis of pentaerythritol diacetals and diketals.
Received 26 September 2009
Received in revised form 15 January 2010
Accepted 21 January 2010
Available online 29 January 2010
Keywords:
SO3H-functionalized ionic liquids
Pentaerythritol diacetals and diketals
Structure–catalytic activity relationship
Ab initio calculation
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
in chemistry [11–14]. The introduction of specific functionality has
led to the identification of functionalized ionic liquids (FILs, or task-
Acetals are biologically important due to their role in the
chemistry of carbohydrates [1–3]. Also, in the fragrance and
pharmaceutical industries, acetals are employed as both interme-
diates and end products [4,5]. Pentaerythritol acetals in general
are applied as plasticizers and vulcanizers for various polymeric
materials, raw materials for the production of valuable resins
and lacquers, physiologically active substances and defoamers
for washing solution defoamers for washing solution containing
anionic surfactants [6,7]. They can be used as potential protective
groups for aldehydes and ketones as well as important derivatives
of carbonyl compounds, since most are crystalline substances and
have sharp melting points. Several publications have described
the preparation of pentaerythritol diacetals under acidic condi-
tions [8–10]. However, many of these reported methods suffer
from drawbacks such as harsh reaction conditions, unsatisfactory
yields, cumbersome product isolation procedures, great excess of
aldehyde (ketones), volatile and hazardous organic solvents, which
have to be recovered, treated and disposed of. Therefore, the design
and synthesis of highly active catalysts aimed at overcoming these
limitations become a significant challenging task.
specific ionic liquids, TSILs) [15]. 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 [16–20].
We have reported the efficient acetalization of aromatic aldehydes
with diols using FILs based on benzimidazolium cations as catalysts
[21]. As an extension to our previous work on FILs catalysis, we
investigate the synthesis of pentaerythritol diacetals and diketals
promoted by SO3H-functionalized ILs (Fig. 1), which posses good
recyclabilities and reusabilities. As far as we known, this is the first
example of synthesis of pentaerythritol diacetals and diketals in ILs.
2. Experimental
2.1. Chemicals and instruments
All chemicals were analytical grade and used as received. The
melting points were determined on Sanyo Gallenkamp MPD350
apparatus. 1H NMR spectra were recorded on Brucker DRX-500
spectrometer. FT-IR measurements were performed using KBr disc
on a NEXUS 670 FT-IR infrared spectrometer (Nicolet).
During the past decades, ionic liquids (ILs) have received consid-
erable attention as environmentally friendly and effective reaction
media for a wide variety of organic reaction and other applications
2.2. Synthesis of SO3H-functionalized ILs
SO3H-functionalized ILs based on trimethylamine, 1-
methylimidazole and pyridine were prepared according to
the procedures reported by Cole et al. [22], Li et al. [23] and Dai
∗
Corresponding author. Tel.: +86 21 62232279; fax: +86 21 62602447.
E-mail addresses: lydai@chem.ecnu.edu.cn, ly dai@yahoo.cn (L. Dai).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.01.011

8
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
Fig. 1. Structures of SO3H-functionalized ILs used in this paper.
Table 1
Synthesis of dibenzalpenlpentaerythritol catalyzed by various SO3H-functionalized
ILs.
Fig. 2. Absorbance spectra of 4-nitroaniline for various SO3H-functionalized ILs in
CH2Cl2.
Entry
ILs
Yield (%)a
Table 2
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
65
84
Calculation and comparison of H0 values of different SO3H-functionalized ILs in
CH2Cl2 (20 ◦C).
Entry
ILs
Amax
[I] (%)
[IH+] (%)
H0
1
2
3
4
5
6
7
8
9
–
2.353
1.689
1.996
1.184
1.393
1.912
1.173
1.537
2.038
0.882
100.0
71.8
84.8
50.3
59.2
81.3
49.8
65.3
86.6
37.5
0
–
[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
10
11
H2SO4 (98%)b
H2SO4 (98%)c
a
Yield refers to isolated pure products: pentaerythritol (10 mmol), benzaldehyde
(20 mmol), SO3H-functionalized IL (3 mmol), T = 100 ◦C, t = 1.5 h.
b
10
Solvent free.
Solvent: benzene.
c
Indicator: 4-nitroanline.
and co-workers [24], respectively. The obtained ILs were identified
by 1H NMR and IR analysis.
by performing ab initio geometry optimizations at the RHF/6-31(d,
p) level using the Gaussian03 program [25]. A vibrational analysis
was performed to ensure the absence of negative frequencies and
verify the existence of a true minimum.
2.3. UV–vis acidity evaluation
Dichloromethane solutions of the ILs were prepared from anhy-
drous CH2Cl2 and the ILs (dried under vacuum at 70 ◦C for 2 h). All
spectra were obtained with an Agilent B453 spectrophotometer.
2.5. General procedure for the synthesis of pentaerythritol
diacetals and diketals
A mixture of pentaerythritol (10 mmol), aldehyde (ketone)
(10 mmol) and IL (3 mmol) was placed into a round bottom flask of
25 mL capacity. The flask was fitted with a condenser and flushed
with nitrogen gas. A nitrogen balloon was fitted at the top of the
2.4. Computational methods
The minimum-energy geometries of SO3H-functionalized ILs
and isolated [PSPy]+, [PSMim]+, [PSTma]+ cations were determined
Table 3
The geometry parameters of isolated cations and SO3H-functionalized ILs.
[PSTma]
14–O13 = 0.956
[PSTma][HSO4]
[PSTma][H2PO4]
[PSTma][OTf]
[PSMim]
[PSMim][HSO4]
H–O bond distance (Å)
H
H11–O10 = 0.981
H14–O13 = 0.992
H14–O13 = 0.970
H14–O13 = 0.952
H14–O13 = 0.970
Distance between the anion
and sulfonic acid group (Å)
H11· · ·O14 = 1.676
H14· · ·O15 = 1.538
H14· · ·O15 = 1.758
H14· · ·O18 = 1.739
H24· · ·O12 = 2.294
H19· · ·O15 = 2.611
H19· · ·O12 = 2.417
H5· · ·O15 = 2.438
H20· · ·O26 = 1.900
H5· · ·O16 = 2.038
H33· · ·O18 = 2.331
H26· · ·O18 = 2.375
H5· · ·O18 = 2.276
H5· · ·O18 = 2.308
H29· · ·O18 = 2.560
H29· · ·O15 = 2.046
Other hydrogen bonds
in ILs (Å)
[PSMim][H2PO4]
14–O13 = 0.969
H14· · ·O16 = 1.962
[PSMim][OTf]
[PSPy]
H25–O24 = 0.950
[PSPy][HSO4]
[PSPy][H2PO4]
H30–O23 = 0.994
H30· · ·O26 = 1.556
[PSPy][OTf]
H–O bond distance (Å)
Distance between the anion
and sulfonic acid group (Å)
H
H14–O13 = 0.965
H14· · ·O18 = 1.814
H25–O24 = 0.976
H25· · ·O28 = 1.700
H25–O24 = 0.963
H25· · ·O26 = 1.732
H30· · ·O15 = 2.737
H27· · ·O15 = 1.631
H5· · ·O15 = 2.480
H27· · ·O15 = 2.057
H5· · ·O18 = 2.326
H27· · ·O18 = 2.644
H30· · ·O15 = 2.455
H8· · ·O26 = 2.001
H8· · ·O29 = 2.492
H19· · ·O26 = 2.386
H16· · ·O28 = 2.608
H16· · ·O26 = 2.754
H8· · ·O28 = 2.265
H8· · ·O29 = 2.270
H16· · ·O29 = 2.415
Other hydrogen bonds
in ILs (Å)

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
9
(3): M.p. (◦C): 155–156. IR (KBr, ꢀ/cm−1): 3034, 2958, 2857,
1496, 1468, 1373, 1160, 1115, 1078, 1030, 1021, 747. 1H NMR
(500 MHz, d6-acetone) ı 3.49–3.93 [m, 6H, CH2C(CH2)3], 4.80 [d,
2H, CH2C(CH2)3], 5.41 (s, 2H, 2PhCH), 7.19–7.58 (m, 8H, 2Ph).
(4): M.p. (◦C): 141–142. IR (KBr, ꢀ/cm−1): 2970, 2856, 1575,
1477, 1384, 1073, 960, 710. 1H NMR (500 MHz, d6-acetone) ı
3.61–3.82 [m, 6H, CH2C(CH2)3], 4.83 [d, 2H, CH2C(CH2)3], 5.48 (s,
2H, 2 PhCH), 7.25–7.48 (m, 8H, 2Ph).
condenser to maintain inert atmosphere. The acetalization was typ-
ically proceed for 1.5–4 h with vigorous stirring at 100 ◦C. After the
reaction, the reaction mixture was cooled to room temperature and
the layers separated. The crude product was isolated by distillation
and purified by recrystallization with ethanol as a solvent to give
the title compound, which was characterized by FT-IR and 1H NMR.
The IL phase contained the produced water, could be easily recov-
ered and reused in the next run after heat treatment under vacuum
at 70 ◦C for 2 h.
(5): M.p. (◦C): 201–202. IR (KBr, ꢀ/cm−1): 2980, 2862, 1596,
1490, 1382, 1201, 1162, 1076, 816, 680. 1H NMR (500 MHz, d6-
acetone) ı 3.63–3.85 [m, 6H, CH2C(CH2)3], 4.82 [d, 2H, CH2C(CH2)3],
5.50 (s, 2H, 2 PhCH), 7.26–7.50 (m, 8H, 2Ph).
IR spectra and 1H NMR spectra data for products 1–12 (Table 4)
are shown as follows:
(1): M.p. (◦C): 43–45. IR (KBr, ꢀ/cm−1): 2958, 2930, 2871, 1110,
1075, 990, 722. 1H NMR (500 MHz, d6-acetone) ı 0.90 (t, 6H, 2CH3),
1.20–1.69 (m, 8H, 2(CH2)2), 3.24–3.41 [m, 6H, CH2C(CH2)3], 4.41 (t,
2H, 2CH), 4.65 [d, 2H, CH2C(CH2)3].
(6): M.p. (◦C): 183–184. IR (KBr, ꢀ/cm−1): 3008, 2828, 1610,
1517, 1460, 1396, 1063, 1034, 830. 1H NMR (500 MHz, d6-acetone)
ı 3.18 (s, 6H, 2OCH3), 3.87–4.01 [m, 6H, CH2C(CH2)3], 4.32 [d, 2H,
CH2C(CH2)3], 4.56 (s, 2H, 2PhCH), 6.81–7.78 (m, 8H, 2Ph).
(7): M.p. (◦C): 212–213. IR (KBr, ꢀ/cm−1): 2973, 2860, 1618,
1470, 1384, 1075, 806. 1H NMR (500 MHz, d6-acetone) ı 2.35 (s,
6H, 2CH3), 3.63–3.83 [m, 6H, CH2C(CH2)3], 4.56 [d, 2H, CH2C(CH2)3],
5.43 (s, 2H, 2PhCH), 7.18–7.36 (m, 8H, 2Ph).
(2): M.p. (◦C): 159–160. IR (KBr, ꢀ/cm−1): 3120, 2987, 2864,
1610, 1504, 1675, 1114, 1087, 1022, 781, 743. 1H NMR (500 MHz,
d6-acetone)
ı 3.51–3.92 [m, 6H, CH2C(CH2)3], 4.82 [d, 2H,
CH2C(CH2)3], 5.55 (s, 2H, 2ArCH), 6.32–6.53 (m, 4H, ˇ-H), 7.42 (d,
2H, ˛-H).
Fig. 3. Molecular structures of isolated [PSPy]+ cation and ILs based on [PSPy]+ cation.

10
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
Fig. 3. (Continued ) .
(8): M.p. (◦C): 185–186. IR (KBr, ꢀ/cm−1): 2983, 2904, 1615,
(12): M.p. (◦C): 114–115. IR (KBr, ꢀ/cm−1): 2935, 2860, 1446,
1371, 1251, 1157, 1086, 918. 1H NMR (500 MHz, d6-acetone) ı
1.20–2.06 [m, 20H, 2(CH2)5], 3.22–3.46 [m, 6H, CH2C(CH2)3], 3.54
[d, 2H, CH2C(CH2)3].
1534, 1354, 1078, 1005, 712, 677. 1H NMR (500 MHz, d6-acetone)
ı 3.66–3.93 [m, 6H, CH2C (CH2)3], 4.85 [d, 2H, CH2C(CH2)3], 5.54 (s,
2H, 2PhCH), 7.58–8.37 (m, 8H, 2Ph).
(9): M.p. (◦C): 226–227. IR (KBr, ꢀ/cm−1): 2985, 2940, 1613,
1529, 1346, 1072, 851, 750, 697. 1H NMR (500 MHz, d6-acetone)
ı 3.73–3.94 [m, 6H, CH2C (CH2)3], 4.85 [d, 2H, CH2C(CH2)3], 5.53 (s,
2H, 2PhCH), 8.04–8.48 (m, 8H, 2Ph).
3. Results and discussion
3.1. Synthesis of dibenzalpenlpentaerythritol in different ILs
(10): M.p. (◦C): 43–45. IR (KBr, ꢀ/cm−1): 2970, 2865, 1470,
1380, 1160, 1075. 1H NMR (500 MHz, d6-acetone) ı 0.88 [s, 6H,
2(CH2)5CH3], 1.28 (m, 16H, 8CH2), 1.35 (s, 6H, 2CH3), 1.66 (t, 4H,
2CH2), 3.71–3.88 [m, 8H, C(CH2O)4].
The synthesis of pentaerythritol diacetals and diketals can be
catalyzed by some other inorganic Lewis acidic catalysts, of which
the concentrated sulfuric acid has the highest catalytic activity. For
the purpose of comparison, the same acetalization reaction was
also carried out with equivalent concentrated sulfuric acid (98%)
as catalyst (Table 1, entries 10, 11). The results demonstrate that
the catalytic performances of the SO3H-functionalized ILs (Table 1,
(11): M.p. (◦C): 147–148. IR (KBr, ꢀ/cm−1): 2980, 2900, 1468,
1378, 1250, 1171, 889, 701. 1H NMR (500 MHz, d6-acetone) ı
1.51 (s, 6H, 2CH3), 3.13–3.29 [m, 6H, CH2C(CH2)3], 4.47 [d, 2H,
CH2C(CH2)3], 7.29–7.70 (m, 10H, 2Ph).

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
11
Fig. 4. Molecular structures of isolated [PSMim]+ cation and ILs based on [PSMim]+ cation.
entries 1–9) are better than that of the concentrated sulfuric acid
(Table 1, entry 10) under the same reaction conditions. Further-
more, there are many drawbacks of using concentrated sulfuric acid
catalyst system, such as heavy environmental pollution, trouble-
some recovery and purification of product, and difficult recovering
of catalyst.
All the ILs give excellent yields of products. 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. Among all
SO3H-functionalized ILs, we found that [PSPy][OTf] might be the
best catalyst for the acetalization, leading to 93% yield of diben-
zalpenlpentaerythritol.
its location, so the [I]/[IH+] (I represents indicator) ratio could be
determined from the differences of measured absorbance after the
addition of Brønsted acidic ILs and Hammett function, H0, could be
calculated using Eq. (1). This value could be regarded as the relative
acidity of the ILs.
ꢀ
ꢁ
[I]
[IH+]
H0 = pK(I)aq + log
(1)
Under the same concentration of 4-nitroanline (10 mg/L,
pka = 0.99) and ILs (5 mmol/L) in dichloromethane, we deter-
mined the H0 values of the SO3H-functionalized ILs. The maximal
absorbance of the unprotonated form of the indicator was observed
at 350 nm in CH2Cl2. When the IL was added, the absorbance of the
unprotonated form of the basic indicator decreased. As shown in
Fig. 2, the absorbance of the unprotonated form of the indicator on
nine SO3H-functionalized ILs decreased as follows: [PSPy][H2PO4]
> [PSTma][H2PO4] > [PSMim][H2PO4] > [PSTma][HSO4] > [PSPy]
[HSO4] > [PSMim][HSO4] > [PSTma][OTf] > [PSMim][OTf] > [PSPy]
[OTf]. After the calculation, we obtained the acidity order of the
nine ILs with the following H0 values (Table 2): [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),
3.2. Determination of H0 values of SO3H-functionalized ILs
The measurement of the acidic scale of these SO3H-
functionalized ILs was conducted on an Agilent 8453 UV-vis
spectrophotometer with a basic indicator according to the litera-
ture reported previously [21,26]. With the increase of acidic scale of
the acidic ILs, the absorbance of the unprotonated form of the basic
indicator decreased, whereas the protonated form of the indicator
could not be observed because of its small molar absorptivity and

12
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
Fig. 4. (Continued ).
which is in good agreement with the sequence of catalytic activity
observed in acetalization. The acidities of the ILs depended both on
the characteristics of the cations and anions. When the cations of
the ILs were the same, the dependence of the acidities of the ILs on
anions was significant. The acidities of the ILs with the conjugate
base of [H2PO4]− is weaker than those of the ILs with the conjugate
bases of [HSO4]− and [OTf]− (Table 2, entries 2–4).
hydrogen bonds C5–H19· · ·O26, C13–H16· · ·O28 (bond distances are
2.386 Å, 2.608 Å) between the anion and alkyl sulfonic acid chain;
most importantly, the strongest interaction between the anion
and the sulfonic acid proton was O24–H25· · ·O28 (bond distance
is 1.700 Å). The other eight SO3H-functionalized ILs also exhibit
similar strong interactions between the anions and the sulfonic
acid groups, which making the H–O bond of the sulfonic acid
group to be lengthened with different levels compared with the
one in the isolated cation. When the cations of the ILs are the
same, the H–O bond distance of SO3H-functionalized ILs decreased
as follows: [Cation][H2PO4] > [Cation][HSO4] > [Cation][OTf]. Tak-
ing ILs based on [PSPy]+ cation as examples, the H–O bond
distance of the sulfonic acid group of these ILs decreased
as follows: [PSPy][H2PO4] (H30–O23 = 0.994 Å) > [PSPy][HSO4]
(H25–O24 = 0.976 Å) > [PSPy][OTf] (H25–O24 = 0.963 Å), which is
reverse to their acidity sequence. The formation of hydrogen bond
was a process of sharing lone electron pair of O or F atom with H
atom. When the hydrogen bond was strong, the lone electron pair
was shared in deep degree, and cause an increase in the electron
density and stability of H atom. As a result, the acidity of H atom
decreased.
3.3. The molecular geometries of SO3H-functionalized ILs
The minimum-energy geometries of SO3H-functionalized ILs
were determined by performing ab initio geometry optimizations
at the RHF/6-31(d, p) level. The fully optimized geometries of these
ILs are presented in Figs. 3–5and Table 3. It is obvious that a strong
hydrogen bond network is presented in the SO3H-functionalized
ILs. Taking IL [PSPy][HSO4] for example, the anion is located on
the side of the propyl sulfonic acid group, and interacts with
the propyl alkyl sulfonic acid group and pyridinium ring in the
form of C–H· · ·O or O–H· · ·O type (hydrogen bonds). There are
five hydrogen bonds in IL [PSPy][HSO4], which are shorter than
the Van der Waals distance of 2.67 Å [27]. These include the
hydrogen bonds C1–H8· · ·O26, C1–H8· · ·O29 (bond distances are
2.001 Å, 2.492 Å) between the anion and pyridinium ring; the
The results obtained from experiments and the minimum-
energy geometries of SO3H-functionalized ILs revealed that the

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
13
Table 4
Synthesis of pentaerythritol diacetals and diketals catalyzed by IL [PSPy][OTf].
Entry
Aldehydes (acetones)
Product
Time (h)
1.5
Yield (%)a
1
91
89
2
3
1.5
1.5
93
4
5
6
7
1.5
1.5
2.5
2.0
94
94
90
92
8
9
1.5
1.5
92
95
10
11
3.0
3.0
4.0
87
86
87
12
a
Yield refers to isolated pure products: pentaerythritol (10 mmol), aldehyde (ketone) (20 mmol), [PSPy][OTf] (3 mmol), T = 100 ◦C. All the products were characterized by
FT-IR, 1H NMR.

14
Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
acidities and catalytic activities of ILs were related to their struc-
tures. The SO3H-functionalized IL with shorter H–O bond has
stronger acidity and higher catalytic activity in the synthesis of
pentaerythritol diacetals and diketals.
(Table 4, entry 6) was treated with pentaerythritol in IL, the
reaction time was longer (2.5 h) and the yield of pentaerythritol
di-4-methoxybenzaldehyde was lower (90%), for the strong donor
methoxy group reduced the reactivity of 4-methoxybenzaldehyde.
The
4-nitrobenzaldehyde > 4-chlorobenzaldehyde > benzaldehyde > 4-
methylbenzaldehyde > 4-methoxybenzaldehyde, which is
consistent with the report [28] that electron-withdrawing
substituents could enhance the rate of acetal formation.
reactivity
order
of
aromatic
aldehydes
is
3.4. Acetalization of various aldehydes (ketones) with
pentaerythritol in IL [PSPy][OTf]
The rate-determining step of acetalization is the formation of
a cation from the protonated hemiacetal. In order to compensate
for the low rate of hemiacetal formation, the medium is suffi-
ciently acidic to promote effective protonation of any hemiacetal
that is formed and sufficiently polar to allow stabilization of
the cationic intermediate. Hence in our case, the use of strongly
acidic and polar SO3H-functionalized ILs gave shorter reaction
time and higher yields than the methods presented in literatures
[9,10]. The acetalization of various aldehydes with pentaerythritol
reported here, showed clean conversion in high yields even when
moderately hindered aldehydes were used (Table 4, entries 1–9).
Ketones showed lower reactivity than aldehydes for this reaction,
for example in IL [PSPy][OTf], dibenzalpentaerythritol (Table 4,
entry 3) was obtained in quantitative yield (93%) for 1.5 h, whereas
acetophenone (Table 4, entry 11) provided 86% yield of product
for 3.0 h. It is worth noting that the substitutional group of the
reactants is crucial to the yields of products, which can be seen
from entries 3 to 9 in Table 4. When 4-methoxybenzaldehyde
3.5. Reusability of IL [PSPy][OTf]
Compared with the traditional acidic catalysts, easy recycling is
an attractive property of the acidic ILs. Consequently, we investi-
gated the catalytic activity of recycled [PSPy][OTf] in the synthesis
of dibenzalpenlpentaerythritol. After the reaction, the mixture was
placed for a while in order to form two phases. The crude prod-
uct was isolated by distillation and the IL phase was conveniently
recovered and reused after heat treatment under vacuum at 70 ◦C
for 2 h. The IL maintained its structure as confirmed by 1H NMR
spectroscopy. As shown in Fig. 6, [PSPy][OTf] could be reused for
8 times at least and there was no obvious decrease in the yield of
dibenzalpenlpentaerythritol, which indicated that [PSPy][OTf] was
high efficient and recyclable catalyst for the synthesis of pentaery-
thritol diacetals and diketals.
Fig. 5. Molecular structures of isolated [PSTma]+ cation and ILs based on [PSTma]+ cation.

Y. Wang et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 7–16
15
Fig. 5. (Continued ).
reusable. Furthermore, this series of SO3H-functionalized ILs have
higher catalytic activities under mild reaction condition without
the addition of any other organic solvents or catalysts. The results
got from the experiments and the minimum-energy geometries
of SO3H-functionalized ILs revealed that the acidities and catalytic
activities of ILs in acetalization were related to their structures. The
IL with shorter H–O bond distance has stronger acidity and higher
catalytic activity in the synthesis of pentaerythritol diacetals and
diketals.
Acknowledgement
The authors gratefully acknowledge Prof. Hua Liang Jiang at
Shanghai Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai, China for providing the Gaussian03 program and helpful
guidance.
Fig. 6. Recycling of the IL [PSPy][OTf].
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16
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