Novel acidic ionic liquids as efficient and recyclable catalysts for the cyclotrimerization of aldehydes

Article abstract of DOI:10.1080/00397911.2010.523804

A mild, efficient, and ecofriendly procedure for cyclotrimerization of aldehydes was realized by using a series of novel Brnsted acidic ionic liquids (BAILs) consisting of double-SO₃H groups in cations as catalysts. Good conversion of aldehydes and selectivity of trialkyl-1,3,5-trioxanes were achieved by using 1mol% of BAILs. In addition, the catalyst system could be recycled and reused at least eight times without apparent loss of activity. Taylor & Francis Group, LLC.

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Novel Acidic Ionic Liquids as Efficient
and Recyclable Catalysts for the
Cyclotrimerization of Aldehydes
Heyuan Song a , Jing Chen a , Chungu Xia a & Zhen Li a
a State Key Laboratory of Oxosynthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, China
Accepted author version posted online: 18 Jul 2011.Published
online: 14 Sep 2011.
To cite this article: Heyuan Song , Jing Chen , Chungu Xia & Zhen Li (2012): Novel Acidic Ionic
Liquids as Efficient and Recyclable Catalysts for the Cyclotrimerization of Aldehydes, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
42:2, 266-273
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Synthetic Communications1, 42: 266–273, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.523804
NOVEL ACIDIC IONIC LIQUIDS AS EFFICIENT
AND RECYCLABLE CATALYSTS FOR THE
CYCLOTRIMERIZATION OF ALDEHYDES
Heyuan Song, Jing Chen, Chungu Xia, and Zhen Li
State Key Laboratory of Oxosynthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China
GRAPHICAL ABSTRACT
Abstract A mild, efficient, and ecofriendly procedure for cyclotrimerization of aldehydes
was realized by using a series of novel Brønsted acidic ionic liquids (BAILs) consisting
of double–SO3H groups in cations as catalysts. Good conversion of aldehydes and selec-
tivity of trialkyl-1,3,5-trioxanes were achieved by using 1 mol% of BAILs. In addition,
the catalyst system could be recycled and reused at least eight times without apparent loss
of activity.
Keywords Acidic ionic liquid; aldehydes; cyclotrimerization; trialkyl-1,3,5-trioxane
INTRODUCTION
Cyclotrimerization of aldehydes is an important reaction in organic synthesis
to produce corresponding 1,3,5-trioxanes, which are widely used in various fields
as constituents of stabilizers in color photography,[1] flavoring materials, and carriers
for scents, repellents, deodorants,[2] and insecticide.[3] Catalysts used for this reaction
were generally proton acids such as H2SO4 and H3PO4;[4,5] Lewis acid such as
ZnCl2;[4] solid acid such as ZrO2, Zr(OH)2, TiO2, SnO2,[5,6] Al2O3, Y-zeolite,[7,8]
sulfonic cation resin,[9] Me3Si-Cl,[10] and Cp2Zr(Otf)2;[11] and Keggintype heteropo-
lyacids such as H3PMo12O40, H3PW12O40, and H4SiW12O40.[12] Although good
results were obtained, the drawback of corrosion and environmental pollution
caused by liquid acid catalysts and the rapid deactivation of solid acids because of
the buildup of coke made them impossible to apply in industry. Therefore, to design
and synthesize new catalysts possessing novel properties of high stability, environ-
mentally friendliness, and reusability has long been pursued.
Received June 21, 2010.
Address correspondence to Zhen Li, State Key Laboratory of Oxosynthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
China. E-mail: zhenli@licp.cas.cn; shy@licp.cas.cn
266

CYCLOTRIMERIZATION OF ALDEHYDES
267
Scheme 1. Structures of novel acidic ILs used in this study.
Ionic liquids (ILs) have attracted interest as environmentally benign media for
catalytic processes because of their favorable properties. Brønsted acidic ILs, which
possess the advantageous characteristics of solid acids and liquid acids, have been
designed to replace traditional mineral liquid acids, such as sulfuric acid and hydro-
chloric acid.[13] In recent years, ILs as media or catalysts in the cyclotrimerization of
aldehydes have been reported,[14,15] affording a new approach for catalytic cyclotri-
merization. With the aim of developing a clean, highly efficient catalyst system for
the cyclotrimerization of aldehydes, we synthesized a series of novel functional acidic
ILs [Bis-BsPy][HSO4]2 (Bs ¼ butyl-sulfonyl, Py ¼ pyridine), [Bis-BsPy][CF3SO3]2,
and [Bis-BsPy][Ts-OH]2 (Ts-OH ¼ p-toluenesulfonic acid) (Scheme 1), consisting of
pyridine-based dications and acid anions, which are highly active, easily recyclable,
and air and moisture stable.
Because of their unique structures, they have quite strong acidity and very poor
solubility in common organic solvents except for ethanol and methanol. These novel
acidic ILs exhibited high activities in cyclotrimerization of aldehydes (Scheme 2)
compared with the normal acidic ILs [Bis-Bpy][HSO4]2 (B ¼ butyl) and [BsPy]
[CF3SO3]. Moreover, the unique immiscibility of the novel acidic ILs with organic
solvents makes the separation of them after reaction very facile.
Scheme 2. Cyclotrimerization of aldehydes catalyzed by ILs.

268
H. SONG ET AL.
The cis-trans configuration of trialkyl-1,3,5-trioxanes was also determined with
isolated triisopropyl-1,3,5-trioxane and triethyl-1,3,5-trioxane as examples by NMR
spectroscopy. There is only one set of signals in the 1H and 13CNMR spectra, which
indicated that the products are all cis, cis isomers.
RESULTS AND DISCUSSION
Originally, a probe reaction of the cyclotrimerization of isobutylaldehyde to
form triisopropyl-1,3,5-trioxane was carried out at 25 ꢀC for 60 min under solvent-
free conditions catalyzed by these acidic ILs, and the results are shown in Table 1.
The isobutylaldehyde conversion was affected by either cations or anions of the
ILs employed, while this effect on the triisopropyl-1,3,5-trioxane selectivity was
slight, giving 99.6–99.9% over all the ILs catalysts. Good conversion of 97.6% was
obtained over –SO3H functionalized IL [Bis-BsPy][HSO4]2 (entry 2, Table 1), while
for [Bis-BPy][HSO4]2, without –SO3H groups in side chains, moderate conversion of
85.2% was achieved (Entry 1, Table 1), which indicated the Brønsted acidity of the
ILs is the main factor for the reaction. Among the –SO3H functionalized ILs, when
the acidic ILs have the same cation, the conversion over IL with CF3SOꢁ3 anion was
the best (Table 1, entries 2–4). It might be that the acidities of IL with CF3SOꢁ3 were
stronger than those with Ts-OHꢁ or HSO4ꢁ. Although it has the same CF3SOꢁ3 anion,
the IL containing a single –SO3H group in the cation moiety gave less conversion,
93.7% (Table 1, entry 5). Even so, to the best of our knowledge, the results obtained
here with our novel acidic ILs are better than that reported in the literature.[14,15]
Because of the excellent catalytic performance in cyclotrimerization of isobuty-
laldehyde, we chose [Bis-BsPy][CF3SO3]2 as the catalyst to investigate further the
effect of the amount of IL, reaction temperature, and reaction time on the reaction,
and the results are listed in Table 2.
The conversion of isobutylaldehyde passed through a maximum and then
decreased as the amount of IL was increased. The greatest conversion was obtained
when the amount of IL was 1.0% (entry 4, Table 2), but the selectivity to triisopropyl-
1,3,5-trioxane was almost unchanged (Table 2, entries 1–5). It revealed that the
cyclotrimerization of isobutyraldehyde needed suitable acidity. When IL=
isobutyraldehyde ¼ 0.1%, the weak acidity in the system was not favorable for the
catalytic reaction. With the increasing of [Bis-BsPy][CF3SO3]2 concentration, the
acidity inside the system went up and the reaction rate increased. However, too much
strong acidivity could decompose triisopropyl-1,3,5-trioxane,[16] so 1.0% of [Bis-BsPy]
[CF3SO3]2 is the best dosage for the reaction.
Table 1. Cyclotrimerization of isobutylaldehyde catalyzed by different ILsa
Entry
IL
Conversion (%)
Selectivity (%)
1
2
3
4
5
[Bis-Bpy][HSO4]2
[Bis-BsPy][HSO4]2
[Bis-BsPy][CF3SO3]2
[Bis-BsPy] [Ts-OH]2
[BsPy][CF3SO3]
85.2
97.6
98.3
90.2
93.7
99.6
99.8
99.9
99.7
99.8
aIL=isobutylaldehyde ¼ 0.5% (molar ratio).

CYCLOTRIMERIZATION OF ALDEHYDES
269
Table 2. Effect of different reaction conditions using [Bis-BsPy][CF3SO3]2
as catalyst
Molar ratio of
Entry IL=isobutylaldehyde
Temperature Time Conversion Selectivity
(ꢀC)
(min)
(%)
(%)
1
2
0.1%
0.2%
0.5%
1.0%
2.0%
1.0%
1.0%
1.0%
1.0%
1.0%
1.0%
1.0%
25
25
25
25
25
0
60
60
60
60
60
60
60
60
60
10
30
120
91.4
94.6
98.3
98.8
90.7
93.2
95.9
81.1
61.6
91.4
98.4
97.7
99.7
99.7
99.8
99.8
99.6
99.7
99.7
99.1
98.5
99.9
99.8
99.8
3
4
5
6
7
10
40
60
25
25
25
8
9
10
11
12
From the data in Table 2, we can see that temperature had a dramatic effect on
the cyclotrimerization of isobutylaldehyde (entries 4 and 6–9). When the temperature
increased from 0 to 25 ꢀC, the conversion of isobutylaldehyde increased from 93.2%
to 98.8%; with further increase of the temperature, the conversion and selectivity
decreased because of the reversible reactions of cyclotrimerizaion of aldehydes and
decomposition of trialkyl-1,3,5-trioxane in the presence of acid with increasing
temperature.[17]
The degree of isobutylaldehyde conversion was also influenced by reaction time
(Table 2, entries 4 and 10–12). When the reactant was stirred only 10 min, the isobu-
tylaldehyde conversion of 91.4% and triisopropyl-1,3,5-trioxane selectivity of 99.9%
could be reached, suggesting the high efficiency of the IL catalyst. After 60 min, the
whole reaction tended to equilibrium with the conversion and the selectivity reached
98.8% and 99.8%.
The applicability of this catalytic system for other aliphatic aldehydes was also
studied, and the results are summarized in Table 3. All of the aliphatic aldehydes
examined were converted into the corresponding trialkyl-1,3,5-trioxane with high
a
Table 3. Cyclotrimerization of several aliphatic aldehydes catalyzed by [Bis-BsPy][CF3SO3]2
Entry
Aldehyde
Conversion (%)
Selectivity (%)
1
CH3CH2CHO
93.4
90.1
98.8
58.2
62.5
63.4
52.2
99.8
99.4
99.8
99.6
99.9
99.8
99.7
2
CH3(CH2)2CHO
(CH3)2CHCHO
(CH3)2CHCH2CHO
CH3(CH2)3CHO
CH3(CH2)4CHO
CH3(CH2)6CHO
3
4
5b
6b
7b
aAldehyde (30 mmol), [Bis-BsPy][CF3SO3]2=aldehyde ¼ 1% (molar ratio), reaction temperature 25 ꢀC,
reaction time 60 min.
b[Bis-BsPy][CF3SO3]2=aldehyde ¼ 2% (molar ratio).

270
H. SONG ET AL.
Figure 1. Recycling of [Bis-BsPy][CF3SO3]2 in the cyclotrimerization of isobutylaldehyde. Reaction con-
ditions: isobutylaldehyde (30 mmol), [Bis-BsPy][CF3SO3]2=isobutylaldehyde ¼ 1% molar ratio, reaction
temperature 25 ꢀC, reaction time 60 min.
selectivity. Table 3 showed that the conversion of aliphatic aldehydes decreased with
the growth of the carbon chain (Table 3, entries 1–2 and 5–7) and the cyclotrimer-
ization of stright-chain aldehydes was more difficult compared with that of
branched-chain aldehydes, which has the same number of carbons (Table 3, entries
2–5). We considered that the reason was the nucleophilic addition feature of the
cyclotrimerization. Therefore, the reaction depends dramatically on the steric
hindrance and the stabilization of the carbenium ion. The increasing length of
the carbon chain enhanced the steric hindrance, which leads to the decrease of
nucleophilic ability, and the branched chain may favor the formation of the stable
carbenium ion and enhanced nucleophilic ability.
One of the greatest advantages of using ILs as catalysts is the convenient
recovery because of unique solubility. The ILs with double –SO3H functional groups
are hardly soluble in common organic solvents except for methanol and ethanol. The
immiscibility of [Bis-BsPy][CF3SO3]2 with n-hexane and the good solubility of
triisopropyl-1,3,5-trioxane in n-hexane make the separation of the catalyst quite
facile. After reaction, the IL was extracted by n-hexane (5 mL ꢂ 3) and vacuumized
for 2 h at 80 ꢀC for another cycle. The reusability of the recycled catalyst is shown in
Fig. 1. It could be seen that [Bis-BsPy][CF3SO3]2 was utilized repeatedly more than
eight times without any apparent loss of conversion and selectivity.
CONCLUSION
In conclusion, a highly effective catalytic cyclotrimerization of aliphatic
aldehydes using 1 mol% novel Brønsted acidic ILs as catalysts was successfully
processed under mild reaction conditions, the conversion and selectivity could be
up to 98.8% and 99.8%, respectively. The IL could be separated by extraction with
n-hexane, and the activity did not change when it was recycled eight times. The
present study shows that the novel double functional acidic ILs have potential appli-
cation in the production of trialkyl-1,3,5-trioxane.

CYCLOTRIMERIZATION OF ALDEHYDES
271
EXPERIMENTAL
Measurements
1H NMR spectra and 13C NMR spectra in D2O were recorded on an Inova
400-MHz instrument using tetramethylsilane (TMS) as internal standard. Elemen-
tary analysis were obtained with an Elementar Vario EL cube instrument. The pro-
ducts were identified and quantitatively analyzed by gas chromatography=mass
spectrometry (GC=MS) (Agilent 7890=5975C) and GC (Agilent 6890 equipped with
a SE-54 capillary column).
[Bis-BsPy][HSO4]2
4,40-Di-(butyl-4-sulfonate) dipyridine was prepared according to a similar
procedure.[18] A mixture of 1,4-butanesultone and 4,40-dipyridine in toluene was
stirred at 80 ꢀC for 12 h (molar ratio of 1,4-butanesultone to 4,40-dipyridine is
2.1:1). After the reaction was completed, the obtained solid was filtered, washed with
toluene and ether, and dried in vacuo. 4,40-Di-(butyl-4-sulfonate) dipyridine was
obtained as a white powdery solid in 89% yield. A stoichiometric amount of
H2SO4 was added dropwise to 4,40-di-(butyl-4-sulfonate) dipyridine. The mixture
was stirred for 8 h at 60 ꢀC to form the IL [Bis-BsPy][HSO4]2. The IL phase was then
washed with toluene and dried in vacuo. The product was a yellow viscous liquid.
1
Yield: 96%. H NMR D2O (d ppm): 1.613 (h, J ¼ 7.7 Hz, 4 H), 2.005 (h, J ¼ 7.6 Hz,
4 H), 2.757 (t, J ¼ 7.6 Hz, 4 H), 4.538 (t, J ¼ 7.4 Hz, 4 H), 8.313 (d, J ¼ 6.4 Hz, 4 H),
9.880 (d, J ¼ 6.8 Hz, 4 H); 13C NMR D2O (d ppm): 20.735, 29.185, 49.781, 61.297,
126.940, 145.328, 149.989. Anal. calcd. for C18H28N2O14S4: C, 34.61; H, 4.52; N,
4.49; S, 20.53. Found: C, 34.74; H, 4.56; N, 4.45; S, 20.43.
[Bis-BsPy][CF3SO3]2, [Bis-Bspy] [Ts-OH]2, and [BsPy][CF3SO3]
The procedure was the same as described for the preparation of [Bis-BsPy]
[HSO4]2, and a yellow viscous liquid was obtained.
[Bis-BsPy][CF3SO3]2. 1H NMR D2O (d ppm): 1.747 (h, J ¼ 7.7 Hz, 4 H),
2.147 (h, J ¼ 7.6 Hz, 4 H), 2.895 (t, J ¼ 7.6 Hz, 4 H), 4.679 (t, J ¼ 7.2 Hz, 4 H),
8.446 (d, J ¼ 6.8 Hz, 4 H), 9.027 (d, J ¼ 7.2 Hz, 4 H); 13C NMR D2O (d ppm):
20.789, 29.260, 49.806, 61.380, 121.102 (JC-F ¼ 315.3 Hz CF3), 126.226, 145.410,
150.095. Anal. calcd. for C20H26N2O12S4F6: C, 32.97; H, 3.60; N, 3.85; S, 17.60.
Found: C, 32.80; H, 3.61; N, 3.51; S,17.58.
[Bis-BsPy] [Ts-OH]2. 1H NMR D2O (d ppm): 1.589 (h, J ¼ 7.7 Hz, 4 H), 1.950
(h, J ¼ 7.7 Hz, 4 H), 2.103 (s, 6 H), 2.738 (t, J ¼ 7.6 Hz, 4 H), 4.464 (t, J ¼ 7.2, 4 H),
7.052 (d, J ¼ 8.0 Hz, 4 H), 7.380 (d, J ¼ 8.0 Hz, 4 H), 8.190 (d, J ¼ 6.4 Hz, 4 H), 8.796
(d, J ¼ 6.8 Hz, 4 H); 13C NMR D2O (d ppm): d 20.339, 20.792, 29.253, 49.795,
61.309, 125.184, 126.783, 129.556, 139.302, 142.253, 145.246, 149.528. Anal. calcd.
for C32H40N2O12S4: C, 49.73; H, 5.22; N, 3.62; S, 16.59. Found: C, 49.79; H, 5.16;
N, 3.58; S, 17.01.

272
H. SONG ET AL.
[BsPy][CF3SO3]. 1H NMR D2O (d ppm): 1.635 (h, J ¼ 7.8 Hz, 2 H), 2.027
(h, J ¼ 7.6 Hz, 2 H), 2.816 (t, J ¼ 7.6 Hz, 2 H), 4.512 (t, J ¼ 7.6 Hz, 2 H), 7.923 (t,
J ¼ 7.2 Hz, 2 H), 8.416 (t, J ¼ 4.6 Hz, 1 H), 8.725 (d, J ¼ 5.6 Hz, 2 H); 13C NMR
D2O (d ppm): 20.779, 29.236, 49.851, 61.108, 117.956 (JC-F ¼ 315.4 Hz CF3),
128.214, 144.157, 145.590. Anal. calcd. for C10H14NO6S2F3: C, 32.88; H, 3.86; N,
3.83; S, 17.55. Found: C, 32.71; H, 3.73; N, 3.87; S, 17.69.
[Bis-Bpy][HSO4]2
A stoichiometric amount of 1-bromobutane was added dropwise to a solution
of 4,40-dipyridine in toluene within an hour (molar ratio of 1-bromobutane to
4,40-dipyridine is 2.1:1), and then the mixture was stirred at 80 ꢀC for 12 h. The
obtained white powdery solid was filtered, washed with toluene and ether, and dried
in vacuo to afford pure 4,40-dibutyl dipyridine bromide. Yield: 92%. The white pow-
der and H2SO4 were mixed in a molar ratio of 1:2 in anhydrous toluene and heated
at 60 ꢀC for 8 h, followed by washing with toluene and drying in vacuo to obtain the
final IL. The product was a yellow viscous liquid. Yield: 91%. 1H NMR D2O
(d ppm): 0.759 (t, J ¼ 7.2 Hz, 6 H), 1.195 (m, J ¼ 7.5 Hz, 4 H), 1.858 (h, J ¼ 7.5 Hz,
4 H), 4.520 (t, J ¼ 7.4 Hz, 4 H), 8.330 (d, J ¼ 6.4 Hz, 4 H), 8.890 (d, J ¼ 6.8 Hz, 4
H). 13C-NMR D2O (d ppm): 12.512, 18.606, 32.447, 61.834, 126.780, 145.272,
149.812; Anal. calcd. for C18H28N2O8S2 : C, 46.54; H, 6.08; N, 6.03; S, 13.80. Found:
C, 46.46; H, 5.93; N, 6.02; S, 13.78.
Typical Procedure
In a 50-mL three-necked flask equipped with a thermometer, a reflux condenser
and a N2-inlet valve, aldehyde (30 mmol) and IL (0.03–0.6 mmol) were added. The
reaction mixture was stirred at 0–60 ꢀC for 10–120 min. After reaction, n-hexane
was added to the flask to dissolve the crystals, the products were identified and
quantitatively analyzed by GC=MS (Agilent 7890=5975C) and GC (Agilent 6890
equipped with a SE-54 capillary column).
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