Mesoporous silica-functionalized dual Br?nsted acidic ionic liquid as an efficient catalyst for thioacetalization of carbonyl compounds in water

Article abstract of DOI:10.1016/j.catcom.2013.07.004

A novel silica-functionalized dual Br?nsted acidic ionic liquid (ionic liquid with two Br?nsted acidic species) has been reported as a highly efficient catalyst for thioacetalization of carbonyl compounds. The reaction was efficiently performed in water a

Full text of DOI:10.1016/j.catcom.2013.07.004

Mesoporous silica–functionalized dual Brønsted acidic ionic liquid as an
efficient catalyst for thioacetalization of carbonyl compounds in water
Majid Vafaeezadeh, Zahra Bahrami Dizicheh, Mohammad Mahmoodi
Hashemi
PII:
DOI:
Reference:
S1566-7367(13)00246-X
doi: 10.1016/j.catcom.2013.07.004
CATCOM 3562
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 May 2013
26 June 2013
1 July 2013
Please cite this article as: Majid Vafaeezadeh, Zahra Bahrami Dizicheh, Mohammad
Mahmoodi Hashemi, Mesoporous silica–functionalized dual Brønsted acidic ionic liquid
as an efficient catalyst for thioacetalization of carbonyl compounds in water, Catalysis
Communications (2013), doi: 10.1016/j.catcom.2013.07.004
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ACCEPTED MANUSCRIPT
Mesoporous silica–functionalized dual Brønsted acidic ionic liquid as an
efficient catalyst for thioacetalization of carbonyl compounds in water
Majid Vafaeezadeh, Zahra Bahrami Dizicheh and Mohammad Mahmoodi Hashemi*
Department of Chemistry, Sharif University of Technology, Tehran, Iran. P.O. Box 11155-3615.
Tel.: +98 21 66005718; Fax: +98 21 66029165. E-mail: mhashemi@sharif.edu
Abstract
A novel silica–functionalized dual Brønsted acidic ionic liquid (ionic liquid with two Brønsted
acidic species) has been reported as a highly efficient catalyst for thioacetalization of carbonyl
compounds. The reaction was efficiently performed in water as an environmentally benign
solvent with good to high yields. Thermal gravimetric analysis showed that the catalyst is stable
at least up to 350 °C. Furthermore, the catalyst can be recycled and reused for six runs of
reaction without appreciable loss of activity. The structure of the catalyst is modelized to gain
more realistic structural insight about the functionalized ionic liquid.
Keywords: Silica–functionalized ionic liquid; Thioacetalization; Organic reaction in water;
Theoretical modeling; Density functional theory.
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1. Introduction
Protecting of carbonyl groups in the form of dithioacetal is a useful method in organic synthesis
[1] as they are quite stable over usual acidic and basic conditions [2,3] The dithioacetals can also
be utilized as precursors of acyl anion [4] or masked methylene functions in carbon–carbon bond
formations [5]. They are routinely prepared by the reaction of the carbonyl compounds with thiol
precursor using either Brønsted or Lewis acids such as BF3.OEt [6], LnCl3 [7], Indium triflate
(In(OTf)3) [8], WCl6 [9], p–toluenesulfonic acid (PTSA) [10]. Milder precursors such as lithium
bromide [11] and N–bromosuccinimide (NBS) [12] have also been reported for this reaction.
The major drawbacks of the mentioned catalysts are using chlorinated organic solvents, releasing
hazardous metal wastes and difficulties in catalyst separation. Recently, Brønsted acidic ionic
liquids have been reported as efficient catalyst for thioacetalization [13–16]. Even though, such
approaches were efficient, but homogeneous ionic liquid systems generally suffer from high
viscosity of ionic liquids and requirement of large amounts of these expensive compounds.
Furthermore, the ionic liquids (especially Brønsted acidic ionic liquids) have some degree of
instability in the presence of air and moisture.
Immobilization of the ionic liquid on the surface of a solid support (generally silica gel) is a
useful way to overcome these problems [17–19]. In these methods the homogeneous ionic liquid
catalytic systems transfer into the heterogeneous processes. The supported ionic liquid catalyst
can be readily used in water as a cheap, economical and environmentally benign solvent [20,21].
Herein, we wish to report a dual Brønsted acidic silica–functionalized ionic liquid catalyst for
thioacetalization of carbonyl compounds in water. The study also focused on finding insight
about the catalyst structure by mimicking the surface of the functionalized silica gel. Various
conformers of the initial and deprotonated structures of the supported ionic liquid are explored.
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The value of the ꢀHacidity of the catalyst was calculated for the deprotonation reaction of the
lowest energy conformers. The intra–structural hydrogen bonds (H–bonds) in the catalyst are
studied in detail using natural bond orbital (NBO) [22] and the quantum theory of atoms in
molecules (QTAIM) analyses [23,24].
2. Results and discussion
The catalyst was prepared by the reaction of the 3–chloropropyl silica and B in the presence of
potassium iodide (Scheme 1). The presence of the KI can facilitate the reaction between B and
3–chloropropyl silica via in situ formation of 3–iodopropyl silica (see electronic supplementary
information for details of the experimental procedure).
<Scheme 1>
FT–IR spectra of pure silica, 3–chloropropyl silica and catalyst 1 are shown in Figure 1. The FT–
IR analysis for the catalyst 1 shows the characteristic bands at 604 and 1177 cm–1 which is
attributed to the S–O and S=O stretching frequencies. The band at 1429 and 1503 cm–1 belong to
C=N of the imidazolium ring of the supported IL proving the successful preparation of the
catalyst 1
.
<Figure 1>
To investigate the thermal stability, thermal gravimetric analysis (TGA) was performed for the
catalyst 1 (Figure 2). The first weight loss around the 70 °C is attributed to the residual
physisorbed water and/or organic solvent which were applied during the preparation of the
catalyst. The largest weight loss which was occurred between 350 to nearly 500 °C is related to
the decomposition of the covalently attached ionic liquid from the surface of the silica gel.
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<Figure 2>
The performance of the catalyst 1 was investigated for thioacetalization of 4–
methoxybenzaldehyde (as a sample reaction) with thiophenol in the water (Table 1). In this
reaction, the corresponding thioacetal was obtained in 96% yield when 5 mol% (proton
equivalent) of the catalyst 1 was applied. Encourage by this result, we then manage
thioacetalization of the various available aldehydes and ketones with different thiol precursor
(thiophenol, 1,2–ethanedithiol and 1,3–propanedithiol).
<Table 1>
As shown in Table 1, various types of aromatic aldehydes with electron
–
donating and electron
–
withdrawing groups as well as cyclohexane 1,4 dione were converted to the corresponding
–
–
thioacetals in high yield. Similarly, a less reactive benzophenone was also successfully converted
to the thioacetal derivative in 54% yield (Entry 11, Table 1).
The recycling experiments were also performed for the catalyst 1 in thioacetalization of 4–
methoxybenzaldehyde and thiophenol at the optimized reaction condition (Entry 1, Table1). For
this purpose, after the first run, the catalyst 1 was simply filtrated, washed with hexane and was
subjected to another reaction with the same condition. Interestingly, it was observed that the
catalyst 1 can be recovered and reused for at least five more consecutive reaction runs (Figure 3).
The average yields for six runs were more than 90%, which clearly demonstrates the practical
reusability of this catalyst. The small decrease in the catalytic activity may be attributed to the
–
small leaching of the supported Brønsted acidic ionic liquid groups and/or losing the HSO4
moieties during the reaction from the surface of the mesoporous catalyst.
<Figure 3>
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Computational methods are fruitful ways to study the functionalized surface of the various
supports [25] to gain useful information about the catalyst structure. Recently, Shanks and co–
worker modelized the surface of a mesoporous propyl sulfonic acid catalyst in the form of
relatively simple fixed –Si(OMe)3 groups and performed the calculation for this structure [26].
However, using of relatively large cage–like structure of silica gel leads to perform more
accurate mimicking of the surface of the catalyst and thus earning more reliable computational
results [27–29]. In this study, we attached the acidic ionic liquid on a Si4O6H3 cage–like cluster
of the silicon–oxygen sequences to modelize the surface. The lowest energy conformers were
selected and optimized at B3LYP/6–311++G** level of calculation with the aim of finding
global minimum energy structures [30]. Vibrational frequencies were calculated for the
optimized geometries to ensure that each structure is the real minima. The optimized structures
of the initial acid catalyst (the overall charge is zero and donated by Ac) and their corresponding
deprotonated anions (the overall charge is –1 and donated by An) are shown in the best
directions according to the energy in Figure 4.
<Figure 4>
–
It is worth mentioning that the sulfonic acid specie is significantly stronger acid than HSO4
moiety. Thus, the first deprotonation of the catalyst 1 was occurred from the sulfonic acid head
–
group rather than HSO4 specie [18]. The An1 has the lowest energy among the other anions. It
is mainly stabilize by the intra–structural hydrogen bonding and electrostatic interaction.
To gain insight about the acidity, the value of the ꢀHacidity of the catalyst 1 was calculated
according to the literature report [31]. In order to understand the accuracy of the presented
calculation method, the gas phase ∆Hacidity was calculated for a known compound,
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methanesulfonic acid, at B3LYP/6–311++G** and the results were compared with the
experimental value. The calculated value of ∆Hacidity for the methanesulfonic acid is 319 kcal/mol
which is in good agreement with experimental value (320.9± 2.2 kcal/mol) [32]. Next, we
calculated the ∆Hacidity of the catalyst 1 from the deprotonation reaction of Ac1 to An1 (the global
minimum energy conformers). The result shows that the ∆Hacidity of the catalyst 1 is 309.4
kcal/mol which is approximately 10 kcal/mol smaller (i.e. stronger acid) than methanesulfonic
acid. It is worth mentioning that methanesulfonic acid is known as a strong Brønsted acid with
pKa= –1.9.
The acidity of catalyst was also investigated measuring Hammett acidity parameter (H0)
according to the literatures [33,34]. The Hammett acidity function expresses the acidity strength
of an acid in aprotic organic solvents by the following equation:
[In]s
H0 = pK (In)aq + log
[HIn]s
where [In] and [HIn] are concentration of the indicator and protonated indicator which can be
determined by UV–vis spectrum. Concentrated sulfuric acid and methane sulfonic acid fully
protonate the indicator and thus no H0 value define for these acids. The value of the H0 for the
catalyst 1 measured to be 1.22 which refers to strong deprotonation potential capacity of the
supported Brønsted acid ionic liquid (Table 2).
<Table 2>
To provide evidence about the nature of the hydrogen bonds, NBO and QTAIM analyses were
performed for Ac1–4 and An1–4. In the NBO analysis, the interaction between the electron pairs
of the H–bond acceptor atoms and the antibonding orbitals of the H–bonds donor is investigated
as a characteristic of Hydrogen bonding. The second order perturbation stabilization energy, E(2),
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is related to the delocalization of electrons in donor–acceptor pair and was calculated from
perturbation theory [22]. The higher value of E(2) indicates a higher delocalization trend of
electron from donor to acceptor orbitals, and thus stronger interaction. The selected values of E(2)
for donor–acceptor (lp–σ*) interactions are listed in Table 3. Charge transfer (ꢀqCT) from donor
to acceptor is another interesting result derived from the NBO analysis. Formation of H–bond
leads to observed charge transfer from the donor sites to the acceptors. The value of charge
transfer can be attributed to the strength of formed hydrogen bonds. As shown in Table 3, shorter
H–bonds generally have higher ꢀqCT values.
The nature of hydrogen bonding can also explored by QTAIM analysis. One of the useful
features of this program is the topological analysis of electron density ρ(r) at the bond critical
points (BCPs) of the optimized structures. The strength of a bond is described by the ρ(r)
parameter which has larger values for stronger bonds. The results of the QTAIM analysis
comprising the values of ρ(r) and V(r) at the BCP and the energies of the H–bonds (E H…O) are
shown in Table 3.
<Table 3>
3. Conclusion
A silica–based dual acidic ionic liquid catalyst has been prepared for thioacetalization of various
carbonyl compounds in water. The catalyst showed high thermal stability and six–time
recyclability without appreciable loss of activity. The advantages of the presented work over
homogeneous ionic liquid system are reducing reaction cost, simple work–up procedure of the
catalyst and highly accessible reactive centres on the surface of mesoporous material. The
structure of the catalyst was also theoretically studied and modelized considering various
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conformers to find global minimum energy of the proposed structure. This modelling gives
useful insight about the structure of the catalyst in comparison of simple paper drawing
structures. To the best of our knowledge this is the first example which the energetic and the
ꢀHacidity of a supported ionic liquid is theoretically explored.
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List of Table and Figure captions
Scheme 1. Schematic illustration to preparation of the catalyst 1.
Figure 1. FT–IR spectra of pure silica, 3–chloropropyl silica and catalyst 1.
Figure 2. TGA of the catalyst 1.
Figure 3. Recyclability of the catalyst 1 for thioacetalization of 4–methoxybenzaldehyde with
1,2–ethanedithiol at optimized reaction condition.
Figure 4. The structures of Ac1–4 and An1–4 optimized at B3LYP/6–311++G**. The lengths of
the selected H–bonds are shown in Å.
Table 1. The data of the catalytic performance for thioacetalization of various carbonyl
compounds in the reflux of the water.
Table 2. Comparison of Hammett acidity function (H0) of the catalyst 1 and other dual Brønsted
acidic ionic liquid (BAIL) based catalysts.
Table 3. The values of the total energy, significant values of E(2) (kcal/mol), ꢀqCT and the results
of the selected bond critical point related to the Ac1 and An1.
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Scheme 1
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Table 1
Entry
1
Substrate
Time (h) Yield (%)a
Product
SPh
SPh
SPh
H3CO
CHO
H3CO
5
5
3
96
91
94
2
3
H3C
H3CO
O
CHO
CHO
H3C
SPh
S
H3CO
S
S
S
S
S
O
4
5
6
7
89
95
93
S
CHO
2.5
3
S
Br
Br
S
S
Cl
CHO
CHO
Cl
S
S
7
2
2
3
2
95
96
92
85
H3C
H3C
S
S
8
H3C
H3C
H3CS
CHO
H3C
H3C
H3CS
S
CHO
CHO
9
S
S
10
S
S
S
11
O
24
54
a Yields refer to the isolated pure products and gave satisfactory
1H NMR spectra.
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Table 2
a
Entry
Catalyst
H2SO4
CH3SO3H
Catalyst 1
Dual BAIL
Poly(Dual BAIL)
H0
Ref.
-
-
This work
33
34
b
1
2
3
4
5
–
b
–
1.22
0.88
1.83
a
Condition for Hammett acidity function: solvent: CH2Cl2,
indicator: 4-nitroaniline (pK(I)aq = 0.99), 5.0×10−4 mol/L.
Catalyst 1: 7.5 ×10−3 mol/L (proton equiv.), 25 °C.
b Full protonation of indicator.
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Table 3
b
c
Structure Total energya Charge transfer
r H…O E(2)
ꢀqCT
BCP
ρ(r) V(r)
-E H…O
Ac1
-3476.039993 lp O45→σ*H41–O40 1.901 8.42 0.164 O45…H41–O40 0.027 -0.0214 6.71
lp O42→σ*H36–O35 1.596 36.86 0.642 O42…H36–O35 0.054 -0.0562 17.63
-3475.549306 lp O4→σ*H52–O51 1.815 11.96 0.205 O4…H52–O51 0.036 -0.0324 10.17
lp O49→σ*H32–C30 2.126 5.47 0.198 O49…H32–C30 0.019 -0.0122 3.83
An1
a The values of total energy are reported in a.u.
b r H...O is the length of H–bonds which are reported in Å.
c
The energy of H–bonds reported in kcal/mol which are calculated according to the following equation: E
=
H...O
V(r)/2×627.5 [35].
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Graphical abstract
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Highlights
•
•
•
Silica supported dual Brønsted acidic ionic liquid.
Water–tolerant catalyst for thioacetalization of carbonyl compounds in water.
Theoretical modeling of the catalyst structure.
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