Increased SBA-15-SO3H catalytic activity through hydrophilic/hydrophobic fluoroalkyl-chained alcohols (RFOH/SBA-15-Pr-SO3H)

Article abstract of DOI:10.1055/s-0034-1380683

A superior catalytic activity for the SBA-15-functionalized sulfonic acid containing fluoroalkyl chain alcohols (R_FOH/SBA-15-Pr-SO₃H adduct) is presented for the synthesis of highly substituted imidazoles. The advantages of this method include low catalyst loading, simple procedure, excellent yields, recyclability of catalyst, and short reaction times.

Full text of DOI:10.1055/s-0034-1380683

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© Georg Thieme Verlag Stuttgart · New York
2015, 26, 1345–1347
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S. Rostamnia, E. Doustkhaha
Letter
Synlett
Increased SBA-15-SO3H Catalytic Activity through Hydrophilic/
Hydrophobic Fluoroalkyl-Chained Alcohols (RFOH/SBA-15–Pr-SO3H)
Sadegh Rostamnia*a
Esmail Doustkhahb
a Organic and Nano Group (ONG), Department of Chemistry, Faculty of Science,
University of Maragheh, P.O. Box 55181-83111, Maragheh, Iran
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rostamnia@maragheh.ac.ir
srostamnia@gmail.com
b Young Researchers and Elite Club, Maragheh Branch, Islamic Azad University,
Maragheh, Iran
Received: 10.03.2015
Accepted after revision: 03.04.2015
Published online: 11.05.2015
ethylene-bridged periodic mesoporous organosilicas (PMO)
have been investigated,19,20 but these were most suitable for
the hydrophobic compounds. Therefore, development of a
MSM–SO3H catalyst resistant to poisoning by water while
maintaining the suitable hydrophilicity remains a target.
DOI: 10.1055/s-0034-1380683; Art ID: st-2015-d0172-l
Abstract A superior catalytic activity for the SBA-15-functionalized
sulfonic acid containing fluoroalkyl chain alcohols (RFOH/SBA-15–Pr-SO3H
adduct) is presented for the synthesis of highly substituted imidazoles.
The advantages of this method include low catalyst loading, simple pro-
cedure, excellent yields, recyclability of catalyst, and short reaction
times.
Key words fluoroalkyl chain alcohols, TFE/SBA-15–Pr-SO3H, imidaz-
ole, multicomponent reaction
Solid Brønsted acid catalysts have received a great deal
of attention for applications in green and recyclable proto-
cols and their versatility has been demonstrated in the var-
ious processes.1–4 Mesoporous silica materials (MSM), such
as SBA-15, offer high surface area, good thermal stability,
and potential for tunability5,6 by which it is possible to
modify the surface activity and nature of of the MSM.7–9
One approach is to functionalize with mercaptopropyltri-
alkoxysilane when a subsequent oxidation process provides
a surface sulfonic acid functionality.10,11
One of the major concerns in the area of sulfonated
MSM is catalyst poisoning with water, and several groups
have addressed this problem (Figure 1).12–16 Karimi et al.
developed an efficient catalytic system by immobilizing a
hydrophobic ionic liquid onto SBA-15.14,17 Co-functionaliza-
tion of a hydrophobic functionality such as a phenyl group
or alkyl chain with a Brønsted acid function is another ap-
proach to increase the hydrophobicity.16,18 Unfortunately,
this method results in pore-size restriction. Wilson et al.
prepared a monolayer of MPTS which produced a highly
functionalized SO3H inside the MSM-type material.13 This
method was efficient in the fabrication of highly acidic
MSM–SO3H. To prevent pore-size decrease, phenylene- and
Figure 1 Silica/SO3H catalyst poisoning with water
Fluorous compounds have the unusual property of be-
ing both hydrophobic and lipophilic and thus can cause an
increase in the acid strength and decrease catalyst poison-
ing as a result of water-repellency of the surface.21–23 We
have demonstrated that confining fluoroalkyl-chain alco-
hols (RFOH) inside SBA-15 (RFOH/SBA-15) produces an effi-
cient catalytic system by increasing mass transfer and wa-
ter repellency.24–26 and that such catalytic systems can be
applied to the synthesis of 1,2,4,5-tetrasubstituted imidaz-
oles by multicomponent reaction. Imidazoles are key struc-
tures in biological systems and an active moiety in several
drugs.27–30 There are many approaches for the synthesis of
substituted imidazoles.31–35 However, these molecules are
usually polar compounds; therefore, an increase in the sur-
face hydrophobicity of the catalyst may lower the diffusibil-
ity and mass transfer into the mesochannels and, as a re-
sult, there needs to be a balance between hydrophobicity
and polarity (Figure 2).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1345–1347

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S. Rostamnia, E. Doustkhaha
Letter
Synlett
The catalytic behavior of the TFE/SBA-15–Pr-SO3H in
the synthesis of imidazoles43 was then compared to non-
RFOH-functionalized catalysis for imidazoles synthesis,
with the condensation of ammonia, diphenylglyoxal (DPG),
benzyl amine, and benzaldehyde being selected as a model
reaction. TFE was compared with ethanol and HFIP was
compared with isopropanol under similar conditions to
demonstrate that functionalization with fluorinated alco-
hols had a positive effect on the catalytic behavior. The TFE-
modified catalyst had better activity than the HFIP-modi-
fied catalyst, and therefore TFE was selected as the opti-
mum RFOH for further studies.
We then studied the catalytic activity of TFE/SBA-15–
Pr-SO3H in the synthesis of tetrasubstituted imidazole de-
rivatives wherein a selection of primary amines and aro-
matic aldehydes where reacted with ammonium acetate as
ammonia source and phenylglyoxal to produce the desired
imidazoles (Table 1).
Trisubstituted imidazoles were similarly synthesized
using solely ammonium acetate (Table 2).
In these reactions, the TFE/SBA-15–Pr-SO3H could be re-
covered by centrifugation, washed with TFE, and reused at
least for eight runs.
In summary, TFE/SBA-15–Pr-SO3H is a water-tolerant
catalyst with enhanced mass transfer as a result of a bal-
ance between surface hydrophobicity and hydrophilicity.
This results in a suitable environment for production of po-
lar imidazole derivatives, and this system represents an
economical, recyclable solid acid catalyst (Scheme 1).
Figure 2 RFOH/SBA-15–Pr-SO3H for the synthesis of imidazoles
TFE/SBA-15–Pr-SO3H was synthesized according to the
literature procedure.36 SBA-15 was prepared by acidic hy-
drolysis of TEOS in the presence of P123 as a template.37
Then, the calcinated SBA-15 was functionalized with
MPTMS in toluene under reflux. Subsequent oxidation of
the SH groups to SO3H was achieved by excess hydrogen
peroxide.38 Finally, to achieve the RFOH/SBA-15–Pr-SO3H
adduct, fluorous alcohols such as trifluoroethanol (TFE) or
hexafluoroisopropanol (HFIP) were introduced into the sili-
ca mesopores by dispersing the TFE/SBA-15–Pr-SO3H in
TFE or HFIP under reflux. The solid material of TFE/SBA-15–
Pr-SO3H was characterized by FT-IR spectroscopy, TGA, SEM
and TEM, and exhibited a well-ordered hexagonal structure
typical of SBA-15 materials. Bands at 800 and ca. 1150 cm–1
in the FT-IR spectrum correlated to Si–O–Si bond vibrations,
with a weak band at ca. 950 cm–1 attributed to functional-
ized Si–OH bonds. SiO–H groups were also evidenced by a
very broad absorption between 3000–3700 cm–1.
Table 1 Synthesis of 1,2,4,5-Tetrasubstituted Imidazoles by TFE/SBA-15–Pr-SO3Ha
TFE/SBA-15–Pr-SO3H
Ph
O
O
N
(2 mol%)
Ph
RNH2
+
+
Ph
Ar
NH3
Ph
Ar
H
N
R
O
70 °C, 50 min
Entry
3a–j
Ar
Ph
R
Yield (%)b
mp (°C) found
mp (°C) reported
1
2
3a
3b
3c
3d
3e
3f
Ph
94
92
95
91
93
88
92
94
95
94
211–216
160–165
152–154
133–135
139–143
169
211–21839
–
4-ClC6H4
4-ClC6H4
Ph
Bn
Bn
Bn
Ph
3
157–15840
134–13539
140–14239
167–17041
–
4
4-HOC6H4
2-ClC6H4
4-O2NC6H4
4-ClC6H4
3-ClC6H4
3-HOC6H4
Ph
5
6
7
3g
3h
3i
cyclohexyl
195–199
145–147
7
8
Bn
Bn
Bn
144–14540
–
9
10
3j
155–155
163–16539
a Reaction conditions: DPG (1 mmol), aldehyde (1 mmol), amine (1 mmol), NH3 (1.1 mmol), TFE/SBA-15–Pr-SO3H (0.035 g) at 70 °C.
b Isolated yield.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 1345–1347

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S. Rostamnia, E. Doustkhaha
Letter
Synlett
Table 2 Synthesis of Trisubstituted Imidazoles Using TFE/SBA-15–
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Pr-SO3Ha
Ph
TFE/SBA-15–Pr-SO3H
(2 mol%)
O
O
N
Ph
+
NH3
+
Ph
Ar
70 °C
70 °C, 50 min
Ph
Ar
H
N
O
H
Entry
4a–f
Ar
Yield
(%)b
mp (°C) found mp (°C)
reported
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1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
Ph
95
91
94
92
91
95
271
274–27641
–
4-ClC6H4
3-ClC6H4
258–262
191–193
228–231
240–242
198
190–19242
231–23342
243–24542
200–20241
(22) Qiu, Z. M. WO 2002072537 A2, 2003.
4-HOC6H4
4-BrC6H4
2-HOC6H4
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a Reaction conditions: DPG (1 mmol), aldehyde (1 mmol), NH3 (2.2 mmol)
70 °C, TFE/SBA-15–Pr-SO3H (0.035 g).
b Isolated yield.
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Scheme 1 Proposed mechanism for the synthesis of imidazoles
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