B. Karimi et al. / Tetrahedron xxx (2014) 1e6
5
catalyst can be also understood from the low leaching of tungstate
species, which was found to be less than 0.5 ppm by ICP-AES.
for removal of un-reacted starting materials to give almost pure
BTMSPI in 78% isolated yields; dH (250 MHz CDCl3), 0.62 ppm (4H, t,
J 8.1 Hz, CH2Si), 2.00 (4H, tt, J 7.1, J 8.1 Hz CH2CH2CH2), 3.60 (18H, s,
OMe), 4.32 (4H, t, J 7.1 Hz; CH2N), 7.46 (2H, d, J 1.7 Hz; CH]CH),
10.00 (1H, s, NCHN); dC (63 MHz CDCl3) 5.81(CH2Si), 24.12
(CH2CH2CH2), 50.77 (OCH3), 51.76 (CH2N), 122.20 (CH]CH), 136.08
(NCHN).
3. Conclusion
We have shown that tungstate ion immobilized on periodic
mesoporous organosilica comprising bridged ionic liquid
¼
(WO4 @PMO-IL) was prepared through a simple ion exchange
technique is a highly recoverable and effective catalyst in the ac-
tivation of 30% H2O2for the selective oxidation alcohols to their
corresponding carbonyl compounds under a base- or acid-free
conditions without the need of any external phase transfer addi-
tives. The studies showed that primary aromatic or aliphatic alco-
hols were selectively converted to the corresponding aldehyde
without any over oxidation to the corresponding carboxylic acids
under the reaction conditions. Moreover, secondary aliphatic al-
cohols were also oxidized to the corresponding ketones with good
to excellent yields under the same reaction conditions. The catalyst
was successfully recovered from the reaction mixture and reused
for the six subsequent reaction cycles with only slight decrease in
its activity and selectivity. N2 sorption analysis, TEM, TGA indicates
that the recovered catalyst showed no significant changes in the
catalyst nanostructural order and the amount of loaded functional
groups. It is believed that the incorporated imidazolium moieties in
the channel wall of PMO-IL not only provide a means of immobi-
lizing tungstate species through simple ion exchange techniques
and preventing their leaching into the reaction medium but they
can also operate as a supported phase transfer catalyst. Further
work on the practical applications of PMO-IL as heterogeneous
ionic liquid and also as innovative support for the immobilization of
other types of transition metal catalysts through anion-exchange
capabilities of the materials is underway in our laboratories.
4.3. Synthesis of PMO-IL
PMO-IL was also synthesized according to our previously re-
ported methods.25e29 In a typical procedure, Pluronic P123 (1.67 g)
was dissolved in a mixture of H2O (10.5 g), HCl (2 M, 46.14 g), and
KCl (8.8 g), and the mixture was stirred at 40 ꢁC until a homogenous
solution obtained. To this end, a pre-mixed of BTMSPI (2 mmol,
0.86 g) and TMOS (18 mmol, 2.74 g) in dried methanol were im-
mediately added to the above-mentioned solution and stirred at
40 ꢁC for 24 h. The resulting mixture was posed in hydrothermal
treatment without stirring at 100 ꢁC for 72 h. The obtained solid
materials containing surfactant was filtered and carefully washed
with deionized water. The surfactant residue was then extracted
from the materials through a Soxhlet apparatus by using ethanol
(100 ml) and concentrate HCl (37%, 3 ml). In a typical extraction, as-
synthesized PMO (1 g) washed four times with acidic ethanol over
12 h, dried under vacuum for 24 h, to afford PMO-IL as a bright
yellow powder.
4.4. Preparation of WO[4 @PMO-IL
¼
WO4 @PMO-IL was synthesized by simple ion exchange tech-
nique according to our previous procedure with slight mod-
ifications.21b,c For a typical method, PMO-IL (0.5 g) was added to
20 ml of deionized water and sonicated for at least 10 min. Na2WO4,
2H2O (0.090 g, 0.26 mmol) in 3 ml of deionized water was gradually
added to the aforementioned suspension and stirred at room
temperature for 5 h. The excess of tungstate solution was filtered
and successfully washed with deionized water (3ꢂ30 ml) and dried
under vacuum at 65 ꢁC for overnight. The total tungstate loading of
the materials was estimated by inductively coupled plasma atomic
4. Experimental section
4.1. Materials
Sodium Hydride 95%, Pluronic P123 (MWavy5800, Aldrich), and
tetramethoxyorthosilicate (TMOS) were obtained from Aldrich.
Imidazole, 3-chloropropyltrimethoxysilan (CPTMS), potassium
chloride, concd-HCl (37%), and Na2WO4, 2H2O were purchased
from Merck. Imidazole was first re-crystallized in distilled CH2Cl2
and dried in a desiccator under vacuum over dry P2O5 for 3 days at
room temperature.
emission spectroscopy (ICP-AES) to be w0.07 mmol gꢀ1
.
4.5. General procedure for oxidation of alcohols to the cor-
responding carbonyl compound with 30% H2O2
In a two necked flask equipped with a condenser, alcohol
4.2. Synthesis of ionic liquid precursor
(1 mmol), 30% H2O2 (5 mmol) were added to an equal mixture of
¼
water and acetonitrile (1.5 ml). Then, WO4 @PMO-IL (0.23 g,
The ionic liquid BTMSPI precursor was prepared by few modi-
fications of our last synthetic reports.24e29 In a typical experiment,
a suspension of sodium imidazolide in dry THF was prepared from
the direct reaction of freshly dried imidazole (2 g) and NaH 95%
(0.77 g) at a flame-dried two-necks flask containing dry THF (60 ml)
under argon atmosphere. CPTMS (5.4 ml) was added to the men-
tioned stirred suspension and the resulting mixture was refluxed
for 30 h. Then, the reaction mixture was allowed to cool to room
temperature followed by the solvent removal under reduced
pressure until an oily mixture containing NaCl obtained. To this
end, 3-chloropropyltrimethoxysilan (5.4 ml) and dry toluene
(60 ml) were then added and the resulting mixture was refluxed for
48 h until a two phase mixture comprising toluene and ionic liquid
(BTMSPI) obtained. Then, toluene phase was removed and dry
CH2Cl2 (60 ml) was added to remove the precipitated NaCl. In the
next stage, CH2Cl2 phase was transferred to the well-dried/two-
necks flask and the volatiles removed by reduced pressure until
the ionic liquid (BTMSPI) and un-reacted starting materials ob-
tained. Finally, ionic liquid was washed by dry toluene (5ꢂ50 ml)
w1.5 mol %) was added to the above solution and the resulting
mixture was stirred at 90 ꢁC for requisite time. The progress of the
reaction was monitored by gas chromatography using standard
addition method. After completion of the reaction, the mixture was
allowed to cool down to the room temperature and the catalyst was
successfully isolated with centrifugation and washed with CH2Cl2
(3ꢂ10 ml) and dried under the vacuum for 12 h. Then, the collected
CH2Cl2 phase was first washed with water, dried over Na2SO4, and
the solvent was concentrated with evaporation under the reduced
pressure to give the corresponding carbonyl compounds. The re-
covered catalyst was used in the recycling procedure in the same
manner as reported in the first run.
Acknowledgements
The authors thank the Institute for Advanced Studies in Basic
Science (IASBS) and the Iran National Science Foundation (INSF)
(INSF90005382) for supporting this work.