Synergistic catalysis within TEMPO-functionalized periodic mesoporous organosilica with bridge imidazolium groups in the aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c6ra15483b

Anchoring 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) within the nanospaces of a periodic mesoporous organosilica with bridged imidazolium groups led to an unprecedented powerful bifunctional catalyst (TEMPO@PMO-IL-Br), which showed enhanced activity in the metal-free aerobic oxidation of alcohols. The catalyst and its precursors were characterized by N₂ adsorption-desorption analysis, transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), solid state electron paramagnetic resonance (EPR) spectroscopy, elemental analysis, transmission electron microscopy (TEM) and high resolution TEM. It was clearly found that the catalytic activity of SBA-15-functionalized TEMPO (TEMPO@SBA-15) not bearing IL, TEMPO@PMO-IL-Cl, PMO-IL-AMP, or individual catalytic functionalities (PMO-IL/TEMPO@SBA-15) was inferior as compared with those obtained from TEMPO@PMO-IL-Br in the metal-free aerobic oxidation of benzyl alcohol, suggesting the critical role of co-supported TEMPO and imidazolium bromide in obtaining high catalytic activity in the described catalyst system. Our observation clearly points to the fact that the combination of imidazolium bromide units in close proximity to TEMPO moieties in the nanospaces of TEMPO@PMO-IL-Br might be indeed one of the key factors explaining the enhanced catalytic activity observed for this catalyst in the oxidation of benzyl alcohol, possibly through a synergistic catalysis relay pathway. A proposed model was suggested for the observed synergistic effect.

Full text of DOI:10.1039/c6ra15483b

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Synergistic Catalysis within the TEMPO-Functionalized Periodic
Mesoporous Organosilica with Bridge Imidazolium Groups in the
Aerobic Oxidation of Alcohols
Received 00th January 20xx,
Accepted 00th January 20xx
Babak Karimi,*a Saleh Vahdati,a Hojatollah Vali,b
DOI: 10.1039/x0xx00000x
Anchoring 2,2,6,6-tetra-methylpiperidine-1oxyl radical (TEMPO) within the nanospaces of
a periodic mesoporous
www.rsc.org/
organosilica with bridged immidazolium group led to an unprecedented poweful bifunctutional catalyst (TEMPO@PMO-IL-
Br), which showed enhanced activity in the metal-free aerobic oxidation of alcohols. The catalyst and its precursors were
characterized by N2 adsorption-desorption analysis, transmission electron microscopy (TEM), small angle X-ray scattering
(SAXS), thermal gravimetric analysis (TGA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), solid state
electron paramagnetic resonance (EPR) spectroscopy, elemental analysis, transmission electron microscopy (TEM) and
high resolution TEM. It was clearly found that the catalytic activity of of SBA-15-functionalized TEMPO (TEMPO@SBA-15)
not bearing IL, TEMPO@PMO-IL-Cl, PMO-IL-AMP, or individual catalytic functionalities (PMO-IL/ TEMPO@SBA-15) was
inferior as compared with those obtained from TEMPO@PMO-IL-Br in the metal-free aerobic oxidation of benzyl alcohol,
suggesting the critical role of co-supported TEMPO and imidazolium bromide in obtaining high catalytic activity in the
described catalyst system. Our observation clearly points to the fact that the combination of imidazolium bromide units in
the close proximity of TEMPO moieties in the nanospaces of TEMPO@PMO-IL-Br might be indeed one of the key factor
explaining the enhanced catalytic activity observed for this catalyst in oxidation of benzyl alcohol possibly through a
synergistic catalysis relay pathway.
A
proposed model was suggested for the observed synergistic effect.
counter ion of imidazolium IL provided the necessary Br2/Br-
redox couple in order to ensure fast electron transfer from
Introduction
alcohol to molecule oxygen through
a catalysis relay
In the past decades imidazolium-based ionic liquids have
received increasing attention due to their unique properties
related to very low vapour pressure, the capability to dissolve
both organic and inorganic reactants, and tuneable structure
and polarity. In this context, significant advances have been
achieved in the application of ionic liquid (ILs) as potential
green replacement media for volatile organic solvents in
organic synthesis and homogeneous catalysis.[1] In particular,
the significance of using functionalized ionic liquid (so called
task specific IL, TSIL) has been well drawn in various types of
chemical transformations and organic synthesis under
homogeneous reaction conditions.2 Among all the systems, the
use of imidazolium ionic liquid functionalized with 2,2,6,6-
tetra-methylpiperidine-1oxyl (TEMPO) is recognized as an
effective approach for the selective aerobic oxidation of
alcohols in combination with NaNO2 as co-catalyst.3, 4 In these
studies, NaNO2 was thought to be a NO-source while halide
mechanism.4 However, several drawbacks like, high cost,
evidence of toxicity even at low concentration,5 unsuitable
viscosity and tedious separation procedure hinder the
widespread applications of homogeneous ionic liquid systems.
For these reasons, the use of ionic liquids as homogeneous
reaction media is often debatable from the view point of green
and sustainable chemistry. Hence, in order to avoid the
intrinsic disadvantages of using homogeneous ionic liquids,
their efficient heterogenization is highly desirable because it
considerably reduces the amount of expensive ILs while
allowing easy separation and recycling of both IL reaction
media and utilized catalyst system from the reaction products.
Inspired by the synergistic effect of ionic liquids in TEMPO-
catalyzed selective alcohol oxidation,3 the possibility of
designing novel catalyst system with improved catalytic
activity by an appropriate combination of IL/TEMPO system on
a suitable support which enable efficient recycling of both
system have been described.6 It was suggested that the high
synergism in these IL/TEMPO Supported systems is most likely
caused by the confinement of bromide counter ion of IL in the
close spatial proximity of immobilized TEMPO in the same
support.6 What is appealing in these systems is that the
described synergistic effect produces immediately a much
better catalytic activity and/or selectivity in comparison with
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their individual counterpart.6 Although, the high cost of
a saturated aqueous solution of NaBr to afford the
homogeneous TEMPO, can be avoided to a great extent by corresponding catalyst TEMPO@PMO-IL-Br.8
these systems, in most cases the use of non-covalent IL in the The N2 adsorption analysis for all materials (PMO-IL, PMO-IL-
form of physisorbed (release and catch system) is potentially AMP and TEMPO@PMO-IL-(Br)) showed a typical type-IV
restricting the efficient IL recovery. We have recently engaged isotherm pattern with a sharp hysteresis loop at P/P0=0.6-0.8,
in developing several periodic mesoporous organosilica (PMO) which is characteristic of highly ordered mesoporous materials
with extraordinary robust bridged imidazolium network so with narrow pore size distribution and 2-dimensional
called PMO-IL. These material was found to be excellent hexagonal pore structure (Figure 1).
supported reaction media and/or versatile support for
immobilization and stabilization of varied transition metal
nanoparticles, in several important synthetic transformation.7
With much compelling evidences, it was shown that these
functional catalysts combines the molecular diversity of the IL-
like structures with highly durable catalytic activity of metal
catalysts in a single solid. Considering the excellent synergy in
combining the IL with TEMPO, we wondered whether the
covalent immobilization of TEMPO onto the surface of PMO-IL
in close proximity of bridged imidazolium group might not only
amplify the catalytic performance of the supported TEMPO but
Figure 1. N2 adsorption–desorption isotherms (left) and BJH pore size distribution
it can also combine the advantages of employing molecular
oxygen through a catalysis relay approach with the possibility
of recycling both IL and TEMPO at the same time.
The main text of the article should appear here with headings
as appropriate.
(right) of PMO-IL (dark blue), PMO-IL-AMP (green) and TEMPO@PMO-IL-Br (red), and
recovered TEMPO@PMO-IL-Br (blue)
Total pore volume and pore diameter was estimated by
measuring the volume of nitrogen adsorbed at P/P0=0.99 and
BJH model based on the adsorption branch of the isotherms,
respectively. PMO-IL has a BET surface area of 578.8 m2g-1, a
regular pore diameter of 10.6 nm and a total pore volume of
1.08 cm3g-1. By comparing of nitrogen adsorption analysis data
of TEMPO@PMO-IL- with PMO-IL itself, surface area and pore
volume decrease to 351 m2g-1 and 0.77 cm3g-1, respectively.
These results confirmed to some extent that the organic
moieties were successfully grafted inside the mesochannels.
Furthermore, the IR spectrum of TEMPO@PMO-IL(Br), beside
the signals that are characteristic of alkylimidazolium groups
[3125 cm-1 (for unsaturated C-H stretching), 3050, 2918 cm-1
(aliphatic C-H stretching), 1620 cm-1 (C=N stretching of
imidazolium ring), 1558 cm-1 (C=C stretching of imidazolium
ring), 1442 cm-1 (C-H deformation vibrations), 700-790 cm-1
(for C-Si stretching vibrations)], exhibits a new band at 1348
cm-1 which is reasonably related to the N-O free radicals
(Figure 2).9
Results and Discussion
To realize this hypothesis, PMO-IL was initially prepared by
hydrolysis
trimethoxysilylpropyl)imidazolium chloride and TMOS in the
presence of Pluronic P123 as template under acidic
and
co-condensation
of
1,3-bis(3-
a
conditions according to our previously reported procedure.7,8
The resulting PMO-IL were then allowed to react with (3-
aminopropyl) trimethoxysilane followed by reductive
amination with 4-oxo-2,2,6,6-tetramethyl-1-piperidinyloxy (4-
oxo-TEMPO) in the presence of NaBH3CN to furnish the
corresponding PMO-IL-supported TEMPO (TEMPO@PMO-IL-Cl)
(Scheme 1).
O
OSi
O
NH2
O
O Si
OMe
PMO-IL-AMP
NH2
Toluene, reflux, 18 h
PMO-IL
1
)
O
O
O
Si
O
4
Si
N N
Cl-
-
O
O
O
2
)
x
M
o
N
-
e
a
T
O
B
E
r
H
M
,
N
,
N
H
P
3
O
d
O
,
,
,
Si
N
Br
Si
R
1
T
a
2
B
h
H
R
R
O
C
N
N
R
R
N
H
R
R
Figure 2. FT-IR spectroscopy of PMO-IL, TEMPO and TEMPO@PMO-IL-Br.
Scheme1. Schematic pathway for the preparation of TEMPO@PMO-IL-Br.
This clearly indicated successful loading of TEMPO on the
surface of PMO-IL. This statement was further supported by
elemental analysis (CHN analysis) and thermogravimetric
analysis (TGA), estimating the loading of TEMPO and IL group
Since the catalytic activity of supported TEMPO can
remarkably be relied on the synergy of anionic part of IL-like
bridged imidazolium groups, the anion Clˉ was exchanged using
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at 0.25 and 0.98 mmol g-1, respectively (Figure S3-S5, ESI).
Moreover, TEM images of TEMPO@PMO-IL clearly show highly
ordered cylindrical pores with uniform size, which is in good
agreement with the results obtained from N2-adsorption-
desorpsion analysis (Figure 3). The additional structural
information about TEMPO@PMO-IL-Br was obtained by small
angle X-ray scattering (SAXS) and solid state electron
paramagnetic resonance (EPR) spectroscopy. The SAXS
patterns exhibited three reflections indexed as (100), (110),
and (200) according to the highly ordered 2D hexagonal
mesostructures (P6mm space group, Figure S6).
Figure 4. Solid state EPR spectrum of TEMPO@PMO-IL-Br
To assign the optimal experimental conditions, the impact of
various nitrite sources such as tert-butylnitrite (TBN) and
NaNO2 in different solvent such as water, toluene,
water/acetic acid (50 mol%), toluene/acetic acid (50 mol%)
and pure acetic acid were examined. Our initial investigations
showed that in the presence of TEMPO@PMO-IL-Br (0.75
mol%) and atmospheric pressure of O2 (622 Torr) in
combination with either NaNO2 or TBN, the oxidation of benzyl
alcohol was quite inefficient (Table 1, entries 1-5). Although
the conversion was improved to some extent by decreasing
the volume of water no yield better than 27% was obtained
(Table 1, entry 5). A more or less the same result was observed
by employing NaNO2 in toluene under otherwise the same
reaction conditions (Table 1, entries 6, 7). However,
a
considerable improvement was achieved when TBN (instead of
NaNO2) with a little bit more AcOH (30 mg) as additives were
employed and a benzaldehyde yield up to 40% was obtained
(Table 1, entries 8, 9). Notably, no significant oxidation of
benzyl alcohol in the absence of TBN, TEMPO@PMO-IL-Br or
AcOH occurred under the same reaction conditions (Table 1,
entries 10-12). Further screening of the reaction condition
Figure 3. a, b) HRTEM images TEMPO@PMO-IL-Br before and after recycling from the
aerobic oxidation of benzyl alcohol respectively (scale bar: 5 nm); c, d) TEM images of revealed that the oxidation of benzyl alcohol could be
TEMPO@PMO-IL-Br with low magnification (scale bar: 100 nm) before and after
efficiently proceeded in the presence of TEMPO@PMO-IL-Br
recycling from the aerobic oxidation of benzyl alcohol respectively
(1.5 mol%), TBN (10 mol%), AcOH (30 mg), in toluene (0.5 ml)
under atmospheric pressure of O2 at 50 °C (Table 1, entries 13,
The solid state EPR spectrum showed a triplet with a g value at
14). It was also noted that the decreasing of either the
≈ 2.007, which is very close to that of sol-gel entrapped
TEMPO@PMO-IL-Br loading or reaction temperature resulted
TEMPO.10 This spectra strongly confirms the presence of N-O
in significantly lower yields (Table 1, entries 10, 13). Notably,
radical moiety in TEMPO@PMO-IL-Br (Figure 4, please also
the use of SBA-15-functionalized TEMPO (TEMPO@SBA-15,
0.33 mmol TEMPO g-1),4b TEMPO@PMO-IL-Cl, PMO-IL-AMP,
see: Figure S7, S8).
We first tested the catalytic activity of PMO-IL-TEMPO in the
and PMO-IL/TEMPO@SBA-15 was shown to afford
aerobic oxidation of benzyl alcohol as a model substrate, in the
considerably lower product yields, demonstrating the critical
presence of molecular oxygen in combination with varied NOx
role of both co-supported TEMPO and ILs in obtaining
sources (Table 1). In this regard, this study aims at identifying
satisfactory catalytic activity (Table 1, entries 15-18). In all of
whether imidazolium groups in close proximity of TEMPO
these cases, the total amounts of the materials were adjusted
inside the nanospaces of our catalyst system in combination
to keep constant the mol% of functionalized TEMPO and
with NO/NO2 may cooperatively bridge the large kinetic gap
immidazolium units introduced into the individual described
between O2 and TEMPO.11
reactions. These results clearly highlight that the combination
of imidazolium bromide units in the close proximity of TEMPO
moieties in the nanospaces of TEMPO@PMO-IL-Br in one of
the key factor explaining the enhanced catalytic activity
observed for this catalyst in oxidation of benzyl alcohol as
compared to that either TEMPO@SBA-15 not bearing IL or
individual catalytic functionalities (PMO-IL/ TEMPO@SBA-15).
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Although, the relative loading of imidazolium groups and aldehydes and ketones showed excellent activity and
itsration with TEMPO functions might be important in selectivity (Table 2, entries 24-30).
enhancing the performance of the present catalyst system, our
Table 2. Aerobic Oxidation of Various Alcohols using TEMPO@PMO-IL-Br
attempts to increase the IL molar ratios in the PMO-IL led to
significantly less ordered PMO-IL as evidence by N2-sorption
and TEM analysis.[7c] Moreover, our investigations showed that
when a sample of PMO-IL having 25% imidazolium moiety was
employed, the resulting TEMPO catalyst system exhibited a
lower activity suggesting that an optimal IL loading around
10% (0.98 mmol g-1) and highly ordered mesoporous structure
of the materials needs to be respected (Table 1, entry 19). We
have also investigated the aerobic oxidation of benzyl alcohol
using a mixture of PMO-IL (40 mg) and homogeneous TEMPO
(1.5 mol%) to further clarify the crucial importance of co-
supported TEMPO and imidazolium bromide in obtaining high
catalytic activity in the described catalyst system (Please see
Teble 1, entries 18, 20).
Time
(h)
1
Yielda
(%)
Entry
R1
R2
1
C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99b
>99
>99b
>99
>99
>99
>99
>99
80
2
2,6-Cl-C6H5
3-Cl-C6H5
8.5
1
3
4
2,4-Cl-C6H5
2-Cl-C6H5
1
5
1
6
4-Me-C6H5
2-Me-C6H5
3-Me-C6H5
4-NO2-C6H5
3-NO2- C6H5
4-SMe-C6H5
1-Naphtyl
4-OMe-C6H5
3-Pyridyl
1.5
3
7
8
1.5
1
Table 1. Optimization of the conditions for aerobic oxidation of benzyl alcohol
9
Entry
Cat.
(mol%)
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
-
Solvent
(ml)
AcOH
(mg)
-
NOx
(10 mol%)
NaNO2
NaNO2
NaNO2
NaNO2
TBN
Yielda
(%)
1.8
5.2
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
1
1
1
2
Water (2)
1.5
1
Water (2)
20
20
20
20
20
20
-
3
Water (1)
1
4
Water (0.3)
Water (0.3)
Toluene (1)
Toluene (0.5)
Toluene (1)
Toluene (1)
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
21
27
5
Thiophenyl
Hydroxymethylfurfural
Furyl
1
5
1
6
NaNO2
NaNO2
TBN
H
1
7
19
3
C6H5
Me
Me
Ethyl
2
8
4-Phenyl-C6H5
C6H5
2
9
20
30
-
TBN
30
40
22
15
>99
75
32b
8c
2
10
11
12
13
14
15
16
TBN
C6H5
cyclohexyl
4
TBN
α-Tetralol
4
30
30
20
30
30
TBN
1-indanol
3
1.5
TBN
PhCH2CH2
H
3
1.5
TBN
CH3(CH2)6
H
4
1.5
TBN
CH3(CH2)5
H
4
1.5
TBN
CH3(CH2)5
Me
Me
H
12
12
6
17
18
19
20
1.5
1.5
1.5
1.5g
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
Toluene (0.5)
30
30
30
30
TBN
TBN
TBN
TBN
7d
48e
24f
41
CH3(CH2)4
80
CH3(CH2)10
94
CH3(CH2)8
H
4
96
Ph-CH=CH
H
21
5
22
95c
aThe reactions were performed using benzyl alcohol at 50 ºC for 1h. bThe
reaction was performed usingTEMPO@SBA-15 (1.5 mol% TEMPO, 45 mg).
cThe reaction was performed in the presence of PMO-IL-AMP (60 mg). dThe
reaction was performed in the presence of PMO-IL (40 mg) and
TEMPO@SBA-15 (1.5 mol% TEMPO, 45 mg). eThe reaction was performed
in the presence of TEMPO@PMO-IL-Cl (60 mg). fThe reaction was perform
in the presence of TEMPO supported on PMO-IL containing 25% IL. gThe
reaction was performed using PMO-IL (40 mg), homogeneous TEMPO (1.5
mol%).
Ph-CH=CH
H
Dicyclopropylcarbinol
1-Octyn-3-ol
2-Norbornano
2-Adamantanol
Ph-CH=CH
5
>99
93
5
9
>99
94
5
Ph
Ph
Ph
Ph
Ph
Ph
6
37
Ph-CH=CH
6
97d, b
81d , b
87d , b
85d , b
94d , b
4-Me-C6H5-CH=CH
4-NO2-C6H6-CH=CH
2,4-Cl-C6H6- CH=CH
3-OMe-C6H6- CH=CH
6
6
6
With this optimal reaction conditions, we then investigated the
scope of this protocol for other alcohols. As illustrated in Table
2, various types of primary and secondary benzylic alcohol
with electron-withdrawing and electron-donating groups were
selectively transformed to the corresponding benzaldehydes
and ketones in excellent yields under the optimal reaction
conditions (Table 2, entries 1–23). Similarly, the
TEMPO@PMO-IL-Br catalyst system for aerobic oxidation of
primary and secondary aliphatic alcohol to corresponding
6
aPurity of the products was analyzed by GC. bPurity of the product was analyzed
by NMR. cThe reactions were performed using 20 mol% TBN. dThe reactions were
performed using 2 mol% cat., 15 mol% TBN and 10 mg AcOH
Generally, catalytic systems based on transition metal are not
always suitable for aerobic oxidation of heteroatom-
functionalized alcohols, because these substrates are
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susceptible to coordinate the metal center and deactivate the The reusability and recovery of the catalyst are important
catalyst. Our catalyst system proved to be efficient for aerobic issues, especially when the reactions employ solid catalysts.
oxidation of alcohols for bearing common heteroatoms, giving The recyclability of TEMPO@PMO-IL was also investigated by
the corresponding aldehydes in excellent yields (Table 2, isolating it from the reaction mixture of aerobic oxidation of
entries 11 and 14-17). Although we observed a lower activity benzyl alcohol, washing with EtOAc and acetone and drying
for highly challenging, sterically hindered alcohols in the (Figure 5). The recycled catalyst was then successfully used in 8
presence of TEMPO@PMO-IL-Br, but excellent result could be consecutive reaction runs with just slight decrease in activity.
still retained by increasing the reaction time (Table 2, entries 2, The recovered catalyst after 8 run was also studied by N2-
35 and 36). Notably, the similar transformations are very adsorption–desorption analyses. The N2 sorption diagram of
difficult (or need longer reaction times) to proceed even in the recovered catalyst TEMPO@PMO-IL interestingly showed a
AcOH as reaction solvent using previously developed type IV isotherm with relatively sharp hysteresis loop (Figure 1,
heterogeneous TEMPO/O2 systems.4b, 12
Of particular interest is that 5-hydroxymethylfurfural (5-HMF), have survived. TEM micrograph of the fresh and recovered
a versatile bio-based platform chemical, can be selectively catalyst after runs demonstrates the high-ordered
S11), which indicates that the high-ordered mesostructures
8
oxidized under the optimized reaction condition (1.5 mol% mesostructure is not destroyed (Figure 3).
TEMPO@PMO-IL, 30 mg AcOH, Toluene, 50 °C) to afford 2,5-
diformylfuran (DFF), an important platform derived from 5-
HMF for the synthesis of the pharmaceuticals, macrocyclic
ligands, and others (Scheme 2).13
H
HO
H
H
TEMPO@PMO-IL (1.5 mol%)
O
O
TBN (10 mol%), AcOH (30 mg)
Toluene (0.5 ml), 50 °C, 1 h
>99%
O
O
O
Scheme 2. Aerobic oxidation of 5-HMF into DFF using TEMPO@PMO-IL-Br.
In addition, acid sensitive propargylic alcohols and
dicyclopropylcarbinol could be effectively oxidized using the
present method and gave excellent yields of the corresponding
α,β-unsaturated carbonyl products (Table 2, entries 31-34, 37-
42).11 However, it was found that the oxidation of more
sensitive alcohols to acid environment such as cinnamyl and
benzylic-allylic alcohols was not successful. In particular, the
Figure 5. Recyclability of TEMPO@PMO-IL-Br in the aerobic oxidation of benzyl alcohol
On the basis of several successful studies underlying the
mechanism for metal-free aerobic oxidation of alcohols using
homogeneous TEMPO in the presence of various NOx sources
under homogeneous conditions and our observarion regarding
the importance of co-supported TEMPO and imidazolium
reaction of (E)-1,3-diphenyl-2-propen-1-ol gave only 37% of
the corresponding carbonyl along with 57% yields of the
respected symmetrical ether under the optimal conditions
(Scheme 3).
4a,
c
bromides usits (Table 1, entries 15-20) ,3d,
a plausible
reaction pathway was proposed and in Scheme 4.
Scheme 3. Symmetrical ether formation in the aerobic oxidation of highly acid sensitive
substrate (E)-1,3-diphenyl-2-propen-1-ol at the optimal condition.
Therefore, it was decided to investigate the oxidation of
corresponding alcohols in the different conditions. It was
found that the use of 20 mol% of TBN and lower amounts of
acetic acid (10 mg) and 15 mol% of TBN and 10 mg of acetic
acid was better suited for the oxidation of cinnamyl alcohol
and
(E)-1,3-diphenyl-2-propen-1-ol, respectively (Table 2,
Scheme 4. Proposed reaction pathway highlighting the role of close proximity of
enteries 32, 38). Similarly, this modified procol could be imidazolium bromide and TEMPO inside the mesopores of TEMPO@PMO-IL-Br in fast
electron transfer from alcohol to molecular oxygen through a synergistic catalysis relay.
equally employed for a set of acid-sensitive allylic acohols,
furnishing the corresponding α,β-unsaturated carbonyl
compounds with in high selectivity (Table 2, entries 39-42).
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In this proposed pathway, the role of close proximity of of TEMPO
moieties with imidazolium bromide units in the mesochannels of
PMO-IL in enhancement of electron transfer from alcohol to O2 by
NO/NO2 couples, immobilized Br-/Br2 and immobilized nitroxyl
radical through a combination of two successive redox cycles is
highlighted.
the third part after 48 h. The reaction mixture was stirred
vigorously for 3 days at ambient temperature under argon
atmosphere. The final product was separated by filtration and
washed three times by water (20 ml), methanol (20 ml), and
acetone (20 ml) and dried under vacuum at room temperature
to give TEMPO@PMO-IL-Cl. The product stored in
refrigerator under inert atmosphere for the next uses.
a
Preparation of TEMPO@PMO-IL-Br.11 In a typical experiment,
a suspension of TEMPO@PMO-IL-Cl (2 g) in saturated aqueous
solution of NaBr (25 ml) was stirred at ambient temperature
for 24 h. The final product was separated by filtration and
thoroughly washed with deionized water (6×50 ml) and
acetone (25 ml), respectively, to remove the generated NaCl
and the excess of NaBr from the surface of the material and
dried under vacuum to give TEMPO@PMO-IL-Br. The final
product stored in a refrigerator under inert atmosphere.
Typical Catalytic Procedure for Oxidation of Alcohols Using
TEMPO@PMO-IL-Br. A mixture of alcohol (1 mmol), TBN (10
mol%), AcOH (30 mg) and catalyst (1.5-2 mol%, 60-80 mg ) in
toluene (0.5-1 mL) was prepared in a flask and charged with
pure oxygen (balloon filled, O2 1 atmosphere). The resulting
mixture was stirred at 50 ºC for the time indicated in Table 2.
The progress of the reaction was monitored by GC. After
completion of the reaction, the solution was diluted with
ethylacetate and the catalyst was separated by centrifuge. The
resulting solution was then dried with sodium sulfate and the
excess solvent was removed under reduced pressure to give
the corresponding carbonyl compounds. The purity of the
products was analyzed by GC or NMR without any
chromatographic purification.
Conclusion
In summary, we have introduced a bifunctional catalyst
compose of TEMPO anchored in the nanospaces of a periodic
mesoporous organosilica with imidazolium bromide network.
Thanks to the close proximity of TEMPO moieties with
imidazolium bromide units in the same solid, while the
materials shows enhanced catalytic activity in the metal-free
aerobic oxidation of various activated and non-activated
alcohols through a synergistic catalysis relay mechanism, the
strategy allows simultaneous recovery of both ionic liquid and
TEMPO catalyst. This unprecedented cooperative effect results
in much superior activity of TEMPO@PMO-IL-Br in oxidation of
alcohols as compared to that either TEMPO@SBA-15 not
bearing IL or individual catalytic functionalities (PMO-
IL/TEMPO@SBA-15)
Experimental Section
Preparation of PMO-IL.7a In a typical synthesis, Pluronic P123
(1.67 g) and KCl (8.8 g) were added to a solution of distilled
water (10.5 g) and HCl (2 M, 46.14 g) with stirring at 40 °C.
After a clear homogeneous solution obtained, a pre-prepared
homogeneous
mixture
of
ionic
liquid
1,3-
bis(trimethoxysilylpropyl) imidazolium chloride (2 mmol, 0.86
g) and tetramethoxysilane (18 mmol, 2.74 g), in super-dry
methanol was rapidly added and stirred at the same
temperature for 24 h. The temperature of resulting mixture
was then raised to 100 °C and the content of flask was
statically maintained at this temperature for 72 h. The
obtained solid material containing surfactant was filtered,
washed with deionized water, and dried at room temperature.
The surfactant residue was then extracted from the materials
through a Soxhlet apparatus by using ethanol (100 ml) and
concentrate HCl (37%, 3 ml) for 24 h.
Acknowledgements
The authors acknowledge IASBS Research Councils and Iranian
National Science Foundation (INSF) through a grant no. G014
and NanoQuebec as well as Natural Sciences and Engineering
Research Council of Canada (H.V.) for support of this work.
Notes and references
1
2
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Preparation of PMO-IL-AMP 4b
In a typical procedure, 3 g of
.
,
synthesized PMO-IL was mixed and refluxed with 3-
aminopropyltrimethoxysilane (1 mmol, 0.23 mL) in dry toluene
for 18 h under argon atmosphere. The white solid materials
were filtered and washed with toluene and ethanol in order to
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was then dried in oven at 105 °C to give the PMO-IL-AMP at a
loading of 0.35 mmol g-1 as determined by thermogravimetric
analysis and confirmed by elemental analysis..
Preparation of TEMPO@PMO-IL-Cl.4b Reductive amination has
been used for the synthesis of TEMPO@PMO-IL-Cl. In a typical
procedure, to a mixture of PMO-IL-AMP (3 g) in super-dry
CH3OH (50 mL), 4-Oxo-TEMPO (1.5 mmol, 0.281 g) was added.
NaBH3CN (2 mmol, 0.125 g) was divided into three portions,
the first part was added after 3 h, second part after 24 h and
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