A highly efficient heterogeneous aerobic alcohol oxidation catalyzed by immobilization of bipyridine copper(I) complex in MCM-41

Article abstract of DOI:10.1016/j.tet.2015.09.054

A heterogeneous copper(I)-catalyzed aerobic oxidation of primary benzylic and allylic alcohols to aldehydes was achieved under air in EtOH at 50°C in the presence of 5 mol % of MCM-41-immobilized bipyridine copper(I) complex [MCM-41-bpy-CuI] and 5 mol % of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) using aqueous ammonia as base, yielding a variety of aromatic and α,β-unsaturated aldehydes in good to excellent yields. This heterogeneous copper catalyst can be easily recovered by a simple filtration of reaction solution and reused for at least 10 consecutive trials without any decreases in activity. The use of recyclable heterogeneous copper catalyst and green reagents, such as air as oxidant and ethanol as solvent, made the system attractive for environmentally sustainable processes.

Full text of DOI:10.1016/j.tet.2015.09.054

Accepted Manuscript
A highly efficient heterogeneous aerobic alcohol oxidation catalyzed by immobilization
of bipyridine copper(I) complex in MCM-41
Hong Zhao, Qiurong Chen, Li Wei, Yuanyuan Jiang, Mingzhong Cai
PII:
S0040-4020(15)30082-X
10.1016/j.tet.2015.09.054
TET 27151
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 27 July 2015
Revised Date: 20 September 2015
Accepted Date: 22 September 2015
Please cite this article as: Zhao H, Chen Q, Wei L, Jiang Y, Cai M, A highly efficient heterogeneous
aerobic alcohol oxidation catalyzed by immobilization of bipyridine copper(I) complex in MCM-41,
Tetrahedron (2015), doi: 10.1016/j.tet.2015.09.054.
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ACCEPTED MANUSCRIPT
A highly efficient heterogeneous aerobic alcohol oxidation
catalyzed by immobilization of bipyridine copper(I) complex
in MCM-41
a,b
a
a
a
a
Hong Zhao, Qiurong Chen, Li Wei, Yuanyuan Jiang and Mingzhong Cai *
a
College of Chemistry & Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, P. R. China
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
b
University, Guangzhou 510006, P. R. China
Fax: (+86)-791-8812-0388; E-mail: mzcai@jxnu.edu.cn
ABSTRACT
A heterogeneous copper(I)-catalyzed aerobic oxidation of primary benzylic and
allylic alcohols to aldehydes was achieved under air in EtOH at 50 ºC in the presence
of 5 mol% of MCM-41-immobilized bipyridine copper(I) complex [MCM-41-bpy-
CuI] and 5 mol% of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) using aqueous
ammonia as base, yielding a variety of aromatic and α,β-unsaturated aldehydes in
good to excellent yields. This heterogeneous copper catalyst can be easily recovered
by a simple filtration of reaction solution and reused for at least 10 consecutive trials
without any decreases in activity. The use of recyclable heterogeneous copper catalyst
and green reagents, such as air as oxidant and ethanol as solvent, made the system
attractive for environmentally sustainable processes.
Keywords: Aerobic alcohol oxidation; Supported copper complex; Bipyridine ligand;
MCM-41; Heterogeneous catalysis
1
. Introduction
The oxidation of alcohols into aldehydes or ketones is a ubiquitous transformation
1
in synthetic organic chemistry and enormous efforts have been devoted to the
2
development of aerobic oxidation methods. The development of copper-catalyzed
oxidation systems with economically and environmentally benign oxidants such as
1

ACCEPTED MANUSCRIPT
oxygen or air is of particular interest because copper is an inexpensive, abundant
3
metal and the reduction product is water. Since Semmelhack first reported the
4
CuCl/TEMPO-catalyzed aerobic oxidation of alcohols to aldehydes or ketones, the
5
copper-catalyzed aerobic alcohol oxidations have received much attention. Marko et
al. reported a CuCl/1,10-Phen/DBADH catalytic system for aerobic oxidation of
2
primary and secondary alcohols into aldehydes and ketones using K CO as a base in
2
3
6
t
toluene. Sheldon et al. described a CuBr /bpy/TEMPO catalytic system with KOBu
2
7
as a base in an acetonitrile/water (2:1) solvent mixture. Koskinen et al. reported a
Cu(OTf) /bpy/TEMPO catalytic system using NMI (N-methylimidazole) and/or DBU
2
8
as a base in MeCN. More recently, Stahl et al. have developed a highly efficient
CuOTf/bpy/TEMPO/NMI catalytic system in which the use of Cu(I) source improved
9
the performance of the catalyst and substrate scope. Although these copper-catalyzed
aerobic alcohol oxidations are highly efficient, the problem with homogeneous
catalysis is the difficulty to separate the catalyst from the reaction mixture and the
impossibility to reuse it in consecutive reactions. It is well known that homogeneous
catalysis might result in heavy metal contamination of the isolated product, which is a
particularly significant drawback for its application in the pharmaceutical industry. In
contrast, heterogeneous catalysts can be easily separated from the reaction mixture by
a simple filtration of the reaction solution and reused in successive reactions provided
that the active sites have not become deactivated. Heterogeneous catalysis also helps
to minimize wastes derived from reaction workup, contributing to the development of
1
0,11
green chemical processes.
From the standpoint of environmentally benign organic
2

ACCEPTED MANUSCRIPT
synthesis, the development of immobilized copper catalysts is challenging and
important. In an ideal system, they can be recovered by simple filtration and re-used
infinitely, and the contamination of products by copper is prevented. In spite of
considerable effort dedicated to heterogeneous copper-catalyzed carbon-carbon and
1
2
carbon-heteroatom bond formation reactions over the last several years, to the best
of our knowledge, the aerobic alcohol oxidation catalyzed by heterogeneous copper
complexes has not been reported until now.
The discovery of mesoporous material MCM-41 has given an enormous stimulus to
research in heterogeneous catalysis and provided a new possible candidate for a solid
13
support for immobilizing homogeneous catalysts. MCM-41 has a regular pore
2
-1
diameter of ca.5 nm and a specific surface area > 700 m g and rich silanol groups in
1
4
the inner walls. To date, functionalized MCM-41-immobilized palladium or rhodium
1
5
complexes have been prepared and successfully used in organic reactions. Very
recently, we have reported the first preparation of 3-(2-aminoethylamino)propyl-
functionalized MCM-41-immobilized copper(I) complexes [MCM-41-2N-CuI(CuCl)]
and found that they are highly efficient and recyclable heterogeneous catalysts for the
1
2d
homo- and heterocoupling of terminal alkynes and C–N or C–S bond formation
1
2h,16
reactions.
In continuing our efforts to develop greener synthetic pathways for
organic transformations, our new approach, described in this paper, was to design and
synthesize a novel MCM-41-immobilized bipyridine copper(I) complex [MCM-41-
bpy-CuI], which was used as an effective copper catalyst for the aerobic alcohol
oxidation under mild conditions.
3

ACCEPTED MANUSCRIPT
2
. Results and discussion
A novel MCM-41-immobilized bipyridine copper(I) complex [MCM-41-bpy-CuI]
was synthesized starting from 3-aminopropyltriethoxysilane, 4,4'-bis(bromomethyl)-
1
7
2
,2'-bipyridine, mesoporous material MCM-41, and CuI according to Scheme 1.
Firstly, the reaction of 3-aminopropyltriethoxysilane with 4,4'-bis(bromomethyl)-2,2'-
bipyridine in THF at 50 °C in the presence of triethylamine for 6 h gave 4,4'-bis[3-
(triethoxysilyl)propylaminomethyl]-2,2'-bipyridine (BTESBPY), which reacted with
MCM-41 in toluene at 100 °C for 24 h, followed by the silylation with Me SiCl in
3
toluene at room temperature for 24 h to generate bipyridine-functionalized MCM-41
[
MCM-41-bpy]. Then MCM-41-bpy reacted with CuI in DMF at room temperature
for 7 h to afford the MCM-41-immobilized bipyridine copper(I) complex [MCM-41-
bpy-CuI] as a pale blue powder.
4

ACCEPTED MANUSCRIPT
X-ray powder diffraction (XRD) patterns of the parent MCM-41 and the modified
material MCM-41-bpy-CuI are displayed in Fig. 1. The small angle X-ray powder
diffraction (XRD) analysis of the MCM-41-bpy-CuI indicated that, in addition to an
intense diffraction peak (100), two higher order diffraction peaks (110) and (200)
were also detected and became weak and diffuse, which could be due to contrast
matching between the silicate framework and organic moieties which are located
inside the channels of MCM-41, therefore the chemical bonding procedure did not
diminish the structural ordering of the parent MCM-41.
(100)
1
(110)
2
(200)
1
2
3
4
5
6
7
2θ (degrees)
Fig. 1 XRD patterns of the parent MCM-41 (1) and MCM-41-bpy-CuI (2).
The N adsorption-desorption isotherms and pore size distributions for MCM-41
2
and MCM-41-bpy-CuI are presented in Fig. 2 and Fig. 3, respectively. The isotherm
of parent MCM-41 in Fig. 2 is of type IV as defined by IUPAC, characteristic of
mesoporous materials according to the classification. Compared to parent MCM-41,
the modified material MCM-41-bpy-CuI also gave reversible type IV N sorption
2
5

ACCEPTED MANUSCRIPT
isotherm, but exhibited a decreased uptake of N owing to the incorporation of the bpy
2
ligand and the further silylation with Me SiCl and coordination with CuI. As shown in
3
Fig. 3, the pore volume and size of MCM-41-bpy-CuI reduced apparently compared
to parent MCM-41, also indicating the organic moieties were introduced into the inner
channels, but the pore still remained a narrow distribution. The parent MCM-41 had
2
-1
surface area of 902 m g and diameter of 2.7 nm, however, MCM-41-bpy-CuI had
2
-1
surface area of 658 m g and diameter of 2.2 nm. These results indicated that the
bipyridine copper(I) complex has been successfully grafted onto MCM-41.
500
450
400
350
300
250
200
150
100
MCM-41
MCM-41-bpy-CuI
50
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure [P/P0]
Fig. 2 N adsorption/desorption isotherms of MCM-41 and MCM-41-bpy-CuI
2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a MCM-41
b MCM-41-bpy-CuI
a
b
-
0.2
1
2
3
4
5
6
7
8
9
10
Pore Diameter [nm]
Fig. 3 Pore size distributions of MCM-41 and MCM-41-bpy-CuI
6

ACCEPTED MANUSCRIPT
Initially, 4-methoxyphenylmethanol was selected as the model substrate to optimize
the oxidation conditions, and the results are presented in Table 1. An 85% yield of the
product was obtained when the oxidation reaction was performed at 30 ºC in EtOH
with aqueous ammonia as a base (Table 1, entry 1). Notably, neither carboxylic acid
nor ester as byproduct was detected. Among the solvents examined [EtOH, MeCN,
MeCN/H O, DMSO, DMF, and toluene], both EtOH and MeCN gave good yields,
2
while other solvents were substantially less effective (Table 1, entries 2-6). We then
turned our attention to investigate the effect of base on the oxidation reaction. When
t
NMI or BuOK was used as a base, good yield was also obtained, but aqueous
ammonia was the best choice due to its low cost (Table 1, entries 1, 7 and 8). Other
bases such as K CO , Na CO and K PO were inferior to aqueous ammonia (Table 1,
2
3
2
3
3
4
entries 9-11). The reaction was very slow in the absence of a base and a low yield was
observed (Table 1, entry 12). When reaction temperature was raised to 50 °C, the
reaction rate was increased obviously and an excellent yield was achieved (Table 1,
entry 13). Finally, the amount of supported copper catalyst was also screened, and 5.0
mol% loading of copper was found to be optimal (Table 1, entry 13), a lower yield
was observed and a longer reaction time was required when the amount of the catalyst
was decreased (Table 1, entry 15). Increasing the amount of copper catalyst could
shorten the reaction time, but did not increase the yield of 4-methoxybenzaldyhyde
(Table 1, entry 16). Thus, the optimized reaction conditions for this oxidation reaction
involved the use of the MCM-41-bpy-CuI (5 mol%), aqueous ammonia (1.0 equiv), in
7

ACCEPTED MANUSCRIPT
EtOH at 50 °C under air for 24 h (Table 1, entry 13).
a
Table 1 The optimization of heterogeneous copper-catalyzed alcohol oxidation
b
Entry Solvent
Base
Temp. (°C) Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
0
EtOH
NH ·H O
30
30
30
30
30
30
30
30
30
30
30
30
50
80
50
50
48
48
48
48
48
48
48
48
48
48
48
72
24
24
48
15
85
81
68
54
46
34
84
79
57
55
50
37
97
92
72
95
3
2
MeCN
NH ·H O
3 2
MeCN/H O (2/1) NH ·H O
2
3
2
DMSO
DMF
NH ·H O
3 2
NH ·H O
3
2
Toluene
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
NH ·H O
3
2
NMI
t
BuOK
K CO
2
3
1
Na CO
2
3
11
K PO
3
4
12
13
14
15
16
None
NH ·H O
3
2
NH ·H O
3
2
c
NH ·H O
3
2
d
NH ·H O
3
2
a
All reactions were performed using 4-methoxyphenylmethanol (0.5 mmol), TEMPO (0.025
b
mmol), base (0.5 mmol) and MCM-41-bpy-CuI (0.025 mmol) in solvent (1.0 mL) under air.
c
d
Isolated yield. 2 mol% copper catalyst was used. 10 mol% copper catalyst was used.
8

ACCEPTED MANUSCRIPT
To examine the scope for this heterogeneous copper-catalyzed aerobic alcohol
oxidation reaction, we have investigated the reactions using a variety of primary
benzylic and allylic alcohols as the substrates under the optimized ambient conditions
(Scheme 2) and the results are summarized in Table 2. To our delight, most tested
substrates could be selectively transformed into the corresponding aldehydes in good
to excellent yields and over oxidation products such as acids or esters were not
detected by GC-MS during the reaction. A number of functional groups were tolerated.
The reactions were particularly efficient for electron-rich and -neutral benzylic
alcohols. For example, benzylic alcohols with electron-donating substituents such as
methyl, methoxy, and dimethylamino groups on the benzene ring underwent the
oxidation reaction very smoothly to afford the corresponding substituted benzalde-
hydes 2b-g in excellent yields (Table 2, entries 2-7). Satisfactory results were also
obtained for most electron-deficient benzylic alcohols except for strongly electron-
deficient 4-nitrobenzyl alcohol and 3-nitrobenzyl alcohol which gave moderate yields
of desired products 2l and 2m on a prolonged reaction time (Table 2, entries 8-13). In
addition, benzylic alcohols carrying an ortho substituent were found to readily partici-
pate in the reaction. For instance, the reactions of 2-methylbenzyl alcohol, 2-methoxy-
benzyl alcohol, 2-aminobenzyl alcohol, and 2-halobenzyl alcohols provided good to
excellent yields of the corresponding ortho-substituted benzaldehydes 2n-s under the
optimized reaction conditions (Table 2, entries 14-19). It is noteworthy to mention
that the oxidation protocol also allowed the smooth transformation of 1-naphthyl-
9

ACCEPTED MANUSCRIPT
methanol and heterocyclic alcohols into the corresponding 1-naphthaldehyde (2t) and
heteroaromatic aldehydes 2u-w in good to excellent yields (Table 2, entries 20-23).
The oxidation reaction of the aryl-substituted primary allylic alcohol, (E)-cinnamyl
alcohol, could also proceed effectively under the standard conditions to give the
desired (E)-cinnamaldehyde (2x) in good yield (Table 2, entry 24), however, the
alkyl-substituted primary allylic alcohols such as geraniol did not work (Table 2, entry
2
5). In addition, the aerobic oxidation of secondary alcohols like 1-phenylethanol into
acetophenone was inhibited, which might be ascribed to the steric effect of the methyl
group in the α-position (Table 2, entry 26). We attempted to carry out the oxidation of
aliphatic alcohols such as 1-octanol, unfortunately, no desired 1-octanal (2a') was
detected under the optimized reaction conditions (Table 2, entry 27). We also
performed the reaction on a gram-scale. 4-Methylbenzyl alcohol (1.221 g, 10 mmol)
was stirred at 50 ºC for 26 h in the presence of MCM-41-bpy-CuI (0.8 g, 0.5 mmol),
TEMPO (80 mg, 0.5 mmol), aqueous ammonia (10 mmol, 25-28%, w/w) in EtOH (20
mL) under air to afford 1.128 g of the desired product (2b) in 94% yield (Table 2,
entry 28).
Table 2 Heterogeneous copper-catalyzed aerobic oxidation of primary alcohols to
a
aldehydes
b
Entry
R
Time (h)
Product
Yield (%)
1
2
3
4
Ph
22
23
24
18
PhCHO (2a)
4-MeC H CHO (2b)
91
96
97
96
4-MeC H4
6
6
4
4-MeOC H4
4-MeOC H CHO (2c)
6 4
6
3,4-Me C H
3,4-Me C H CHO (2d)
2 6 3
2
6
3
1
0

ACCEPTED MANUSCRIPT
5
6
7
8
9
0
3,4-(MeO) C H
24
30
24
26
24
24
24
48
48
24
24
24
24
48
24
36
22
36
36
48
48
48
48
26
3,4-(MeO) C H CHO (2e)
98
94
92
92
94
93
91
58
55
94
93
91
90
79
91
83
98
65
78
83
0
2
6
3
2
6
3
3,4-CH O C H
3,4-CH O C H CHO (2f)
2 2 6 3
2
2
6
3
4-Me NC H
4-Me NC H CHO (2g)
2 6 4
2
6
4
4-ClC H4
4-ClC H CHO (2h)
6 4
6
4-BrC H4
4-BrC H CHO (2i)
6 4
6
1
4-FC H4
4-FC H CHO (2j)
6 4
6
11
4-AcNHC H4
4-AcNHC H CHO (2k)
6 4
6
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
4-O NC H
4-O NC H CHO (2l)
2 6 4
2
6
4
4
3-O NC H
3-O NC H CHO (2m)
2 6 4
2
6
2-MeC H4
2-MeC H CHO (2n)
6 4
6
2-MeOC H4
2-MeOC H CHO (2o)
6 4
6
2-ClC H4
2-ClC H CHO (2p)
6 4
6
2-BrC H4
2-BrC H CHO (2q)
6 4
6
2-H NC H
2-H NC H CHO (2r)
2 6 4
2
6
4
2,4-Cl C H
2,4-Cl C H CHO (2s)
2 6 3
2
6
3
1-naphthyl
2-thienyl
2-furyl
1-naphthaldehyde (2t)
2-thiophenaldehyde (2u)
2-furaldehyde (2v)
3-pyridinaldehyde (2w)
(E)-PhCH=CHCHO (2x)
geranial (2y)
3-pyridyl
(E)-PhCH=CH
geraniol
1-phenylethanol
1-octanol
acetophenone (2z)
1-octanal (2a')
0
0
c
4-MeC H4
4-MeC H CHO (2b)
94
6
6
4
a
All reactions were performed using 0.5 mmol of alcohol, 0.5 mmol of aqueous ammonia
25-28%, w/w), 5 mol% of TEMPO and 5 mol% of MCM-41-bpy-CuI in EtOH (1.0 mL) at 50 °C
(
b
c
under air. Isolated yield. Reaction run on 10 mmol scale.
To verify whether the observed catalysis was due to the heterogeneous catalyst
MCM-41-bpy-CuI or to a leached copper species in solution, we performed the hot
11

ACCEPTED MANUSCRIPT
1
8
19
filtration test and three-phase test. We focused on the aerobic oxidation reaction of
benzyl alcohol. We filtered off the MCM-41-bpy-CuI complex after 10 h of reaction
time and allowed the filtrate to react further. The catalyst filtration was performed at
the reaction temperature (50 °C) in order to avoid possible recoordination or
precipitation of soluble copper upon cooling. We found that, after this hot filtration,
no further reaction was observed and no copper could be detected in the hot filtered
solution by ICP-AES analysis. These results suggest that the Cu is not being leached
out from the solid surface of the catalyst during the oxidation reaction. To perform
three-phase test, we have chosen benzylic alcohol, p-(hydroxymethyl)benzaldehyde
(PHB) that is anchored on the surface of the 3-aminopropyl-functionalized MCM-41
20
[
MCM-41-NH ] to give PHB-MCM-41 as shown in Scheme 3. A solution of p-tolyl-
2
methanol (0.5 mmol), aqueous ammonia (0.5 mmol, 25-28%, w/w), TEMPO (0.025
mmol) in EtOH (2 mL) was stirred in the presence of MCM-41-bpy-CuI (0.025 mmol)
and PHB-MCM-41 (0.5 g) at 50 ºC under air for 24 h. The reaction mixture was
cooled to room temperature, then filtered and washed with EtOH. The collected
1
filtrate was purified and analyzed by GC and H NMR, which showed about 96% of
p-tolylmethanol converted to 4-methylbenzaldehyde as expected for the aerobic
oxidation reaction of benzylic alcohols. The residue obtained from the filtration was
then hydrolyzed by 2 N aqueous HCl under refluxing condition. It was neutralized
1
and extracted with ethyl acetate and finally analyzed by GC and H NMR. The only
compound obtained in this process was p-(hydroxymethyl)benzaldehyde (PHB). The
expected product of p-(hydroxymethyl)benzaldehyde in the oxidation reaction is
1
2

ACCEPTED MANUSCRIPT
terephthalaldehyde and it was not detected from the liquid phase test. However, it was
found that the oxidation of p-(hydroxymethyl)benzaldehyde (PHB) could proceed
smoothly under the optimized reaction conditions to give terephthalaldehyde in 89%
yield after 30 h. So it is clearly evident that the p-(hydroxymethyl)benzaldehyde
remains unreacted inside the mesopores of PHB-MCM-41. Hence, the p-(hydroxyl-
methyl)benzaldehyde has not participated in the oxidation reaction, while anchored on
the functionalized MCM-41. If there will be any leaching of copper species from
MCM-41-bpy-CuI, the anchored p-(hydroxylmethyl)benzaldehyde would have also
participated in the above oxidation reaction. However, this is not the case for our
present catalyst.
Recently, Stahl et al. have reported the detailed mechanistic studies of copper-
catalyzed aerobic oxidation reaction of alcohols, this heterogeneous copper-catalyzed
aerobic oxidation of primary benzylic and allylic alcohols may proceed through a
1
3

ACCEPTED MANUSCRIPT
mechanism analogous to that proposed for the Cu(I)/bpy/TEMPO catalytic system
2
1
(
Scheme 4). First, NH coordinates to a copper(I) species of MCM-41-bpy-CuI to
3
provide an MCM-41-immobilized bipyridine copper(I) salt (A), which was oxidized
by O in the presence of TEMPOH to give an MCM-41-immobilized bipyridine
2
II
Cu –OH intermediate (B). The latter undergoes a ligand exchange reaction with
II
alcohol leading to an MCM-41-immobilized bipyridine Cu –alkoxide intermediate
(C). Then the newly generated intermediate (C) could be oxidized by TEMPO
through a hydrogen abstraction mechanism, to afford the corresponding aldehyde and
regenerate the MCM-41-immobilized bipyridine copper(I) salt (A).
MCM-41-bpy-CuI
NH3
MCM-41
OH
N
O
OH
N
+
I
R
H
1
/2O +
2
N
N
CuI
NH3
(
A)
O
N
O
N
MCM-41
MCM-41
I
I
N
N
CuII
N
N
CuII
(C)
(B)
O
OH
H N
3
H N
3
H
R
H
H O
2
R
OH
Scheme 4.
For any heterogeneous catalytic system, it is very important to know its ease of
1
4

ACCEPTED MANUSCRIPT
separation and possible reuse. We next investigated the recyclability of the MCM-41-
bpy-CuI by using the aerobic oxidation reaction of 4-methoxyphenylmethanol (0.5
mmol) in the presence of MCM-41-bpy-CuI (0.025 mmol) and TEMPO (0.025 mmol)
in EtOH (1.0 mL) with aqueous ammonia (0.5 mmol) as base at 50 ºC under air for 24
h. Each time, after completion of the reaction, the catalyst was separated by simple
filtration and washed with ethanol and diethyl ether. After being air-dried, it can be
reused directly without further purification. As shown in Fig. 4, the recovered copper
catalyst was used in the next run, and almost consistent activity was observed for 10
consecutive cycles. In addition, copper leaching in the immobilized catalyst was also
determined. The copper content of the catalyst was found by ICP analysis to be 0.62
mmol/g after ten consecutive runs, only 1.6% of copper had been lost from the
MCM-41 support. The high stability and excellent reusability of this heterogeneous
copper catalyst should result from the chelating action of bidentate bipyridine ligand
on copper and the mesoporous structure of the MCM-41 support. The result is
important from industrial and environmental points of view. The high catalytic
activity and excellent reusability of the MCM-41-bpy-CuI make it a highly attracttive
heterogeneous copper catalyst for the parallel solution phase synthesis of diverse
libraries of compounds.
3
. Conclusions
In summary, we have developed a novel, practical and environmentally friendly
catalyst system for the aerobic oxidation reaction of primary benzylic and allylic
1
5

ACCEPTED MANUSCRIPT
alcohols by using an MCM-41-immobilized bipyridine copper(I) complex [MCM-41-
bpy-CuI] as catalyst. This catalytic system encompassed a broad substrate scope,
oxidizing a range of primary benzyl alcohols and (E)-cinnamyl alcohol to the
corresponding aldehydes in good to excellent yields under mild conditions using air as
the ultimate oxidant. The use of recyclable heterogeneous copper catalyst and green
reagents, such as air as oxidant and ethanol as solvent, made the system attractive for
environmentally sustainable processes.
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Reused times
Fig. 4 Recycle of the MCM-41-bpy-CuI catalyst
4
4
. Experimental
.1. Physical measurements and materials
All chemicals were obtained from commercial suppliers and used as received,
1
7
unless otherwise noted. 4,4'-Bis(bromomethyl)-2,2'-bipyridine and mesoporous
2
2
material MCM-41 were prepared according to literature procedures. All solvents
were distilled and dried before use. All oxidation products were characterized by
1
6

ACCEPTED MANUSCRIPT
comparison of their spectra and physical data with authentic samples. IR spectra were
1
determined on a Perkin-Elmer 683 instrument. H NMR spectra (400 MHz) were
recorded on a Bruker Avance 400 MHz spectrometer with TMS as an internal
1
3
standard in CDCl as solvent. C NMR spectra (100 MHz) were recorded on a
3
Bruker Avance 400 MHz spectrometer in CDCl as solvent. Copper content was
3
determined with inductively coupled plasma atom emission Atomscan16 (ICP-AES,
TJA Corporation). Nitrogen adsorption/desorption isotherms were obtained using a
Bel Japan Inc. Belsorp-HP at 77 K. Prior to gas adsorption measurements materials
were degassed for 6 h at 423 K. X-ray powder diffraction patterns were obtained on
Damx-rA (Riguka).
4
.2. Preparation of 4,4'-bis[3-(triethoxysilyl)propylaminomethyl]-2,2'-bipyridine
(BTESBPY)
Et N (1.52 g, 15 mmol) and 3-aminopropyltriethoxysilane (3.32 g, 15 mmol) were
3
added to a solution of 4,4'-bis(bromomethyl)-2,2'-bipyridine (0.513 g, 1.5 mmol) in
THF (10 mL), and the mixture was stirred at 50 °C under Ar for 6 h. After cooling the
solution to room temperature, hexane (30 mL) was added and the mixture was filtered
through a short MgSO column to remove the ammonium salt. The clear solution was
4
then concentrated and dried under vacuum at 100 °C for 24 h to produce the title
1
compound as a pale yellow viscous oil in 92% yield. H NMR (400 MHz, C D ): δ =
6
6
8
1
1
.78 (s, 2H), 8.57 (d, J = 4.2 Hz, 2H), 7.08 (d, J = 4.2 Hz, 2H), 3.76 (q, J = 7.0 Hz,
2H), 3.51 (s, 4H), 2.42 (t, J = 7.0 Hz, 4H), 1.68-1.62 (m, 4H), 1.14 (t, J = 7.0 Hz,
13
8H), 0.74-0.68 (m, 4H). C NMR (100 MHz, C D ): δ = 156.8, 151.3, 149.3, 123.2,
6
6
1
7

ACCEPTED MANUSCRIPT
1
20.7, 58.5, 53.1, 52.4, 23.9, 18.6, 8.4. Anal. Calcd. for C H N O Si : C, 57.86; H,
30
54
4
6
2
8
.74; N, 8.99. Found: C, 57.63; H, 8.52; N, 8.78.
4
.3. Preparation of MCM-41-bpy
A solution of BTESBPY (2.368 g, 3.8 mmol) in dry chloroform (18 mL) was added
to a suspension of 1.881 g of the MCM-41 in dry toluene (180 mL). The mixture was
stirred for 24 h at 100 °C. Then the solid was filtered and washed by CHCl (2 × 20
3
mL), and dried in vacuum at 160 °C for 5 h. The dried white solid was then soaked in
a solution of Me SiCl (3.1 g) in dry toluene (100 mL) at room temperature under
3
stirring for 24 h. Then the solid was filtered, washed with acetone (3 × 20 mL) and
diethyl ether (3 × 20 mL), and dried in vacuum at 120 °C for 5 h to obtain 2.963 g of
-1
hybrid material MCM-41-bpy. The nitrogen content was found to be 2.91 mmol g by
elemental analysis.
4
.4. Preparation of MCM-41-bpy-CuI complex
In a small Schlenk tube, 1.00 g of the above-functionalized MCM-41 (MCM-41
-bpy) was mixed with CuI (0.15 g, 0.78 mmol) in dry DMF (10 mL). The mixture was
stirred at room temperature for 7 h under an argon atmosphere. The solid product was
filtered by suction, washed with DMF and acetone and dried at 40 °C/26.7 Pa under
Ar for 5 h to give 1.069 g of a pale blue copper complex (MCM-41-bpy-CuI). The
-1
-1
nitrogen and copper contents were found to be 2.67 mmol g and 0.63 mmol g ,
respectively.
4
.5. General procedure for the aerobic alcohol oxidation
1
8

ACCEPTED MANUSCRIPT
Under an air atmosphere, a Schlenk tube was charged with MCM-41-bpy-CuI (40
mg, 0.025 mmol), alcohol (0.5 mmol), TEMPO (4 mg, 0.025 mmol), aqueous
ammonia (0.5 mmol, 25-28%, w/w) and EtOH (1.0 mL). The mixture was stirred at
5
0 °C for 18-48 h. After completion of the reaction, the reaction mixture was cooled
to room temperature, diluted with ethyl acetate (10 mL), and filtered. The MCM-41-
bpy-CuI complex was washed with EtOH (2 × 5 mL), and Et O (5 mL) and reused in
2
the next run. The filtrate was concentrated under reduced pressure and the residue was
purified by flash column chromatography on silica gel (petroleum/ethyl acetate = 15:1
to 10:1) to provide the desired product.
5
h
1
4
.5.1. Benzaldehyde (2a). Oil. H NMR (400 MHz, CDCl ): δ = 9.98 (s, 1H), 7.84 (d,
3
1
3
J = 7.2 Hz, 2H), 7.61-7.57 (m, 1H), 7.49 (t, J = 7.6 Hz, 2H). C NMR (100 MHz,
-1
CDCl ): δ = 192.4, 136.5, 134.5, 129.8, 129.1. IR (film, cm ): 1708, 1596, 1203, 736.
3
4
1
4
.5.2. 4-Methylbenzaldehyde (2b). Oil. H NMR (400 MHz, CDCl ): δ = 9.96 (s, 1H),
3
1
3
7
.78 (d, J = 8.0 Hz, 2H), 7.33 (d, J = 7.6 Hz, 2H), 2.44 (s, 3H). C NMR (100 MHz,
-1
CDCl ): δ = 191.8, 145.4, 134.3, 129.8, 129.7, 21.8. IR (film, cm ): 2926, 1702, 1605,
3
1168, 808.
5
h
1
4
.5.3. 4-Methoxybenzaldehyde (2c). Oil. H NMR (400 MHz, CDCl ): δ = 9.89 (s,
3
1
3
1
H), 7.84 (d, J = 8.8 Hz, 2H), 7.00 (d, J = 8.8 Hz, 2H), 3.89 (s, 3H). C NMR (100
-1
MHz, CDCl ): δ = 190.8, 164.6, 132.0, 129.9, 114.3, 55.6. IR (film, cm ): 1682, 1595,
3
1
4
1
256, 1157, 1022, 830.
2
3
1
.5.4. 3,4-Dimethylbenzaldehyde (2d). Oil. H NMR (400 MHz, CDCl ): δ = 9.94 (s,
3
13
H), 7.65 (s, 1H), 7.61 (d, J = 7.6 Hz, 1H), 7.29 (d, J = 7.6 Hz, 1H), 2.34 (s, 6H). C
1
9

ACCEPTED MANUSCRIPT
NMR (100 MHz, CDCl ): δ = 192.1, 137.5, 134.7, 133.5, 130.6, 130.2, 127.7, 20.1,
3
-1
1
9.5. IR (film, cm ): 1691, 1607, 1242, 908, 731.
2
3
1
4
.5.5. 3,4-Dimethoxybenzaldehyde (2e). Oil. H NMR (400 MHz, CDCl ): δ = 9.86
3
(s, 1H), 7.47 (d, J = 8.4 Hz, 1H), 7.42 (d, J = 1.2 Hz, 1H), 6.99 (d, J = 8.0 Hz, 1H),
1
3
3
1
7
4
3
7
1
1
4
7
6
1
.98 (s, 3H), 3.95 (s, 3H). C NMR (100 MHz, CDCl ): δ = 190.7, 154.6, 149.7,
3
-1
30.2, 126.6, 110.5, 109.3, 56.1, 56.0. IR (film, cm ): 1682, 1590, 1513, 1268, 1137,
28.
2
4
.5.6. 3,4-Methylenedioxybenzaldehyde (2f). White solid, m.p. 36–37 °C (ref. m.p.
1
5–36 °C). H NMR (400 MHz, CDCl ): δ = 9.82 (s, 1H), 7.42 (d, J = 8.0 Hz, 1H),
3
1
3
.34 (s, 1H), 6.94 (d, J = 8.0 Hz, 1H), 6.08 (s, 2H). C NMR (100 MHz, CDCl ): δ =
3
-1
90.1, 153.1, 148.7, 132.0, 128.5, 108.3, 107.0, 102.1. IR (KBr, cm ): 1686, 1604,
490, 1447, 1254, 1037.
2
5
.5.7. 4-(Dimethylamino)benzaldehyde (2g). White solid, m.p. 72–73 °C (ref. m.p.
1
3–75 °C). H NMR (400 MHz, CDCl ): δ = 9.75 (s, 1H), 7.75 (d, J = 8.4 Hz, 2H),
3
1
3
.73 (d, J = 8.0 Hz, 2H), 3.09 (s, 6H). C NMR (100 MHz, CDCl ): δ = 190.3, 154.3,
3
-1
32.0, 125.1, 111.0, 40.1. IR (KBr, cm ): 2904, 2795, 1659, 1589, 1231, 810.
5
h
4
.5.8. 4-Chlorobenzaldehyde (2h). White solid, m.p. 44–45 °C (ref. m.p. 46–47 °C).
1
H NMR (400 MHz, CDCl ): δ = 9.99 (s, 1H), 7.83 (d, J = 8.0 Hz, 2H), 7.52 (d, J =
3
13
8
.0 Hz, 2H). C NMR (100 MHz, CDCl ): δ = 190.8, 141.0, 134.8, 130.9, 129.5. IR
3
-1
(
KBr, cm ): 1706, 1592, 1285, 909, 734.
5
h
4
.5.9. 4-Bromobenzaldehyde (2i). White solid, m.p. 57–58 °C (ref. m.p. 56–57 °C).
1
H NMR (400 MHz, CDCl ): δ = 9.98 (s, 1H), 7.76 (d, J = 8.0 Hz, 2H), 7.70 (d, J =
3
2
0

ACCEPTED MANUSCRIPT
13
8
.0 Hz, 2H). C NMR (100 MHz, CDCl ): δ = 191.1, 135.1, 132.5, 131.0, 129.8. IR
3
-1
(
KBr, cm ): 1686, 1586, 1572, 1203, 808.
2
3
1
4
1
1
2
4
1
7
1
1
4
1
8
1
.5.10. 4-Fluorobenzaldehyde (2j). Oil. H NMR (400 MHz, CDCl ): δ = 9.97 (s,
3
13
H), 7.94-7.89 (m 2H), 7.22 (t, J = 8.4 Hz, 2H). C NMR (100 MHz, CDCl ): δ =
3
1
3
2
90.3, 166.6 (d, J = 255.1 Hz), 133.2, 132.2 (d, J = 9.6 Hz), 116.4 (d, J =
C-F
C-F
C-F
-1
2.2 Hz). IR (KBr, cm ): 1701, 1598, 1506, 1231, 908, 731.
2
6
.5.11. 4-Acetamidobenzaldehyde (2k). White solid, m.p. 157–158 °C (ref. m.p.
1
56–157 °C). H NMR (400 MHz, CDCl ): δ = 9.93 (s, 1H), 7.86 (d, J = 8.0 Hz, 2H),
3
1
3
.79 (s, 1H), 7.72 (d, J = 8.0 Hz, 2H), 2.24 (s, 3H). C NMR (100 MHz, CDCl ): δ =
3
-1
91.9, 169.6, 145.3, 131.6, 131.3, 119.0, 24.7. IR (KBr, cm ): 3264, 1690, 1674,
597, 1533, 831.
5
h
.5.12. 4-Nitrobenzaldehyde (2l). Yellow solid, m.p. 104–105 °C (ref. m.p.
1
05–106 °C). H NMR (400 MHz, CDCl ): δ = 10.17 (s, 1H), 8.41 (d, J = 7.6 Hz, 2H),
3
1
3
.09 (d, J = 7.6 Hz, 2H). C NMR (100 MHz, CDCl ): δ = 190.1, 151.2, 140.1, 130.4,
3
-1
24.3. IR (KBr, cm ): 1709, 1604, 1531, 1345, 905.
.5.13. 3-Nitrobenzaldehyde (2m). Yellow solid, m.p. 57–58 °C (ref. m.p. 58–59 °C).
2
4
4
1
H NMR (400 MHz, CDCl ) δ (ppm): 10.14 (s, 1H), 8.72 (s, 1H), 8.50 (d, J = 7.6 Hz,
3
1
3
1
H), 8.25 (d, J = 7.6 Hz, 1H), 7.79 (t, J = 8.0 Hz, 1H),. C NMR (100 MHz, CDCl )
3
-1
δ (ppm): 189.6, 148.9, 137.5, 134.6, 130.4, 128.5, 124.4. IR (KBr, cm ): 1705, 1616,
1
4
1
535, 1353, 908, 730.
.5.14. 2-Methylbenzaldehyde (2n). Oil. H NMR (400 MHz, CDCl ): δ = 10.28 (s,
2
4
1
3
13
H), 7.80 (d, J = 7.6 Hz, 1H), 7.50-7.26 (m, 3H), 2.68 (s, 3H). C NMR (100 MHz,
2
1

ACCEPTED MANUSCRIPT
-1
CDCl ): δ = 192.6, 140.6, 134.3, 133.5, 132.0, 131.7, 126.3, 19.4. IR (film, cm ):
3
1
4
1
695, 1601, 908, 731.
.5.15. 2-Methoxybenzaldehyde (2o). Oil. H NMR (400 MHz, CDCl ): δ = 10.47 (s,
2
3
1
3
1
3
H), 7.82 (d, J = 7.6 Hz, 1H), 7.57-7.53 (m, 1H), 7.04-6.97 (m, 2H), 3.93 (s, 3H). C
NMR (100 MHz, CDCl ): δ = 189.7, 161.9, 135.9, 128.6, 125.1, 120.7, 111.7, 55.7.
3
-1
IR (film, cm ): 1689, 1602, 1248, 909, 728.
2
3
1
4
1
1
9
4
1
1
4
1
.5.16. 2-Chlorobenzaldehyde (2p). Oil. H NMR (400 MHz, CDCl ): δ = 10.50 (s,
3
1
3
H), 7.93 (d, J = 7.6 Hz, 1H), 7.56-7.37 (m, 3H). C NMR (100 MHz, CDCl ): δ =
3
-1
89.6, 137.9, 135.0, 132.6, 130.6, 129.4, 127.2. IR (film, cm ): 1698, 1592, 1267,
09, 732.
2
3
1
.5.17. 2-Bromobenzaldehyde (2q). Oil. H NMR (400 MHz, CDCl ): δ = 10.38 (s,
3
1
3
H), 7.94-7.91 (m, 1H), 7.67-7.44 (m, 3H). C NMR (100 MHz, CDCl ): δ = 191.7,
3
-1
40.6, 135.2, 133.9, 129.9, 127.9, 127.0. IR (film, cm ): 1696, 1587, 1265, 1199, 907.
2
3
1
.5.18. 2-Aminobenzaldehyde (2r). Oil. H NMR (400 MHz, CDCl ): δ = 9.90 (s,
3
H), 7.51 (d, J = 7.6 Hz, 1H), 7.34 (t, J = 7.4 Hz, 1H), 6.77 (t, J = 7.4 Hz, 1H), 6.68
1
3
(
d, J = 8.0 Hz, 1H), 6.13 (br, 2H). C NMR (100 MHz, CDCl ): δ = 194.1, 150.0,
3
-
1
1
4
7
7
1
4
35.8, 135.2, 118.9, 116.4, 116.2. IR (film, cm ): 3324, 1719, 1663, 1480, 747.
2
3
.5.19. 2,4-Dichlorobenzaldehyde (2s). White solid, m.p. 69–70 °C (ref. m.p.
1
1–73 °C). H NMR (400 MHz, CDCl ): δ = 10.42 (s, 1H), 7.88 (d, J = 8.4 Hz, 1H),
3
1
3
.49 (s, 1H), 7.38 (d, J = 8.4 Hz, 1H). C NMR (100 MHz, CDCl ): δ = 188.5, 141.1,
3
-
1
38.5, 130.9, 130.5, 130.3, 128.0. IR (KBr, cm ): 1678, 1574, 1557, 1373, 822.
2
3
1
.5.20. 1-Naphthaldehyde (2t). Oil. H NMR (400 MHz, CDCl ): δ = 10.41 (s, 1H),
3
2
2

ACCEPTED MANUSCRIPT
9
.26 (d, J = 8.4 Hz, 1H), 8.11 (d, J = 8.4 Hz, 1H), 8.00 (d, J = 7.2 Hz, 1H), 7.93 (d, J
1
3
=
8.0 Hz, 1H), 7.72-7.58 (m, 3H). C NMR (100 MHz, CDCl ): δ = 193.3, 136.3,
3
-
1
1
1
4
7
1
4
35.2, 133.8, 131.6, 130.6, 129.0, 128.4, 126.9, 124.9, 124.8. IR (film, cm ): 1689,
573, 1217, 1170, 909.
5
d
1
.5.21. 2-Thiophenaldehyde (2u). Oil. H NMR (400 MHz, CDCl ): δ = 9.95 (s, 1H),
3
1
3
.79-7.76 (m, 2H), 7.24-7.21 (m, 1H). C NMR (100 MHz, CDCl ): δ = 182.8, 144.2,
3
-1
36.0, 134.9, 128.2. IR (film, cm ): 1661, 1516, 1418, 1213, 908, 725.
2
3
1
.5.22. 2-Furaldehyde (2v). Oil. H NMR (400 MHz, CDCl ): δ = 9.68 (s, 1H), 7.71
3
1
3
(
s, 1H), 7.27 (s, 1H), 6.62 (s, 1H). C NMR (100 MHz, CDCl ): δ = 177.9, 152.9,
3
-
1
1
4
9
48.1, 121.2, 112.6. IR (film, cm ): 1669, 1568, 1463, 1392, 1017, 747.
5
d
1
.5.23. 3-Pyridinaldehyde (2w). Oil. H NMR (400 MHz, CDCl ): δ = 10.13 (s, 1H),
3
13
.09 (s, 1H), 8.86 (d, J = 4.4 Hz, 1H), 8.19 (d, J = 7.6 Hz, 1H), 7.52-7.48 (m, 1H). C
-1
NMR (100 MHz, CDCl ): δ = 190.6, 154.6, 152.0, 135.7, 131.5, 124.0. IR (film, cm ):
3
1
4
7
707, 1589, 1246, 907, 727.
2
3
1
.5.24. (E)-Cinnamaldehyde (2x). Oil. H NMR (400 MHz, CDCl ): δ = 9.71 (d, J =
3
.6 Hz, 1H), 7.58-7.56 (m, 2H), 7.48 (d, J = 16.0 Hz, 1H), 7.45-7.42 (m, 3H), 6.72
1
3
(
dd, J = 7.6, 16.0 Hz, 1H). C NMR (100 MHz, CDCl ): δ = 193.8, 152.8, 134.0,
3
-1
1
31.3, 129.1, 128.6, 128.5. IR (film, cm ): 1670, 1624, 1449, 1119, 970, 745.
Acknowledgements
We thank the National Natural Science Foundation of China (No. 21272044) and
Scientific Research Fund of Education Department of Jiangxi Province (KJLD13022)
2
3

ACCEPTED MANUSCRIPT
for financial support.
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Graphical Abstract
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Supporting Information for:
A highly efficient heterogeneous aerobic alcohol oxidation
catalyzed by immobilization of bipyridine copper(I) complex
in MCM-41
a,b
a
a
a
a
Hong Zhao, Qiurong Chen, Li Wei, Yuanyuan Jiang and Mingzhong Cai *
a
Key Laboratory of Functional Small Organic Molecule, Ministry of Education and
College of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang
30022, P. R. China
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b
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University, Guangzhou 510006, P. R. China
Fax: (+86)-791-8812-0388; E-mail: mzcai@jxnu.edu.cn
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H NMR and C NMR spectra of compounds 2a-2x.
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H NMR and C NMR of compound 2a
2