Highly selective copper-catalyzed oxidation of benzyl alcohols to aromatic aldehydes in water at room temperature

Article abstract of DOI:10.1002/aoc.3473

Water-soluble 2N2O–Cu(II) complexes were synthesized and used for the catalytic oxidation of benzylic alcohols to the corresponding aldehydes in pure water resulting in high yields of up to 94%. Importantly, the catalytic system exhibits a wide substrate scope, high functional group tolerance and can be reused directly for subsequent reaction cycles. Using this catalytic system, the product 4-methylbenzaldehyde can be produced on a multi-gram scale and in the complete absence of any organic solvent, surfactant or phase transfer reagent. Copyright

Full text of DOI:10.1002/aoc.3473

Full paper
Received: 5 December 2015
Revised: 31 January 2016
Accepted: 9 February 2016
Published online in Wiley Online Library: 28 March 2016
(wileyonlinelibrary.com) DOI 10.1002/aoc.3473
Highly selective copper-catalyzed oxidation of
benzyl alcohols to aromatic aldehydes in water
at room temperature
Jianglong Wu, Yan Liu, Xiaowei Ma, Ping Liu*, Chengzhi Gu and Bin Dai
Water-soluble 2N2O–Cu(II) complexes were synthesized and used for the catalytic oxidation of benzylic alcohols to the corre-
sponding aldehydes in pure water resulting in high yields of up to 94%. Importantly, the catalytic system exhibits a wide substrate
scope, high functional group tolerance and can be reused directly for subsequent reaction cycles. Using this catalytic system, the
product 4-methylbenzaldehyde can be produced on a multi-gram scale and in the complete absence of any organic solvent, sur-
factant or phase transfer reagent. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site
Keywords: aldehydes; copper; oxidation; benzylic alcohols; water
protocol for the catalytic oxidation of benzylic alcohols to alde-
hydes using 2N2O–Cu(II) complex in water. It has the following ad-
Introduction
The oxidation of benzylic alcohols is of synthetic significance, par-
ticularly for the preparation of carbonyl compounds, representing
versatile synthetic intermediates used in the production of pharma-
vantages compared with the reported methods: water is used as
sole solvent without addition of organic solvents or phase transfer
reagents; the reaction is carried out at room temperature; the
product aldehydes can be separated by simple extraction without
complicated work-up procedures; the catalyst is easily available
with high catalytic activities and can be easily reused at least two
times; and the catalytic system can be applied to large-scale
reactions.
[1]
ceuticals, flavors, fragrances and aniline dyes. The selective oxida-
tion of alcohols to the corresponding aldehydes is important in
[
2]
fundamental research and industrial manufacturing. General oxi-
dation processes of benzylic alcohols are usually carried out in the
presence of stoichiometric oxidants such as manganic oxides, chro-
mium oxides, hypervalent organoiodine reagents and other transi-
tion metals. However, over-oxidation to carboxylic acids needs to
be avoided in most cases and the formation of harmful gases to-
Results and discussion
[
3]
gether with process complications limit widespread applications.
Optimization of reaction conditions
Therefore, the oxidation of benzylic alcohols using catalytic systems
seems to be ideal, in terms of both atom economy and
In initial experiments we chose benzyl alcohol (1.0mmol) as a
model substrate for the oxidative reaction catalyzed by complexes
1–4 (1mol%; Fig. 1) in combination with (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl (TEMPO; 5 mol%) in the presence of
[4,5]
sustainability.
Some transition metal catalysts may also be com-
bined with oxygen ligands, resulting in cost-efficient, environmen-
[6–9]
tally friendly and readily available species.
Transition metal catalysts have already been reported that fea-
ture remarkable catalytic activities for this synthetic route. The reac-
tion media usually include organic solvents or mixed solvents, and
expensive metals such as palladium, ruthenium or gold are usually
1 atm neat oxygen and K CO at 80 °C in pure water (5ml). The cor-
2 3
responding yields were determined using GC-MS. As evident from
Table 1, in the complete absence of a copper source the desired
product can only be obtained in low yield (Table 1, entry 1). Simi-
[10–16]
required.
Therefore, replacement of relatively rare, precious
larly, when Cu(OAc)
21% can be obtained (Table 1, entry 2).
2
ꢀH
2
O is used as the sole catalyst, a yield of only
elements with more abundant and cheaper metals offers a substan-
tial saving of costs. Among active metal catalysts reported for
applications in aerobic oxidations, those based on nonprecious,
earth-abundant copper have recently received great attention, par-
Complex 3 displays a high catalytic activity for this oxidative reac-
tion (Table 1, entry 5), resulting in the formation of the desired
product in 94% yield. Conversely, complexes 1, 2 and 4 have low
[17–29]
ticularly due to their low cost and environmental sustainability.
However, reports of homogeneous complex-catalyzed transforma-
[30–32]
tion reactions in pure water are scarce.
To the best of our
* Correspondence to: Ping Liu, School of Chemistry and Chemical Engineering/Key
Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan,
Shihezi University, Shihezi 832003, China. E-mail: liuping1979112@aliyun.com
knowledge, water-soluble salen–Cu(II)-catalyzed conversion reac-
tions of benzylic alcohols to aldehydes in water have never been
reported.
School of Chemistry and Chemical Engineering/Key Laboratory for Green
Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University,
Shihezi 832003, China
In an effort to continue our studies on copper-catalyzed reactions
[33]
in water, in this paper we describe a highly efficient and practical
Appl. Organometal. Chem. 2016, 30, 577–580
Copyright © 2016 John Wiley & Sons, Ltd.

J. Wu et al.
a
Table 2. Oxidation of benzyl alcohol catalyzed by complex 3
b
Entry
R/5
Product 6
Yield (%)
Figure 1. Structures of complexes 1–4.
1
2
3
4
5
6
7
8
9
4-Me/5b
6b
6c
6d
6e
6f
94
92
78
92
32
83
83
91
90
87
90
catalytic activity (Table 1, entries 3, 4 and 6). The results obtained
show that the electronic effects and steric hindrance of the ligands
exhibit an obvious effect on the catalytic activity. Therefore, we se-
lected complex 3 as the catalyst to further investigate the effects of
other reaction conditions and focused on factors involving catalyst
loadings, solvents, bases and reaction temperature. Interestingly,
upon decreasing the amount of solvent from 5 to 2 ml, the product
yield still remains constant at 99% (Table 1, entry 7). Worth noting in
this context is the fact that the presence of 1 atm air as oxygen
source and oxidant proves to be sufficient for this transformation (
Table 1, entry 8). When the catalyst loading is significantly reduced
to 0.1%, the yield of the product decreases to 56% (Table 1, entry 9).
4-OMe/5c
2
4-NO /5d
4-Br/5e
3-F/5f
3-OMe/5 g
2-Me/5 h
2-F/5i
6 g
6 h
6i
2-Cl/5j
6j
1
0
1
2-Cl, 4-Cl/5 k
3-F, 4-OMe/5 l
6 k
6 l
1
a
Reaction conditions: benzyl alcohol (1 mmol), K
TEMPO, H O (2 ml), room temperature, 10 h.
Isolated yield.
CO
2 3
(1 mmol), 5%
2
b
a
Table 1. Optimization of oxidative reactions
Experiments using a variety of different bases showed that Na CO ,
2
3
pyridine and Et N appear to be the preferred bases over NaOH,
3
K PO and KOH, resulting in the formation of the desired product
3
4
in 79–95% yields (Table 1, entries 10–15). The reaction can also be
performed with 2.5mol% TEMPO under modified reaction condi-
tions and results in a product yield of 95% (Table 1, entry 16). More-
over, when the reaction is carried out at room temperature, the
product can be obtained in 99% yield (Table 1, entry 17). When
the reaction time is significantly reduced to 2 h, the yield of the
product decreases to 40% (Table 1, entry 18). Finally, a combination
b
Entry
Catalyst
TEMPO
%)
Base
H
2
O solvent Yield (%)
(ml)
(
1
2
—
5
K
2
CO
CO
3
5
5
6
Cu(OAc)
2
ꢀH
2
O
5
K
2
3
21
(1%)
2 3
of complex 3 (1mol%), TEMPO (5mol%), K CO (1 equiv.) at room
3
4
5
6
7
Complex 1 (1%)
Complex 2 (1%)
Complex 3 (1%)
Complex 4 (1%)
Complex 3 (1%)
5
5
5
5
5
K
2
CO
CO
CO
CO
CO
3
5
5
5
5
2
60
22
temperature in H O (2ml) are found to be the optimized reaction
2
K
2
3
conditions for the oxidation of benzyl alcohol.
K
2
3
94
Upon determination of the optimized reaction conditions, we an-
alyzed the scope of various benzyl alcohol substrates suitable for
this reaction type. First, the oxidative reactions of p-substituted
benzyl alcohol were explored in the presence of 1 mol% complex
K
2
3
71
K
2
3
>99
c
(
88)
d
8
Complex 3 (1%)
Complex 3 (0.1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
Complex 3 (1%)
5
5
K
2
CO
CO
NaOH
PO
KOH
Na CO
Pyridine
Et
CO
CO
CO
3
2
2
2
2
2
2
2
2
2
2
2
95
56
3
in water. As evident from Table 2, substrates bearing electron-
9
K
2
3
withdrawing or electron-donating groups afford the desired prod-
ucts 6b–6e with good to excellent yields (78–94%; Table 2, entries
10
11
12
13
14
15
16
17
18
5
8
5
K
3
4
28
1–4). The only exception is that the oxidation of (3-fluorophenyl)
5
Trace
79
methanol to form compound 6f proceeds with a low yield (32%;
Table 2, entry 5). Interestingly, (3-methoxyphenyl)methanol results
in the formation of product 6 g with a high yield of 83% (Table 2,
entry 6). In addition, even though the benzylic alcohol features a
high steric demand, the corresponding products 6 h–6j can be ob-
tained with yields ranging between 83 and 91% (Table 2, entries 7–
5
2
3
5
95
5
3
N
85
2.5
5
K
2
3
95
e
f
K
2
3
>99
40
5
K
2
3
a
Reaction conditions: benzyl alcohol (1 mmol), K
TEMPO, H O as solvent, 80 °C, 10 h.
GC-MS yield.
2 3
CO (1 mmol), 5%
2
b
c
Isolated yield.
d
Air as oxidant.
e
Room temperature.
f
Reaction time of 2 h.
Scheme 1. Oxidations of secondary alcohols catalyzed by complex 3.
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 577–580

Highly selective oxidation of benzyl alcohols in water
the oxidation of the alcohols, the oxoammonium ion itself is reduced
to hydroxylamine. The subsequent oxidation of hydroxylamine re-
generates the oxoammonium ion. Finally, Cu(II) is regenerated by ox-
idation of Cu(I) with oxygen, producing water as a byproduct
resulting in the completion of the catalytic cycle. To verify the pro-
posed mechanism, another model reaction was carried out under ar-
gon atmosphere. Unfortunately, under these conditions the desired
product can be obtained only in trace amounts. This finding illus-
trates that the presence of oxygen is crucial for the reaction and pro-
vides further evidence for the proposed mechanism as being
reasonable.
Scheme 2. Gram-scale synthesis of 6b.
Table 3. Recycling experiments
Entry
1
2
3
Yield (%)
93
70
58
9
). Furthermore, the oxidations of polysubstituted benzyl alcohols Conclusions
were also attempted under the standard reaction conditions de-
scribed. The compounds (2,4-dichlorophenyl)methanol and (3-
fluoro-4-methoxyphenyl)methanol can be oxidized to the desired
aldehydes 6 k and 6 l with yields of 87 and 90%, respectively (
Table 2, entries 10 and 11).
Furthermore, we evaluated this catalyst for the oxidations of
secondary alcohols at 80°C, which also produce the corresponding
ketones in 38 and 37% yields, respectively (Scheme 1).
An efficient copper-catalyzed protocol for the oxidation of benzylic
alcohols to aldehydes in water has been developed. This approach
represents an important complement to traditional procedures and
may therefore have important industrial applications. Further stud-
ies are currently underway in our laboratory to explore other reac-
tion types for which this or a similar catalytic system may prove
suitable.
Considering the broad application range and large demand of al-
dehyde in many fields, especially as a raw material for making phar-
macological products and drug stabilizers, we tried to conduct a
scale-up preparation study of aldehyde based on the optimized
conditions obtained above. It is noteworthy that the reaction was
performed with 10 mmol of p-tolylmethanol, and gives a yield of
Experimental
Materials and instrumentation
Chemicals were obtained commercially and used as received. NMR
spectra were recorded with a Bruker DPX–400 spectrometer using
tetramethylsilane as the internal standard. EI mass spectra were re-
corded with a GC-MS (Agilent 7890A/5975C) instrument. All prod-
ucts were isolated by short chromatography on a silica gel (200–
84%, corresponding to 1.021 g of product 6b (Scheme 2).
To evaluate the reusability of complex 3 as catalyst, recycling
studies were carried out for the oxidative reaction of benzyl alcohol
in water at room temperature. After the reaction was completed,
the product was extracted with ethyl ether and the water phase
containing the catalyst was loaded with the reactants, TEMPO and
base for the next run. As evident from Table 3, the catalyst can be
recycled two times. However, a significant loss of catalytic activity
is observed in the third run.
300 mesh) column using petroleum ether (60–90 °C), unless other-
wise noted. Benzyl alcohols were of analytical grade, purchased
from Adamas-beta Pharmaceuticals, Inc. Compounds described in
the literature were characterized using H NMR spectra compared
1
[
34]
to reported data.
Based on related reports,
a reasonable mechanism for the
oxidation of benzylic alcohols is shown in Scheme 3. Here, the
nitroxyl radical is first oxidized to the oxoammonium ion (I) by the
Cu(II) complex. Then, the oxoammonium ion serves as catalytic ox-
idant and oxidizes the alcohol substrates (5) via an ionic transition
state (II) to the corresponding carbonyl compounds (6). During
Synthesis of complexes 1–4
A solution of 2 mmol of substituted amine and 2 mmol of sodium 3-
formyl-4-hydroxybenzenesulfonate in 10 ml of 50% ethanol was
stirred at 60 °C for 5 h, and then 1 mmol of Cu(OAc)
of 50% ethanol was added, and the mixture stirred at 60°C for
h. After cooling to room temperature, the reaction solution was
2 2
ꢀH O in 10ml
2
diluted with 50 ml of acetone and filtered. The solid product was
collected and dried under vacuum to afford the desired complexes
1–4.
2 2 8 2
Complex 1: yield 75%. Anal. Calcd for C26H18CuN Na O S (%): C,
4
9
7.31; H, 2.75; N, 4.24; S, 9.72. Found (%): C, 46.57; H, 2.77; N, 4.20; S,
+
.69. MS (negative); m/z: [(Mꢁ Na)] , calculated, 659.0; found, 659.6.
ꢁ
1
IR (KBr, cm ): 3441 (OH), 1612 (C¼N), 1184 (SO
3
Na).
Complex 2: yield 70%. Anal. Calcd for C H CuN Na O S (%): C,
28
22
2
2 8 2
4
9
8.87; H, 3.22; N, 4.07; S, 9.32. Found (%): C, 48.43; H, 3.28; N, 3.98; S,
+
.27. MS (negative): m/z: [(Mꢁ Na)] , calculated, 664.0; found, 664.2.
ꢁ
1
IR (KBr, cm ): 3433 (OH), 1628 (C¼N), 1169 (SO Na).
3
Complex 3: yield 72%. Anal. Calcd for C28H22CuN Na O S (%): C,
2 2 10 2
4
8
6.70; H, 3.08; N, 3.89; S, 8.91. Found (%): C, 46.80; H, 2.54; N, 3.84; S,
+
.79. MS (negative): m/z: [(Mꢁ Na)] , calculated, 690.6; found, 690.2.
ꢁ
1
IR (KBr, cm ): 3448 (OH), 1616 (C¼N), 1184 (SO Na).
3
Complex 4: yield 75%. Anal. Calcd for C26H14Cl CuN Na O S (%):
4 2 2 8 2
Scheme 3. Proposed reaction mechanism.
C, 39.14; H, 1.77; N, 3.51; S, 8.04. Found (%): C, 39.05; H, 1.81; N, 3.45;
Copyright © 2016 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/aoc
Appl. Organometal. Chem. 2016, 30, 577–580

J. Wu et al.
+
[
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12] B. Guan, D. Xing, G. Cai, X. Wan, N. Yu, Z. Fang, L. Yang, Z. Shi, J. Am.
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ꢁ
1
7
72.2. IR (KBr, cm ): 3430 (OH), 1608 (C¼N), 1184 (SO
3
Na).
1
981, 703, 3522.
General procedure for catalytic reaction
A solution of benzyl alcohol (1mmol), complex 3 (1mol%), TEMPO
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Catalyst recycling for oxidative reaction
When the reaction was completed, the mixture was extracted with
ethyl ether (2ml). Benzyl alcohol (1mmol), TEMPO (5mol%) and
K CO (1.0mmol) were added to the aqueous phase that was sep-
2 3
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temperature.
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Appl. Organometal. Chem. 2016, 30, 577–580