Recyclable copper-catalyzed ambient aerobic oxidation of primary alcohols to aldehydes in water using water-soluble PEG-functionalized pyridine triazole as ligand

Article abstract of DOI:10.1016/j.catcom.2017.07.010

A water-soluble PEG-PyTa ligand, conveniently derived from commercially available reagents via a simple synthetic approach, could efficiently promote copper/2, 2, 6, 6-tetramethyl piperidine-1-oxyl (TEMPO)-catalyzed aerobic oxidation of primary alcohols to aldehydes in water under room temperature. A number of primary benzylic, allylic alcohols and heteroaryl methanols were selectively converted into their corresponding aldehydes with excellent yields. The oxidation products could be isolated by simple extraction, and the residual aqueous phase containing the catalyst Cu(OTf)₂/PEG-PyTa could be reused at least 7 times.

Full text of DOI:10.1016/j.catcom.2017.07.010

Accepted Manuscript
Recyclable copper-catalyzed ambient aerobic oxidation of
primary alcohols to aldehydes in water using water-soluble PEG-
functionalized pyridine triazole as ligand
Nan Sun, Xiaonan Zhang, Liqun Jin, Baoxiang Hu, Zhenlu Shen,
Xinquan Hu
PII:
S1566-7367(17)30303-5
doi: 10.1016/j.catcom.2017.07.010
CATCOM 5129
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
6 June 2017
4 July 2017
14 July 2017
Please cite this article as: Nan Sun, Xiaonan Zhang, Liqun Jin, Baoxiang Hu, Zhenlu Shen,
Xinquan Hu , Recyclable copper-catalyzed ambient aerobic oxidation of primary alcohols
to aldehydes in water using water-soluble PEG-functionalized pyridine triazole as ligand.
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ACCEPTED MANUSCRIPT
Recyclable copper-catalyzed ambient aerobic oxidation of primary alcohols
to aldehydes in water using water-soluble PEG-functionalized pyridine
triazole as ligand
Nan Sun*, Xiaonan Zhang, Liqun Jin, Baoxiang Hu, Zhenlu Shen, Xinquan Hu*
The College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China
E-mail: sunnan@zjut.edu.cn and xinquan@zjut.edu.cn

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ABSTRACT:
A water-soluble PEG-PyTa ligand, conveniently derived from commercially available reagents via a
simple synthetic approach, could efficiently promote copper/2, 2, 6, 6-tetramethyl piperidine-1-oxyl
(TEMPO)-catalyzed aerobic oxidation of primary alcohols to aldehydes in water under room
temperature. A number of primary benzylic, allylic alcohols and heteroaryl methanols were selectively
converted into their corresponding aldehydes with excellent yields. The oxidation products could be
isolated by simple extraction, and the residual aqueous phase containing the catalyst
Cu(OTf)2/PEG-PyTa could be reused at least 7 times.
Keywords: PEG-functionalized pyridine triazole ligand (PEG-PyTa); copper/TEMPO-catalyzed
aerobic oxidation of alcohols; Aqueous homogeneous catalysis.

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1
. Introduction
The selective oxidation of alcohols to their corresponding aldehydes or ketones is one of most
important processes in industry and academia. [1, 2] Traditional methods to accomplish this
transformation rely on the stoichiometric amount of metallic or organic oxides, which are expensive,
toxic and tend to generate large amounts of hazardous waste. [3] To overcome these inherent problems,
significant efforts have been devoted to developing catalytic oxidation alternatives using molecular
oxygen as the terminal oxidant during the past decade. [4-8] Among them, quite a lot reports have been
focusing on synergistic catalytic systems of simple copper salts and TEMPO, initially described by
Semmelhack and co-workers. [9] To improve the catalytic efficiency, Sheldon, Koskinen and others
introduced a variety of ligands to stabilize copper catalysts. [10-20] Recently, Stahl reported a more
general and efficient Bipy-Cu(I)/TEMPO/N-methyl imidazole catalytic system, where a detailed
mechanism was also investigated. [21, 22] Despite of a number of efficient copper/TEMPO-related
catalytic systems available for aerobic alcohol oxidation, most of these reactions occurred in organic
solvents for high conversion. In the view of green chemistry, in recent years, exploring of these
catalytic systems in the environmentally benign solvents has been attracted increasing interests.
Water is an extremely cheap, non-toxic and non-flammable solvent. It is obviously advantageous to use
water as solvent for organic reactions, which include low cost, work-up procedure simplification and
minimum environmental impact. [23] More importantly, the concern about the potential explosion
hazard associated with the combination of O2 and the flammable organic solvents also could be
completely eliminated when an aerobic oxidation reaction was implemented in water. [24] More

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recently, several water-compatible copper/TEMPO-related catalytic systems for aerobic alcohol
oxidation have been reported. [25-32] However, many of them suffered from one or more limitations,
such as the requirement of heating [25, 27, 28], elevated O2 pressure [25], long reaction time [26] and
the presence of special phase transfer agents [29, 31]. Poly(ethylene glycols) (PEG), a kind of less
expensive, non-toxic and biocompatible polymers and also being widely used as green solvents and
phase transfer regents in various organic synthesis, are well-known water-soluble supports for catalysts.
[33, 34] However, the application of PEG-supported ligand for the aerobic alcohol oxidations in water
was rare reported to date. Previously, Toy and co-workers reported a PEG-functionalized 2,
2
’-bipyridine as water-soluble ligand for the copper/TEMPO-catalyzed aerobic oxidation of alcohols,
o
however, CH3CN was crucially required as co-solvent under elevated reaction temperature (80 C).
[35]
Due to our interest in developing practical and efficient aqueous catalytic systems for transition
metal-catalyzed reactions, we reported herein the development of a new PEG-functionalized pyridine
triazole (PEG-PyTa) ligand (Scheme 1) and its application as water soluble N, N-bidentate ligand for
copper/TEMPO-catalyzed aqueous aerobic alcohols oxidation reactions. With a balloon of oxygen
under room temperature, the in-situ generated catalytic species could efficiently catalyze a wide range
of primary alcohols to their corresponding aldehydes in water. Moreover, after extracting the desired
oxidation products, the residual aqueous phase could be reused as recovery catalyst for at least 7 times.

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Scheme1. The synthetic route for PEG-PyTa ligand
2
. Experimental
.1. Synthesis of PEG-PyTa ligand
2
The PEG-PyTa was synthesized via a three-step reaction sequence as described in Scheme 1. The
detailed experimental procedure was as following.
To a solution of 20.0 g of PEG2000 (10 mmol) and 6.8 g of methanesulfonyl chloride MSCl (60 mmol)
o
in 250 mL of dry CH2Cl2, was dropwisely added 4.7 g of pyridine (60 mmol) at 0 C under N2
atmosphere. After completion of addition, the mixture was stirred at room temperature for 24 h. The
reaction was quenched with 100 mL of ice-water and pH was adjusted to 7 with a 20 % aqueous NaOH
solution (about 15 mL). Then the organic layer was separated, washed with brine, dried with Na2SO4
and filtered. After removal of solvent (CH2Cl2) under vacuum, the residue was precipitated with 150
mL of methyl tert-butyl ether (MTBE) to afford 20.7 g of PEG-OMS (93%) as a white solid.
A mixture of 17.8 g of PEG-OMS (8 mmol) and 3.1 g of NaN3 (48 mmol) was stirred in 150 mL of
o
DMF at 65 C for 12 h. The resulting mixture was cooled to room temperature and most of DMF was
removed under vacuum. 100 mL of CH2Cl2 was added to dissolve the residue, washed with brine, dried

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with Na2SO4 and followed a similar procedure as described above to afford 15.1 g of PEG-N3 (91%) as
a white solid.
To 50 mL of 1:1 of CH3OH/H2O, were sequentially added 10.3 g of PEG-N3 (5 mmol), 1.24 g of
2
-ethynylpyridine (12 mmol), 0.50 g of CuSO4·5H2O (2.0 mmol) and 0.40 g of sodium ascorbate (2.0
o
mmol). The mixture was stirred at 45 C for 24 h. Then the solution was concentrated under vacuum
and 50 mL of 10% NH3·H2O was added. After stirred for 12 h, the mixture was extracted twice with 50
mL of CH2Cl2, dried with Na2SO4 and followed a similar procedure as described above to afford 9.8 g
1
of PEG-PyTa (87%) as a white solid. H NMR (500 MHz, MeOD): δ=8.60 (d, J=4.7 Hz, 1H), 8.52 (s,
1
H), 8.09 (d, J=8.0 Hz, 1H), 7.96-7.92 (m, 1H), 7.40-3.38 (m, 1H), 4.69 (t, J=5.0 Hz, 2H), 3.97 (t,
1
3
J=5.0 Hz, 2H), 3.65-3.63 (m, 92H, CH2 of PEG chain); C NMR (125 MHz, MeOD): δ=151.2, 150.5,
48.5, 138.8, 125.0, 124.4, 121.3, 71.5-71.4 (C of PEG chain), 70.3, 51.6. MALDI-TOF-MS (m/z)
1
+
+
calcd. for C110H203N8O47 [Mn=47+H] 2388.37, found: 2388.48, C110H202N8O47Na [Mn=47+Na] 2410.37,
found: 2410.39.
2
.2. General procedure for aerobic oxidation of alcohols in water
To a 48 mL tube, were added Cu(II) or Cu(I) salt (0.05 mmol), PEG-PyTa (0.025 mmol) and H2O (3.0
mL). The mixture was stirred for 30 min at room temperature and a clear dark-blue solution was
observed. Then alcohols (1.0 mmol), TEMPO (0.05 mmol), and K CO (0.2 mmol) were sequentially
2
3
added, followed by connecting a balloon of oxygen. The reaction mixture was stirred at room
temperature until the reaction completed based on GC analysis. After that, the reaction mixture was
extracted with MTBE (3 mL × 3) and the extracts were combined, dried over anhydrous Na2SO4 and
concentrated under vacuum. Finally, the residue was purified by flash chromatography on silica to

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afford the desired aldehydes.
3
. Results and discussion
.1. Optimization of the reaction conditions
3
The performance of PEG-PyTa on the copper/TEMPO-catalyzed aerobic alcohol oxidation in water
was investigated with benzyl alcohol as a model substrate (Table 1). As previous reports, base is
essential for the reaction. Therefore, we started our aerobic oxidation reaction in water with 5 mol%
CuCl2, together with 5 mol% TEMPO and 20 mol% K2CO3 as catalyst system under a balloon of O2.
After stirred at room temperature for 6 h, only 27% of benzaldehyde was observed according to GC
analysis (entry 1, Table 1). However, the conversion could be significantly improved when adding
PEG-PyTa as ligand. In the presence of 2.5 mol% PEG-PyTa, the conversion was increased to 83%
under the same reaction conditions (entry 2, Table 1). This positive result promoted us for further
optimizations. With PEG-PyTa as ligand, copper sources, including Cu(II) salts (CuBr2, CuSO4,
Cu(NO3)2, Cu(OTf)2, Cu(TFA)2 and Cu(OAc)2) and Cu(I) salts (CuBr and CuCl), were first screened. It
was found that Cu source has an obvious effect on the reaction conversion and Cu(II) salts generally
exhibited better catalytic activity than Cu(I) salts (entries 3-10, Table 1). To our delight, quantitative
conversion was achieved when Cu(OTf) was employed (entries 6, Table 1). Next, the effect of base
2
was also investigated. Na2CO3 afforded a similar result as K2CO3 (entry 11, Table 1). However, either
stronger base (KOH and K3PO4) or weaker base (NaHCO3) dramatically decreased the conversion
(entries 12-14, Table 1). We also attempted to lower down the amount of Cu(OTf)2, TEMPO and
K2CO3. Unfortunately, decreasing each of them resulted in lower conversions (entries 15-19, Table 1).

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Noteworthy, increasing PEG-PyTa could not improve the reaction, which hinted that Cu(II) and PyTa
formed a 1 : 1 complex (entries 18 and 19, Table 1). In addition, we also tested air to replace O2. Under
the same reaction conditions, only a 39% conversion was observed. However this lower conversion
could be compensated by elongating the reaction time (99% conversion after 18 h, entry 20, Table 1).
The different reaction rates with balloons of air and O2 were contributed to their different oxygen
concentrations in water (10.6 mg/L under a balloon of air vs 29.4 mg/L under a balloon of O2,
determined by Dissolved Oxygen Meter). Noteworthy, we found that increasing the reaction
o
temperature to 40 C remarkably decreased the reaction rate (entry 21, Table 1). Such negative effect
might also be ascribed to the relative lower oxygen concentration in water at higher temperature. As
expected, no reaction occurred without TEMPO (entry 22, Table 1). Furthermore, kinetic study
revealed that 6 h was necessary for the complete reaction (See Figure 2S). Finally, the optimal
conditions were concluded as: Cu(OTf)2 (5 mol%), PEG-PyTa (2.5 mol%), TEMPO (5 mol%) and
K2CO3 (20 mol%) in water with a balloon of O2 under room temperature.
a
Table 1 Optimizing the reaction conditions of the oxidation of benzyl alcohol
b
Cu source
mol%)
Base
PEG-PyTa
(mol%)
--
TEMPO
Conv.
(%)
27
Entry
(
(mol%)
(mol%)
1
2
3
CuCl2 (5)
K2CO3 (20)
K CO (20)
5
5
5
CuCl (5)
2.5
83
2
2
3
CuBr2 (5)
K2CO3 (20)
2.5
86

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4
5
6
7
8
9
CuSO4 (5)
Cu(NO3)2 (5)
Cu(OTf)2 (5)
Cu(TFA)2 (5)
Cu(OAc)2 (5)
CuCl (5)
K2CO3 (20)
K2CO3 (20)
K2CO3 (20)
K2CO3 (20)
K2CO3 (20)
K2CO3 (20)
K2CO3 (20)
Na2CO3 (20)
KOH (20)
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.5
2.5
2.5
2.5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
5
5
5
5
5
84
90
100
79
60
56
1
0
CuBr (5)
66
11
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Cu(OTf)2 (3)
Cu(OTf)2 (3)
99
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
10
K3PO4 (20)
KHCO3(20)
K2CO3 (10)
K2CO3 (5)
49
57
54
43
K2CO3 (20)
K2CO3 (20)
K2CO3 (20)
K CO (20)
93
86
86
c
2
0
Cu(OTf) (5)
39 (99)
46
2
2
3
d
e
2
1
Cu(OTf)2 (5)
Cu(OTf)2 (5)
K2CO3 (20)
K2CO3 (20)
2
2
trace
a
Reaction conditions: Benzyl alcohol (1 mmol), H2O (3 mL), a balloon of O2, room temperature.
GC conversions. Selectivities to benzaldehyde in all cases were 100% and no benzoic acid was
b

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observed.
c
d
e
Under a balloon of air, the conversion in parentheses was obtained after 18 h.
o
Reaction at 40 C.
Without TEMPO.
3
.2. Substrate scope
With the optimal conditions in hands, we then explored the substrate scope of the newly developed
Cu(OTf)2/PEG-PyTa/TEMPO/K2CO3 catalytic system for the aerobic oxidation of primary alcohols
and the results were summarized in Table 2. To our delight, most of the tested substrates, including
substituted benzyl alcohols, 1-naphthalenemethanol, heteroaryl methanols (2-thiophenemethanol and
3
-pyridinemethanol) and cinnamyl alcohol, could be smoothly transformed into their corresponding
aldehydes with high yields by adjusting the reaction time, no acids and esters were observed during the
reaction. A wide range of functional groups such as OMe, SMe, F, Cl, Br, OH and NO2 could be
well-tolerated, some of which could be utilized for further derivation. For substituted benzyl alcohols,
there appeared to be no obvious relationship between the yields and the position or electronic property
of the substituents on the phenyl ring, just the oxidation time varied with different substrates. For
example, much longer reaction time (24 h) was needed for complete reaction of 2-methylbenzyl
alcohol than that for 4-methylbenzyl alcohol (entry 2 vs entry 4, Table 2). However, no obviously
different reaction activity was observed between 2- and 4-bromobenzyl alcohols (entry 11 vs entry 12,
Table 2). Generally, for an aqueous organic reaction, water-solubility of substrates plays an important
role on the reaction rate. Therefore, we speculated that such irregular results might be caused by their
different solubility in water. Unfortunately, 4-cyanobenzyl alcohol and 2-pyridinemethanol could not

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transform to their corresponding aldehydes which could be ascribed to their strong coordination ability
and thus deactivating the catalyst. Furthermore, the newly developed catalytic system failed to the
oxidation of aliphatic primary alcohol (entry 21, Table 2).
a
Table 2 Aerobic oxidation of primary alcohols to aldehydes
b
c
Entry
R
Time (h)
Conv. (%)
100
Yield (%)
d
1
2
3
4
5
6
7
C6H5
6
6
87 (99)
4-CH3C6H4
3-CH3C6H4
2-CH3C6H4
4-CH3OC6H4
3-CH OC H
>99
93
95
94
96
94
93
6
97
24
6
>99
>99
6
>99
3
6
4
3,4-(CH3O)2C6H3
4-CH3SC6H4
6
94
8
9
12
18
6
100
100
95
95
91
95
4
-FC6H4
1
0
4
-ClC6H4
1
1
1
2
4-BrC6H4
2-BrC6H4
12
12
>99
100
96
91

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1
1
3
4
4-(OH)C6H4
4-CNC6H4
18
100
--
92
--
1
2
1
1
1
5
6
7
4-(NO2)C6H4
1-naphthyl
18
24
100
94
95
93
2
-thienyl
12
100
92
1
1
2
8
9
0
3-pyridyl
2-pyridyl
12
12
100
93
--
trace
C6H5CH=CH
6
100
99
--
2
1
n-butyl
12
N.R.
a
Reaction conditions: Substrate (1 mmol), Cu(OTf)2 (5 mol%), PEG-PyTa (2.5 mol%), TEMPO (5
mol%), K2CO3 (20 mol%), H2O (3 mL), a balloon of O2, room temperature.
Determined by GC.
b
c
d
Isolated yields.
Benzaldehyde was volatile and some of them were lost during work-up. The yield in parentheses was
determined by GC with naphthalene as internal standard.
3
.3. Recycling study
As mentioned previous, one of the important advantages of using water-soluble catalyst for aqueous
organic reactions is the simple product isolation by convenient solvent extraction, while leaving the
catalyst in aqueous phase for potential reuse. Therefore, the recyclability of catalyst
(Cu(OTf)2/PEG-PyTA) for aqueous oxidation reaction was examined using benzyl alcohol as the
tested substrate under the optimal conditions. After the reaction completed, TEMPO and the resulted

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benzaldehyde were extracted with MTBE. To the residual aqueous phase, was added fresh benzyl
alcohol, TEMPO and K2CO3. To our delight, as the results illustrated in Table 3, the catalyst did not
lose any activity for four cycles. Despite that reduced catalytic activity was observed in subsequent
cycles, complete conversion still could be achieved at the fifth and sixth cycles and 96% conversion for
the seventh cycle after prolonging the reaction time to 12 h. In this regard, the catalyst
Cu(OTf)2/PEG-PyTa could be reused for at least 7 times.
a
Table 3 Reusability of Cu(OTf) /PEG-PyTa catalyst for the oxidation of benzyl alcohol
2
b
Cycle(n)
Time
Conv. (%)
100
Fresh
6
1
2
3
4
5
6
7
6
6
100
100
6
97
6
100
6 (12)
6 (12)
6 (12)
87 (100)
76 (100)
56 (96)
a
Reaction conditions: Benzyl alcohol (1 mmol), Cu(OTf)2 (5 mol%), PEG-PyTa (2.5 mol%), TEMPO
5 mol%), K2CO3 (20 mol%), H2O (3 mL), a balloon of O2, room temperature.
(
b
Conversions of benzyl alcohol were determined by GC.
4
. Conclusion
In summary, we have successfully developed a novel PEG-functionalized pyridine triazole ligand from

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commercially available PEG2000 and 2-ethynyl pyridine via a simple synthetic approach. It was shown
that PEG-PyTa could be utilized as water-soluble bidentate N, N-ligand to promote the aqueous
aerobic Cu(OTf)2/TEMPO-catalyzed oxidation of primary alcohols under room temperature in water
without any organic co-solvent or phase transfer reagent. Under optimal conditions, a wide range of
functionalized aldehydes could be achieved from their corresponding primary alcohols in excellent
yields. The catalytic system could be reused at least 7 times. This newly developed protocol provided
an efficient, practical and environmental benign method for the synthesis of various aldehydes.
Acknowledgements
This work was supported by the National Natural Science Foundations of China (21473160). We also
thank the support from “Zhejiang Key Course of Chemical Engineering and Technology”of Zhejiang
University of Technology.
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Highlights
An efficient and recyclable catalytic system for the aerobic alcohol oxidation was developed.
A variety of aldehydes were synthesized from primary alcohols in neat water under room temperature.
A novel PEG-functionalized pyridine triazole was designed and synthesized.