Base-free copper-catalyzed aerobic oxidation of benzylic alcohols with N-benzylidene-N,N-dimethylthane-1,2-diamine and TEMPO

Article abstract of DOI:10.1016/j.catcom.2011.07.019

Selective oxidation of alcohols using N-benzylidene-N,N-dimethylthane-1,2- diamine, CuBr₂ and TEMPO as the catalytic system was developed. Catalyzed by this simple catalytic system in the absence of any external base, various benzylic alcohols

Full text of DOI:10.1016/j.catcom.2011.07.019

Catalysis Communications 14 (2011) 92–95
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Base-free copper-catalyzed aerobic oxidation of benzylic alcohols with
N-benzylidene-N,N-dimethylthane-1,2-diamine and TEMPO
a
b,
Qiufen Wang a, Ying Zhang a, Gengxiu Zheng a, , Zhongzhen Tian , Guanyu Yang
⁎
⁎
a
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
b
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidation of alcohols using N-benzylidene-N,N-dimethylthane-1,2-diamine, CuBr2 and TEMPO as
the catalytic system was developed. Catalyzed by this simple catalytic system in the absence of any external
base, various benzylic alcohols could be oxidized to their corresponding aldehydes with excellent yields in
CH3CN/H2O (v/v=1/1) under 0.2 MPa O2 at 80 °C.
Received 27 December 2010
Received in revised form 19 July 2011
Accepted 25 July 2011
Available online 30 July 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Aerobic oxidation
Alcohol
N-benzylidene-N,N-dimethylthane-1,2-
diamine
Copper
TEMPO
1. Introduction
In continuation of our interest in exploring green oxidation
methods [24,25], our attention was focused on developing a new
Oxidation of alcohols totheir correspondingcarbonyl products is one
of fundamental transformations in organic chemistry [1]. Various
investigations have showed that 2,2,6,6-tetramethyl-piperidyl-1-oxyl
(TEMPO) could efficiently and selectively catalyze the oxidation of
alcohols with molecular oxygen [2–4]. Its catalysis always relied on the
assistance of various co-catalysts, especially the transition metal
compounds [5–7]. Among them, Cu(II) complexes with N-donor
ligands, including bipyridine [8,9], 1,10-phenanthroline [10], 2-N-
arylpyrrolecarbaldimine [11], 3-(2-hydroxy-4-nitrophenylhydrazo)
pentane-2,4-dione [12], triethanolamine [13], pyrazole-pyridine [14],
DMAP [15], N,N-bis-(2-pyridylmethyl)-1,3-diaminopropan-2-ol [16],
1,4-diazabicyclo[2.2.2]octane [17] and N-alkyldiethanolamine [18], etc.,
proved to be the prime catalysts. Recently, this Cu/TEMPO system was
extended interestingly to the use of bifunctional catalysts bearing both
TEMPO and Cu complex moieties in a 1,3,5-triazine ring [19,20]. For
thesecatalytic oxidations, a new mechanismwasproposed, in which the
key step was the simultaneous coordination of TEMPO and the
deprotonated alcohol on the Cu center [10,11,21,22]. However, an
additional base was always needed for alcohol deprotonation. Sot-BuOK
[8,9,14,19,20], NaOH [10,16,23], K2CO3[11–13,18], etc., were used in
different systems, respectively.
Cu/TEMPO system by using simple ligands and also attempting to
avoid additional base. It was encouraging to find that in the absence of
additional base a three-component catalytic system consisting of N-
benzylidene-N,N-dimethylthane-1,2-diamine, CuBr2 and TEMPO
could catalyze effectively the selective aerobic oxidation of alcohols
under mild conditions. As far as we are aware, the catalytic properties
of such Cu complexes for aerobic oxidation of alcohols have not been
studied yet. Moreover, it was notable that this type of Schiff base was
easily prepared and inexpensive. Herein, the catalytic performance of
this catalytic system was investigated.
2. Experimental
2.1. Generals
The chemical reagents used were of analytical grade. Elemental
analyses were carried out with a Perkin-Elmer 2400 elemental analyzer.
FT-IR spectra were recorded on a PE-SPECTRUM ONE Spectrometer in
KBr pellets. 1H NMR spectra were acquired on a Bruker Biospin-AVANCE
III 400 MHz Digital NMR Spectrometer. Mass spectra were collected on a
JEOL JMS-SX 102 mass spectrometer. UV spectra were recorded with a
PE-lamada-35 spectrophotometer in CH3CN/ water (1/1). Perkin-Elmer
Clarus 500 GC-MS detector equipped with a PE-5MS capillary column
was used to identify reaction products. GC measurements were
conducted using an Agilent 7890 gas chromatograph equipped with a
flame ionization detector and a KB-Wax capillary column. Quantitation
⁎
Corresponding authors. Tel.: +86 531 8276 5841; fax: +86 531 8276 5841.
E-mail addresses: chm_zhenggx@ujn.edu.cn (G. Zheng), yangguanyu@zzu.edu.cn
(G. Yang).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.07.019

Q. Wang et al. / Catalysis Communications 14 (2011) 92–95
93
of components was performed by using acetophenone as an internal
standard.
ν(C=N) 1608 (s), ν(C-N) 1339 (s), ν(Cu-N) 547 (s). m/z (%): 485
(20, M+-1), 275 (49, M+- L4), 212 (100, L4). UV (λ, nm): 270,
312, 585.
2.2. Synthesis and characterization of ligands and complexes
L5
Red oil (yield 89%). Anal. Calc. (%): C 59.71, H 6.83, N 18.99;
found: C 59.65, H 6.77, N 19.04. IR (cm−1): ν(C=N)1621 (s),
ν(C-N) 1347 (s). 1H NMR (d-DMSO, ppm): 2.23 (s, 6H, CH3);
2.74 (t, 2H); 3.56 (t, 2H); 7.80 (d, 1H); 8.09 (d, 1H); 8.22 (s,
1H). UV (λ, nm): 281, 324.
All ligands were synthesized by condensation reaction of N,N-
dimethylethylenediamine and the corresponding aldehydes [26,27].
Two equivalent ligand (typically, 1.76 g L1) reacted with CuBr2
(1.12 g) in 10 mL CH3CN under refluxing for 3 h to give the deposited
corresponding Cu complex, which was collected by filtration, washed
by ether, and then vacuum dried. They were characterized by 1H NMR,
IR, Elemental analyses, MS and UV, respectively, and were presented
in Fig. 1 where the ligands were described as typified by L1, and the
complexes as Cu(L1)2, respectively.
Cu(L5)2 Blue solid (yield 86%). Mp 127–128 °C. Anal. Calc. (%): C
39.68, H 4.54, N 12.62; found: C 39.81, H 4.53, N 12.57. IR
(cm−1): ν(C = N) 1614 (s), ν(C-N) 1342 (s), ν(Cu-N) 549(s).
m/z (%): 506 (20, M+-1), 285 (47, M+- L5), 222 (100, L5).
UV (λ, nm): 282, 320, 600.
L1
Brown oil (yield 90%). Anal. Calc. (%): C 74.96, H 9.15, N 15.89;
found: C 74.86, H 9.23, N 15.92. IR (cm−1): ν(C = N) 1628 (s),
ν(C-N) 1332 (s). 1H NMR (d-DMSO, ppm): 2.26 (s, 6H, CH3);
3.39 (t, 2H); 3.63 (t, 2H); 7.29 (d, 1H); 7.61(d, 1H); 8.27 (s,
1H). UV (λ, nm): 267, 321.
2.3. Oxidation experiments
Typically, benzyl alcohol (3 mmol), TEMPO (0.09 mmol), CuBr2
(0.06 mmol), L1 (0.12 mmol) and CH3CN/H2O (v/v=1/1, 6 mL) were
added into a 70-mL Teflon-lined stainless-steel reactor. After the air
was replaced with O2 for three times, the reactor was heated to 80 °C
under stirring, and then charged by 0.2 MPa O2. After reaction ended,
the reaction mixture was cooled to room temperature, and extracted
with ethyl acetate. The organic phase was measured by GC analysis.
Cu(L1)2 Blue solid (yield 87%). Mp 151–152 °C. Anal. Calc. (%): C 45.88,
H 5.60, N 9.73; found: C 45.92, H 5.71, N 9.66. IR (cm−1):
ν
(C=N) 1604 (s), ν(C-N) 1328 (s) , ν(Cu-N) 533 (s). m/z (%): 416
(20, M+-1), 239 (48, M+- L1), 176 (100, L1). UV (λ, nm): 265,
310, 571.
L2
Yellow oil (yield 86%). Anal. Calc. (%): C 69.87, H 8.80, N 13.58;
found: C 69.53, H 8.98, N 13.43. IR (cm−1): ν(C = N) 1638 (s),
ν(C-N) 1342 (s). 1H NMR (d-DMSO, ppm): 2.18 (s, 6H, CH3);
3.71 (s, 3H, CH3); 2.72 (t, 2H); 3.64 (t, 2H); 6.92 (d, 1H); 7.61
(d, 1H); 8.12 (s, 1H). UV (λ, nm): 270, 324.
3. Results and discussion
3.1. Comparison of the activities of catalysts
The oxidation of benzyl alcohol in CH3CN/H2O (v/v=1/1) was
chosen as a model reaction to test initially the catalytic potential of the
Cu(L)2 s (Table 1). As could be seen, all of pre-made Cu(L)2 s in
combination with TEMPO gave above 83% conversions in 2 h, and
benzaldehyde was the sole product (entries 1–5). In particular, Cu
(L1)2 realized a complete transformation. Since the Cu(L)2 s could be
easily prepared just by heating CuBr2 and Ls, CuBr2 and Ls were
directly employed in the oxidation, which realized approximate
conversions to those of the pre-made Cu(L)2 s under the same
conditions (entries 6–10). It was reasonably thought that Cu(L)2 s
could be in situ formed during the oxidations when CuBr2 and Ls were
used directly.
Cu(L2)2 Blue solid (yield 89%). Mp 160–161 °C. Anal. Calc. (%): C 45.33,
H 5.71, N 8.81; found: C 45.23, H 5.78, N 8.43. IR (cm−1):
ν
(C=N) 1624 (s), ν(C-N) 1334 (s), ν(Cu-N) 542 (s). m/z (%): 477
(22, M+-1), 269 (47, M+- L2), 206 (100, L2). UV (λ, nm): 269,
315, 582.
L3
Yellow oil (yield 88%). Anal. Calc. (%): C 75.74, H 9.53, N 14.72;
found: C 75.58, H 9.68, N 14.41. IR (cm−1): ν(C = N) 1634 (s),
ν(C-N) 1336 (s). 1H NMR (d-DMSO, ppm): 2.17 (s, 6H, CH3);
2.33 (s, 3H, CH3); 3.39 (t, 2H); 3.63 (t, 2H); 7.24 (d, 1H); 7.61
(d, 1H); 8.28 (s, 1H). UV (λ, nm): 268, 323.
Cu(L3)2 Blue solid (yield 84%). Mp 159–160 °C. Anal. Calc.: C 47.73; H
6.01; N 9.28; found: C 47.52, H 6.23, N 9.41. IR (cm−1):ν(C=N)
1623 (s), ν(C-N) 1328 (s), ν(Cu-N) 538 (s). m/z (%): 444 (20,
M+-1), 253 (50, M+- L3), 190 (100, L3). UV (λ, nm): 267, 314,
580.
Table 1
a
Oxidation of benzyl alcohol using different Cu catalysts
.
L4
Yellowish oil (yield 84%). Anal. Calc. (%): C 62.70, H 7.18, N
13.30; found: C 62.82, H 7.03, N 13.55. IR (cm−1): ν(C=N)
1619 (s), ν(C-N) 1347(s). 1H NMR (d-DMSO, ppm): 2.18(s, 6H,
CH3); 2.72 (t, 2H); 3.53 (t, 2H); 7.30 (d, 1H); 7.56 (d, 1H); 8.15
(s, 1H). UV (λ, nm): 272, 326.
Entry
Catalyst
Conv. (%)
1
2
3
4
5
6
7
8
Cu(L1)2
100
86
99
93
83
100
95
99
94
85
Cu(L2)2
Cu(L3)2
Cu(L4)2
Cu(L5)2
Cu(L4)2 Blue solid (yield 86%). Mp 135–136 °C. Anal. Calc. (%): C 40.98,
CuBr2 +L1
CuBr2 +L2
CuBr2 +L3
CuBr2 +L4
CuBr2 +L5
CuBr2
H 4.69, N 8.69; found: C 40.82, H 4.53, N 8.85. IR (cm−1):
9
R
10
11
12
13b
14b
15c
16d
b1
2
4
L1
CuBr2 +L1
Cu(L1)2
L1: R = H
b1
98
N
N
N
N
L2: R = CH3O
CuBr2 +L1
CuBr2 +L1
Cu
35, 100e
_
L3: R = CH3
L4: R = C1
L5: R = NO2
2Br
a
Reaction conditions: 3 mmol benzyl alcohol, 3 mol% TEMPO, 2 mol% Cu(L)2 or the
combination of 2 mol% CuBr2 and 4 mol%L, 3 mL CH3CN, 3 mL H2O, 80 °C, 0.2 MPa O2,
2 h.
b
TEMPO-free.
with 2 mmol K2CO3.
Air instead of O2.
for 12 h.
c
R
d
e
Fig. 1. Schematic structures of Cu(L)2 complexes.

94
Q. Wang et al. / Catalysis Communications 14 (2011) 92–95
In sharp contrast, the reaction occurred barely in the absence of any
2/1 to 1/2, the oxidation of benzyl alcohol achieved moderate or
excellent conversions (entries 6–14). The decrease of catalytic
activity, as previously reported [11], was not observed.
component (entries 11–14), displaying that all three components were
indispensable. Moreover, both L2 bearing a strong electron-donating
substituent and L5 bearing a strong electron-withdrawing substituent
gave relatively low conversions (entries 2, 5, 7, 10), showing the
substituents in the ligands had no obvious electronic effects on their
catalytic activities.
3.3. Effect of temperature
In general, the Cu/TEMPO system needs an additional base for
better activity. Actually, in the presence of K2CO3 our oxidation
realized 98% conversion (entry 15). S. Mannam et al. found that when
1,4-diazabicyclo[2.2.2]octane was used as the ligand, the conversion
of the oxidation of alcohol without any base was similar to the data of
using K2CO3 as a base [17]. They assumed that the ligand might
display dual roles: as a base to deprotonate the hydroxy and as an N-
donor ligand to coordinate Cu. So it might be supposed that the amino
group of Ls had adequate basicities for deprotonation, and the
oxidation could occur without any external base.
Fig. 2 displayed the influence of reaction temperature on catalytic
activity of Cu/L1/TEMPO. The conversions were found to have
corresponding responses with the rise of temperatures. At ambient
temperature, only 44% of benzyl alcohol was oxidized. The conversion
could be up to 78% when the temperature was increased to 50 °C. The
preferable temperature was 80 °C, which resulted in 100% conversion.
Moreover, according to GC-MS analysis, no overoxidation occurred as
benzoic acid was not identified in all cases.
In addition, when air was used as the oxidant instead of O2, only 35%
benzyl alcohol converted under the same conditions, however, a
complete conversion was obtained by prolonging time to 12 h (entry
16). This implied that for the present system the efficient oxidation
relied on the considerable concentrations of O2 in the reaction solution.
3.4. Oxidation of various alcohols
Cu/L1/TEMPO was further employed to catalyze the aerobic
oxidation of various alcohols in CH3CN/H2O (1/1) (Table 3). It was
notable that the corresponding carbonyl compounds were the sole
products in all cases. As could be seen, cinnamyl alcohol and all
primary benzylic alcohols with either electron-donating or electron-
withdrawing substituents could be oxidized efficiently to the
corresponding aldehydes with excellent conversions (entries 2–9).
However, in the series of the methoxy-substituented benzyl alcohols,
the p-substituted one gave the lowest conversion (83%) (entries 2–4).
This phenomenon was also found in the oxidation using 2-N-
arylpyrrolecarbaldimine as ligand before [11]. In conjunction with
the relatively low conversions of other p-substituted benzylic
alcohols, such as p-methylbenzyl alcohol, p-chlorobenzyl alcohol
and p-nitrobenzyl alcohol, the author proposed that the influence of
the remote p-substituent appeared to be more related to steric effects
than electronic ones. When Cu/L1/TEMPO was employed, the
complete oxidation of p-chlorobenzyl alcohol was achieved (entry
5). Meanwhile, the conversion of p-methylbenzyl alcohol was even
higher than that of m-methylbenzyl alcohol (entries 6–7). It could be
suggested that the reactivity of benzylic alcohols had unobvious
relationships with both steric and electronic effects of the substituents
when Cu/L1/TEMPO was used. However, aliphatic alcohols could not
be oxidized effectively, which was similar to the previous Cu/TEMPO
systems [8–18].
3.2. Influence of the solvent
The oxidations of the Cu/TEMPO systems reported before were
conducted in various solvents, in particular CH3CN/H2O (2/1)
[8,9,16,19,20]. And the solvent showed obvious influences on the
activities. When pyrazole-pyridine was used as ligand, H2O led to
precipitation of the Cu catalysts and thus the oxidation was retarded,
whereas a much higher conversion was obtained in neat CH3CN [14].
In contrast, for the oxidation in water using 2-N-arylpyrrolecarbaldi-
mine as ligand, even small amounts of organic solvents, such as
CH3CN, EtOH, MeOH, or CH3COCH3, could deactivate the catalytic
system [11]. Hence, in order to explore which solvent would be
suitable for the present Cu/L1/TEMPO, different solvent pairs were
studied (Table 2). The results showed that the rate of CH3CN and H2O
influenced the catalytic activity, and the preferable ratio was 1/1
(entry 2). However, no matter whether neat CH3CN or H2O was used
as solvent, the oxidations slowly took place (entries 3 and 4). The
external tests indicated that the pre-made Cu(L1)2 was dissoluble in
H2O but less dissoluble in CH3CN, whereas benzyl alcohol was
miscible in CH3CN but less dissoluble in H2O. These meant that a
more homogeneous reaction system was obtained and the oxidation
proceeded faster when CH3CN/H2O (1/1) was used. Interestingly,
using methanol, ethanol or acetone as cosolvents with the ratio from
100
100
Table 2
a
Oxidation of benzyl alcohol with Cu(L1)2/TEMPO in different solvent
.
78
80
Entry
Solvent (v/v)
Conv. (%)
1
2
3
4
5
6
7
8
Acetonitrile/Water (2/1)
Acetonitrile/Water (1/1)
Acetonitrile/Water (1/2)
Acetonitrile
81
100
94
32
59
92
81
87
90
94
94
96
95
81
60
44
40
20
0
Water
Methanol/Water (1/1)
Methanol/Water (1/2)
Methanol/Water (2/1)
Ethanol/Water (1/1)
Ethanol/Water (2/1)
Ethanol/Water (1/2)
Acetone/Water (1/1)
Acetone/Water (1/2)
Acetone/Water (2/1)
9
10
11
12
13
14
20
30
40
50
60
70
80
90
T(°C)
Fig. 2. Oxidation of benzyl alcohol at a different temperature. The reaction conditions
were as described in Table 1.
a
The reaction conditions were as described in Table 1.

Q. Wang et al. / Catalysis Communications 14 (2011) 92–95
95
Table 3
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o-Methoxylbenzyl alcohol
m-Methoxylbenzyl alcohol
p-Methoxylbenzyl alcohol
p-Chlorobenzyl alcohol
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p-Methylbenzyl alcohol
p-Nitrobenzyl alcohol
Cinnamyl alcohol
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7
a
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In summary, a new catalytic system consisting of L1, CuBr2 and
TEMPO was developed for the efficient aerobic oxidation of various
benzylic alcohols to their corresponding aldehydes under mild condi-
tions. The CH3CN/H2O (1/1) was the preferable solvent. Interestingly,
unlike most of the previously reported Cu/TEMPO systems, Cu/L1/
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Acknowledgements
We gratefully acknowledge the financial support from the
Development Programs in Science and Technology of Shan Dong
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