Selective Oxidation of Benzyl Alcohols to Aldehydes with a Salophen Copper(II) Complex and tert-Butyl Hydroperoxide at Room Temperature

Article abstract of DOI:10.1080/00397911.2015.1015034

(Figure Presented) An efficient and selective oxidation of benzyl alcohols has been developed using a salophen copper(II) complex as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant in the presence of base. Moderate to excellent yields of the corresponding benzaldehydes were obtained at room temperature without the carboxylic acids being formed.

Full text of DOI:10.1080/00397911.2015.1015034

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Selective Oxidation of Benzyl Alcohols
to Aldehydes with a Salophen Copper(II)
Complex and tert-Butyl Hydroperoxide
at Room Temperature
a
a
Tingting Chen & Chun Cai
a
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
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To cite this article: Tingting Chen & Chun Cai (2015) Selective Oxidation of Benzyl Alcohols to
Aldehydes with a Salophen Copper(II) Complex and tert-Butyl Hydroperoxide at Room Temperature,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 45:11, 1334-1341, DOI: 10.1080/00397911.2015.1015034
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Synthetic Communications , 45: 1334–1341, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2015.1015034
SELECTIVE OXIDATION OF BENZYL ALCOHOLS TO
ALDEHYDES WITH A SALOPHEN COPPER(II) COMPLEX
AND tert-BUTYL HYDROPEROXIDE AT ROOM
TEMPERATURE
Tingting Chen and Chun Cai
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
GRAPHICAL ABSTRACT
Abstract An efficient and selective oxidation of benzyl alcohols has been developed using a
salophen copper(II) complex as the catalyst and tert-butyl hydroperoxide (TBHP) as the
oxidant in the presence of base. Moderate to excellent yields of the corresponding
benzaldehydes were obtained at room temperature without the carboxylic acids being
formed.
Keywords Benzyl alcohols; salophen copper(II) complex; selective oxidation
INTRODUCTION
As one of the most important raw materials, aldehydes are widely used
in organic synthesis for the preparation of fine chemicals both in laboratory and
[
1]
industry. Traditional methods for the synthesis of aldehydes mainly rely on the
[
2]
[3]
[4]
[5]
oxidation of alcohols, organic halides, amines, alkenes, and the methyl group
[
6]
on the aromatic ring. Among these methods, the oxidation of alcohols is con-
venient and effective because of atom economy, available raw materials, and relative
greater yields. Many transition-metal compounds have been used for this purpose,
such as vanadium (V), cobalt (Co), copper (Cu),
ruthenium (Ru), rhodium (Rh), palladium (Pd), iron (Fe), and chromium
[
7]
[
8]
[9]
[10]
[11]
manganese (Mn),
[
12]
[13]
[14]
[15]
Received August 27, 2014.
Address correspondence to Chun Cai, Chemical Engineering College, Nanjing University of
Science and Technology, 200 Xiaolingwei, Nanjing 210094, China. E-mail: c.cai@njust.edu.cn
1
334

SELECTIVE OXIDATION OF BENZYL ALCOHOLS
1335
Scheme 1. Synthesis of the salophen copper(II) complex.
[
16]
[17]
(Cr).
In contrast to other metals, Cu is an abundant metal with less toxicity.
There are several reported methods in which Cu has been used as a cheap and
“
green” catalyst for the oxidation of alcohols.
Copper catalytic systems employing 2,2,6,6-tetramethylpiperidine N-oxyl
(
TEMPO) or dialkylazodicarboxylate as cocatalysts have emerged as some of the
[
18]
most effective methods in the oxidation of alcohols to aldehydes.
The first-
generation copper-catalyzed aerobic oxidation protocol, which was developed by
[
19]
Riviere and Jallabert,
(
employed equivalents of CuCl and 1,10-phenanthroline
[20]
Phen). Subsequently, Marko and coworkers
developed a series of catalytic
systems for aerobic alcohol oxidation using the catalytic amount of CuCl and Phen
as the catalyst in combination with dialkylazodicarboxylates as redox-active cocata-
lysts, which exhibited the broadest scope of alcohol oxidation. On the other hand,
[21]
the Cu=TEMPO system was also widely used for the oxidation of alcohols.
The
related catalytic system, which consists of a copper salt with 2,2'-bipyridine as a
ligand and TEMPO as a cocatalyst, could also enable the mild aerobic oxidation
[
22]
of primary alcohols to aldehydes.
Meanwhile, salen-type Schiff bases, including the salen and salophen Schiff
bases, can be prepared by condensation between aldehydes and amines in different
[
23]
reaction conditions.
They are able to stabilize many different metals in various
oxidation states with four coordinating sites and control the performance of metals
[
24]
in a large variety of useful catalytic transformations. Schiff base metal complexes
[
25]
are of great importance for catalysis,
such as the asymmetric epoxidation of
[
26]
[27]
alkenes and oxidation of alcohols. Transition-metal Schiff base complexes that
catalyzed oxidation of alcohols were attractive, especially for their more accessible
[
16b]
synthesis conditions and versatile coordination structures.
Salen-Cu(II) could
be used in the oxidation of primary alcohols to the carboxylic acids in good
[27]
yields.
However, when combined with TEMPO, the salen-Cu(II) complex
could selectively oxidized the alcohols to the corresponding aldehydes at reflux
[28]
temperature.
Because the system described requires the toxic TEMPO and high
temperature, a mild, “green,” and efficient catalytic system is still desirable.
Herein, we developed an efficient method for the oxidation of benzyl alcohols
to the corresponding aldehydes using a salophen copper(II) complex (Scheme 1) as
the catalyst and tert-butyl hydroperoxide (TBHP) as the source of oxygen in the
presence of base. To our delight, excellent yields and good selectivity were achieved
for a variety of benzyl alcohols with no trace of corresponding carboxylic acid.
RESULTS AND DISCUSSION
The oxidation of benzyl alcohol was first investigated as the model reaction
and the results are summarized in Table 1. Very poor yields were obtained when

1336
T. CHEN AND C. CAI
a
Table 1. Optimizing the conditions for the oxidation of benzyl alcohol
b
Entry
Solvent
TBHP (equiv.)
Catalyst (mol%)
Base (equiv)
Yield (%)
1
2
3
4
5
6
7
8
9
PhMe
PhMe
PhMe
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.6
0.6
—
—
Cs
2
CO
—
3
(2.0)
36
34
68
80
88
93
39
62
92
91
58
27
36
92
94
95
95
92
93
95
95
96
95
93
90
66
90
99
97
85
83
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
0.5
0.75
1.0
2.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
Cs
2
CO
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(2.0)
(1.0)
(0.6)
DMF
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
DMSO
MeCN
Acetone
MeOH
2 2
CH Cl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
c
d
e
—
0.8
1.0
1.1
1.4
1.7
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
NaOH (2.0)
NaOH (1.0)
NaOH (0.6)
NaOH (0.4)
NaOH (0.6)
NaOH (0.6)
f
g
a
Reaction conditions: benzyl alcohol (0.5 mmol), TBHP, salophen copper(II) complex, and base were
stirred under room temperature in air overnight.
b
Isolated yields.
c
The reaction was performed under argon.
d
The reaction was performed in air without the oxidant TBHP.
e
O
2
was used as the oxidant.
was used as the catalyst.
CuCl was used as the catalyst.
f
Cu(OAc)
2
g
no catalyst or base was used (Table 1, entries 1 and 2), which indicated that both of
them are necessary for the oxidation.
To optimize the reaction conditions, the oxidant amount was varied from 0.6 to
2
equiv with other conditions remaining constant (Table 1, entries 6 and 10–18). A
trend of increasing yield with oxidant amount was observed up to 1.1 equiv TBHP.
There was still no carboxylic acid formed even when the oxidant amount was up to

SELECTIVE OXIDATION OF BENZYL ALCOHOLS
1337
2
.0 equiv (Table 1, entry 6). It was worth noting that even when 0.6 equiv of TBHP
was used for the oxidation (Table1, entry 10), the reaction provided the product in
1% yield. It is possible that oxygen may participate in the reaction as the reaction
9
was conducted in the open air. The assumption was proved by the results from the con-
trolled experiments (Table 1, entries 11 and 12). Under the same reaction conditions, O2
was used as the oxidant but only 36% yield was obtained (Table 1, entry 13). The load-
ing amount of catalyst was also examined (Table 1, entries 6 and 19–23). The best yield
was achieved when the amount of the catalyst was 2 mol%. The type and amount of the
bases also played important influences on the reaction and 0.6 equiv NaOH resulted in
the best yield (Table 1, entries 21 and 24–29). Meanwhile, other Cu sources such as Cu
(
OAc) and CuCl exhibited less catalytic activity than the salophen Cu(II) complex
2
(Table 1, entries 30 and 31).
To determine the application scope of this catalytic system, a wide range of
benzyl alcohols were oxidized under the optimized conditions (Table 2). All the
benzyl alcohols employed were converted into the corresponding aldehydes
with excellent yields and selectivity, and no carboxylic acids were detected with
a
Table 2. Oxidation of primary alcohols under optimized conditions
b
Entry
R
Yield (%)
1
2
3
4
5
6
7
8
9
4-Cl
4-Br
4-NO
97
97
85
83
87
95
96
96
98
69
41
69
84
85
96
97
93
92
95
2
4-CN
4-CF
4-Me
4-OMe
2-Cl
3
2-Br
2-I
2-NH
2-Br-4,5-(OMe)
3-NO
10
11
12
13
14
15
16
17
18
19
20
21
2
2
2
3-CF
3-Cl
3-Br
3
3-Me
2,4-Cl
1-Phenylethanol
2-Phenylethanol
n-Octanol
2
c
6
c
4
a
Reaction conditions: alcohols (0.5 mmol), salophen copper(II) com-
plex (2 mol%), TBHP (1.1 equiv), and NaOH (0.6 equiv) in acetonitrile
were stirred under room temperature in air overnight.
b
Isolated yield.
c
GC yield.

1338
T. CHEN AND C. CAI
Figure 1. Proposed mechanism for the oxidation of benzyl alcohols.
high-performance liquid chromatography (HPLC) after the reaction completed. No
obvious influence of the electronic effects was found in the reaction, and both benzyl
alcohols containing the electron-withdrawing and electron-donating groups could
produce the reaction with satisfactory yields (Table 2, entries 1–7). 2-Iodobenzyl
alcohol provided a poor yield, which may be explained by the steric effects of
the iodo substituent (Table 2, entry 10). In the case of 2-aminobenzyl alcohol
(Table 2, entry 11), an undesired by-product of imine was afforded, leading to a poor
yield. The aliphatic primary alcohol exhibited low conversion and poor reactivity
(Table 2, entries 20 and 21), but there was still no trace of carboxylic acid.
Finally, a probable mechanism for this reaction has been proposed (Fig. 1).
The role of the base is to deprotonate the alcohol and accelerate the formation
of the benzyloxy-Cu(II) complex 1 by favoring the coordination of the resulting
[
22b,29c]
alcoholate to the salophen copper(II) complex.
The aldehydes were then
obtained by the reaction of the benzyloxy-Cu(II) complex with the oxidant TBHP
with release of the by-product H O and the tert-butoxy-Cu(II) complex 2. Finally,
2
the tert-butoxy-Cu(II) complex exchanged with the starting alcohol to release the
[
29]
tert-butyl alcohol and completed the catalytic cycle.
CONCLUSION
In summary, a mild and selective oxidation method of benzyl alcohols to the
corresponding aldehydes has been established when using the TBHP as the oxidant
and a salophen copper(II) complex as the catalyst in the presence of NaOH at room
temperature. In this protocol, a variety of benzyl alcohols are oxidized to the corre-
sponding aldehydes in moderate to excellent yields and no overoxidation takes place.

SELECTIVE OXIDATION OF BENZYL ALCOHOLS
1339
EXPERIMENTAL
All reagents were purchased from commercial sources and used without
treatment; 70% TBHP in water was used. The products were purified by column
1
chromatography over silica gel. H NMR spectra were recorded on a Bruker
AMX500 (500 MHz) spectrometer and tetramethylsilane (TMS) was used as a
reference. A Nicolet IS-10 spectrometer was recorded for IR spectroscopy.
Preparation of Salophen H2
O-Phenylenediamine (108 mg, 1 mmol) in 5 mL MeOH was added to a stirred
mixture of salicylaldehyde (244 mg, 2 mmol) in 10 mL MeOH. The resulting orange
mixture was stirred overnight at room temperature. The solid product was collected
by filtration, washed with cool alcohol, and dried in vacuo (256 mg, yield:
1
8
1%); H NMR (500 MHz, CDCl ): d 13.05 (s, 2 H), 8.63 (s, 2 H), 7.46–7.31
3
(m, 6 H), 7.27–7.20 (m, 2 H), 7.05 (d, J ¼ 8.2 Hz, 2 H), 6.92 (t, J ¼ 7.4 Hz, 2 H).
Preparation of Salophen Copper(II) Complex
Solution of salophen H ligand (189 mg, 0.5 mmol) in EtOH (10 mL) and Cu
2
.
(
OAc) H O (99 mg, 0.5 mmol) in water (1 mL) were mixed and refluxed with
2
2
vigorous stirring for 2 h. The resulting solution was then cooled to room temperature
and filtered. After filtration, the solid product was washed with H O, MeOH, and
2
Et O subsequently, then dried in vacuo to afford the desired copper complex
2
–1
(
139 mg, yield 71%). IR: v (cm ) ¼1602, 1519, 1334.
Typical Procedure for the Oxidation of Alcohols
Alcohol (0.5 mmol), salophen copper(II) complex (2 mol%), NaOH (0.6 equiv),
and 70% TBHP in water (1.1 equiv) were dissolved in acetonitrile (5 mL), and the
homogeneous solution was stirred at room temperature in air overnight. After
completion of the reaction, the solvent was evaporated under reduced pressure.
The residue was purified over silica gel by column chromatography (10–25% EtOAc
in hexane). All the products were known compounds and were identified by comparison
of their physical and spectra data with those of authentic samples.
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website.
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