Aerobic oxidation of alcohols by copper(I)/benzoxazine ligand/TEMPO under mild and base-free conditions

Article abstract of DOI:10.1134/S1070363215080277

A novel benzoxazine ligand with an imidazole moiety was synthesized. This ligand in combination with Cu(OTf) and TEMPO efficiently catalyzed aerobic oxidation of active primary alcohols and also showed good to excellent activity in the oxidation of secondary alcohols under mild and base-free conditions.

Full text of DOI:10.1134/S1070363215080277

ISSN 1070-3632, Russian Journal of General Chemistry, 2015, Vol. 85, No. 8, pp. 1965–1972. © Pleiades Publishing, Ltd., 2015.
Aerobic Oxidation of Alcohols
by Copper(I)/Benzoxazine Ligand/TEMPO
under Mild and Base-Free Conditions1
Y. C. Zhang, R. Huang, F. L. Lü, X. H. Cao, and J. Q. Zhao
School of Chemical Engineering and Technology, Hebei University of Technology,
Tianjin 300130, People’s Republic of China
e-mail: zhaojq@hebut.edu.cn
Received May 5, 2015
Abstract—A novel benzoxazine ligand with an imidazole moiety was synthesized. This ligand in combination
with Cu(OTf) and TEMPO efficiently catalyzed aerobic oxidation of active primary alcohols and also showed
good to excellent activity in the oxidation of secondary alcohols under mild and base-free conditions.
Keywords: benzoxazine ligand, copper(I) trifluoromethanesulfonate, aerobic oxidation, alcohols, TEMPO,
base-free
DOI: 10.1134/S1070363215080277
The oxidation of alcohols to the corresponding
carbonyl compounds is one of the most important and
ubiquitous reactions in organic synthesis [1–3]. In
particular, this transformation is crucial for the
synthesis of versatile intermediates for fragrances and
food additives [4–6]. Classical oxidation methods
require stoichiometric amounts of oxidants such as
chromates [7–10], manganese dioxide [11–13], or
selenium dioxide [14], which are sometimes toxic or
hazardous. From the environmental and economic
standpoints, molecular oxygen is an ideal oxidant
which generates water as the only by-product in the
oxidation process [15–20]. Catalytic oxidation systems
employing molecular oxygen as a primary oxidant in
combination with metal ions are particularly attractive
[21–28]. Among the metals, copper is more attractive
since it is an abundant metal on the earth’s crust and
copper ions are metal centers of enzymes such as
galactose oxidase and tyrosinase [29, 30]. Thus, a
series of copper-based catalysts or catalyst systems
have been developed for the oxidation of alcohols to
aldehydes and ketones with molecular oxygen. In
1984, Semmelhack and co-workers [31] were the first
to report aerobic oxidation of benzylic and allylic
alcohols with 10% CuCl/TEMPO in DMF as solvent.
Aliphatic alcohols proved to be substantially less
reactive and required 2 equiv of CuCl2 to achieve good
yields. Since then a number of copper/TEMPO catalyst
systems have been proved to be highly efficient for the
transformation of a broad range of alcohols to
aldehydes and ketones [32–38]. It has been found that
chelating nitrogen-containing ligands have beneficial
effect on oxidation reactions. Notable examples
include a (bipy)CuBr2/TEMPO catalyst system that
employs t-BuOK as a catalytic base in MeCN/H2O as
solvent [35–37], and (bipy)Cu(OTf)2/TEMPO catalyst
system [38]. Recently, more copper-based catalyst
systems have been reported [39–49]. Stahl et al. [50]
made a breakthrough in the development of copper/
TEMPO systems. They disclosed a (bipy)Cu(I)/
TEMPO/NMI catalyst system (NMI = N-methyl-
imidazole) and found that replacement of Cu(II) by
Cu(I) significantly enhanced the reaction rate. The
catalytic system exhibits high rates and selectivity
even with unactivated aliphatic alcohols. The other
advantage of Stahl’s catalyst system is the use of N-
methylimidazole (NMI) instead of t-BuOK as a base.
However, all the catalyst systems are complicatedly
composed of four components: copper salt, ligand,
base, and TEMPO, which is unfavorable for the
isolation of the product.
Having been inspired by Stahl’s catalyst system, we
design a bidentate benzoxazine ligand containing an
imidazole moiety which is expected to act both as
ligand and base to simplify the catalytic system for the
aerobic oxidation of alcohols. Encouragingly, the
1 The text was submitted by the authors in English.
1965

1966
ZHANG et al.
Scheme 1.
Bu-t
Bu-t
O
OH
N
MeOH, reflux
+ 2 CH2O +
· 2 HCl
NH2
N
HN
N
t-Bu
t-Bu
NH
L
Scheme 2.
Cu(OTf) (5 mol %), TEMPO (5 mol %)
CHO
L (5 mol %), CH2Cl2, O2, rt
OH
benzoxazine ligand in conjugation with Cu(OTf) and
TEMPO has shown excellent catalytic properties in the
aerobic oxidation of various alcohols under mild and
base-free conditions. Herein, we report the preparation
of this novel ligand and the performance of the
catalytic system based thereon for the oxidation of
alcohols with molecular oxygen.
adhesives, glass fiber reinforce materials, coatings,
phenolic resins, etc. [54–57]. Herein, the benzoxazine
ring in combination with imidazole moiety will be
used as a bidentate ligand in the copper-based catalytic
system for the aerobic oxidation of alcohols.
Based on Stahl’s catalytic system [50] bidentate
ligand L in combination with Cu(OTf) and TEMPO
[Cu(OTf)/L/TEMPO] was tested in the aerobic
oxidation of benzyl alcohol as a model substrate
(Scheme 2, Table 1). The reaction was carried out
under atmospheric oxygen pressure at room temperature.
The benzoxazine ligand containing an imidazole
moiety was readily synthesized by the Mannich
condensation [51–53] of 2,4-di-tert-butylphenol with
histamine dihydrochloride and aqueous formaldehyde
in the presence of sodium hydroxide by heating for 40
h under reflux (Scheme 1). The structure of 6,8-di-tert-
butyl-3-[2-(1H-imidazol-4-yl)ethyl]-3,4-dihydro-2H-
As shown in Table 1, the use of benzoxazine ligand
L is essential. Only 13.5% of benzyl alcohol was
converted to benzaldehyde in 6.5 h when no benz-
oxazine ligand was added (Table 1, no. 1). The
presence of TEMPO and Cu(OTf) is crucial for the
oxidation too. Only 16.2% of benzaldehyde was
formed in 6.5 h without TEMPO (Table 1, no. 2), and
almost no reaction was observed in the absence of Cu
(OTf) (Table 1, no. 3). The results indicated that each
component of the catalytic system is essential in the
aerobic oxidation of benzyl alcohol. It
is worth noting that, as we expected, the novel
catalytic system is active without any extra base added.
The complete conversion of benzyl alcohol into
benzaldehyde is attained in 6.5 h in the absence of base
(Table 1, no. 4).
1
1,3-benzoxazine was characterized by H NMR, 13C
NMR and HRMS (ESI) data. Generally, the studies of
benzoxazines are mainly focused on synthetic
polymers which can be used as insulation materials,
Table 1. Effect of the composition of the Cu(OTf)/L/
TEMPO catalytic system on the aerobic oxidation of benzyl
alcohola
Entry Cu(OTf), TEMPO, L, Conversion,b Selectivity,b
no. mol % mol % mol %
%
%
1
2
3
4
5
5
–
5
5
–
5
5
–
5
5
5
13.5
16.2
3.0
>99
>99
>99
>99
Several other copper salts including copper(I) and
copper(II) salts were also evaluated as catalyst
precursors in the aerobic oxidation of benzyl alcohol.
As shown in Table 2, Cu(OTf) worked well and
showed the highest efficiency among all the copper
salts tested, and the conversion of benzyl alcohol
reached up to 90.2 % after 4 h (Table 2, no. 1). Both
Cu(OAc)2·H2O and CuCl led to less active catalysts
99.4
a
Reaction conditions: benzyl alcohol, 1 mmol; CH2Cl2, 1 mL;
room temperature, atmospheric oxygen pressure, reaction time
6.5 h. The conversion and selectivity were determined by GLC
b
(peak area normalization method).
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AEROBIC OXIDATION OF ALCOHOLS
1967
Table 3. Effect of solvent on the copper(I)/L/TEMPO-
Table 2. Effect of the copper salt in the copper/L/TEMPO
catalyzed aerobic oxidation of benzyl alcohola
catalyst on the aerobic oxidation of benzyl alcohola
Entry
no.
Conversion,b
%
Selectivity,b
%
Copper salt
Cu(OTf)
Entry
no.
Conversion,b Selectivity,b
Solvent (v/v)
%
%
1
2
3
4
5
6
90.2
86.1
40.1
35.2
28.0
16.8
>99
>99
>99
>99
>99
>99
1
2
3
4
5
CH3CN
11.7
10.5
90.3
47.5
6.2
>99
>99
>99
>99
>99
Cu(OTf)2
Cu(OAc)2·H2O
CuCl
CH3CN–H2O (2/1)
CH2Cl2
CH2Cl2–H2O (2/1)
CH2Cl2–CH3CN (1/1)
CuCl2
CuBr2
a
a
Reaction conditions: benzyl alcohol, 1 mmol; CH2Cl2, 1 mL;
copper salt, 5 mol %; TEMPO, 5 mol %; L, 5 mol %; room
temperature, atmospheric oxygen pressure, reaction time 4 h.
The conversion and selectivity were determined by GLC (area
normalization method).
Reaction conditions: benzyl alcohol, 1 mmol; solvent, 1 mL;
copper salt, 5 mol %; TEMPO, 5 mol %; L, 5 mol %; room
temperature, atmospheric oxygen pressure, reaction time 4 h.
The conversion and selectivity were determined by GLC (area
normalization method).
b
b
with only 40.1% and 35.2% conversion after 4 h,
respectively (Table 2, nos. 3, 4). Copper(II) chloride
and bromide also produced unfavorable results with
only 28.0% and 16.8% conversion, respectively
(Table 2, nos. 5, 6). Cu(OTf)2 afforded 86.0% con-
version of benzyl alcohol in the same reaction time
(Table 2, no. 2), i.e., it exhibited lower efficiency than
Cu(OTf) as shown in Stahl’s catalytic system [50]. The
observed differences in the catalytic activity are likely
to be related to the propensity for dissociation of the
anion from the catalytic copper complex. Chloride,
bromide, and acetate ions are more strongly
The effect of temperature on the reaction was
evaluated in the range from 15 to 35°C. Generally, the
reaction rate increased with rise in temperature, and
the selectivity for benzaldehyde remained constant.
The catalytic system Cu(OTf)/L/TEMPO was then
applied to the oxidation of various alcohols, including
benzylic, allylic, heterocyclic, and aliphatic alcohols,
and the results are summarized in Table 5. Various
primary benzylic alcohols, including those bearing
electron-withdrawing and electron-donating groups,
were selectively converted to the corresponding
aromatic aldehydes in excellent yields. Slightly
different times were required to complete the reaction
with para-substituted substrates (Table 5, nos. 1–3, 6,
8), but the difference between the substrates with
–
coordinated to copper than OTf [37]. Consequently,
the alcoholate enters the copper coordination sphere
–
more easily when OTf is the counterion rather than
other ions [50].
Based on a series of experiments dichloromethane
was chosen as solvent. Initially, the reaction was run in
anhydrous acetonitrile which was employed as solvent
in Stahl’s catalytic system [50]. However, only 11.7%
conversion of benzyl alcohol was obtained in 4 h using
that solvent (Table 3, no. 1). Then the oxidation
reaction was performed in a 2 : 1 (v/v) acetonitrile/
water mixture. However, no improvement was
achieved (Table 3, no. 2). The poor results could be
ascribed to the insolubility of the catalytic system in
acetonitrile-based solvents. Finally, the reaction was
run in dichloromethane where it proceeded smoothly.
The conversion of benzyl alcohol reached 90.3% in 4 h
(Table 3, no. 3). Furthermore, the addition of water or
acetonitrile dramatically retarded the reaction (Table 3,
nos. 4, 5).
Table 4. Effect of temperature on the copper(I)/L/TEMPO-
catalyzed aerobic oxidation of benzyl alcohola
Entry Temperature, Time, Conversion,b Selectivity,b
no.
ºC
15
20
25
30
35
h
%
%
1
9
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
2
8
3
7
4
3
5
2.5
a
Reaction conditions: benzyl alcohol, 1 mmol; solvent, 1 mL;
copper salt, 5 mol %; TEMPO, 5 mol %; L, 5 mol %;
atmospheric oxygen pressure.
The conversion and selectivity were determined by GLC (area
normalization method).
b
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1968
ZHANG et al.
Table 5. Aerobic oxidation of various alcohols to carbonyl compounds catalyzed by Cu(OTf)/L/TEMPOa
Cu(OTf), TEMPO, L
OH
R2
O
CH2Cl2, O2
R1
R1
R2
Selec-
Selec-
tivity,b
%
Entry
no.
Time, Conver-
h
Entry
no.
Time, Conver-
h
Substrate
OH
tivity,b
%
Substrate
sion,b %
sion,b %
1c
2
7
>99 (92)
>99
12
9
>99
(91)
>99
OH
6.5
4
>99
>99 (86)
>98
>99
13
14
15
8
4
>97
(89)
>99
OH
OH
S
3
4
>99
>99
30.1
>99
>99
>99
OH
OH
OH
H3CO
N
O
33
3.5
OH
OCH3
OH
5
8
>99
>99
16
6
>99
(94)
>99
OCH3
HO
6
7
6.5
8
>99 (90)
>99
>99
>99
17
18
11
55.3
51.1
79.9
71.7
OH
OH
Cl
27.5
OH
OH
OH
Cl
8
4.5
>99
>99
19
24
80.0
95.8
OH
F
OH
9
7
>99 (87)
>99
>99
>99
20
21
24
24
90.8
17.4
95.6
99
OH
OH
F
10
6.5
OH
F
F
OH
11
6
>99
>97
22
24
84.5
99
OH
F
H3CO
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 8 2015

AEROBIC OXIDATION OF ALCOHOLS
Selec-
1969
Table 5. (Contd.)
Selec-
tivity,b
%
Entry
Time, Conver-
h
Entry
no.
Time, Conver-
h
Substrate
tivity,b
%
Substrate
no.
sion,b %
sion,b %
OH
23
28.5
72.3
99 26
24
32.2
99
99
99
OH
OCH3
OH
24
25
24
67.3
23.0
96.2 27
99.0 28
12
24
99.0
54.9
OH
OH
F
32.5
OH
OCH3
a
Reaction conditions: alcohol, 1 mmol; atmospheric oxygen pressure. Primary alcohols: Cu(OTf), 5 mol %; TEMPO, 5 mol %; L, 4 mol %;
CH2Cl2, 1 mL; room temperature. Secondary alcohols: Cu(OTf), 5 mol %; TEMPO, 5 mol %; L, 5 mol %; CH2Cl2, 2 mL; temperature
b
40°C. The conversion and selectivity were determined by GLC (area normalization method); the isolated yields are given in
parentheses; all products were identified by 1H NMR. c The amount of benzyl alcohol was 40 mmol.
electron-withdrawing and electron-donating groups
was not very obvious. This means that the electronic
effect is not the main factor in the reaction. In contrast,
the position of the substituent on the benzene ring
appreciably affects the reactivity of benzylic alcohols.
ortho-Substituted benzyl alcohols showed poor reac-
tivity compared to meta- and para-substituted analogs
due to steric hindrance (Table 5, nos. 3–5). The
proposed catalytic system also showed good efficiency
in the oxidation of some heterocyclic substrates, and
good to excellent yields of the corresponding
aldehydes were obtained under the optimal reaction
conditions (Table 5, nos, 13, 15). However, primary
alcohol bearing vicinal chelating group [50] like
pyridin-2-ylmethanol proved to be less reactive, and
the substrate conversion was incomplete even under
increased loadings of the catalytic components, or/and
extended reaction time, or/and elevated temperature
(Table 5, no. 14). Allylic alcohols like cinnamyl
alcohol and geraniol were completely converted into
the corresponding aldehydes in 9 and 6 h, respectively,
without enhancing reaction conditions (Table 5,
nos. 12, 16). It is clear that the aerobic oxidation of
benzylic and allylic alcohols to the corresponding
carbonyl compounds is fast and efficient. However,
aliphatic alcohols were still less reactive than benzylic
or allylic alcohols resulting in lower conversions and
yields, as observed with other copper-based catalytic
systems [32, 34, 36, 37, 50]. For 1-octyl alcohol and
cyclohexylmethanol, only ~50% conversion and less
than 80% selectivity were observed even under
prolonged reaction time or/and increased amounts of
the catalyst components (Table 5, nos. 17, 18). The low
reactivity of aliphatic alcohols can be attributed to the
instability of TEMPO in the presence of aliphatic
aldehydes and oxygen [31]. The major reason for the
reduced selectivity is aldol condensation reaction,
which is inferred from the results of analysis of the
reaction mixture. It is very important to note that this
system is also effective in the aerobic oxidations of
secondary benzylic alcohols which are poor substrates
for some copper-based catalyst systems [22, 31, 36, 37,
50, 58, 59]. Various secondary benzylic alcohols,
including those bearing electron-withdrawing and
electron-donating groups, were effectively and
selectively converted to the corresponding ketones
under relatively higher loadings of benzoxazine ligand
(5 mol %) and higher reaction temperature (40°C)
(Table 5, nos. 19, 20, 22–24). 1-(2-Methylphenyl)
ethanol with an ortho substituent showed poor
reactivity compared to those with meta and para
substituents due to steric hindrance (Table 5, no. 21).
1-Hydroxyindan was completely converted into indan-
1-one in 12 h. However, only 54.9% conversation was
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 8 2015

1970
ZHANG et al.
attained in the case of 1,2,3,4-tetrahydronaphthalen-1-
ol even after extending the reaction time to 24 h.
Secondary aliphatic alcohols showed poor reactivity
compared to secondary benzylic alcohols (Table 5,
nos. 25, 26). In general, the benzoxazine ligand-
mediated catalytic system has broad applicability in
the aerobic oxidations of both primary and secondary
alcohols under base-free conditions.
believe that initially Cu(I)–benzoxazine complex A is
generated in situ via coordination of benzoxazine
ligand L to Cu(OTf). As noted above, the catalytic
activity of the catalytic system is related to steric
hindrance of the substrate, which is consistent with the
results observed in the (bipy)Cu/TEMPO-catalyzed
oxidation of alcohols [35, 36, 50]. Taking into account
these data, as well as the mechanisms for (bipy)Cu/
TEMPO-catalyzed oxidation of alcohols described in
[35, 36], especially the mechanism for (bipy)Cu(OTf)/
TEMPO-catalyzed oxidation of alcohols recently
proposed by Stahl [60], copper(II) alkoxide complex B
might be formed in the catalytic cycle. Once the
complex is formed, a hydrogen atom is abstracted from
the Cu(II) alkoxide by TEMPO to give the aldehyde
[60].
To demonstrate scalability of this catalytic system,
the reaction with benzyl alcohol as substrate was
carried out on a multigram scale. The same results
were obtained when the amount of benzyl alcohol was
increased to 40 mmol (Table 5, no. 1).
It is difficult to propose a detailed mechanism for
the novel catalytic system at this stage. However, we
+
+
Bu-t
Bu-t
O
O
TfO–
N
TfO–
N
t-Bu
t-Bu
CuII
O
N
CuI
N
H
NH
NH
H
R
B
A
In conclusion, a novel benzoxazine ligand with an
imidazole moiety has been synthesized. This ligand in
combination with Cu(OTf) and TEMPO showed
excellent performance in the oxidation of various
alcohols to the corresponding carbonyl compounds
under very mild base-free conditions. In all cases,
benzylic and allylic alcohols were selectively oxidized
to the corresponding aldehydes, and no over-oxidized
products (carboxlylic acids) were detected. The novel
catalytic system not only offers the advantages of
performing aerobic oxidation of active primary
alcohols under mild and base-free conditions, but also
has certain catalytic activity for the selective oxidation
of secondary alcohols.
were used as received. Solvents were purified by
standard methods and dried if necessary.
1
The H and 13C NMR spectra were recorded on a
Bruker AC-P 400 spectrometer using TMS as internal
standard. The mass spectra (ESI) were recorded on an
LCQ Advanced high-resolution spectrometer. The
reaction mixtures were analyzed on a Shandong Lunan
Ruihong Gas Chromatograph (SP-6800A) equipped
with a flame ionization detector and a SE-30 column
(30 m × 0.5 mm); detector temperature 280°C, oven
temperature 130–220°C (depending on the alcohol),
carrier gas inlet pressure 0.05–0.07 MPa (depending
on the alcohol), injection split ratio 10 : 1.
6,8-Di-tert-butyl-3-[2-(1H-imidazol-4-yl)ethyl]-
3,4-dihydro-2H-1,3-benzoxazine (L). A solution of
2.40 g (60 mmol) of sodium hydroxide in 20 mL of
MeOH was added dropwise to a solution of 6.19 g
EXPERIMENTAL
2,4-Di-tert-butylphenol was purchased from
Aldrich. Histamine hydrochloride was obtained from
Shanghai Yuanye Bio-Technology Co., Ltd.
Formaldehyde solution and sodium hydroxide were
purchased from Tianjin Fengchuan Chemical Reagent
Technologies Co., Ltd. Alcohols were obtained from
Alfa Aesar China (Tianjin) Co., Ltd. All the chemicals
(30 mmol) of 2,4-di-tert-butylphenol, 5.52
g
(30 mmol) of histamine dihydrochloride, and 5.00 mL
(60 mmol) of aqueous formaldehyde in 60 mL MeOH.
The mixture was heated for 40 h under reflux with
vigorous stirring under atmospheric pressure, the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 85 No. 8 2015

AEROBIC OXIDATION OF ALCOHOLS
solvent was evaporated under reduced pressure, and
1971
REFERENCES
the residue was extracted with ethyl acetate. The
extract was dried over anhydrous sodium sulfate, the
drying agent was filtered off, and the filtrate was
concentrated under reduced pressure. The residue was
subjected to silica gel column chromatography (n-
hexane–ethyl acetate, 2 : 1 by volume) to obtain 6,8-
di-tert-butyl-3-[2-(1H-imidazol-4-yl)ethyl]-3,4-di-
hydro-2H-1,3-benzoxazine as a white solid. Yield 2.15
g (21%), mp 201.2°C. 1Н NMR spectrum (CD3OD), δ,
ppm: 1.27 s and 1.37 s (9H each, t-Bu), 2.71 t (2H,
4′-CH2, J = 4 Hz), 2.90 t (2H, 3-CH2, J = 6 Hz), 3.58 s
(2H, 4-H), 3.90 s (2H, 2-H), 6.91 d and 7.19 d (1H
each, 5-H, 7-H, J = 4 Hz), 7.53 s (1H, imidazole). 13С
NMR spectrum (CDCl3), δС, ppm: 22.16, 29.57, 31.60,
31.68, 34.13, 34.83, 49.30, 50.47, 61.11, 120.61,
123.04, 123.44, 133.91, 135.60, 140.61, 154.32. Mass
spectrum: m/z 341.2553. C21H31N3O. Calculated: M
341.2467.
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alcohols. A 5-mL two-necked, round-bottom flask
equipped with a magnetic stirrer and an oxygen
balloon was charged in succession with 0.0106 g
(0.05 mmol) of Cu(OTf), 0.0078 g (0.05 mmol) of
TEMPO, 0.0136 g (0.04 mmol) of benzoxazine ligand
L, and 1 mL of methylene chloride. The corresponding
alcohol, 1 mmol, was then added at 25°C under
stirring, and oxygen from the balloon was introduced
through a three-way valve. The progress of the reaction
was monitored by GLC using a suitable column.
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Typical procedure for the oxidation of secondary
alcohols. A 5-mL two-necked, round-bottom flask
equipped with a magnetic stirrer and an oxygen
balloon was charged in succession with 0.0106 g
(0.05 mmol) of Cu(OTf), 0.0078 g (0.05 mmol) of
TEMPO, 0.0171 g (0.05 mmol) of benzoxazine ligand
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