Transition metal-free catalytic oxidation of aromatic alcohols with molecular oxygen in the presence of a catalytic amount of N-bromosuccinimide

Article abstract of DOI:10.1016/j.molcata.2014.03.011

A highly efficient N-bromosuccinimide (NBS)-mediated transition metal-free catalytic system has developed for the efficient aerobic oxidation of aromatic alcohols. Various aromatic alcohols are successfully oxidized to the corresponding aldehydes or keton

Full text of DOI:10.1016/j.molcata.2014.03.011

Journal of Molecular Catalysis A: Chemical 391 (2014) 1–6
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Transition metal-free catalytic oxidation of aromatic alcohols with
molecular oxygen in the presence of a catalytic amount of
N-bromosuccinimide
Xinli Tonga,∗, Yongfa Suna, Yongtao Yana, Xuan Luob, Jinbiao Liua, Zhidong Wua
a Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of
Technology, No. 391 Binshuixi Road, Tianjin 300384, PR China
b Technical Center for Safety of Industrial Products, Tianjin Entry-Exit Inspection and Quarantine Bureau, No. 33 Youyi Road, Tianjin 300201, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 April 2013
Received in revised form 6 March 2014
Accepted 8 March 2014
Available online 25 March 2014
A highly efficient N-bromosuccinimide (NBS)-mediated transition metal-free catalytic system has devel-
oped for the efficient aerobic oxidation of aromatic alcohols. Various aromatic alcohols are successfully
oxidized to the corresponding aldehydes or ketones under mild condition. For instance, benzyl alcohol
is oxidized to benzaldehyde with 99% conversion in 94.5% selectivity with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ)–NaNO2–NBS system under 0.3 MPa of O2 at 90 ◦C for 2 h. The effects of reaction time,
catalyst amount and solvents are investigated in detail, and a possible reaction mechanism is proposed.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Oxidation
Aromatic alcohols
Oxygen
Transition metal-free
N-Bromosuccinimide
1. Introduction
of alcohols under mild conditions. Though these catalyst systems
could efficiently perform oxidation, transition metal ions were not
The selective oxidation of alcohols into carbonyl compounds
is one of the most important transformations in both laboratory
and industrial synthetic chemistry [1–3]. Many oxidizing reagents
including permanganate and dichromate have been traditionally
employed in order to accomplish this transformation [4–7]. For
instance, Helquist’s group reported that catalytic 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) (20 mol%) catalyzed alcohol
oxidation with 6 equiv of Mn(OAc)3 as oxidant [7]. Electron-rich
benzylic alcohols are oxidized efficiently to their correspond-
ing carbonyls, but less electron-rich benzylic alcohols remain
unchanged. However, these are all hazardous reaction process and
produce a large amount of toxic waste. From both economic and
environmental point of view [8,9], the catalytic and selective oxi-
dation with H2O2 or molecular oxygen as the oxidant becomes
a promising orientation in this research field. Up to now, sev-
eral efficient catalytic oxidation systems have been developed
using molecular oxygen as the terminal oxidant [10–12]. Therein,
catalysts containing transition metal Ru [13–15], Cu [16–21], Pd
[22–39], Co [30] were widely used in the catalytic aerobic oxidation
easily removed and produce new toxic waste. They are referred as
being not green from the point of environment. Recently, the recy-
clable heterogeneous catalyst and transition metal-free catalytic
systems are considered as the promising and environment-friendly
catalytic technology in the oxidation processes. For example, in
the heterogeneous catalytic process, the supported Au nanoparticle
catalystshas been reportedas highly active, selectiveand recyclable
catalysts for the oxidation of alcohols into corresponding alde-
hydes and ketones [31,32]. On the other hand, several transition
metal-free catalyst systems based on the organic radical 2,2,6,6-
tetramethyl piperidine-N-oxide (TEMPO) have been used for the
oxidation of alcohols [33–37]. Particularly, Liu et al. [33] reported
using TEMPO–NaNO2–Br2 as the catalytic system for the efficient
oxidation of aromatic alcohols. Galli’s group [34] described lac-
case/TEMPO mediator system as the catalyst, in which the primary
benzyl and allyl alcohols were converted into the corresponding
aldehydes in high yields. Notably, secondary benzyl alcohols were
also efficiently oxidized to ketones, while tertiary benzyl alcohols
kept unchanged. Tromp et al. [35] further reported the kinetic
parameters and the catalytic reaction mechanism for oxidation of
benzyl alcohol with laccase/TEMPO system. Hu’s group [36] found
a TEMPO-based catalyst system that could oxidize benzylic alco-
hols and hetero- aromatic analogues in water. Therein, 88–98%
∗
Corresponding author. Tel.: +86 22 6024259; fax: +86 22 60214259.
E-mail addresses: tongxinli@tju.edu.cn, tongxli@sohu.com (X. Tong).
http://dx.doi.org/10.1016/j.molcata.2014.03.011
1381-1169/© 2014 Elsevier B.V. All rights reserved.

2
X. Tong et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 1–6
yields of benzylic aldehydes and ketones were obtained when a cat-
alytic amount of TEMPO, 1,3-dibromo-5,5-dimethylhydantoin and
NaNO2 was employed. In addition, Hu’s group [37] further reported
using TEMPO-HBr-tert-butyl nitrite (TBN) as the catalytic system
for the aerobic alcohols oxidation, in which up to a 16,000 turnover
number could be achieved for oxidation of benzylic alcohol to benz-
aldehyde. Moreover, using the catalytic amounts of DDQ and TBN,
selective oxidations of non-hindered benzylic alcohols were also
successfully performed under mild conditions [38].
Scheme 1. The aerobic oxidation of 1 with transition metal-free catalytic systems.
Through the careful considerations on the present “new” cat-
alyst systems for oxidation of alcohols with molecular oxygen, it
can be found that the numerous catalysts have an unavoidable
relationship with the stoichiometric oxidant in the past oxidation
systems. For example, metallic manganese salt has been devel-
oped as the catalyst for aerobic oxidation process while it was
initially employed as an oxidant long years ago [7,39,40]. Besides,
the occurrence of TEMPO as a catalyst is closely related to the
oxoammonium cation that can directly oxidize primary alcohols to
the corresponding aldehydes as a stoichiometric oxidant [41,42].
Herein, the TEMPO can be converted to oxoammonium cation via
the single electron oxidation by the molecular halides [43]. So, it can
be concluded that the extensive development of these new oxida-
tion catalyst should be attributed to the continuous research on
stoichiometric oxidation reaction systems in the recent years.
As inspired by the above progress of catalyst, we consider N-
bromosuccinimide (NBS) can be employed as a catalyst component
for the oxidation of alcohols with molecular oxygen though it or it
analogues was always regarded as an oxidant for the oxidation reac-
tion [44–47]. Thus, we have investigated the oxidation of aromatic
alcohols with NBS as a catalyst in the presence of DDQ and NaNO2.
It is found that primary alcohols can be efficiently and selectively
converted to the corresponding aldehydes under mild condition.
then is transferred into a flask and a saturated aqueous NaHSO3
solution is added. The obtained liquid mixtures were stirred for 2 h
under N2 atmosphere and left in a refrigerator for 3 h, and then the
resulting solid was collected by filtration. The solid mixtures are
washed with an aqueous solution of sodium chloride to remove
1 and by-products. In the following, the washed solid was trans-
ferred into a two-neck flask and the HCl solution is added under
N2 atmosphere. The reaction is performed with stirring for 1 h at
room temperature and for 2 h at 50 ◦C. The product is regenerated
and extracted with diethyl ether. After being dried, the diethyl ether
layer was distilled to obtain pure 2 as a liquid. As a result, the purity
is more than 99% from GC analysis. The difference between the GC
yield and separated yield is less than 10%.
3. Results and discussion
3.1. Oxidation of benzyl alcohol with different catalysts
Initially, the oxidation of 1, as shown in Scheme 1, is selected as a
model to investigate the catalytic performance of different catalyst
systems. As a result, it is found that the conversion of 1 is only 2.7%
for 2 h in the absence of any catalyst at 90 ◦C under 0.3 MPa of O2
in dichloromethane (CH2Cl2) (Table 1, entry 1). The conversion is
elevated to 4.5% when 3 mol% NBS is added in the reaction (Table 1,
entry 2).
2. Experimental
2.1. Reagents
Moreover, the conversion of 1 is 14.3% or 23.1% when the
NBS–DDQ or NBS–NaNO2 is used as the catalyst, respectively
(Table 1, entries 3 and 4). To our surprise, 99% conversion and
94.5% selectivity for 2 are obtained under similar reaction condi-
tions when a catalytic amount of NBS–DDQ–NaNO2 is employed as
the catalyst system (Table 1, entry 5). Furthermore, the combina-
tion of DDQ–NaNO2 and single DDQ are also investigated, and it is
found that only 11.8% and 9.2% conversions of 1 are obtained for 2 h
(Table 1, entries 6 and 7). Besides, the oxidation process is studied
with a sole and large amount of NBS (10 mol%, 20 mol%, 30 mol%
or 50 mol%) in order to further reveal the character of NBS as the
catalytic component, and the conversions of 1 are 13.7%, 67.4%,
60.8% and 80.3%, respectively (Table 1, entries 8–11), in which the
highest selectivity for benzaldeyde is only 86.2% in the presence of
50 mol% NBS. This exhibited that the combination of NBS, DDQ and
NaNO2 is most efficient to the oxidation of 1 with molecular oxygen,
which is superior to large amount of single NBS in oxidation. Also,
the catalytic systems including analogues of NBS and NaNO2–DDQ
were investigated for the oxidation of 1. It is found that a 37.6%,
73.8% and 21.0% conversions of benzyl alochol are obtained at
90 ◦C for 2 h when NBS is replaced by N-iodosuccinimide (NIS),
N-chlorosuccinimide (NCS) and succinimide, respectively (Table 1,
entries 12–14). All these results show that NBS plays an important
role for obtaining a good catalytic activity.
Benzyl alcohol (1), Succinimide, benzaldehyde (2), NBS, NCS,
NIS, DDQ, and NaNO2 are of analytical grade and purchased from
Alfa Aesar, A Johnson Matthey Company. Dichloromethane and
other solvents are purified by distillation. All other reagents are
analytical grade and obtained from commercial. Oxygen (purity
of 99%) supplied in a high-pressure cylinder was used through a
reducing valve without further treatment.
2.2. General procedure for oxidation of aromatic alcohols
All oxidation experiments are performed in a 120 mL autoclave
equipped with the magnetic stirring and automatic temperature
control. A typical procedure for the oxidation of 1 is as follows: a
CH2Cl2 (10 mL) solution of 1 (1.0 g, 9.2 mmol), NBS (3.0 mol%), DDQ
(7.0 mol%), and NaNO2 (10.0 mol%) is charged into the reactor, and
the atmosphere inside is replaced with the pure oxygen after the
reactor is sealed. Under stirring, oxygen is charged to 0.3 MPa at
room temperature and the autoclave is preheated to 90 ◦C, and then
kept for 2 h. After reaction, the autoclave was cooled and the mix-
ture is analyzed by GC and GC–MS after the excess gas (unreacted
oxygen and little nitrogen oxide) is purged.
2.3. Analysis and separation of reaction products
The products are analyzed with the internal standard technique
by gas chromatography using a flame ionization detector (all prod-
ucts are determined on GC–MS). Moreover, for oxidation of 1, the
product is separated from the product solutions as follows: the
reaction mixture is first distilled to remove the solvent CH2Cl2, and
3.2. The effect of reaction time and kinetic investigations
The effect of reaction time is investigated and presented in Fig. 1.
The time is recorded once the temperature adds up to 90 ◦C, in
which the conversion and selectivity for 2 are 22.9% and 68.9%.

X. Tong et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 1–6
3
Table 1
Oxidation of 1 with molecular oxygen with different catalystsa.
c
Entry
Catalyst (mol%)
Conversion (%)b
Selectivity (%)b
TOF (h−1
)
1
2
3
4
5
6
7
8
No
NBS (3)
2.7
4.5
34.8
66.3
49.5
69.3
94.5
48.2
39.6
67.4
68.9
72.4
86.2
81.6
91.5
67.2
–
0.8
2.4
3.9
16.5
–
–
0.7
1
NBS (3), DDQ (5)
NBS (3), NaNO2 (10)
NBS (3), NaNO2 (10), DDQ (7)
DDQ (3), NaNO2 (10)
DDQ (3)
NBS(10)
NBS(20)
NBS (30)
NBS (50)
14.3
23.1
99.0
11.8
9.2
13.7
39.4
60.8
80.3
37.6
73.8
21.0
9
10
11
12
13
14
1
0.8
6.3
12.2
3.5
NIS (3), NaNO2 (10), DDQ (3)
NCS (3), NaNO2 (10), DDQ (8)
Succinimide(3),NaNO2(10),DDQ(3)
a
Reaction conditions: 1.0 g (9.2 mmol) of 1, a certain amount of catalyst (with the molar ratio of catalyst and substrate), in 10 mL of CH2Cl2, under 0.3 MPa of O2, reaction
time 2 h, temperature 90 ◦C.
b
The results are obtained with internal standard technique by GC.
The values of TOF (turnover frequency) are obtained from the formula: TOF = the mole amount of reactant/(the mole amount of NBS · reaction time).
c
This indicates that the oxidation can proceed during heating period.
After first 10 min, the conversion and the selectivity reach to 33.8%
and 79.5%. In the following, the conversion and the selectivity for
2 gradually increase from 10 min to 120 min. The highest conver-
sion of 1 and yield of 2 is 99.0% and 93.6%. However, after 120 min,
the yield of 2 almost unchanged. These results show that the con-
version and product selectivity increase along with the increase
of time before 120 min, and then keeps invariable in the reaction
system. Furthermore, it can also be seen that the concentration of
1 is gradually decreased along with the increase of reaction time
during 10–120 min. Meanwhile, the nearly linear relationship of
concentration of 1 and reaction time indicated that the oxidation
of 1 should be a zero order reaction.
suitable amount of DDQ is very useful not only to the conversion
but also to the selectivity of 2.
3.4. The effect of solvent
Different solvents, including protic and aprotic ones such as
water (H2O), CH2Cl2, dimethyl sulfoxide (DMSO), N,N-dimethyl
formamide (DMF), acetonitrile (CH3CN), and tetrahydrofuran
(THF), have been employed in the oxidation of 1 for 2 h at 90 ◦C
(Fig. 3). Herein, only CH2Cl2 exhibits special qualification to the oxi-
dation process. For other solvents, it can be seen that protic solvent
(H2O) is poor to aprotic solvents (DMSO, THF, DMF and CH3CN),
which is probably due to catalyst components being more easily
solved in the latter solvents.
3.3. The effect of the amount of DDQ
3.5. The oxidation of various aromatic alcohols with molecular
oxygen
Fig. 2 shows the effect of different DDQ amounts in the oxi-
dation of 1. It can be found that both the conversion and yield
are increased along with the amount of DDQ being increased from
1.0 mol% to 7.0 mol%; meanwhile, the selectivity of 2 is also gradu-
ally increased. The maximum conversion and yield of 2 is 99% and
94% in the presence of 7.0 mol% DDQ, respectively. However, the
conversion and yield keep almost unchanged when the amount of
DDQ was increased to 9.0 mol%. Thus, it can be concluded that a
In the following, the oxidations of a variety of aromatic alcohols
are also successfully performed using this transition metal-free cat-
alytic system. As a result, 100% or 99% conversion in 99% selectivity
is obtained after 2 h when 4-methyl benzyl alcohol or 4-ethoxy
benzyl alcohol is oxidized with the NBS–DDQ–NaNO2 system at
90 ◦C (Table 2, entries 1 and 2). Moreover, 3-methoxy benzyl alco-
hol is similarly oxidized with 100% conversion and 98% selectivity
for 3-methoxy benzaldehyde (Table 2, entry 3). In case of 3-chloro
Fig. 1. Effect of reaction time in the oxidation of 1.
Fig. 2. Effect of different DDQ amounts in oxidation of 1.

4
X. Tong et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 1–6
Table 2
The oxidation of other primary and secondary alcoholsa.
c
Entry
1
Reactant
Time (h)
2
Conversion (%)b
100
Selectivity (%)b
99
TOF (h−1
16.7
)
2
2
99
99
16.5
3
4
4
2
100
39
98
8.2
6.5
100
5d
6e
7
2
2
2
94
65
97
99
48
3.2
2.6
100
16.7
8e
6
2
76
100
2.1
9f
+
28
62
99
99
4.7
10.3
a
Reaction conditions: reaction substrate (10.0 mmol), 3.0 mol% NBS, 7.0 mol% DDQ and 10.0 mol% NaNO2, in 10 mL of CH2Cl2, under 0.3 MPa of O2 (room temperature),
temperature 90 ◦C.
b
The results are obtained by GC with internal standard technique.
c
The values of TOF are obtained from the formula: TOF = the mole amount of reactant/(the mole amount of NBS · reaction time).
d
The amount of the used NBS is 15.0 mol%.
e
The amount of the used NBS is 6.0 mol%.
The added mol number of benzyl alcohol and 1-phenyl ethanol is 5 mmol: 5 mmol in the reaction.
f
benzyl alcohol, only 39% conversion in 100% selectivity is obtained
which is probably due to the electron withdrawing effect of the
chloro group (Table 2, entry 4). On the other hand, cinnamyl alcohol
is oxidized with 94% conversion and 99% selectivity for cinnamyl
aldehyde using this catalyst system (Table 2, entry 5). Furthermore,
a 65% conversion and 48% selectivity for furfural are obtained in
the oxidation of furfuryl alcohol with molecular (Table 2, entry 6).
This shows a promising method for aerobic oxidation of hetero-
cyclic substrates. Besides, 1-phenyl ethanol and 4-fluoro-1-phenyl
ethanol are also efficiently oxidized to acetophenone and 1-(4-
fluorophenyl) ethanone, in which 98% and 76% conversion are
obtained at 90 ◦C, respectively (Table 2, entries 7 and 8). Base on the
experimental data, the turnover frequency (TOF) values are related
to the structure of substrates and their substituent groups. In the
oxidation of withdrawing group substituted aromatic alcohol or
heterocyclic alcohol, the TOF value is low. For the oxidation of ben-
zyl alcohols para-substituted by donating groups, TOF value can
reach more than 16.5 h−1 under mild conditions.
Furthermore, in order to reveal the difference between oxida-
tion of primary alcohol and second alcohol in this oxidation system,
the 1:1 (molar ratio) of benzyl alcohol and 1-phenyl ethanol was
used as the reactants for the catalytic oxidation under the same con-
ditions. It is found that the conversion of benzyl alcohol was 28%
and the conversion of 1-phenyl ethanol was 62% after the reaction
was carried out for 2 h, which indicated second alcohol was more
easily oxidized than primary alcohol with the NBS–DDQ–NaNO2
catalytic system.
3.6. The reaction mechanism for the oxidation of alcohols
Based on the above experimental data and phenomena during
the oxidation reaction, a hypothetical reaction mechanism is pro-
posed for the oxidation of 1 (Scheme 2). First, a mutual function
comes forth between the bromide cation of NBS and the oxygen
atom of DDQ, and an active complex is generated. The following
step involves the cracking of C–H bond and O–H bond through

X. Tong et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 1–6
5
Scheme 2. Proposed reaction mechanism for the oxidation of 1.
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Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2014.03.011.
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