Nitrogen dioxide-catalyzed aerobic oxidation of benzyl alcohols under cocatalyst and acid-free conditions

Article abstract of DOI:10.1016/j.catcom.2017.08.003

Nitrogen dioxide is usually considered as a mediator between dioxygen and the catalysts for the aerobic oxidation of alcohols. Here, we report that nitrogen dioxide has an ability to catalyze this reaction, which not only avoids the use of the cocatalysts or the acids in traditional approaches, but also reveals a method for the present transformation with a single component catalyst. A series of primary and secondary benzyl alcohols underwent this transformation to give the targeted products in low to high yields.

Full text of DOI:10.1016/j.catcom.2017.08.003

Accepted Manuscript
Nitrogen dioxide-catalyzed aerobic oxidation of benzyl alcohols
under cocatalyst and acid-free conditions
Fangping Ren, Xinzhe Tian, Yun-Lai Ren, Shuang Zhao, Jianji
Wang, Bo Zhao
PII:
DOI:
Reference:
S1566-7367(17)30334-5
doi: 10.1016/j.catcom.2017.08.003
CATCOM 5152
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
30 May 2017
18 July 2017
3 August 2017
Please cite this article as: Fangping Ren, Xinzhe Tian, Yun-Lai Ren, Shuang Zhao, Jianji
Wang, Bo Zhao , Nitrogen dioxide-catalyzed aerobic oxidation of benzyl alcohols under
cocatalyst and acid-free conditions. The address for the corresponding author was captured
as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/
j.catcom.2017.08.003
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ACCEPTED MANUSCRIPT
Nitrogen dioxide-catalyzed aerobic oxidation of benzyl alcohols
under cocatalyst and acid-free conditions
Fangping Ren,a Xinzhe Tian,a,c Yun-Lai Ren, a,* Shuang Zhao,a Jianji Wang,b,c* Bo
Zhaoa
a School of Chemical Engineering & Pharmaceutics, Henan University of Science and Technology, Luoyang,
Henan 471003, P. R. China. renyunlai@126.com
b School of Chemical and Environmental Sciences, Henan Normal University, Xinxiang, Henan 453007, P. R.
China E-mail address: Jwang@htu.cn
c College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, P. R. China.
* Corresponding author. E-mail address: Jwang@htu.cn (J. Wang). renyunlai@126.com (Y.-L. Ren).

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ABSTRACT
Nitrogen dioxide is usually considered as a mediator between dioxygen and the
catalysts for the aerobic oxidation of alcohols. Here, we report that nitrogen dioxide
has an ability to catalyze this reaction, which not only avoids the use of the
cocatalysts or the acids in traditional approaches, but also reveals a method for the
present transformation with a single component catalyst. A series of primary and
secondary benzyl alcohols underwent this transformation to give the targeted products
in low to high yields.
Keywords:
Oxidation
Benzyl alcohols
Nitrogen dioxide
Oxygen

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1. Introduction
The selective oxidation of alcohols to aldehydes or ketones is considered to be an
important and fundamental transformation in modern organic synthesis because the
resulting products not only serve as the indispensable intermediates for the synthesis
of many important fine chemicals, pharmaceuticals and agrochemicals, but also are
the structural motifs frequently occurring in numerous drugs, vitamins and fragrances
[1]. As a result, considerable efforts have been made to develop effective methods
[2,3] for this transformation by using various oxidants, e.g. chromium-based reagents,
permanganate, hypervalent iodine compounds, ruthenium tetroxide (RuO4) and
nitrogen dioxide [4]. Compared with other oxidants, the inexpensive and green
oxygen is a more attractive alternative from both economic and green chemistry
viewpoints [5-8]. However, the molecular oxygen is less reactive towards most
organic molecules because it exists in the inactive triplet ground state. Thus an
efficient activation of the molecular oxygen is usually crucial for its successful utility
in the catalytic aerobic oxidation [9-14].
Cocatalysis
NOx
Catalysis
NOx
RCH2OH
RCHO
RCH2OH
RCHO
Catalyst
O2
O2
Previous methods
This work
Scheme 1. The aerobic oxidation of alcohols in the presence of NOx.
Recently, NOx has attracted considerable attention as an effective cocatalyst to
activate molecular oxygen in the aerobic oxidation of alcohols (Scheme 1). One
example is the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/HCl/NaNO2 catalyst
system [15,16], in which TEMPO plays a role of the catalyst, and the in situ generated

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NO from HCl and NaNO2 serves as a mediator between dioxygen and the catalyst.
Other
representative
catalyst systems
include
AZADO/NaNO2
[17],
DDQ/AcOH/NaNO2 [18], (TEMPO)/Br2/NaNO2 [19], 1-Me-AZADO/HNO3/NaNO2
[20], nanoshell carbon/HNO3 [21], HNO3/NaNO2/amberlyst-15 [22,23], HBr/NaNO2
[24], 5-F-AZADO/AcOH/NaNO2 [25]. By all appearance, it should be desirable to
simplify these sophisticated multi-component catalyst systems from environmental
and economic perspectives. So we had attempted to develop a single component
catalyst for the aerobic oxidation, and it was found that NO2 had an ability to catalyze
the aerobic oxidation of alcohols under cocatalyst and acid-free conditions, which
provided an effective and practical method for the synthesis of arylaldehydes and
arylketones.
2. Experimental
2.1. Materials and instruments
Commercially available compounds were purchased and used without
purification unless otherwise stated, and the quality and suppliers of the chemicals are
listed in Table S1 in the supporting information. Benzhydrol was synthesized by the
reaction between the benzophenone and LiAlH4.
GC-MS spectra was recorded on an Agilent 6890/5973N gas
1
13
chromatography-mass spectrometry instrument. H-NMR and C-NMR spectra were
recorded on a Bruker 500 MHz instrument with chemical shifts reported in ppm
relative to the internal standard tetramethylsilane. Gas chromatography analyses were

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performed on a Varian CP-3800 instrument with a FID detector and a CP-WAX 57CB
FS capillary chromatographic column (25 m × 0.32 mm).
2.2. General procedure for the aerobic oxidation of various substrates
To a dried 45 mL tube equipped with a magnetic stirring, 2 mL acetonitrile, 0.5
mmol substrate and 0.046 mmol NO2 were sequentially added (note: the air in the
tube was not removed). Then the reaction tube was sealed and stirred magnetically at
a constant-temperature to perform the reaction for 5 h. Once the reaction time was
reached, GC analysis of the mixture provided the GC yields of the products. Then the
crude product from another parallel experiment was purified by silica gel
chromatography to give the desired product.
3. Results and discussion
5%
CO2H
1 atm air, 9.2 mol% NO2
CH3CN, 140 oC, 5 h
CH2OH
CHO
no
CH2OH
74%
O2N
Scheme 2. The selective oxidation of benzyl alcohol.
Our investigation began with the aerobic oxidation of benzyl alcohol under the
condition that 0.5 mmol benzyl alcohol was stirred with 9.2 mol% nitrogen dioxide in
2 mL acetonitrile at 140 oC. As shown in Scheme 2, the reaction proceeded efficiently
to give the benzaldehyde product in 74% yield, along with a small amount of benzoic
acid by-product resulting from further oxidation of the targeted product. Although
nitrogen dioxide had been reported to have an ability to nitrify the benzene ring
[26-28], no nitration product was observed under the present condition. It was rational

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to assume that the residual oxygen in the reaction tube served as the oxidizing reagent.
Indeed, the removal of oxygen from the reaction system resulted in a remarkable
decrease in the product yield (Table 1, entry 3 vs entry 18).
Table 1. Aerobic oxidation of benzyl alcohol catalyzed by nitrogen dioxide.[a]
Air, NO2
Solvent, 5 h
CH2OH
CHO
Loading of
Entry
Solvent
Temperature (oC) Yield[b] (%)
NO2 (mol%)
-
1
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
THF
140
140
140
140
120
40
trace
40
74
67
72
8
2
4.6
3[c]
4[c]
5[c]
6
9.2
13.8
13.8
9.2
7
9.2
60
18
34
36
65
70
10
15
5
8
9.2
80
9
9.2
100
120
160
140
140
140
140
140
140
140
10
11[c]
12
13
14
15
16
17[d]
18[e]
9.2
9.2
9.2
9.2
toluene
9.2
H2O
9.2
DMF
43
53
50
21
9.2
CH2Cl2
9.2
CH2Cl2
9.2
acetonitrile
[a] Reaction conditions: 0.5 mmol benzyl alcohol, NO2, 2 mL solvent, 5 h, 1 atm air. [b] Determined by GC with
[c]
chlorobenzene as an internal standard.
Yield of benzoic acid by-product was respectively 5% (entry 3), 16%
(entry 4), 6% (entry 5) and 9% (entry 11), while only trace amount of benzoic acid was observed under other
condition. [d] 10mg amberlyst-15 was added. [e] Ar atmosphere.
The reaction was highly dependent on the loading amount of NO2. As shown in
Table 1 (entry 1), the oxidation did not occur in the absence of NO2, while an addition
of 4.6 mol% NO2 allowed reaction to give the targeted product in 40% yield (Table 1,
entry 2). The yield of the targeted product greatly increased with increasing NO2

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loading level up to 9.2 mol% but decreased as the loading level increased further
(Table 1, entries 1-4), which suggested that 9.2 mol% was the optimal loading amount.
It was also worth noting that an increase of the catalyst loading facilitated further
oxidation of the targeted product to carboxylic acid. For example, when the loading
amount of NO2 was changed from 9.2 mol% to 13.8 mol%, the yield of benzoic acid
as the overoxidation product increased from 5% to 16% (Table 1, entries 3 and 4).
Subsequently, we optimized other critical parameters such as temperature,
solvent, additive, etc. The reaction temperature had a significantly effect on the results
o
(Table 1, entries 3 and 6-11). The reaction at 40 C gave a very low yield of 8%, while
o
the reaction temperature of 120 C allowed the desired product to be obtained in 65%
yield. When 9.2 mol% catalyst was used, 140 oC was the optimal temperature in terms
o
o
of the yield. An increase of the reaction temperature from 140 C to 160 C led to an
increase in the yield of the benzoic acid by-product (Table 1, entry 3 vs entry 11).
Among the screened solvents (Table 1, entries 3 and 12-16), acetonitrile turned out to
be the most effective one, while the reactions using tetrahydrofuran, water or toluene
were very sluggish. The presence of a Bronsted acid could often facilitate the
oxynitride species-catalyzed aerobic oxidative reactions [26,29], which prompted us
to add a Bronsted acid into the reaction system to improve the present reaction results.
However, the presence of the Bronsted acid had almost no effect on the aerobic
oxidation. For example, an addition of amberlyst-15 gave a similar yield with that in
the absence of the acid (Table 1, entry 16 vs entry 17).
With the optimized results in hand, we set out to evaluate the scope of the

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present aerobic oxidation protocol. In most cases, the obtained isolated yields were
significantly lower than those from gas chromatography or liquid chromatography
under our conditions of separation and analysis, possibly due to the loss of the product
in the course of the chromatographic purification. Generally, the present reactions
were compatible with various groups, e.g. alkyl, chloro, bromo, nitro and
trifluoromethyl groups (Table 2, entries 2-8). The benzyl alcohols with alkyl
substituents were all good substrates (Table 2, entries 2 and 3). ortho-Methylbenzyl
alcohol underwent this transformation in 70% GC yield, and similar result was
obtained in the case of para-methylbenzyl alcohol, which revealed that the steric
hindrance seemed to have a little influence on the reaction. When the benzyl alcohols
with electron-withdrawing substituents were employed as the substrates, all the
aerobic oxidation reactions proceeded smoothly with moderate to good GC yields
(Table 2, entries 4-8). However, the reaction of the benzyl alcohol with a strong
electron-rich substituent (methoxy group) gave the targeted product in a very low
yield, and 71% substrate was recovered (Table 2, entry 9), which was rational to
assume that the catalytic species was destroyed by the reaction between the
electron-rich phenyl ring and NO2 to prevent the oxidation from proceeding [26-28].
Indeed, 7% 4-methoxy-3-nitrobenzyl alcohol product from nitration of the substrate
was observed in the case of 4-methoxybenzyl alcohol (Table 2, entry 9).
Table 2. Aerobic oxidation of various benzyl alcohols catalyzed by nitrogen dioxide. [a]
OH
OH
1 atm air, 9.2 mol% NO2
CH3CN, 140 oC, 5 h
R
R
Ar
Ar

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Entry
1
Substrate
Product[b]
GC yield[c] (%) Isolated yield (%)
CH2OH
CH2OH
CHO
74
70
71
69
87
70
96
92
16
72
93
54
57
60
62
63
64
89
73
–
Me
Me
2
CHO
Me
Cl
Me
CHO
CH2OH
3
CH2OH
Cl
CHO
4
Cl
Cl
5
Cl
Br
CH2OH
CH2OH
Cl
Br
CHO
CHO
6
O2N
F3C
O2N
F3C
CH2OH
CH2OH
CHO
7
8
CHO
9[d]
10
11
MeO
MeO
CH2OH
CH2OH
CHO
CHO
56
–
N
N
CH2OH
N
N
CHO
12[e]
13
60
6
52
CH2OH
CHO
–
OH
OH
O
O
14
15
16
88
85
–
67
70
92
OH
O
OH
O
–
55
17
F
F
F
F
O
O
O
-
18
19
72
OH
OH
O
26
–
CH
CH
[a] Reaction conditions: 0.5 mmol substrate, 0.046 mmol (9.2 mol%) NO2, 2 mL acetonitrile, 140 oC, 5 h, 1 atm air.
[b]
[c]
The product was characterized by 1H-NMR, 13C-NMR or mass spectra.
Determined by GC with
[d]
chlorobenzene as an internal standard.
7% 4-methoxy-3-nitrobenzyl alcohol and 71% substrate were observed
after reaction. [e] 0.069 mmol NO2, 120 oC.

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Another kind of excellent substrates was naphthalenemethanols. For example,
2-naphthalenemethanol was selectively oxidized to the aldehyde product in 72% GC
yield (Table 2, entry 10). 2-Pyridinemethanol underwent this transformation in 93%
GC yield, while the reaction of 4-pyridinemethanol gave the targeted product in only
6% yield under the same condition (Table 2, entries 11 and 13). When
2-pyridinemethanol was used as the substrate (Table 2, entry 11), we had done our
best to isolate the targeted product (determined by GC-MS), but it was puzzling that
no purified product was obtained. The same phenomenon was also observed in the
case using 4-methoxybenzyl alcohol or 1-phenyl-2-propyn-1-ol as the substrate (Table
2, entries 9 and 19), thus the isolated yields were not described in these cases.
We found that the NO2 catalyst could catalyze the oxidation of the secondary
benzyl alcohols. Whether aryl alkyl alcohols or diaryl alcohols underwent this
transformation to give the ketone products in moderate to high yields (Table 2, entries
14-17).
Very
interestingly,
both
2-hydroxy-1,2-diphenylethanone
and
1-phenyl-2-propyn-1-ol were smoothly oxidized to the ketone products (Table 2,
entries 18 and 19).
Air, 9.2 mol% NO2
CH3CN, 140 oC, 5 h
74%
6%
CH2OH
CH2OH
CHO
CHO
Air, 9.2 mol% NO2
CH3CN, 140 oC, 5 h
2 equiv. 2,4,6-tri-tert-butylphenol
(radical scavenger)
Scheme 3. Effect of the radical scavenger on the reaction.
To gain insight into the reaction mechanism, we performed several control
experiments where the well-known radical scavenger 2,4,6-tri-tert-butylphenol [22]
was added into the reaction system. As shown in Scheme 3, the presence of

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2,4,6-tri-tert-butylphenol led to a dramatic decrease in the yield of the aldehyde
product. In addition, the addition of the Bronsted acid had almost no effect on the
present reaction (Scheme 4), which is different from the ionic aerobic oxidation where
the presence of a Bronsted acid can often facilitate the reaction [22,23,29]. These
experimental results seem to be consistent with a free radical-based mechanism
[23,30]. However, only based on these results, it can not be excluded that the present
reaction undergoes an ionic pathway because that it is also possible that the presence
of 2,4,6-tri-tert-butylphenol blocks the ionic reactions [30].
Air, 9.2 mol% NO2
CH2Cl2,140 oC, 5 h
53%
50%
CH2OH
CH2OH
CHO
CHO
Air, 9.2 mol% NO2
CH2Cl2, 140 oC, 5 h
1 equiv. amberlyst-15 (acid )
Scheme 4. Effect of the Bronsted acid on the reaction.
According to previous literatures [31], the reaction between nitrogen dioxide and
alcohols can provide HNO3 product that has an ability to catalyze aerobic oxidation of
the benzyl alcohol [22], thus a HNO3-catalyzed pathway is possible (for the detailed
mechanism, see Ref. 22). Another possible mechanistic pathway involves an
alpha-hydroxyl radical because that it is known that nitrogen dioxide [30] or the
nitroxyl radical generated from NHPI (N-hydroxyphthalimide) [32] is prone to
abstract hydrogen from an organic molecule to leave an alpha-hydroxyl radical. As
shown in Scheme S1 (see the supporting information), the reaction starts with the
reaction between benzyl alcohol and NO2 [31]. The resulting radical can react with
dioxygen to provide the aldehyde product based on previous literatures (Pathway A in

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Scheme S1) [32,33]. It is also possible that the resulting radical is combinated with
another molecule of NO2 to give an α-hydroxynitrite intermediate [34], followed by
the formation of the aldehyde product from a thermal decomposition of the
nitrocarbinol (Pathway B in Scheme S1) [34,35].
4. Conclusions
In conclusion, nitrogen dioxide was found to have an ability to catalyze the
aerobic oxidation of benzyl alcohols, which not only avoids the use of the cocatalysts
or the acids in traditional approaches [15-25], but also reveals a strategy for the
present transformation with a single component catalyst. The distinctive nitrogen
dioxide catalyst can be easily separated from the final products, and is heavy metal
waste-free. The present reactions were compatible with various groups, e.g. alkyl,
chloro, bromo, nitro, trifluoromethyl and methoxy groups. A series of primary and
secondary benzyl alcohols underwent this transformation to give the targeted products
in low to high yields.
Acknowledgements
The authors wish to thank the financial supports from the Open Project of State
Key Laboratory for Oxo Synthesis and Selective Oxidation (Grant No.
OSSO-2015-21), the Program for Science ＆ Technology Innovation Talents in
Universities of Henan Province (Grant No.15HASTIT004) and the Research Planning
Project of Basic and Advanced Technology of Henan Province (Grant
No.162300410020).

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Graphical abstract
Air
OH
O
9.2 mol% NO2
R
R
Ar
Ar
+ No acid and additive
+ Unique and low-cost catalyst
+ Green and inexpensive oxidant
Nitrogen dioxide is usually considered as a mediator between dioxygen and the
catalysts for the aerobic oxidation of alcohols. Here, we report that nitrogen dioxide
has an ability to catalyze this reaction, which not only avoids the use of the
cocatalysts or the acids in traditional approaches, but also reveals a method for the
present transformation with a single component catalyst. A series of primary and
secondary benzyl alcohols underwent this transformation to give the targeted products
in low to high yields.

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Research Highlights
► A practical method for selective oxidation of benzyl alcohols was developed. ►
Nitrogen dioxide as a distinctive catalyst was used. ► The reactions proceeded
smoothly under cocatalyst and acid-free conditions.