Bromine-catalyzed aerobic oxidation of alcohols

Article abstract of DOI:10.1002/asia.200900609

We have demonstrated that a simple and environmentally benign catalytic system HBr/NaNO₂ is very effective for the selective oxidation of alcohols under balloon pressure of O₂. Furthermore, the aerobic oxidation of alcohols has been achieved under balloon pressure of air rather that pure O₂, with this HBr/NaNO₂/HNO₃ catalytic system. (Chemical equation presented).

Full text of DOI:10.1002/asia.200900609

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DOI: 10.1002/asia.200900609
Bromine-Catalyzed Aerobic Oxidation of Alcohols
Muhammet Uyanik,[a] Ryota Fukatsu,[a] and Kazuaki Ishihara*[a, b]
Dedicated to the 150th anniversary of Japan–UK diplomatic relations
The oxidation of alcohols to the corresponding carbonyl
compounds is one of the most fundamental and important
transformations in synthetic organic chemistry.[1] Molecular
oxygen is the most ideal oxidant because it is both atom-
economically and environmentally benign. To date, numer-
ous transition-metal-catalyzed aerobic oxidations of alcohols
have been developed.[1,2] Recently, the TEMPO (a nitroxy
Scheme 1. IBS-catalyzed selective oxidation of alcohols.
radical)-catalyzed aerobic oxidation of alcohols has also
been reported.[3] However, there is a strong need for cheap-
er, more efficient, more chemoselective, and greener meth-
ods for such transformations, particularly in the pharmaceut-
ical industry.[4]
the presence of catalytic amounts of Br2 and NaNO2
(Scheme 2). The catalytic mechanism they proposed for this
reaction involves three redox cycles (Scheme 3). PhIO2 is
the active oxidant that oxidizes the alcohol to the corre-
sponding carbonyl compound, and is reduced to dihydroxy-
iodobenzene (PhI(OH)2). PhI(OH)2 is reoxidized to PhIO2
with Br2, which is reduced to HBr. The oxidation of NO
with O2 produces NO2, which re-oxidizes HBr to Br2. In ad-
dition, HNO3 produced by dissolving NO2 in water can also
oxidize HBr to Br2.
Over the past two decades, hypervalent iodine compounds
have been the focus of considerable attention, owing to
their mild and chemoselective oxidizing properties and their
environmentally benign character in contrast to toxic-metal
reagents.[5] Very recently, we reported a highly-efficient and
chemoselective oxidation of various alcohols to carbonyl
compounds, such as aldehydes, carboxylic acids, and ketones
with powdered Oxone (2KHSO5·KHSO4·K2SO4) in the pres-
ence of catalytic amounts of 2-iodoxybenzenesulfonic acid
(IBS), which is generated in situ from 2-iodobenzenesulfonic
acid (Scheme 1).[6]
On the other hand, Liu and co-workers reported a hyper-
valent iodine-catalyzed oxidation of alcohols using molecu-
lar oxygen as a terminal oxidant.[7] Accordingly, the aerobic
oxidation of a broad range of primary and secondary alco-
hols to aldehydes and ketones, respectively, in water was ef-
fectively catalyzed by iodoxybenzene (PhIO2, 1 mol%) in
Scheme 2. PhIO2-catalyzed aerobic oxidation of alcohols.[7]
[a] Dr. M. Uyanik, R. Fukatsu, Prof. Dr. K. Ishihara
Graduate School of Engineering
Nagoya University
Furo-cho, Chikusa-ku, Nagoya, 464-8603 (Japan)
Fax : (+81)52-789-3222
Scheme 3. Mechanism proposed by Liu and co-workers for the PhIO2-
catalyzed aerobic oxidation of alcohols.[7]
E-mail: ishihara@cc.nagoya-u.ac.jp
[b] Prof. Dr. K. Ishihara
Japan Science and Technology (JST), CREST
Furo-cho, Chikusa-ku, Nagoya, 464-8603 (Japan)
We were interested in Liuꢀs oxidation system.[7] Unfortu-
nately, however, the oxidation of benzyl alcohol (1a) gave
only a trace amount of benzaldehyde (2a) under both Liuꢀs
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900609.
456
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Chem. Asian J. 2010, 5, 456 – 460

conditions and more severe conditions (higher temperature
(55–1008C) and a reaction time up to 24 h) [Eq. (1)]. How-
ever, when a large amount of bromine (50 mol%) was used,
brominated compounds were produced together with small
amounts of carbonyl compounds [Eq. (2)]. Thus, we con-
cluded that Liuꢀs oxidation could not be reproduced.[8]
mine source (such as Br2, LiBr, HBr, and so forth) under
UV or visible irradiation.[11] However, it is difficult to selec-
tively oxidize primary alcohols to the corresponding alde-
hydes under these conditions.[11] The same group also report-
ed the aerobic oxidation of primary benzylic alcohols to the
corresponding benzaldehyde in the presence of a catalytic
amount (10 mol%) of molecular iodine under visible irradi-
ation.[11g] However, it is difficult to oxidize aliphatic alcohols
under these conditions.[11g]
To find a more efficient aerobic oxidation of alcohols, ini-
tially, we optimized the reaction conditions in the presence
of catalytic amounts of a bromine source (Table 1). A mix-
ture of 1a (5 mmol), Br2 (2 mol%), and NaNO2 (1 mol%)
in acetonitrile (5 mL) was heated at 708C even in the pres-
ence of 1 mol% of p-toluenesulfonic acid (TsOH) to give
2a and 3a in 83% and 5% yield, respectively (entry 2). In
sharp contrast, 2a was obtained in only 10% yield in the ab-
sence of TsOH under the same conditions (entry 1). A small
amount of water was added as a co-solvent to dissolve in-
soluble TsONa, which is generated in acetonitrile
(entry 3).[13] Oxidation did not occur in the absence of bro-
mine or NaNO2 and in the presence of iodine in place of
bromine (entries 4–6). Thus, bromine and NaNO2 were es-
sential for the present aerobic oxidation. Next, we screened
acid catalysts. Strong acids, such as TFA, Tf2NH, HCl, and
H2SO4, showed catalytic activities similar to those of TsOH
(entries 7–10). On the other hand, a relatively weak acid
Barton and co-workers reported the PhIO2-mediated oxi-
dation of benzylic alcohols to the corresponding aldehydes
in organic solvents, and the reaction rate is accelerated in
protic solvent such as acetic acid.[9] We confirmed that the
PhIO2-promoted oxidation of 1a did not take place in water
under neutral (PhIO2 was not dissolved) or acidic (PhIO2
was completely dissolved) conditions [Eq. (3)]. In contrast,
if 1 equiv of HBr was added to the suspension of 1a and
PhIO2 in water, a brown homogeneous solution was ob-
tained immediately, and 2a was obtained in 76% yield with
a small amount of benzoic acid (3a) [Eq. (4)]. Additionally,
we performed the oxidation of 1a with bromine (2 mol%)
and NaNO2 (1 mol%) in acetonitrile instead of water, and
2a was obtained in 10% yield in the absence of PhIO2
[Eq. (5)]. These results suggest that the actual oxidant for
the oxidation of alcohol should be bromine rather than
PhIO2, and PhIO2 might oxidize BrÀ to Br2.[7]
Table 1. Optimization of aerobic oxidation reaction conditions.[a]
Entry Acid
Acid/Br2/NaNO2 Solvent
[mol%]
2a (3a) Conv.
N
[%][b]
1
2
3
4
5
6
7
8
None
–/2/1
1/2/1
1/2/1
1/2/–
1/–/1
1/2/1[e]
2/1/1
1/2/1
1/2/1
1/2/1
CH3CN
10 (0)
83 (5)
85 (5)
<5 (<1)
<1 (<1)
<1 (<1)
77 (2)
91 (8)
79 (4)
80 (4)
6 (<1)
86 (14)
78 (4)
34 (<1)
94 (3)
39 (1)
4 (0)
12 (0)
39 (4)
TsOH
TsOH
TsOH
TsOH
TsOH
CF3SO3H
Tf2NH
HCl[f]
CH3CN[c]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN/H2O[d]
CH3CN
9
10
11
12
13
14
15
16
17
18
19
20
H2SO4
C6F5CO2H 2/1/1
HBr[g]
HBr[g]
HBr[g]
HBr[g]
HBr[g]
HBr[g]
HBr[g]
HBr[g]
HBr[g]
5/–/1
4/–/1
3/–/1
5/–/1
5/–/1
5/–/1
5/–/1
5/–/1
10/–/2[h]
THF
EtOAc
ClCH2CH2Cl
Benzene
CH3CN
In fact, numerous bromine-catalyzed aerobic oxidations
of alcohols in the presence of acid additives and/or light
sources have been reported to date.[10–12] Minisci and co-
workers developed a bromine-catalyzed selective aerobic
oxidation of primary benzyl alcohols to the corresponding
aldehydes in aqueous 1,2-dichloroethane.[10] However, large
amounts of bromine (5–40 mol%) and HNO3 (32–65 mol%)
were required.[10] In contrast, Ito and co-workers reported
aerobic oxidation of primary and secondary alcohols to the
corresponding carboxylic acids and ketones, respectively, in
the presence of a catalytic amount (7–20 mol%) of a bro-
82 (6)
[a] Unless otherwise noted, 1a (5 mmol), acid, NaNO2 and Br2 were
placed in a Schlenk apparatus equipped with a condenser, and the mix-
ture was magnetically stirred at 708C under balloon pressure of oxygen.
[b] 1H NMR analysis. [c] TsONa was precipitated. [d] CH3CN/H2O (40:1
v/v). [e] I2 was used instead of Br2. [f] An aqueous 2n HCl solution (pre-
pared freshly from 12n HCl) was used. [g] An aqueous 48% HBr solu-
tion was used. [h] An aqueous 69% HNO3 solution was used instead of
NaNO2.
Chem. Asian J. 2010, 5, 456 – 460
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www.chemasianj.org
457

K. Ishihara et al.
COMMUNICATION
such as pentafluorobenzoic acid was inefficient (entry 11).
Furthermore, we found that an aqueous 48% HBr solution
was effective as both an acid and a “Br” source. The aerobic
oxidation was rapid in the presence of HBr (5 mol%) and
NaNO2 (1 mol%), however, a large amount of 3a was also
produced in an acetonitrile–water solvent system (entry 12).
In addition, the oxidation rate was strongly dependent on
the catalytic amounts of HBr and NaNO2 (entries 12–15).
Oxidation was very slow in the presence of 3 mol% of HBr
and 1 mol% of NaNO2, and gave 2a in only 34% yield
(entry 14). The selectivity of 2a was increased under nearly
nonaqueous conditions,[14] and 2a and 3a were obtained in
94% and 3% yields, respectively (entry 15). Acetonitrile
was superior to other solvents such as tetrahydrofuran, ethyl
acetate, dichloroethane, and benzene (entries 15–19). Nota-
bly, HNO3 could be also used instead of NaNO2 (entry 20).
To explore the generality of the present aerobic oxidation
of alcohols, various primary benzylic alcohols 1 were exam-
ined as substrates under optimized conditions [HBr (5 or
10 mol%), NaNO2 (1 or 2 mol%) in acetonitrile at 70 or
808C under balloon pressure of pure O2] (Table 2). Most of
benzyl alcohols with the electron-donating or electron-with-
drawing groups were selectively oxidized to the correspond-
ing aldehydes 2 (entries 1–10). However, the oxidation of 4-
nitrobenzyl alcohol 1k was very slow and the product selec-
tivity was low under these conditions (entry 11). Unfortu-
nately, the aerobic oxidations of aliphatic primary alcohols
were not successful under these conditions.
Next, the oxidation of secondary alcohols was examined
(Table 3). The oxidation of 1-phenylethanol 4a in acetoni-
trile[14] gave acetophenone 5a in 38% yield with byproducts
including a-bromoacetophenone (entry 1). Br2 was con-
sumed for the a-bromination of enolizable ketones that are
initially formed under these conditions with the inhibition of
catalysis.[10,15] Fortunately, we found that the use of a small
amount of water was effective for the selective oxidation of
4a to 5a in high yield (entry 2). The oxidation of benzyl al-
cohol 4 was faster in the presence of two-fold amounts of
catalysts (10 mol% HBr and 2 mol% NaNO2) and gave 5 in
high yields (entries 3–5). Furthermore, the oxidation of sec-
ondary aliphatic alcohols 4 gave the corresponding ketones
5 in high yields under these conditions (entries 6–9).
Finally, we found that air could also be used as an oxygen
source instead of pure O2 under our conditions (Table 4).
Notably, the addition of a catalytic amount of HNO3 signifi-
cantly accelerated the reaction rate (entry 2 versus entry 1).
Thus, air oxidation of 1a in the presence of HBr
(10 mol%)/NaNO2 (2 mol%)/HNO3 (2 mol%) gave 2a in
90% yield (entry 2). The use of twice amounts of NaNO2 or
HNO3 was not effective (entries 3 and 4). Additionally, 2a
was obtained in 46% yield in the presence of HCl as an
acid additive instead of HNO3. These results suggest that
the addition of both NaNO2 and HNO3 was essential for the
air oxidation of alcohols under these conditions, however
the additional role of HNO3 was not clear. Noteworthy, air
oxidation of 4g gave ketone 5g in 99% yield under these
modified conditions (entry 4).
Table 2. Aerobic oxidation of primary benzylic alcohols 1 to aldehydes
2.[a]
Entry
Product
2
HBr/NaNO2
[mol%]
Temp.
[8C]
Time
[h]
Yield
[%][b]
N
1
2
3
4
5
2a
2b
2c
2d
2e
5/1
70
80
80
80
80
24
22
20
46
2
90
88
82
80
87
5/1
10/2
10/2
5/1
Our proposed mechanism for the present aerobic oxida-
tion is similar to that of Liu (Scheme 3),[7] although Br2 is
the active oxidant that oxidizes the alcohol to the corre-
sponding carbonyl compounds (Cycle I, Scheme 4). The re-
action of NaNO2 or HNO3 with HBr initially gives NOBr or
6
2 f
5/1
80
3
85
7
8
2g
2h
10/2
10/2
80
80
48
16
72
87
9
10
11
2i
2j
10/2
10/2
10/2
80
80
80
6
96
Scheme 4. Proposed mechanism for the bromine-catalyzed aerobic oxida-
tion of alcohols.
48
72
77
2k
58[c]
NO2Br which decomposes into NO or NO2 and Br2
[Eqs. (6)–(9)].[15,16] NO2 is generated also by the oxidation of
NO with O2, and oxidizes HBr to Br2 [Eqs. (10) and (11),
Cycle II].[16] In addition, HNO3 can be also produced by dis-
solving NO2 in water [Eq. (12)].[16]
[a] Unless otherwise noted, 1 (5 mmol), NaNO2 and aqueous 48% HBr
solution were placed a Schlenk apparatus equipped with a condenser,
and the mixture was magnetically stirred at 70 or 808C under balloon
pressure of oxygen. [b] Yield of isolated product of 2. [c] 4-Nitrobenzoic
acid 3j was obtained in 19% yield (1H NMR).
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Chem. Asian J. 2010, 5, 456 – 460

Bromine-Catalyzed Aerobic Oxidation of Alcohols
In conclusion, we found that a simple and environmental-
ly benign catalytic system consisting of HBr/NaNO2 was
very effective for the aerobic oxidation of alcohols. Primary
benzylic alcohols could be selectively oxidized to the corre-
sponding aldehydes under balloon pressure of O2 in acetoni-
trile. Secondary alcohols were oxidized to the corresponding
ketones in the presence of a small amount of water. Further-
more, the aerobic oxidation of alcohols could also be ach-
ieved under balloon pressure of air instead of pure O2 with
this HBr/NaNO2/HNO3 catalytic system.
Experimental Section
Table 3. Aerobic oxidation of secondary alcohols 4 to ketones 5.[a]
Representative procedure for the aero-
bic oxidation of benzyl alcohols: To a
Schlenk apparatus equipped with
a
condenser and Teflon-coated stirrer
chip were added successively NaNO2
Entry Product
5
HBr/NaNO2 [mol%] Solvent
Temp. [8C] Time [h] Yield [%][b]
1
2
3
5a
5a
5a
10/2
5/1
10/2
CH3CN
80
44
28
5
38[d]
85[e]
76
(3.5 mg,
0.05 mmol),
acetonitrile
CH3CN/H2O[c] 80
CH3CN/H2O[c] 80
(5.0 mL), and 1a (520 mL, 5.0 mmol).
The flask was charged with oxygen,
and commercial aqueous 48% HBr so-
lution (28 mL, 0.25 mmol) was added
rapidly. The resulting mixture was
heated to 708C and stirred for 24 h
under balloon pressure of oxygen. The
resulting mixture was cooled to room
temperature and poured into saturated
NaHSO3 solution, and the organic
layer was washed with saturated
NaHCO3 solution. The aqueous layer
was extracted with diethyl ether and
the combined organic layers were
dried over anhydrous MgSO4 and
evaporated under vacuo. The residue
was chromatographed on SiO2 to give
2a (478 mg, 4.5 mmol) in 90% yield.
4
5
5b
5c
10/2
10/2
CH3CN/H2O[c] 80
CH3CN/H2O[c] 80
4
72
74
24
6
7
8
5d
5e
10/2[f]
10/2
CH3CN/H2O[c] 80
CH3CN/H2O[c] 80
24
24
48
72[f]
75
5 f[g] 10/2
5g[h] 10/1
CH3CN
CH3CN
80
80
95[e]
9
22
95
[a] See footnote for Table 2. [b] Yield of isolated product of 5. [c] CH3CN/H2O (40:1 v/v). [d] 2-Bromoaceto-
phenone and unidentified products were obtained. [e] 1H NMR analysis. [f] An aqueous 69% HNO3 solution
(2 mol%) was used as additive. 5d was obtained in 66% yield (1H NMR analysis) in the absence of HNO3.
[g] Product via exo-norborneol 4 f. [h] Product via borneol 4g.
Acknowledgements
Financial support for this project was
provided
by
JSPS.KAKENHI
(20245022), the New Energy and In-
dustrial Technology Development Organization (NEDO) and the Global
COE Program of MEXT.
Table 4. Bromine-catalyzed air-oxidation of alcohols 1a and 4.
Keywords: acid · aerobic · alcohol · bromine · oxidation
Entry
Product
HBr/NaNO2
[mol%]
HNO3
ACHTUNGTRENNUNG
[mol%][b]
Time
[h]
Yield
[%][c]
U
1
2
3
4
5
5
6
2a
2a
2a
2a
2a
5a
5g
10/2
10/2
10/4
10/0
10/2
20/4
10/2
0
2
0
4
24
24
24
24
24
24
48
47[d]
86
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COMMUNICATION
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Received: November 1, 2009
Published online: February 1, 2010
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Chem. Asian J. 2010, 5, 456 – 460