Aerobic oxidative esterification of benzyl alcohols with catalytic tetrabromomethane under visible light irradiation

Article abstract of DOI:10.1016/j.tetlet.2012.07.091

We report a useful method for the facile synthesis of aromatic esters from benzyl alcohols with molecular oxygen and catalytic tetrabromomethane in alcohol under visible light irradiation with a fluorescent lamp. This is the first metal-free reaction using molecular oxygen as the terminal oxidant.

Full text of DOI:10.1016/j.tetlet.2012.07.091

Tetrahedron Letters 53 (2012) 5306–5308
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Aerobic oxidative esterification of benzyl alcohols with catalytic
tetrabromomethane under visible light irradiation
⇑
Tomoya Nobuta, Akitoshi Fujiya, Shin-ichi Hirashima, Norihiro Tada, Tsuyoshi Miura, Akichika Itoh
Gifu Pharmaceutical University 1-25-4, Daigaku-nishi, Gifu 501-1196, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a useful method for the facile synthesis of aromatic esters from benzyl alcohols with molecular
oxygen and catalytic tetrabromomethane in alcohol under visible light irradiation with a fluorescent
lamp. This is the first metal-free reaction using molecular oxygen as the terminal oxidant.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 28 May 2012
Revised 12 July 2012
Accepted 20 July 2012
Available online 27 July 2012
Keywords:
Esterification
Tetrabromomethane
Visible light
Aerobic
Benzyl alcohol
Aromatic esters have always attracted a great deal of interest in
organic synthesis as intermediates such as liquid crystal polymers,
cosmetics, pharmaceuticals, agrochemicals, and food additives be-
cause of their versatility. In general, aromatic esters have been syn-
thesized for the esterification of carboxylic acid, which is obtained
by the oxidation of alcohols with heavy metals such as Cr or Mn.1
Recently, the direct oxidative esterification of alcohols has at-
tracted a great deal of attention from the viewpoint of reducing en-
ergy consumption, labor, and solvents. These reactions involve the
use of stoichiometric amounts of oxidants such as Ca(OCl)2,2 isoc-
yanuric chloride,3 I2,4 KI/t-BuOOH,5 NaIO4/LiBr,6 PhI(OAc)2/I2,7 and
PhIO/KBr8 or transition metal catalysts such as Mo,9 Mn,10 Pd,11
Ir,12 and Ru.13 On the other hand, the aerobic catalytic oxidative
esterification of alcohols has been developed in recent years.
Molecular oxygen is photosynthesized by plants and is a more effi-
cient oxidant than other oxidants such as toxic heavy metals or
complex organic reagents, theoretically producing only water as
the end product of oxidation. However, these reactions require
transition metal reagents such as Au14 or Pd.15 Thus, more environ-
mental friendly and economical methods of oxidative esterification
of alcohols are needed.
because it does not use heavy metals, whereas it makes use of
molecular oxygen and inexpensive reagents. However, this reac-
tion has limited substrate scope. When electron-poor substrates
were used, a xenon lamp was required, which resulted in low
yields. Furthermore, only methyl carboxylates were obtained by
this reaction, because other alcohols, such as ethanol or propanol
that is used as solvent, are more easily oxidized than methanol.
Therefore, we studied the oxidative esterification of benzyl
alcohols under visible light irradiation from general purpose fluo-
rescent lamp. In this Letter, we report the details of the aerobic
photooxidative synthesis of aromatic esters from benzyl alcohols
(Scheme 1).
Table 1 shows the reaction conditions for the aerobic photoox-
idative synthesis of 4-tert-butyl methylbenzoate (2a) from 4-tert-
butyl benzylalcohol (1a) as the test substrate using methanol as
solvent under an O2 atmosphere irradiated with four 22 W fluores-
cent lamps. Among the bromine sources examined, 0.3 equiv of
CBr4 were found to afford the desired product 2a most efficiently
(entries 1–10). The excellent yield of 2a was obtained by prolong-
ing the reaction time to 20 h (entry 11). It is noted that using of two
fluorescent lamps resulted in low yield (entry 12). The fact that 2a
was not obtained without CBr4, irradiation, and molecular oxygen
shows that this reaction needs them (entries 13–15). It is noted
We have developed various aerobic photooxidation reactions
under oxygen atmosphere and light irradiation.16 Recently, we
reported the aerobic photooxidative synthesis of aromatic methyl
esters from methyl aromatics in the presence of catalytic CBr4.17
This facile and efficient method is of interest in green chemistry
O
O2, hν, CBr4
Ar
OH
Ar
OR
ROH
⇑
Corresponding author. Tel./fax: +81 58 230 8108.
E-mail address: itoha@gifu-pu.ac.jp (A. Itoh).
Scheme 1.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.091

T. Nobuta et al. / Tetrahedron Letters 53 (2012) 5306–5308
5307
Table 1
O2, hν (VIS)
CBr4 (0.3 equiv)
CHO
CO2Me
Study of reaction conditions for aerobic photooxidation
(1)
O2, hν (VIS)
MeOH, rt, 10 h
CO2Me
OH
bromine source
5b (0.3 mmol)
2b (71%)
tBu
tBu
MeOH, rt
1a (0.3 mmol)
2a
OMe
OMe
O2, hν (VIS)
CBr4 (0.3 equiv)
Yielda (%)
CO2Me
Entry
Bromine source (equiv)
Time (h)
(2)
(3)
1
2
3
4
5
6
7
8
LiBr (0.3)
NaBr (0.3)
KBr (0.3)
MgBr2 (0.3)
CaBr2 (0.3)
Aq HBr (0.3)
Br2 (0.3)
CBr4 (0.3)
CBr4 (0.2)
CBr4 (0.1)
CBr4 (0.3)
CBr4 (0.3)
—
10
10
10
10
10
10
10
10
10
10
20
20
20
20
20
20
0
0
0
0
0
MeOH, rt, 10 h
(0.3 mmol)
CO2H
(87%)
6b
7b
2b
2b
O2, hν (VIS)
CBr4 (0.3 equiv)
CO2Me
0
34
74
0
MeOH, rt, 10 h
(0.3 mmol)
(37%)
9b
10c
11
12d
13
14e
15f
16g
0
Scheme 2. Study of reaction mechanism.
94
10
0
0
3
CBr4 (0.3)
CBr4 (0.3)
CBr4 (0.3)
hν
hν
CBr4
Br
Br2
O2
1)
2)
74
Br
HBr
HBr Br
OO
OH
1H NMR analysis.
a
b
c
d
e
f
Ar
Ar
OH
Ar
Ar
OH
Ar
Starting material 1a was recovered in 86% yield.
Starting material 1a was recovered in 98% yield.
The reaction was carried out using two fluorescent lamps.
The reaction was carried out in the dark.
The reaction was carried out under an N2 atmosphere.
The reaction was carried out under an air atmosphere.
1
8
HBr Br2
H2O
OOH
OH
OH
OH
ROH
Ar CHO
g
5
Br
HBr
O2
HBr Br
OR
OR
6
OR
OR
RO OO
Ar
Ar
Ar
OR
Table 2
Aerobic photooxidative synthesis of aromatic esters from benzyl alcoholsa
HBr Br2
ROH
O2, hν (VIS)
CBr4 (0.3 equiv)
RO OOH
Ar OR
RO OH
Ar OR
substrate
(0.3 mmol)
product
CO2Me
Ar CO2R
ROH, rt, 20 h
2
CO2Me
CO2Me
Scheme 3. Plausible path for aerobic photooxidation of benzyl alcohol to aromatic
esters.
MeO
F
tBu
Cl
2b (73%)
2i (41%)b
CO2Me
2a (94%)
CO2Me
CO2Me
the electron-donating or electron-withdrawing group in the ben-
zene ring (2a–2h). Methyl 4-methoxybenzoate (2i) was obtained
in moderate yield in spite of prolonging the reaction time. Both
1-naphthalenemethanol (1j) and 2-naphthalenemethanol (1k)
were suitable substrates for this reaction affording the correspond-
ing esters 2j and 2k in high yields, respectively. Unfortunately, ali-
phatic alcohol was a poor substrate and resulted in low yield (2l).
Interestingly, ethanol and propanol can be used under these oxida-
tive esterification conditions giving the aromatic esters 3a and 4a
in good yields, respectively.
In order to examine the intermediates of this reaction, benzal-
dehyde (5b) and benzaldehyde dimethyl acetal (6b) were sub-
jected to the same aerobic photooxidation conditions for 10 h,
and methyl benzoate (2b) was obtained in good yields (Scheme
2, Eqs. 1 and 2). In addition, the esterification of carboxylic acid
(7b) was slow under these conditions, and methyl ester (2b) was
obtained in 37% yield (Scheme 2, Eq. 3). Therefore, these results
suggest that the reaction proceeds with aldehyde (5b) and dem-
ethyl acetal (6b) as intermediates.
MeO2C
Cl
2c
2d
(82%)
(61%)
2e
(85%)
CO2Me
CO2Me
Cl
CO2Me
O2N
2h
(93%)
2f
(92%)
2g
(66%)
CO2Me
CO2Me
CO2Me
10
c
2l
(5%)
2k
(93%)
b
2j
(84%)
CO2Et
CO2Pr
tBu
tBu
b
3a
(76%)
4a
(63%)
a
b
c
Isolated yields.
The reaction was carried out for 36 h.
Determined by 1H NMR.
Scheme 3 shows a plausible path of this oxidation, which is pos-
tulated by considering all the above mentioned results and the
necessity of molecular oxygen and continuous irradiation in this
reaction. The first step involves the abstraction of the hydrogen
radical from the benzyl position of benzyl alcohols 1 with bromine
radical, generated from CBr4 under light irradiation, to produce the
radical species 8. This is followed by oxygenation to afford alde-
that air instead of molecular oxygen can be also used in this oxida-
tive esterification (entry 16).
Table 2 presents the scope and limitation of the aerobic photo-
oxidative synthesis of aromatic esters from benzyl alcohols under
the optimized reaction conditions.18 In general, the corresponding
aromatic esters were obtained in good to high yields regardless of

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T. Nobuta et al. / Tetrahedron Letters 53 (2012) 5306–5308
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In conclusion, we have developed an aerobic photooxidative
synthesis method of aromatic esters in the presence of a catalytic
amount of CBr4. This method is of great value from the viewpoint
of green chemistry and organic synthesis because it uses inexpen-
sive bromine source, harmless visible light irradiated from a
general purpose fluorescent lamp, and molecular oxygen as the
terminal oxidant. Further application of this photooxidation reac-
tion to other reactions is now in progress in our laboratory.
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