Aerobic oxidation under visible light irradiation of a fluorescent lamp with a combination of carbon tetrabromide and triphenyl phosphine

Article abstract of DOI:10.1016/j.tetlet.2007.10.132

A combination of CBr₄-Ph₃P, a non-metal method, enables us to carry out aerobic photo-oxidation of alcohols and aromatic methyl groups to the corresponding carboxylic acids under irradiation of vis from a general-purpose fluorescent lamp. Aliphatic primary- and secondary alcohols, benzyl alcohols, and methyl groups at the aromatic nucleus generally afforded the carboxylic acids directly in good to high yield.

Full text of DOI:10.1016/j.tetlet.2007.10.132

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 9096–9099
Aerobic oxidation under visible light irradiation of a
fluorescent lamp with a combination of carbon
tetrabromide and triphenyl phosphine
Taichi Sugai and Akichika Itoh*
Gifu Pharmaceutical University 5-6-1, Mitahora-higashi, Gifu 502-8585, Japan
Received 25 August 2007; revised 23 October 2007; accepted 26 October 2007
Available online 30 October 2007
4 3
Abstract—A combination of CBr –Ph P, a non-metal method, enables us to carry out aerobic photo-oxidation of alcohols and
aromatic methyl groups to the corresponding carboxylic acids under irradiation of vis from a general-purpose fluorescent lamp.
Aliphatic primary- and secondary alcohols, benzyl alcohols, and methyl groups at the aromatic nucleus generally afforded the
carboxylic acids directly in good to high yield.
ꢀ
2007 Elsevier Ltd. All rights reserved.
The notion of green chemistry is becoming well estab-
lished, and the development of environmentally benign
processes is the goal of various research projects. A
have been no reports about the application of photo-
oxidation with vis by a general-purpose fluorescent lamp
1
to fine chemistry. Although a combination of CBr and
4
method using molecular oxygen is one way consistent
with this notion due to its high atom economy or E-fac-
PPh has been widely used as brominating reagents of
hydroxyl groups due to its easy generation of a bromo
3
1
2
,3
tor as an oxidant. With this background in mind, we
have been engaged in aerobic oxidation of alcohols and
a methyl group at the aromatic nucleus under irradia-
anion, there have been no reports about the application
of it to fine chemistry as an oxidizing reagent so far.
Thus, we now report our detailed study for visible light
aerobic oxidation of alcohols and an aromatic methyl
group in the presence of a catalytic amount of
CBr –Ph P.
4
tion of UV with several bromo sources. We believe that
the bromo radical, formed by continuous aerobic photo-
oxidation of the bromo anion, effects these reactions.
This is the driving force of our further studies on this
oxidation with a new bromo source, which releases the
bromo anion more easily under a non-metal condition,
4
3
Table 1 shows the results of our study of reaction condi-
tions for the aerobic oxidation conducted with benzyl-
alcohol (1, 0.3 mmol) as test substrate in the presence
of additives in typical solvents equipped with an oxygen
balloon. Among the solvents and the amount of the
and we have found that a combination of CBr and
4
PPh is suitable for oxidation of alcohols and a methyl
3
group at the aromatic nucleus. In addition, surprisingly,
this oxidation was found to proceed smoothly even
under irradiation of visible light (vis) by a general-purpose
fluorescent lamp instead of UV by a high-pressure mer-
cury lamp. The effective use of vis is a most important
research topic at this time of hoped-for development
of new energy conversion and energy-using technology,
and our new oxidation method is interesting since there
5
additives examined, acetonitrile, and 0.1 equiv of
CBr and Ph P was found to be suitable for the reaction,
4
3
respectively.
Table 2 shows the results for oxidation of several sub-
strates under the reaction conditions mentioned above.
Benzyl alcohols were found to give the corresponding
benzoic acids in high yield regardless of an electron-
donating or electron-withdrawing group at the aromatic
nucleus (entries 1–8). Furthermore, 3-thiophenemetha-
nol (17), which is a heterocyclic substrate, afforded
3-thiophenecarboxylic acid (18) in 80% yield (entry 9).
Aliphatic primary alcohols generally afforded the
Keywords: Aerobic oxidation; Carbon tetrabromide; Fluorescent lamp;
Triphenylphosphine; Visible light.
*
Corresponding author. Tel.: +81 58 237 3931x222; fax: +81 58 237
979; e-mail: itoha@gifu-pu.ac.jp
5
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.132

T. Sugai, A. Itoh / Tetrahedron Letters 48 (2007) 9096–9099
9097
in which ethyl acetate was the most suitable solvent,7
an electron-donating group at the aromatic nucleus
accelerates the reactions, and affords the corresponding
carboxylic acids in high yield; however, electron-with-
drawing groups at the aromatic nucleus retarded the
reactions, and yields of the products were zero to mod-
est (entries 12–19). On the other hand, N-heterocyclic
compounds, 3-pyridinemethanol and picoline, were also
employed in this reaction; however, only the starting
Table 1. Study of reaction conditions for the photo-oxidation of
benzylalcohol (1)
a
O -balloon, fluorescent lamp
CO H
2
2
OH
CBr -Ph P
4
3
solvent (5 mL), 10 h
1
(0.3 mmol)
2
b
Entry CBr
4
/PPh
3
(equiv)
Solvent
Yield of 2 (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
0.1/0.1
0.1/0.1
0.1/0.1
0.1/0.1
0.1/0.1
0.1/0.1
0.1/0.1
0.1/0.01
0.1/0.05
0.05/0.05
0.01/0.01
—/0.1
EtOAc
Hexane
Acetone
MeOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
74
c
0
0
8
material was recovered quantitatively.
c
0
To examine the mechanism of this reaction, at first, 1
was subjected to irradiation for 5 h with a fluorescent
lamp and then reacted in the dark for 5 h, and lowering
100
d
0
0
e
9
of the yield shows the necessity of continuous irradia-
93
96
98
0
0
0
tion to complete this oxidation, and this reaction does
not involve an auto-oxidation path. Also, this reaction
includes bromine formation because the color of the
reaction mixture immediately changed to yellow under
irradiation of the lamp. We present in Scheme 1 what
we assume is a plausible path of this oxidation, which
is postulated by considering all of the results mentioned
above, and the necessity of a catalytic amount of CBr4,
1
1
1
1
0.1/—
a
A solution of alcohol (0.3 mmol) additive in dry solvent (5 mL) in
Pyrex test tube was irradiated with fluorescent lamp (22 W · 4) for
10 h.
b
c
All yields are for pure, isolated products.
A trace amount of benzaldehyde was detected.
The reaction was carried out in the dark.
Ph P and of molecular oxygen in this reaction. Regard-
3
ing the oxidation of alcohols, the radical species 33 is
thought to be generated by abstraction of a hydrogen
radical with a bromo radical, formed by continuous
aerobic photo-oxidation of the bromo anion from
d
e
2
The reaction was carried out under N .
CBr (Eqs. 1–3). Solvation effect of ethyl acetate or ace-
corresponding carboxylic acids in good yield; however,
secondary alcohols were found to give the correspond-
4
tonitrile on the phosphonium salt is thought to be suit-
able for liberating the ‘naked’ bromo anion, and the
bromo radical is generated comparatively easily even
ing carbonyl compounds in low yield (entries 10 and
6
1
1). Regarding oxidation of the aromatic methyl group,
a
Table 2. Aerobic photo-oxidation with fluorescent lamp
O -balloon, fluorescent lamp
2
CBr (0.1 equiv)-Ph P (0.1 equiv)
substrate
0.3mmol)
4
3
product
(
solvent (5 mL), 10 h
b
c
Entry
Substrate
Solvent
Product
Yield (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Benzylalcohol (1)
AN
AN
AN
AN
AN
AN
AN
AN
AN
EA
EA
EA
EA
EA
EA
EA
EA
EA
EA
Benzoic acid (2)
4- Bu-benzoic acid (4)
100
94
87
99
96
83
88
70
80
94
t
t
4- Bu-benzylalcohol (3)
4-Chlorobenzylalcohol (5)
4-Methoxybenzylalcohol (7)
4-Nitrobenzylalcohol (9)
sec-Phenethylalcohol (11)
1-Naphthalenemethanol (13)
2-Naphthalenemethanol (15)
3-Thiophenemethanol (17)
1-Dodecanol (19)
4-Chlorobenzoic acid (6)
4-Methoxybenzoic acid (8)
4-Nitrobenzoic acid (10)
Acetophenone (12)
1-Naphthoic acid (14)
2-Naphthoic acid (16)
3-Thiophenecarboxylic acid (18)
1-Dodecanoic acid (20)
Dodecan-2-one (22)
1
1
1
1
1
1
1
1
1
1
d
2-Dodecanol (21)
55
Toluene (23)
Benzoic acid (2)
99
90
100
90
86
60
0
t
t
4- Bu-toluene (24)
4- Bu-benzoic acid (4)
4-Methoxytoluene (25)
4-Phenyltoluene (26)
4-Chlorotoluene (28)
4-Cyanotoluene (29)
4-Nitrotoluene (31)
4-Methoxybenzoic acid (8)
4-Phenylbenzoic acid (27)
4-Chlorobenzoic acid (6)
4-Cyanobenzoic acid (30)
4-Nitrobenzoic acid (10)
1-Naphthoic acid (14)
1-Methylnaphthalene (32)
22
a
A typical procedure follows: A solution of alcohol (0.3 mmol), CBr
irradiated with fluorescent lamp (22 W · 4) for 10 h.
AN; acetonitrile, EA; ethyl acetate.
All yields are for pure, isolated products.
The reaction was carried out for 24 h.
4 3
(0.1 equiv), Ph P (0.1 equiv) in dry ethyl acetate (5 mL) in Pyrex test tube was
b
c
d

9098
T. Sugai, A. Itoh / Tetrahedron Letters 48 (2007) 9096–9099
+
-
Mizugaki, T.; Kaneda, K. J. Mol. Catal. A: Chem. 2004,
12, 161–170; (c) Baucherel, X.; Gonsalvi, L.; Arends, I.
CBr + PPh
Ph P - CBr + Br
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
4
3
3
3
2
W. C. E.; Ellwood, S.; Sheldon, R. A. Adv. Synth. Catal.
2004, 346, 286–296; (d) Matsumura, Y.; Yamamoto, Y.;
Moriyama, N.; Furukubo, S.; Iwasaki, F.; Onomura, O.
Tetrahedron Lett. 2004, 45, 8221–8224; (e) Uozumi, Y.;
Nakao, R. . Angew. Chem., Int. Ed. 2003, 42, 194–197; (f)
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Merinero, J. A. V. J. Org. Chem. 2002, 67, 7493–7500; (h)
Cicco, S. R.; Latronico, M.; Mastrorilli, P.; Suranna, G.
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140; (i) Jenzer, G.; Schneider, M. S.; Wandeler, R.; Mallat,
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Sakaguchi, S.; Iwahama, T. Adv. Synth. Catal. 2001, 343,
hν, O2
Br-
Br
RCH OH + Br
RCHOH + HBr
2
33
hν
2
HBr + 1/2 O2
Br + H O
2 2
3
3
+ Br2
+ Br
R-CHO + HBr + Br
34
3
3
4
5
R-CO + HBr
3
5
3
93–427, and references cited therein.
+
Br2
R-COBr Br
+
3. For recent examples of oxidation of alcohols to the
corresponding aldehydes with molecular oxygen, see: (a)
Ohkubo, K.; Suga, K.; Fukuzumi, S. Chem. Commun.
2006, 2018–2020; (b) Guan, B.; Xing, D.; Cai, G.; Wan,
X.; Yu, N.; Fang, Z.; Yang, L.; Shi, Z. J. Am. Chem. Soc.
3
6
36
+ H O
R-CO H + HBr
2
2
Scheme 1. Plausible path of the aerobic photo-oxidation of alcohols.
2
005, 127, 18004–18005; (c) Mu, R.; Liu, Z.; Yang, Z.;
Liu, Z.; Wu, L.; Liu, Z.-L. Adv. Synth. Catal. 2005, 347,
333–1336; (d) Schultz, M. J.; Hamilton, S. S.; Jensen, D.
1
under irradiation of vis. Bromine, then, was formed by
aerobic photo-oxidation of hydrogen bromide, which
is generated in Eq. 3 (Eq. 4). We believe that the oxida-
tion was not observed when using 3-pyridinemethanol
and picoline as substrate since they trap this hydrogen
R.; Sigman, M. S. J. Org. Chem. 2005, 70, 3343–3352; (e)
Liu, R.; Liang, X.; Dong, C.; Hu, X. J. Am. Chem. Soc.
2004, 126, 4112–4113; (f) Mori, K.; Hara, T. i.; Mizugaki,
T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004, 126,
10657–10666; (g) Marko, I. E.; Gautier, A.; Dumeunier,
R.; Doda, K.; Philippart, F.; Brown, S. M.; Urch, C. J.
Angew. Chem., Int. Ed. 2004, 43, 1588–1591; (h) Stahl, S.
S. Angew. Chem., Int. Ed. 2004, 43, 3400–3420, and
references cited therein; (i) Iwasawa, T.; Tokunaga, M.;
Obora, Y.; Tsuji, Y. J. Am. Chem. Soc. 2004, 126, 6554–
1
0
11
bromide at the N-atom. Aldehyde 34 was afforded
by abstraction of a hydrogen radical with bromine
(
Eq. 5), and the re-generated bromo radical abstracted
a hydrogen radical from 34 to give radical species 35,
which was transformed to acyl bromide 36 (Eqs. 6 and
6
555; (j) Jensen, D. R.; Schultz, M. J.; Mueller, J. A.;
Sigman, M. S. Angew. Chem., Int. Ed. 2003, 42, 3810–
813.
1
2
7
). The carboxylic acid was formed by reaction with
water, generated in Eq. 4, although there was a possibil-
ity that the dissolved water in the solvent effected the
reaction (Eq. 8).
3
4. (a) Kuwabara, K.; Itoh, A. Synthesis 2006, 1949–1952; (b)
Hirashima, S.-I.; Hashimoto, S.; Masaki, Y.; Itoh, A.
Tetrahedron 2006, 62, 7887–7891; (c) Hirashima, S.-I.;
Itoh, A. Synthesis 2006, 1757–1759; (d) Itoh, A.; Hashim-
oto, S.; Kodama, T.; Masaki, Y. Synlett 2005, 2107–2109;
In conclusion, we have found a facile method for aero-
bic oxidation of alcohols and a methyl group at the aro-
matic nucleus directly to the corresponding carboxylic
acid in the presence of a catalytic amount of CBr4–
(
2
e) Itoh, A.; Hashimoto, S.; Masaki, Y. Synlett 2005,
639–2640; (f) Itoh, A.; Hashimoto, S.; Kuwabara, K.;
Kodama, T.; Masaki, Y. Green Chem. 2005, 7, 830–832;
g) Itoh, A.; Kodama, T.; Hashimoto, S.; Masaki, Y.
Ph P under visible light irradiated from a general-
3
(
purpose fluorescent lamp. This new form of oxidation
reaction is interesting due to the non-use of metals and
halogenated solvents, waste reduction, and the use of
Synthesis 2003, 2289–2291.
5
6
7
8
. Among our study, acetonitrile was a suitable solvent for
alcohols and ethyl acetate was suitable for a methyl group
at the aromatic nucleus, respectively, see Supplementary
data.
1
3
molecular oxygen.
. The radical species of secondary alcohols are, in general,
generated easier than that of primary ones. Unfortunately,
we do not have any direct data to explain this result, and
the study is now in progress in our laboratory.
. Since transesterification of the substrate alcohols occurred
when using ethyl acetate as solvent, we generally used
acetonitrile when using alcohols as substrates and ethyl
acetate when using aromatic methyl group as substrates.
. We also examined with 4-methyl benzyl alcohol to
study the selectivity between alcohol and methyl group,
and we could obtain 4-methylbenzoic acid in 60%
yield as the main product. Alcohol is relatively easily
oxidized than methyl group; however, the selectivity is not
so clear.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2
007.10.132.
References and notes
. Larock, R. C. Comprehensive Organic Transformations: A
1
Guide to Functional Group Preparations; Wiley-VCH: New
York, 1999.
9
. Only 19% of 2 was obtained.
2
. For recent examples of oxidation of alcohols to the
corresponding carboxylic acids with molecular oxygen,
see: (a) Figiel, P. J.; Sobczak, J. M.; Ziolkowski, J. J.
Chem. Commun. 2004, 244–245; (b) Ebitani, K.; Ji, H.-B.;
1
0. Although we examined this reaction with 3-methylpyrrole
as test substrate, the complex mixture was obtained
instead of the corresponding carboxylic acid.

T. Sugai, A. Itoh / Tetrahedron Letters 48 (2007) 9096–9099
9099
1
1
1. The reaction is thought to proceed through an aldehyde as
an intermediate since 2 was obtained in 88% yield when
benzaldehyde was subjected to the same aerobic photo-
oxidation conditions.
2. (a) Minisci, F.; Porta, O.; Recupero, F.; Punta, C.;
Gambarotti, C.; Pierini, M.; Galimberti, L. Synlett 2004,
Synlett 1990, 347–348; (c) Mark o´ , I. E.; Mekhalfia, A.
Tetrahedron Lett. 1990, 31, 7237–7240.
13. This oxidation method is also applicable to mol scale
conditions, and 4, for example, was obtained in 77% yield
when using 24 as substrate under similar conditions
mentioned above.
2
203–2205; (b) Mark o´ , I. E.; Mekhalfia, A.; Ollis, W. D.