One-pot epoxidation of alkenes using aerobic photoperoxidation of toluenes

Article abstract of DOI:10.1016/j.tetlet.2015.12.027

We developed an aerobic photooxidative synthesis of peroxybenzoic acids from toluenes using anthraquinone-2-carboxylic acid (AQN-2-CO₂H) as a photocatalyst. We also found a one-pot epoxidation reaction of alkenes using 4-tert-butylperoxybenzoic acid produced in situ by aerobic photooxidative synthesis.

Full text of DOI:10.1016/j.tetlet.2015.12.027

Tetrahedron Letters 57 (2016) 230–232
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-pot epoxidation of alkenes using aerobic photoperoxidation
of toluenes
a
a
a
a
b
Miyabi Taguchi , Yoshitomo Nagasawa , Eiji Yamaguchi , Norihiro Tada , Tsuyoshi Miura ,
Akichika Itoh
a
a,
⇑
Gifu Pharmaceutical University, 1-25-4, Daigaku-nishi, Gifu 501-1196, Japan
Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We developed an aerobic photooxidative synthesis of peroxybenzoic acids from toluenes using anthra-
Received 14 October 2015
Revised 25 November 2015
Accepted 7 December 2015
Available online 11 December 2015
2
quinone-2-carboxylic acid (AQN-2-CO H) as a photocatalyst. We also found a one-pot epoxidation reac-
tion of alkenes using 4-tert-butylperoxybenzoic acid produced in situ by aerobic photooxidative
synthesis.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Epoxidation
Photooxidation
Anthraquinone
Green chemistry
Peroxycarboxylic acids are strong and versatile oxidants which
have been used for various reactions such as epoxidation of
Previous reports
O
O
O
1
2
3
[Ox]
[Ox]
ref. 7, 8
alkenes, Baeyer–Villiger oxidation, and oxidation of sulfides,
4
5
Ar
H
ref. 6
Ar
OOH
Ar
X
amines, and iodides. The general syntheses methods of peroxy-
X = Cl, OH
6
benzoic acids are peroxidation of aldehydes and nucleophilic acyl
substitution with benzoyl halide7 or benzoic acid8 (Scheme 1).
Although these reactions are reliable methods, their syntheses
require extreme reactions, such as oxidation of toluene, ethylben-
zene, or cumene, to prepare the substrates. In addition, these
compounds are explosive. A milder and more efficient method of
synthesis for peroxybenzoic acids produced from non-explosive
materials in situ is therefore desired.
This work
O
O , hν (VIS)
cat. AQN-2-CO
2
R2
Me
OH
R1
2
H
O
O
R2
R1
Scheme 1. Aerobic photooxidative synthesis of peroxybenzoic acid and its appli-
cation to epoxidation of alkenes.
Photochemical reactions using irradiation with visible light are
notable in organic chemistry as environment-friendly methods
because of their high selectivity and mild conditions. In our labo-
developed, probably because toluenes have a low degree of oxida-
tion and are relatively stable compared with aldehydes or other
acyl groups. Herein, we report the direct peroxidation of toluenes
by aerobic photooxidation using anthraquinone-2-carboxylic acid
9
ratory, catalytic aerobic photooxidation reactions using irradiation
with visible light have been developed.10 Recently, we found an
aerobic photooxidation reaction of toluenes to produce carboxylic
acids in the presence of a catalytic amount of 2-chloroan-
thraquinone (2-Cl-AQN) under visible light irradiation using a
(
2
AQN-2-CO H) as the photocatalyst and its application to an epox-
idation reaction of alkenes (Scheme 1).
We first investigated the aerobic photooxidation conditions for
1
1
fluorescent lamp.
In this reaction, we assumed that the
1
2
a in the presence of a catalytic amount of AQN-2-CO H with vis-
corresponding peroxybenzoic acid was the key intermediate in
the reaction. To the best of our knowledge, no precedent for the
direct synthesis of peroxybenzoic acid from toluene has been
ible light irradiation at room temperature leading to 2a (Table 1).
Among the solvents screened (entries 1–5), ethyl acetate was the
most suitable solvent for peroxidation (entry 1). Control experi-
ments revealed that the reaction did not proceed without either
visible light irradiation or an oxygen atmosphere (entries 6 and
7, respectively). Neither the peroxybenzoic acid (2a) nor the
⇑
Corresponding author.
E-mail address: itoha@gifu-pu.ac.jp (A. Itoh).
http://dx.doi.org/10.1016/j.tetlet.2015.12.027
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

M. Taguchi et al. / Tetrahedron Letters 57 (2016) 230–232
231
Table 1
Me
O
2
, hν (VIS)
AQN-2-CO H (0.2 equiv)
AcOEt (5.0 mL), 20 h
Optimization of the peroxidation condition
nBu
O
2
5-decene (4a)
tBu
nBu
(1)
(2)
10 h
O
1a
5a
0%
O
2
, hν (VIS)
AQN-2-CO H (0.05 equiv)
solvent (5 mL), 20 h
(4.0 equiv)
Me
R
2
Me
O
2
, hν (VIS)
tBu
tBu
AQN-2-CO H (0.2 equiv)
2
5-decene (4a)
5a
tBu
AcOEt (2.5 mL)
MeCN (2.5 mL)
20 h
10 h
1a
2a : R = OOH
a : R = OH
1a
3
(4.0 equiv)
80%
Entry
Solvent
Yielda (%)
Scheme 2. One-pot epoxidation of alkene with in situ generated peroxybenzoic
acid.
2
a
3a
1
2
3
AcOEt
MeCN
AcOEt/MeCN
THF
65
31
50
0
2
0
23
32
28
0
0
0
b
Table 3
4
5
One-pot epoxidation of various alkenes
CHCl
3
bc
6
AcOEt/MeCN
AcOEt/MeCN
AcOEt/MeCN
R
bd
standard
condition
R'
time (h)
7
0
0
0
0
O
4
be
1a
R
8
R'
5
a
The yields were determined by 1H NMR analysis of crude reaction mixtures
using 1,1,2,2-tetrachloroethane as an internal standard.
Yielda (%)
Entry
Product
5
Time (h)
b
3
AcOEt (2.5 mL), CH CN (2.5 mL).
The reaction was conducted in the dark.
The reaction was conducted under Ar.
c
d
e
nBu O
1
2
3
4
nBu
O
5a
5b
5c
5d
7
24
12
12
86
62
82
82
2
The reaction was conducted without AQN-2-CO H.
Ph
Ph
O
Me
4
O
Table 2
Exploration of peroxidation of the methyl group on toluenes
-tBuC
6
H
4
O
5
6
5e
5f
9
91
82
O
Ph
Ph
Ph
OH
Ph
O
2
, hν (VIS)
Me
AQN-2-CO H (0.05 equiv)
2
R
O
20
AcOEt (5 mL), 20 h
R'
R'
O
1
2 : R = OOH
7
5g
20
99
3
: R = OH
Ph
Entry
R
Yielda (%)
a
Isolated yields.
2
3
1
2
3
4
MeO
H
Cl
CN
(1b)
(1c)
(1d)
(1e)
57
21
7
22
15
11
Trace
aldehyde with molecular oxygen in situ, under photoirradiation
6
c,14
conditions is an important and a promising method,
although
Trace
it requires aldehydes which are unstable and expensive. Thus, we
applied these peroxybenzoic acid derived from toluenes.
a
The yields were determined by 1H NMR analysis of the crude reaction mixture
using 1,1,2,2-tetrachloroethane as an internal standard.
In an optimization study, the conditions were examined with 4-
tert-butyltoluene (1a) under a molecular oxygen atmosphere with
visible light irradiation at room temperature for 20 h, followed by
reaction with 5-decene (4a) as a model substrate under an argon
atmosphere at room temperature for 10 h. When ethyl acetate
was used as solvent, no epoxide was obtained (Scheme 2, Eq. 1).
However, a mixed solvent system of ethyl acetate and acetonitrile
afforded a 50% yield of peroxybenzoic acid (Table 1, entry 2) and
was found to be a suitable solvent system for this epoxidation reac-
tion (Scheme 2, Eq. 2). These results and entries 1 and 2 in Table 1
suggest that the photoperoxidation requires AcOEt and the epoxi-
dation requires MeCN, respectively.
Next, we conducted epoxidation of the aliphatic and aromatic
alkenes (Table 3). The reaction times of the epoxidation were
determined by monitoring of TLC. The internal and terminal ali-
phatic alkenes (4a, 4b) produced the corresponding epoxides (5a,
5b) in good to high yields (entries 1 and 2). In addition, aromatic
alkenes (4c–4g) were also converted into desired epoxides (5c–
5g) in good to high yields (entries 3–7).
benzoic acid (3a) was formed when the reaction was conducted
without AQN-2-CO H (entry 8). These results suggest that all these
2
conditions are crucial for the present oxidation system.
Several toluenes (1) having electron donating or withdrawing
groups were explored as substrates (Table 2). 4-Methoxytoluene
(
(
1b) gave a 57% yield of the corresponding peroxybenzoic acid
2b) with a 22% yield of benzoic acid (3b) as a by-product (entry
1
5
1
1
). In contrast, toluenes with electron withdrawing groups (1d,
e) (Cl and CN) resulted in low yields (entries 2–4). The substrate
scope and limitations of this reaction are similar to those of previ-
ous oxidation reactions of toluenes.11
With the optimized conditions in hand, we investigated a new
one-pot epoxidation reaction via peroxidation of 4-tert-butyl-
toluene (1a) with molecular oxygen as the terminal oxidant. Much
effort has been made to develop epoxidation of alkenes with
molecular oxygen as the terminal oxidant because its theoretical
waste product is water. Although there have been many reports
on the metal-catalyzed epoxidation of alkenes with molecular oxy-
gen as a terminal oxidant,12 only a limited number of metal-free
methods have been reported, and, moreover, these methods
mainly use N-hydroxyphtalimide.13 Furthermore, epoxidation of
alkenes using peroxybenzoic acid, prepared by oxidation of an
In summary, we have developed an aerobic photoperoxidation
reaction of toluenes and applied this to one-pot epoxidation of
alkenes. With requisites such as harmless visible light, molecular
oxygen as oxidant, and an environment-benign organocatalyst,
this method is of great value from the viewpoint of green
chemistry.

2
32
M. Taguchi et al. / Tetrahedron Letters 57 (2016) 230–232
(
2
d) Harman, D. G.; Ramachandran, A.; Gracanin, M.; Blanksby, S. J. J. Org. Chem.
006, 71, 7996.
Acknowledgments
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(a) Zuo, Z.; Ahneman, D. T.; Chu, L.; Terrett, J. A.; Doyle, A. G.; MacMillan, D. W.
C. Science 2014, 345, 437; (b) Beatty, J. W.; Stephenson, C. R. J. J. Am. Chem. Soc.
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We thank the Gifu Pharmaceutical University and the Asahi
Glass Foundation for their financial support.
Supplementary data
1
0. (a) Kariya, A.; Yamaguchi, T.; Nobuta, T.; Tada, N.; Miura, T.; Itoh, A. RSC Adv.
Supplementary data (synthesis, characterization data and 1H
and 1 C NMR spectra data of all compounds) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
2014, 4, 13191; (b) Itoh, I.; Matsusaki, Y.; Fujiya, A.; Tada, N.; Miura, T.; Itoh, A.
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T.; Nakayama, T.; Uno, B.; Itoh, A. Synlett 2014, 1453; (d) Nagasawa, Y.;
Matsusaki, Y.; Hotta, T.; Nobuta, T.; Tada, N.; Miura, T.; Itoh, A. Tetrahedron Lett.
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5. Representative procedure of the epoxide is as follows: Dry ethyl acetate (2.5 mL)
and acetonitrile (2.5 mL) solution of the 4-tert-butyltoluene (89.0 mg,
0.60 mmol) and AQN-2-CO
2
H (7.6 mg, 0.03 mmol) in a pyrex tube equipped
with O -balloon was stirred and irradiated externally with fluorescent lamp,
2
7
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which was set up at a distance of 0.5 cm, for 20 h. Then, an alkene was added to
the reaction mixture at room temperature. The reaction mixture was
concentrated under reduced pressure, and purified by purification with
preparative TLC.
8