Hydroxide ion as electron source for photochemical Birch-type reduction and photodehalogenation

Article abstract of DOI:10.1016/j.tetlet.2008.03.123

The photochemical Birch-type reduction of arenes and the photodehalogenation of haloarenes by a hydroxide ion that acted as an electron source occurred in 2-PrOH. The efficiency of these photoreactions was dependent on the nature of the substrate, the concentration of NaOH, and the solvent used. These photoreactions provide an environmentally friendly method for the reduction of aromatic rings and dehalogenation.

Full text of DOI:10.1016/j.tetlet.2008.03.123

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3400–3404
Hydroxide ion as electron source for photochemical
Birch-type reduction and photodehalogenation
*
Yasuharu Yoshimi , Akihiro Ishise, Hiromu Oda, Yousuke Moriguchi,
Hiroki Kanezaki, Yukari Nakaya, Kayoko Katsuno, Tatsuya Itou,
Sho Inagaki, Toshio Morita, Minoru Hatanaka *
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan
Received 17 March 2008; accepted 24 March 2008
Available online 28 March 2008
Abstract
The photochemical Birch-type reduction of arenes and the photodehalogenation of haloarenes by a hydroxide ion that acted as an
electron source occurred in 2-PrOH. The efficiency of these photoreactions was dependent on the nature of the substrate, the concentra-
tion of NaOH, and the solvent used. These photoreactions provide an environmentally friendly method for the reduction of aromatic
rings and dehalogenation.
Ó 2008 Elsevier Ltd. All rights reserved.
9
,10
One of the current challenges with regard to organic
synthesis is the development of new and cleaner processes
that minimize the use of hazardous substances. Although
the reduction of aromatic rings.
Moreover, in dehalo-
genation, tributyltin hydride is generally employed as a
reagent in the presence of a catalytic amount of AIBN in
1
1
1,12
several environmentally friendly methods have been
reported, one of the most efficient approaches is the use
of photoreaction since light is a clean and powerful reagent
toluene.
However, a less toxic and more stable reagent
is desirable so that the reactions can be carried out in con-
ventional solvents at room temperature. The photochemi-
cal electron transfer using a suitable electron source can
provide alternative methods for these reactions. The photo-
2
–5
to transform organic molecules. In particular, a photo-
chemical electron transfer reaction between electron
donor–acceptor pairs can produce reactive species under
1
3,14
reduction of arenes
haloarenes
and the photodehalogenation of
by using amines as an electron source have
6
–8
15,16
mild conditions.
The reduction of aromatic rings and the dehalogenation
of haloarenes are the useful synthetic transformations of
organic molecules. In addition, the dehalogenation of halo-
arenes is now considered to be more important for the
decontamination of aromatic halides since haloarenes are
deleterious environmental pollutants. Both the abovemen-
tioned reactions are usually carried out by using metals
or organic metals. For example, Birch reduction using
alkaline metals (Li and Na) as an electron source in liquid
ammonia has been one of the most common methods for
already been reported, although the yields of products were
significantly low because of the formation of adducts with
amines.
During the course of our study on an organic photo-
chemical reaction using a photosensitive surfactant in
1
7,18
water,
we found that the naphthyl group of the surfac-
tant was photoreduced in aqueous NaOH solution to pro-
duce a Birch-type reduction product. This result indicates
that the hydroxide ion could serve as an electron source
for the photochemical reaction. Although the electron
transfer (ET) between the hydroxide ion and the dinitro-
1
9–24
25–28
*
arenes
or the excited nitrobenzene
has already
Corresponding authors. Tel.: +81 776 27 8633; fax: +81 776 27 8747
been reported, these investigations have been carried out
mainly from a mechanistic viewpoint. In this Letter, we
(
Y.Y.).
E-mail address: yoshimi@acbio2.acbio.fukui-u.ac.jp (Y. Yoshimi).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.123

Y. Yoshimi et al. / Tetrahedron Letters 49 (2008) 3400–3404
3401
report the photoreduction of arenes and the photodehalo-
genation of haloarenes using NaOH in the presence of a
hydrogen donor such as 2-PrOH. This photoreaction tech-
nique can serve as a safe, convenient, and environmentally
friendly method for these reactions.
ophenanthrene 2 in 82% isolated yield (Table 1, entry 1).
The efficiency of this photoreduction was highly dependent
on both the nature of the substrate and the concentration
of NaOH; further, the use of KOH or tetrabutylammo-
nium hydroxide instead of NaOH resulted in a low yield
3
0
The photoreduction of arenes was carried out in Pyrex
vessels (>300 nm) with a 500 W high-pressure mercury
of the reduction product. The addition of cysteine
(6 mM) to this solution accelerated the photoreduction
and increased the yield of 2, even with a low concentration
of NaOH (entry 2). Instead of 2-PrOH, the use of EtOH or
MeOH or aqueous CH CN (H O/CH CN = 1:1) as a
2
9
lamp under an argon atmosphere at room temperature.
The results are summarized in Table 1. The excitation of
a 2-propanol solution of phenanthrene 1 (5 mM) and
NaOH (200 mM) for 6 h exclusively afforded 9,10-dihydr-
3
2
3
solvent in the presence of NaOH (40 mM) and cysteine
Table 1
Photoreduction of arenes by NaOH
R
R
hν, NaOH, additive
-PrOH
2
R
R
hν, NaOH, additive
-PrOH or H2O or
2
2
-PrOH:H2O=1:1
a
b
Entry
1
Arenes
Solvent
2-PrOH
NaOH (mM)
200
Irradn time (h)
Additive
—
Products (Yield (%))
6
2
(82)
1
(5 mM)
2
3
1 (5 mM)
2-PrOH
2-PrOH
40
40
8
4
Cysteine 6 mM
—
2 (91)
CN
CN
+
2 (4)
3
(5 mM)
4 (70)
CH
3
CH
3
4
5
6
2-PrOH
2-PrOH
2-PrOH
200
100
200
24
2
—
—
—
5
(5 mM)
6 (60)
+
8 (50)c
9 (14)c
7
(2 mM)
CH3
CH3
CH3
2
+
1
1
1
0 (2 mM)
11 (32)
12 (17)
OCH3
OCH3
OCH3
7
8
2-PrOH
200
500
3
—
—
+
3 (2 mM)
14 (38)
15 (25)
CO2H
CO2H
CO2H
CO2H
+
+
2-PrOH/H
2
O = 1:1
18
6 (2 mM)
17 (51)
18 (3)
19 (7)
(
continued on next page)

3402
Y. Yoshimi et al. / Tetrahedron Letters 49 (2008) 3400–3404
Table 1 (continued)
a
b
Entry
9
Arenes
Solvent
2-PrOH
NaOH (mM)
Irradn time (h)
Additive
Products (Yield (%))
16 (2 mM)
16 (2 mM)
150
500
6
3
—
17 (12) + 18 (34) + 19 (14)
17 (59) + 18 (2)
CO2H
10
H
2
O
Cysteine 40 mM
CO2H
d
11
2
2-PrOH/H O = 1:1
500
0.7
—
2
0 (2 mM)
21 (32)
a
Irradiation was carried out with a 500 W high-pressure mercury lamp at room temperature.
Isolated yield.
b
c
Determined by GC analysis using authentic samples.
d
Irradiation was carried out with a 120 W low-pressure mercury lamp at room temperature.
(
6 mM) resulted in a low yield of 2 (33%, 21%, and 12%,
(10 mM) and NaOH (50 mM) was irradiated to give the
2
9
respectively). Thus, the efficient photoreduction required
a good hydrogen donor such as 2-PrOH. The photoreac-
tion of 9-cyanophenanthrene 3 mainly yielded the reduc-
tion product 4, but was accompanied by the decyanation
product 2 (entry 3). Furthermore, the photoreaction of
corresponding dehalogenated product (Table 2), and fur-
ther irradiation yielded the Birch reduction products. The
presence of electron withdrawing group such as the cyano
group on the aromatic ring increased the reactivity of the
photoreaction (entry 5). In the case of 1,4-dibromonaph-
thalene 22e, successive dehalogenation proceeded to pro-
vide a good yield of naphthalene 7 (entry 6). While the
addition of cysteine was not effective in this photoreaction,
the addition of t-dodecanethiol increased the yield of
dehalogenated products (entries 3 and 9). Instead of
9
-methylphenanthrene 5 yielded the reduction product 6
(
entry 4). When naphthalene derivatives were subjected to
photoreaction, dihydro- and tetrahydronaphthalenes were
formed (entries 5–10), and the regioselectivity of the photo-
reduction was dependent on both the substituent and the
reaction conditions. The photoreduction of 1-methylnaph-
thalene 10 and 1-methoxynaphthalene 13 having an elec-
tron-donating group gave the corresponding 5,8-dihydro-
and 5,6,7,8-tetrahydronaphthalenes (entries 6 and 7), while
2-PrOH, the use of MeOH or EtOH or aqueous CH CN
3
as a solvent decreased the yield of dehalogenated products,
irrespective of the presence and absence of t-dodecanethiol.
Thus, the photodehalogenation also required a good
hydrogen donor such as 2-PrOH.
1
-carboxynaphthalene 16 in aqueous 2-PrOH solution
yielded 1,4-dihydro-, 3,4-dihydro-, and 1,2,3,4-tetrahydro-
naphthalenes 17–19 (entry 8). It should be noted that the
reduction product distribution changed with the solvent
used. The use of 2-PrOH as a solvent mainly produced
the corresponding 3,4-dihydronaphthalene 18 (entry 9),
and the addition of cysteine (40 mM) in water yielded the
corresponding 1,4-dihydronaphthalene 17 as the major
product (entry 10). The tetrahydronaphthalenes could
be produced by the secondary photoreduction of the corres-
ponding 1,2-dihydro-, 3,4-dihydro-, 5,6-dihydro-, and 7,8-
dihydronaphthalenes with the hydroxide ion. In fact, the
photoreaction of isolated 3,4-dihydronaphthalene 18
The fluorescence of 1 was not quenched by the hydrox-
ide ion, and the formation of the reduction product 2 was
quenched by molecular dioxygen. In addition, the higher
efficiency of dehalogenation than that of Birch-type reduc-
tion could be attributed to the heavy atom effect. From
these results, we propose that the ET from the hydroxide
ion to the excited triplet state of arenes or haloarenes is
effected, leading to the formation of the radical anion. In
the case of the photoreaction of 1-bromonaphthalene
22a, the anion radical of 22a generates through the ET
between the hydroxide ion and the excited triplet state of
22a (Scheme 1). The dehalogenation of the anion radical
of 22a produces radical 27 to yield naphthalene 7 through
the abstraction of a hydrogen atom from 2-PrOH. Further,
7 is photoreduced by the hydroxide ion to form the anion
radical, and the successive protonation of 2-PrOH gener-
ates radical 28. Similarly, the hydrogen abstraction of
radical 28 gives the reduction product 8. Although an
enhancement in the efficiency of these photoreactions was
observed by the addition of thiols, the reason for the
enhancement is not clear at this stage.
3
1
(
5 mM) with NaOH (150 mM) in 2-PrOH for 6 h yielded
tetrahydronaphthalene 19 in 45% isolated yield. In addi-
tion, benzoic acid 20 was also reduced by irradiation in
aqueous 2-PrOH solution using a 120 W low-pressure lamp
with quartz vessels to yield 1-carboxy-1,4-dihydrobenzene
2
1 in 32% isolated yield (entry 11). Thus, arenes such as
phenanthrene, naphthalene, and benzene derivatives can
be photoreduced by the hydroxide ion in the presence of
a hydrogen donor such as 2-PrOH.
Next, the photochemical dehalogenation of haloarenes
by the hydroxide ion was examined. A low power light
source (a 100 W high-pressure mercury lamp) was adequate
for this photoreaction under the same conditions. The
In conclusion, we found that the hydroxide ion can be
used as an electron source in the photoreduction of
aromatic rings and photodehalogenation of haloarenes,
although the mechanistic details of photoreaction by the
hydroxide ion have not yet been elucidated. Compared to
2
-PrOH solution containing haloarenes 22a–f, 24, and 25

Y. Yoshimi et al. / Tetrahedron Letters 49 (2008) 3400–3404
3403
Table 2
Br
Photodehalogenation of haloarenes by NaOH
-
hν
1
3
+ OH
2
2a
22a
22a
R
R
Intersystem crossing
ISC)
Electron transfer
(ET)
(
hν, NaOH (50 mM)
2
2a
2-PrOH
H
X
+
-
-
Br
OH
1
0 mM
22a
Dehalogenation
Hydrogen abstraction
2
2
2
2
2
2
2a; X = Br, R = H
2b; X = Br, R = CH
2
7
7
3
2c; X = Br, R = OCH
2d; X = Br, R = CN
2e; X = Br, R = Br
2f; X = Cl, R = H
3
-
+
hν
1
3
+ OH
ET
+ H
7
7
7
7
ISC
Protonation
2
8
Entry Haloarenes
10 mM)
Irradn
time (h)
Additive
—
Products
(Yield (%))
a
b
H
(
+
OH
28
1
2
22a
22b
1
Hydrogen abstraction
8
c
7
(97)
Scheme 1. Plausible mechanism of the photohalogenation and
photoreduction.
CH
3
1.5
—
Acknowledgment
1
0 (50)
This work was supported by Mitsubishi Chemical
Corporation Award in Synthetic Organic Chemistry,
Japan.
3
4
22b
22c
1
t-dodecanethiol 10 (84)
0 mM
1
OCH
3
1.5
—
Supplementary data
1
3 (77)
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2008.03.123.
CN
5
22d
0.5
—
References and notes
2
3 (86)
c
6
7
22e
22f
1
1
—
—
7 (84)
7 (85)
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4
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3
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could serve as an electron source for the photoreduction.
2
2
631.