782
Chemistry Letters Vol.37, No.7 (2008)
Photoreduction of 1,2-Diarylcyclopropanes by Hydroxide Ion
Yasuharu Yoshimi,Ã Naohisa Wada, Koichi Sasamoto, Tatsuya Itou, and Minoru HatanakaÃ
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering,
University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507
(Received April 25, 2008; CL-080430; E-mail: yoshimi@acbio2.acbio.fukui-u.ac.jp)
Table 1. Photoreduction of 1a–1c by NaOH in i-PrOHa
The photoreduction of 1,2-diarylcyclopropanes 1 by NaOH
yielded the corresponding 1,3-diarylpropanes 2 in i-PrOH. The
efficiency of the photoreduction was dependent on the electron
deficiency of 1 and the excitation wavelength, solvent, and
additive. This is a rare example of a cyclopropane serving as
an electron acceptor to afford a reduction product.
O
(254 nm)
NaOH
h
ν
+
i-PrOH
R
R
R
R
R
R
3a-3c
2a-2c
1a; R = H
1b; R = OCH3
1c; R = CN
R
+
The photochemistry of cyclopropanes has been actively
investigated owing to their characteristic reactivity.1 In particu-
lar, cyclopropanes can be utilized as good electron donors in a
photoinduced electron-transfer (PET) reaction because of their
low oxidation potentials. While PET reactions of cyclopropanes
with a variety of electron acceptors have been developed,2 the
only reported example of the PET of cyclopropanes with an elec-
tron donor is with tertiary amines.3 In the latter case, the photo-
reaction of electron-deficient cyclopropanes such as alkoxycar-
bonylcyclopropane derivatives yields an amine adduct as the
major product along with a reduction product through the forma-
tion of the radical anion.
We have recently reported that the photochemical Birch-
type reduction of arenes and the photodehalogenation of halo-
arenes using a hydroxide ion as an electron source.4 The photo-
reaction by the hydroxide ion could offer many advantages from
economical, environmental, and safety standpoints. In this letter,
we report on the photoreduction of 1,2-diarylcyclopropanes by
the hydroxide ion in i-PrOH. This is a rare example of a cyclo-
propane serving as an electron acceptor to afford a reduction
product. In addition, this photoreaction techinique can provide
an environmentally friendly method for the reduction of arylcy-
clopropanes, compared to the conventional reduction techinique
using H2 and Pd/C.5
Initially, the photoreduction of 1,2-diarylcyclopropanes
1a–1c with NaOH by 254-nm light in i-PrOH was examined.
Irradiation was carried out in quartz vessels with a 120-W
low-pressure mercury lamp under an argon atmosphere at room
temperature. The excitation of a i-PrOH solution of 1,2-diphen-
ylcyclopropane (1a) (2 mM) and NaOH (250 mM) for 2 h afford-
ed reduction product 2a and alcohol adduct 3a in 26% and 13%
isolated yields, respectively (Entry 1 in Table 1);6 however,
in the absence of NaOH, 2a was not obtained (Entry 4). Lower
concentrations of NaOH caused a decrease in the yield of 2a
(Entries 2 and 3). Thus, the yield of the reduction product
2a was highly dependent on the concentration of NaOH. In the
photoreaction, the OH adduct was not obtained. When 1,2-
di(4-methoxyphenyl)cyclopropane (1b) having an electron-
donating group was subjected to photoreaction, the yield of
reduction product 2b was very low (Entry 5). In contrast, the
photoreaction of 1,2-di(4-cyanophenyl)cyclopropane (1c) hav-
ing an electron-withdrawing group yielded reduction product
2c as the major product (Entry 6). Even with low concentrations
4a-4c
R
Product yields/%b,c
Entry
1
NaOH/mM
Recovery of 1/%b
2
3
4
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1b
1c
1c
1c
250
100
2
26
13
0
0
3
38
40
33
13
8
5
4
18
0
0
4
4
4
0
9
5
12
15
46
55
57
68
trace
9
0
250
250
100
2
trace
14
15
aThe photoreaction was carried out with 0.8 mmol (2 mM) of 1 and
NaOH in quartz vessels using a 120-W low-pressure mercury lamp
under an argon atmosphere for 2 h. Isolated yield. Characterization
data of these products are shown is Supporting Information.11
b
c
NaOH (100 or 2 mM), similar yields of 2c were obtained (Entries
7 and 8). Thus, the photoreduction of electron-deficient 1,2-
diarylcyclopropane 1c by NaOH smoothly proceeded in i-PrOH
to yield the corresponding 1,3-diarylpropane 2c.
Next, the effects of the excitation wavelength, solvent, and
additive on the photoreduction of 1c were examined. Prolonged
irradiation of the i-PrOH solution containing 1c and NaOH by
254-nm light decreased the yield of 2c, since the secondary
photoreduction of the benzene group of 2c by NaOH could take
place, as reported earlier.4 In fact, the photoreaction of 2c
(2 mM) with NaOH (250 mM) in i-PrOH for 2 h recovered 2c
in only 20% yield. In order to prevent the secondary photoreduc-
tion of the product 2c, the excitation was carried out for 3 h
at a longer wavelength (313 nm) using a 400-W high-pressure
mercury lamp with Pyrex vessels. Under such conditions, the
yield of the reduction product 2c was slightly improved
(Entry 1 in Table 2). Instead of i-PrOH, the use of EtOH as
the solvent decreased the yield of 2c (Entry 2), while the use
of MeOH did not afford the reduction product (Entry 3). When
thiols such as cysteine and t-dodecanethiol were added, higher
yields of 2c were obtained (Entries 4 and 5), and Entry 5 shows
the highest yield of 2c.
From a mechanistic viewpoint, we examined the deuterium
incorporation experiment of 1c using 2-propanol-2d1 as a
solvent (Scheme 1). The ꢀ-position to the aryl group of 2c
was not deuterated, indicating that i-PrOH did not serve as a
hydrogen donor.
Copyright Ó 2008 The Chemical Society of Japan
Yoshimi, Yasuharu
