Cycloaddition of o-quinodimethanes generated from 1,2-bis(bromomethyl)benzenes by constant-current electroreduction

Article abstract of DOI:10.1246/bcsj.75.2693

The constant-current electroreduction of 1,2-bis(bromomethyl)benzenes with dienophiles in NH₄NO₃-MeOH gave Diels-Alder cycloadducts in moderate-to-good yields.

Full text of DOI:10.1246/bcsj.75.2693

c. Jpn.
e Chemical
ciety of Ja-
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Short Articles
Bull. Chem. Soc. Jpn., 75, 2693–2694 (2002) 2693
e Chemical
ciety of Ja-
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with dienophiles other than maleic anhydrides using constant-
current electrolysis, which is more convenient than the con-
stant-potential method. In this paper, we report that the Diels–
Alder cycloaddition of electrogenerated o-quinodimethanes
with dienophiles, like methyl acrylate and methyl vinyl ketone,
was effected by the constant-current electroreduction of 1,2-
bis(bromomethyl)benzenes in NH NO –MeOH.
Cycloaddition of o-Quino-
dimethanes Generated from
1
,2-Bis(bromomethyl)benzenes by
Constant-Current Electroreduction
02
4
3
9
9
3
4
Initially, we surveyed the conditions for the constant-current
electroreduction of 1,2-bis(bromomethyl)benzene (1a) in the
presence of methyl acrylate (5 molar amounts) as a dienophile
at a current of 0.2 A. The results are summarized in Table 1.
Naoki Kise,* Ryosuke Mimura, and Nasuo Ueda
SJA8
9-2673
739
0
Department of Biotechnology, Faculty of Engineering,
Tottori University, Tottori 680-8552
2
The electoreduction in DMF or MeOH containing Et NOTs
4
produced considerable amounts of dimethyl adipate and poly-
meric compounds, though the yields of cycloadduct 3a were
low (runs 1, 2). In contrast, when the reduction was carried out
in NH NO –MeOH with a Pb cathode, the adduct 3a was ob-
4 3
(
Received July 19, 2002)
0
0
.
2
tained in 67% yield, and no dimethyl adipate was formed (run
3). This result showed that only 1a was reduced and methyl
acrylate was not reduced at all under the conditions. Using
DMF or EtOH as a solvent, the yield of 3a decreased (runs 4,
5). As a supporting electrolyte, NH NO gave a better result
The constant-current electroreduction of 1,2-bis(bro-
2
2
4 3
momethyl)benzenes with dienophiles in NH NO –MeOH
gave Diels–Alder cycloadducts in moderate-to-good yields.
4
3
than the other ammonium salts (runs 6–8). Among the other
cathode materials, only a carbon-felt cathode afforded a yield
of 3a comparable with that obtained from a Pb cathode (runs
o-Quinodimethanes are useful intermediates in organic syn-
thesis. The reductive dehalogenation of 1,2-bis(halometh-
1
yl)benzenes 1 seems to be the most convenient and simplest
method to generate o-quinodimethanes 2 (Scheme 1). For this
9
–12). When the reduction was carried out using a Pt, Zn, or
Cu cathode, the formation of dimethyl adipate was observed,
and the yield of 3a somewhat decreased. This result suggests
that the Pb cathode was suitable for the selective reduction of
2
3
4
Ⅱ 5
purpose, several reducing agents such as Zn, Fe, Cu, Cr ,
6
7
Ni and NaI have been reported. On the other hand, electrore-
duction is expected to be a versatile method for the reduction
of 1,2-bis(halomethyl)benzenes without using such reducing
agents under milder conditions. Covitz has reported that the
electroreduction of 1,2-bis(bromomethyl)benzene in DMF us-
ing a Hg cathode gave poly(o-phenyleneethylene) in 50%
1
a, although the reduction peak potential of 1a could not be
measured by cyclic voltammetry using a Pb cathode, unfortu-
nately. Next, we optimized the amounts of methyl acrylate and
current for electroreduction in NH NO –MeOH using a Pb
4 3
cathode. As shown in Table 2, 5 molar amounts of methyl
8
yield. Recently, Utley and his co-workers have reported that
the electroreduction of 1,2-bis(bromomethyl)benzenes and
4
maleic anhydrides in Et NBr–DMF using a Hg cathode pro-
Table 1. Constant-Current Electroreduction of 1,2-Bis(bro-
momethyl)benzene (1a) and Methyl Acrylate
duced the corresponding Diels–Alder cycloadducts in moder-
a)
9
ate-to-good yields (Scheme 2). However, dienophiles other
than maleic anhydrides, such as methyl acrylate and methyl vi-
9a
nyl ketone, brought about poor yields of the cycloadducts;
a
constant-potential electrolysis was employed in this electrore-
duction. We therefore investigated more efficient conditions
for the electroreduction of 1,2-bis(bromomethyl)benzenes
c)
Electricity Yield of
b)
Run
Solvent
Cathode
(
F/mol)
3.0
6.5
3.0
3.0
2.5
3.5
3.0
3.5
4.0
5.5
4.0
3.0
3a/%
30
29
67
50
33
49
64
50
42
23
46
65
1
2
3
4
5
6
7
8
9
Et
Et
4
NOTs/DMF
NOTs/MeOH
Pb
Pb
Pb
Pb
Pb
Pb
Pb
Pb
Pt
4
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
4
4
4
4
4
4
4
4
4
4
NO
NO
NO
3
3
3
/MeOH
/EtOH
/DMF
Cl/MeOH
Br/MeOH
OAc/MeOH
Scheme 1.
NO
NO
NO
NO
3
3
3
3
/MeOH
/MeOH
/MeOH
10
11
12
Zn
Cu
/MeOH Carbon-Felt
a) Electroreduction of 1a was carried out in the presence of
mol. amt. of methyl acrylate at 0.2 A. b) 0.2 M Electrolyte
in solvent. c) Isolated yields.
5
Scheme 2.

2
694 Bull. Chem. Soc. Jpn., 75, No. 12 (2002)
© 2002 The Chemical Society of Japan
Table 2.
Methyl Acrylate in 0.2 M NH
Cathode
Constant-Current Electroreduction of 1a and
same conditions as in run 3 (Table 1). These results show that
the constant-current electroreduction of 1 in NH NO –MeOH
4 3
is also effective for the cycloaddition of electrogenerated o-
quinodimethanes with dienophiles other than maleic anhy-
drides. This electrochemical method provides alternative
means for the reductive generation of o-quinodimethanes from
4
NO
3
/MeOH Using a Pb
a)
Run Methyl Acrylate/mol. amt. Current/A Yield of 3a/%
1
2
3
4
5
6
7
8
1.25
2.5
5.0
0.2
0.2
0.2
0.2
0.2
0.1
0.4
0.6
27
44
67
60
59
63
45
32
1
,2-bis(bromomethyl)benzenes and their subsequent cycload-
dition with a variety of dienophiles without using metal reduc-
ing agents.
10.0
20.0
5.0
5.0
5.0
Experimental
Typical Procedure for Electroreduction.
.2 M NH NO in MeOH (40 mL) was put into a divided cell of a
0 mL beaker (4.5 cm diameter, 6 cm height) equipped with a Pb
A solution of
0
5
4
3
a) Isolated yields.
2
2
cathode (5 × 10 cm ), a Pt anode (2 × 2 cm ), and a cylindrical
ceramic diaphragm (2.5 cm diameter, 7 cm height). To the
catholyte (outside the diaphragm) was added 1a (1.31 g, 5 mmol)
and methyl acrylate (2.25 mL, 25 mmol). Electroreduction was
carried out at a constant current of 0.2 A at 25 °C until 1448 C of
electricity (3 F/mol) had passed. The catholyte was poured into
water and extracted with Et O. The product 3a (0.64 g, 67%
2
yield) was isolated by column chromatography on silica gel (hex-
ane–AcOEt).
Table 3. Constant-Current Electroreduction of 1 with Dieno-
a)
philes in 0.2 M NH
4
3
NO /MeOH Using a Pb Cathode
b,c)
Run
1
1
Dienophile
Adduct (3) Yield of 3/%
1a
83 (58)
42
2
3
4
5
6
1a
1a
1a
1a
1a
Determination of Products. Cycloadducts 3a–i were con-
firmed by a comparison of their spectroscopic data with those of
d)
d)
41
(23)
2
,5
authentic samples prepared by reported methods.
63 (35)
48
References
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2
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a) Electroreduction was carried out under the same conditions
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3
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8
9
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acrylate and 0.2 A of current (4 mA/cm ) gave the best yield of
3
a.
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bis(bromomethyl)benzenes 1 with other dienophiles under the
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