Electrochemistry of aminoazines and nitrones: Electrochemical reductions of 2-amino-1,4-pyrazine with nitrones to form amide compounds and electrochemical oxidations of anilines with nitrones to form imine compounds and benzaldehydes

Article abstract of DOI:10.3987/COM-01-9191

Electrochemical reductions of 2-amino-1,4-pyrazine in the presence of nitrone derivatives afforded two types of amide compounds, one of which was derived from both the pyrazine and the nitrones and the other from only the nitrones. The analogous reactions

Full text of DOI:10.3987/COM-01-9191

HETEROCYCLES, Vol. 55, No. 6, 2001, pp. 1071 - 1080, Received, 1st March, 2001
ELECTROCHEMISTRY OF AMINOAZINES AND NITRONES:
ELECTROCHEMICAL REDUCTIONS OF 2-AMINO-1,4-PYRAZINE
WITH NITRONES TO FORM AMIDE COMPOUNDS AND
ELECTROCHEMICAL
NITRONES TO
BENZALDEHYDES
OXIDATIONS
FORM IMINE
OF
ANILINES
WITH
AND
COMPOUNDS
Katsuhiro Saito,* Ayako Kawamura, Takashi Kanie, Yosuke Ueda, and
Satoru Kondo
Department of Applied Chemistry, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya 466-8555, J apan
Abstract-----Electrochemical reductions of 2-amino-1,4-pyrazine in the
presence of nitrone derivatives afforded two types of amide compounds,
one of which was derived from both the pyrazine and the nitrones and
the other from only the nitrones. The analogous reactions but using
2-aminopyridine or aniline instead of the pyrazine resulted in the
recovery of the starting materials.
On the other hand,
electrochemical oxidations of the aminoazine derivatives in the
presence of nitrones did not afford any isolatable products. However,
the reaction using aniline derivatives formed imine compounds
accompanied by benzaldehyde derivatives.
Electrochemistries of amine derivatives have attracted much attention of chemists from
theoretical and synthetic view points.1 However, researches on electrochemistries of amines
conjugated with any heterocyclic moieties seem to be rare. As a series of our researches on
the electrochemistries of heterocyclic compounds, such as azepines2 or azulanones,3 we
intended to study electrochemical behaviors of amines conjugated with heterocyclic
moieties, i.e., aminoazine derivatives. The authors have also researched on cycloaddition
reactions of nitrone derivatives with sterically hindered cyclic compounds.4 These facts
prompted us to study the reactions of aminoazines with nitrones under the electrochemical
reaction conditions. Here the results are discussed.

10
8
1.4
1.2
1
1.232
1.056
1.095
first cycle
second cycle
pyrazine (1)
pyridine (2)
1.465
6
0.8
0.6
0.4
0.2
0
4
2
0
-0.2
-0.4
-0.6
-2
-4
0.318
-0.534
-0.728
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
potential/V
potential/V
Figure 1. Cyclic Voltammograms of 1 and 2.
Figure 2. Cyclic Voltammogram of 3.5
Cyclic voltammetries of 2-amino-1,4-pyrazine (1), 2-aminopyridine (2), and aniline (3) were
measured as shown in the Figures 1 and 2. The oxidation potentials of 1, 2, and 3 were
1.465, 1.232, and 1.056 (second cycle: 1.095) V vs. Ag/AgCl (0.1 M KCl aq.) respectively, and
the reduction potential of 1 was –0.728 V vs. Ag/AgCl (0.1 M KCl aq.). 2-Aminopyridine
(2) showed no clear reduction potential. Aniline (3) seemed to polymerize at 1.056 V,6
consequently, the reduction peaks are considered to be resulted from polyaniline. These
potentials suggested that compound (1) is reduced most easily and 3 is oxidized most easily
among the amines used in this work.
X
X
X
N
NH2
+ e-
H
N
N
N
H
O
+
+
N
O
N
N
O
Ph
Ph
(1)
(4)
(5)
(6)
a: X=OMe, b: X=Me, c: X=H,
d: X=Cl, e: X=Br
X
X
N
NH2
+ e-
H
O
+
+
N
N
N
O
O
Ph
Ph
(6)
(6)
(2)
(4)
X
X
NH2
+ e-
H
O
N
Ph
Ph
(3)
(4)
Scheme 1.

An anhydrous acetonitrile solution of 1 and an equimolar amount of p-methoxy-
diphenylnitrone (4a) was electrochemically reduced in the presence of tetrabutylammonium
perchlorate (TBAP) as a supporting electrolyte with a platinum gauze as a cathode and a
platinum wire as an anode with a constant current (–30 mA) at 0 ℃ under a nitrogen
stream.7 After the evaporation of the solvent, the reaction mixture was chromatographed
on silica gel to give 13 % yield of an amide compound (5a) derived from both 1 and 4a, as
well as 22 % yield of another type of an amide compound (6a) derived from the nitrone (4a).
The analogous reactions using various types of nitrone derivatives (4b, c, d, e) afforded the
corresponding amide compounds 5b (11 %), 5c (20 %), 5d (19 %), 5e (23 %), 6b (12 %), 6c
(4 %), 6d (34 %), and 6e (6 %), respectively.8, 9
Similar electrochemical oxidation reactions using 2 or 3 instead of 1 afforded the amide
compounds (6) derived from the nitrones but did not form the amide compounds of the type
of 5. The yields of 6 were 8 % (6b) and 10 % (6e) in the reactions of 2, and 25 % (6a) and
2 % (6e) in the reactions of 3.
X
N
NH2
- e-
- e-
- e-
recovery
recovery
O
+
+
N
N
N
Ph
(1)
(4)
(4)
(4)
X
N
NH2
O
Ph
(2)
X
X
X
NH2
N
O
+
+
C
H
N
Y
Y
OHC
Ph
(3)
(7)
(8)
a: Y=OMe, b: Y=H,
c: Y=Cl
Scheme 2.
On the other hand, the electrochemical oxidation reactions of 1, 2, and 3 demonstrated the
opposite order of the reactivity in these compounds, comparing to that in the
electrochemical reduction reactions.

An anhydrous acetonitrile solution of 1 and two mole equivalents of 4a was
electrochemically oxidized in the presence of TBAP with a platinum gauze as an anode and
a platinum wire as a cathode at +1.50 V vs. Ag/AgCl (0.1 M KCl aq.) at 0 ℃ under a
nitrogen stream. The same workup of the reaction mixture as above afforded no isolable
product and the starting material was recovered. Similar electrooxidation reaction using
2 instead of 1 resulted in the similar result as the case of 1.
On the other hand, aniline derivatives (3) afforded imine derivatives (7) and benzaldehyde
derivatives (8) as reaction products. Thus, an electrochemical oxidation of an anhydrous
acetonitrile solution of p-methoxyaniline (3a) and a nitrone derivative (4a) at +0.83 V vs.
Ag/AgCl (0.1 M KCl aq.) at 0 ℃ gave an imine derivative (7aa) in 24 % yield together with
46 % yield of a benzaldehyde derivative (8aa). The same reaction using aniline (3b) and 4a
afforded 7ba and 8ba in 30 and 54 % yields, respectively. A reaction with p-chloroaniline
(3c) and 4a or a reaction with 3a and 4e gave 7ca, 8ca, 7ae, and 8ae in 19, 78, 17, and 58 %
yields, respectively.10
The difference in the reactivity between the aniline moiety and the pyridine moiety was
also demonstrated by the following experiments. The electrochemical oxidation of
3-aminoquinoline (9) with p-chloronitrone (4d) afforded no isolatable compound, but the
analogous reaction using 8-aminoquinoline (10) formed the corresponding imine derivative
(11), though the yield was poor (6 %).
N
NH2
N
N
N
NH2
(10)
(9)
(11)
Cl
Figure 3.
The fact that the electrooxidaion of the imine derivatives (7) under the reaction conditions
of 3 with 4 formed the benzaldehyde derivatives (8) showed that 8 were secondary products
derived from 7 under the reaction conditions.
NH2
CHO
Br
- e
N
+
MeCN-TBAP-(Pt)
Me
Me
Br
(7de)
(3d)
(8e)
Scheme 3.

The formation of the amide compound (5) is considered to proceed through anion
intermediates (13-16) as follows. An electrochemical one electron reduction of 1 forms an
anion radical intermediate (12), which then generates 13 leaving a hydrogen radical. A
nucleophilic attack of 13 on the imine-carbon atom of 4 affords 14. An attack of the oxygen
anion to the carbon atom to cleave the C-N bond forms 15. A proton migration in 15
generates 16, which then affords the final product (5) through the cleavage of the N-O
bond.11
N
NH2
N
NH
NH2
N
+ e-
- H
N
N
N
(13)
(12)
(1)
Ar
HN
O
O
Ar
O
Ar
HN
O
N
Ar
NH
Ph
N
H
N
Ph
HN
Ph
H
N
5
N
Ph
N
NH
N
N
N
N
(14)
(15)
(16)
N
Scheme 4.
The formation mechanism of the imine derivative (7) is considered to be as follows. An
electrochemical one electron oxidation of 3 forms a cation radical of aniline (17), which then
generates a radical intermediate (18), leaving a proton. An attack of the radical (18) at
the imine carbon atom of the nitrone (4) forms a cation radical intermediate (19), which
then affords the final product (7) through an elimination of a hydroxyamine derivative.12, 13
- e-
- H+
NH2
NH
NH2
Ph
Ph
Ph
(3)
(17)
(18)
O
OH
Ar
Ar
N
N
+ H+
7
(4)
Ph
Ph
N
Ph
H
NH
Ph
(19)
(18)
Scheme 5.

ACKNOWREDGEMENT
The authors are indebted to Prof. T. Ohtsuka of Hokkaido University for his kind
suggestions concerning the electrochemistry of aniline.
EXPERIMENTAL
Electrolysis was performed in a two compartment glass cell. The working electrode was a
platinum gauze of a size of 5 cm depth and 12 cm width. The counter electrode was a
platinum wire and the reference electrode was a silver wire. The controlled potential and
current power were supplied from a Yanaco Potentio/Garvanostatic Electrolyser VE-9
apparatus. Acetonitrile was distilled from calcium hydride. Wakogel C200 was used for
column chromatography. NMR and MS spectra were measured with Varian GEMINI
2000/300 and Hitachi 220A spectrometers, respectively. IR spectra were measured with a
J ASCO ET/IR-5300 spectrophotometer.
Typical measurements and reactions are below.
Measurement of Cyclic Voltammetry of 1.
Cyclic voltammograms were measured with a Hokutodenko HZ-3000 potentiostat. Cyclic
voltammogram of 1 was measured with 1.0×10-3 M solution in anhydrous acetonitrile with
0.1 M TBAP as a supporting electrolyte at a scan rate of 0.2 V/s on a wire platinum
electrode.
Electrochemical Reduction of 1 and 4a.
A solution of 1 (95 mg, 1.0 mmol), 4a (229 mg, 1.0 mmol), and TBAP (7.0 g, 20 mmol) in
o
acetonitrile (200 mL) was electrolyzed under a nitrogen stream at 0 C at –30 mA. After
removing the solvent by distillation, the resulting residue was chromatographed on silica
gel to give 5a (29 mg, 13 %, hexane-ethyl acetate (4:6)) and 6a (51 mg, 22 %, hexane-ethyl
acetate (7:3)).
Electrochemical Reduction of 2 and 4b.
A solution of 2 (95 mg, 1.0 mmol), 4b (212 mg, 1.0 mmol), and TBAP (7.0 g, 20 mmol) in
acetonitrile (200 mL) was electrolyzed under the same reaction conditions as above to give
6b (15 mg, 8 %, hexane-ethyl acetate (7:3)).
Electrochemical Reduction of 3 and 4b.
A solution of 3 (100 mg, 1.1 mmol), 4b (212 mg, 1.0 mmol), and TBAP (7.0 g, 20 mmol) in
acetonitrile (200 mL) was electrolyzed under the same reaction conditions as above to give
6b (60 mg, 25 %, hexane-ethyl acetate (7:3)).

Electrochemical Oxidation of 1 and 4a. A solution of 1 (96 mg, 1.0 mmol), 4a (228 mg, 1.0
mmol), and TBAP (7.0 g, 20 mmol) in acetonitrile (200 mL) was electrolyzed under a
o
nitrogen stream at 0 C at +1.50 V vs. Ag/AgCl (0.1 M aq. KCl) After removing the solvent,
the resulting residue was chromatographed on silica gel to give the recovery of 4a (206 mg,
90 %, hexane-ethyl acetate (4:6)).
Electrochemical Oxidation of 2 and 4a.
A solution of 2 (94 mg, 1.0 mmol), 4a (228 mg, 1.0 mmol), and TBAP (7.0 g, 20 mmol) in
acetonitrile (200 mL) was electrolyzed at +1.50 V vs. Ag/AgCl (0.1 M aq. KCl) under the
same reaction conditions as above. The same treatments as above gave the recovery of 4a
(204 mg, 89 %, hexane-ethyl acetate (4:6)).
Electrochemical Oxidation of 3b and 4a.
A solution of 3b (186 mg, 2.0 mmol), 4a (454 mg, 2.0 mmol), and TBAP (7.0 g, 20 mmol) in
acetonitrile (200 mL) was electrolyzed at +0.83 V vs. Ag/AgCl (0.1 M aq. KCl) under the
same reaction conditions as above to give 7ba (86 mg, 30 %, hexane-ethyl acetate (8:2)) and
8ba (101 mg, 54 %, hexane-ethyl acetate (7:3)).
Electrochemical Oxidation of 9 and 4d.
A solution of 9 (145 mg, 1.0 mmol), 4d (232 mg, 1.0 mmol), and TBAP (7.0 g, 20 mmol) in
acetonitrile (200 mL) was electrolyzed at +0.83 V vs. Ag/AgCl (0.1 M aq. KCl) under the
same reaction conditions as above. The same treatments as above gave the recovery of 4d
(209 mg, 90 %, hexane-ethyl acetate (4:6)).
Electrochemical Oxidation of 10 and 4b.
A solution of 10 (288 mg, 2.0 mmol), 4b (423 mg, 2.0 mmol), and TBAP (7.0 g, 20 mmol) in
acetonitrile (200 mL) was electrolyzed at +0.83 V vs. Ag/AgCl (0.1 M aq. KCl) under the
same reaction conditions as above to give 11b (6 mg, 6 %, hexane-ethyl acetate (8:2)) and 8b
(33 mg, 72.9 %, hexane-ethyl acetate (7:3)).
Electrochemical Oxidation of 7de.
A solution of 7de (569 mg, 2.1 mmol), and TBAP (7.0 g, 20 mmol) in acetonitrile (200 mL)
was electrolyzed at +0.83 V vs. Ag/AgCl (0.1 M aq. KCl) under the same reaction conditions
as above to give 3d (78 mg, 52 %, hexane-ethyl acetate (7:3)) and 8e (214 mg, 83 %,
hexane-ethyl acetate (8:2)).
Electrochemical Oxidation of 4b
A solution of 4b (423 mg, 2.0 mmol), and TBAP (7.0 g, 20 mmol) in acetonitrile (200 mL)
was electrolyzed at +0.83 V vs. Ag/AgCl (0.1 M aq. KCl) under the same reaction conditions
as above. The same treatments as above gave the recovery of 4b (414 mg, 98 %,
hexane-ethyl acetate (5:5)).

REFERENCES AND NOTES
1. Y. Matsuda, A. Shono, C. Iwakura, Y. Ohshiro, T. Agawa, and H. Tamura, Bull. Chem. Soc.
J pn., 1971, 44, 2960; S. Wawzonek and T. W. McIntyre, J . Electrochem. Soc., 1972, 119,
1350; T. Fuchigami, Y. Fujita, and T. Nonaka, J . Electroanal. Chem., 1990, 284, 115; D.
Larumbe, I. Gallardo, and C. P. Andrieux, ibid., 1991, 304, 241.
2. S. Kondo, H. Suzuki, T. Hattori, T. Ido, and K. Saito, Heterocycles, 1998, 48, 1151.
3. T. Ido, S. Kondo, and K. Saito, Heterocycles, 1999, 50, 63; K. Saito, T. Ido, Y. Awadu, T.
Kanie, and S. Kondo, ibid., 2000, 53, 519.
4. K. Saito, A. Yoshino, H. Watanabe, and K. Takahashi, Heterocycles, 1990, 34, 497.
5. The cyclic voltammogram of 3 showed no reproducibility.
6. S. Yonezawa, K. Kanamura, and Z. Takehara, J . Electrochem. Soc., 1993, 140, 629.
7. The attempted electrochemical reductions of 1 and 4 under controlled potential
conditions (4a: –1.65 V vs. Ag/AgCl (0.1 M aq.KCl), 4b: –1.60 V vs. Ag/AgCl (0.1 M
aq. KCl)) failed to afford any isolatable products.
1
8. The structures of 5 were determined with their spectral properties, especially HNMR
spectral properties, and confirmed by the coincidence of these properties with these of the
authentic samples.
Sambaiah, T. Reddy, and K. Kondal, Indian J . Chem., Sect. B, 1992, 444-5.
1
5a: HNMR (CDCl3) δ ppm: 1.82 (s, 3H), 7.33 (d, 2H, J =8.0 Hz), 7.94 (d, 2H, J =8.0 Hz),
8.24 (dd, 1H, J =2.7 Hz, J =1.5 Hz), 8.37 (d, 1H, J =2.7 Hz), 9.10 (br s, 1H), 9.73 (s, 1H,
J =1.5 Hz).
5b: MS m/z (rel intensity): 213.0 (49, M+), 118.9 (100), 90.9 (55). IR (KBr): 3241, 3240,
1676, 1531, 1261 cm-1. 1HNMR (CDCl3) δ ppm: 2.44 (s, 3H), 7.32 (d, 2H, J =8.0 Hz),
7.84 (d, 2H, J =8.0 Hz), 8.24 (dd, H, J =2.7 and 1.5 Hz), 8.37 (d, H, J =2.7 Hz), 8.65 (bs,
1H), 9.73 (d, 1H, J =1.5 Hz). 13CNMR (CDCl3) δ ppm: 21.6, 127.4, 129.6, 130.5,
137.3, 140.2, 142.1, 143.5, 148.4, 165.4.
1
5c: HNMR (CDCl3) δ ppm: 7.52 (t, 3H, J =8.0 Hz), 7.96 (d, 2H, J =8.0 Hz), 8.28 (dd, 1H,
J =2.5 and 1.4 Hz), 8.38 (d, 1H, J =2.5 Hz), 8.82 (br s, 1H), 9.72 (s, 1H, J =1.4 Hz).
5d: MS m/z (rel intensity): 233.0 (44, M+), 138.9 (100), 110.6 (45). 1HNMR (CDCl3) δ
ppm: 7.13 (d, 2H, J =8.8 Hz), 7.90 (d, 2H, J =8.8 Hz), 8.29 (dd, 1H, J =2.5 Hz, J =1.6 Hz),
8.41 (d, 1H, J =2.5 Hz), 8.54 (bs, 1H), 9.71 (s, 1H, J =1.6 Hz).
1
5e: HNMR (CDCl3) δ ppm: 7.66 (d, 2H, J =8.6 Hz), 7.83 (d, 2H, J =8.6 Hz), 8.24 (dd, 1H,
J =2.6 and 1.5 Hz), 8.39 (d, 1H, J =2.6 Hz), 8.65 (br s, 1H), 9.68 (s, 1H, J =1.5 Hz).
1
9. The structures of 6 are determined with their spectral properties, especially HNMR

spectral properties, and confirmed by the coincidence of these properties with these of the
authentic samples.
SDBS Web; http://www.asit.go.jp/RIODB/SDBS/(2001.04.25).
1
6a: HNMR (CDCl3) δ ppm: 3.87 (s, 3H), 7.03 (dd, 2H, J =8.0 and 2.0 Hz), 7.09 (t, 1H,
J =7.7 Hz), 7.33 (t, 2H, J =7.7 Hz), 7.84 (d, 2H, J =7.7 Hz), 7.98 (dd, 2H, J =8.0 and 2.0
Hz), 9.42 (br s, 1H).
1
6b: HNMR (Acetone-d6) δ ppm: 2.46 (s, 3H), 7.10 (t, 1H, J =7.2 Hz), 7.24 (dd, 2H, J =8.0
and 2.0 Hz), 7.33 (t, 2H, J =7.2 Hz), 7.86 (d, 2H, J =7.2 Hz), 8.03 (dd, 2H, J =8.0 and
2.0 Hz), 9.46 (br s, 1H).
1
6c: HNMR (CDCl3) δ ppm: 7.12 (t, 1H, J =8.0 Hz), 7.37 (t, 2H, J =8.0 Hz), 7.54 (dd, 2H,
J =6.8 and 2.0 Hz), 7.58 (t, 1H, J =6.8 Hz), 7.87 (d, 2H, J =8.0 Hz), 8.00 (dd, 2H, J =6.8
and 2.0 Hz), 9.52 (br s, 1H).
6d: 1HNMR (Acetone-d6) δ ppm: 7.13 (t, 1H, J =7.7 Hz), 7.36 (t, 2H, J =7.7 Hz), 7.56 (dd,
2H, J =6.6 and 1.9 Hz), 7.84 (d, 2H, J =7.7 Hz), 8.02 (dd, 2H, J =6.6 and 1.9 Hz), 9.62
(br s, 1H).
6e: MS m/z (rel intensity): 276.8 (70, M+), 274.7 (71), 184.9 (99), 182.9 (100). 1HNMR
(Acetone-d6) δ ppm: 7.13 (t, 1H, J =7.7 Hz), 7.36 (t, 2H, J =7.7 Hz), 7.72 (dd, 2H,
J =6.6 and 2.0 Hz), 7.84 (d, 2H, J =7.7 Hz), 8.65 (dd, 2H, J =6.6 and 2.0 Hz), 9.68 (br s,
1H). IR (KBr): 3341, 1659, 1532, 1258 cm-1.
1
10.The structures of 7 were determined with their spectral properties, especially HNMR
spectral properties, and confirmed by the authentic samples synthesized by other
method.
M. Nakamura, K. Komatsu, Y. Gondo, K. Ohta, and Y. Ueda, Chem. Pharm. Bull., 1967,
5851.
1
7aa: HNMR (CDCl3) δ ppm : 3.83 (s, 3H), 3.87 (s, 3H), 6.95 (dd, 4H), 7.21 (d, 2H), 7.86
(d, 2H), 8.42 (s, 1H). 13NMR (CDCl3) δ ppm: 55.4, 55.5, 114.1, 114.3, 122.1, 129.5,
130.2, 145.3, 157.9, 157.9, 162.0.
7ba: 1HNMR (CDCl3) δ ppm : 2.36 (s, 3H), 3.86 (s, 3H), 6.97 (d, 2H), 7.12 (d, 2H), 7.18 (d,
2H), 7.84 (d, 2H), 8.39 (s, 1H). 13CNMR (CDCl3) δ ppm: 21.0, 55.4, 114.1, 120.8,
129.4, 129.7, 130.4, 135.3, 149.7, 159.0, 162.1.
1
7ca: HNMR (CDCl3) δ ppm : 3.87 (s, 3H), 6.98 (d, 2H), 7.19-7.21 (m, 3H), 7.37-7.39 (m,
2H), 7.85 (d, 2H), 8.38 (s, 1H). 13CNMR (CDCl3) δ ppm: 55.4, 114.2, 120.9, 125.6,
129.1, 129.3, 130.5, 152.3, 159.7, 162.2.
7ae: 1HNMR (CDCl3) δ ppm : 3.82 (s, 3H), 6.92 (d, 2H), 7.22 (d, 2H), 7.60 (d, 2H), 7.78 (d,
2H), 8.44 (s, 1H).

11.The formation of 6 is considered to proceed as follows. An electrochemical reduction of
4 forms a radical intermediate (20), which cyclizes to generate an anion intermediate (21)
through an elimination of hydrogen radical. The subsequent ring opening reaction in 21
can afford the final product (6).
+ e-
O
O
O
-H
Ar
H
Ar
H
Ar
H
6
N
N
N
Ph
Ph
Ph
(4)
(20)
(21)
12.A referee suggested the following mechanism.
O
Ph
O
Ar
Ar
N
N
N
7
+
PhNOH
(4)
Ph
Ph
H
NH
Ph
(19)
(18)
13.The benzaldehyde derivative (8) is considered to be formed as follows.
An
electrochemical oxidation of 7 forms a cation radical (22). In the presence of a trace
amount of water, 22 affords a cation radical (23), which then affords 8 and aniline
derivative (3).
H
H
C
-
- e-
N
Ar
MeCN, ClO4
Ar'
H
3 + 8
Ar
N
Ar'
C
7
O
H2O
H
(22)
(23)
M. Masui and H. Ohmori, J . Chem. Soc., Perkin Trans. Ⅱ, 1973, 1112.