Anodic oxidation of benzil hydrazones in the presence of halide ions

Article abstract of DOI:10.1246/bcsj.75.2059

Benzil hydrazones were subjected to electrolytic oxidation in MeOH containing halide ion source, such as KI and KBr. The results show that the reaction products were dependent on both the electrolytes and the substituents. In the presence of KI, benzoylphenyldiazomethanes were obtained, whereas in the presence of KBr, benzil dimethyl acetals were obtained.

Full text of DOI:10.1246/bcsj.75.2059

c. Jpn.
Bull. Chem. Soc. Jpn., 75, 2059–2060 (2002) 2059
e Chemical
ciety of Ja-
n
Short Articles
e Chemical
ciety of Ja-
n
Table 2.
Indirect Electrochemical Oxidation of Benzil
Anodic Oxidation of Benzil
Hydrazones in the Presence of
Halide Ions
Hydrazones in the Presence of KBr
M. Okimoto et al.
02
59
60
SJA8
09-2673
2717
Hydrazone
X
Current passed
Product yielda)
%
Mitsuhiro Okimoto* and Yukio Takahashi
F mol−l
4.2
3.8
4.0
4.2
1a
H
7a
7b
7c
7d
77
81
83
75
Department of Applied Chemistry and Circumstance
Engineering, Kitami Institute of Technology,
Koen-cyo Kitami 090-8507
1b
1c
1d
Me
OMe
Cl
a) Isolated yields.
(Received March 20, 2002)
02
02
Benzil hydrazones were subjected to electrolytic oxida-
tion in MeOH containing a halide ion source, such as KI and
KBr. The results show that the reaction products were depen-
dent on both the electrolytes and the substituents. In the pres-
ence of KI, benzoylphenyldiazomethanes were obtained,
whereas in the presence of KBr, benzil dimethyl acetals were
obtained.
The anodic oxidation of nitrogen-containing organic com-
pounds, such as hydrazones, can be widely used as a clean syn-
thetic method.1 To date, the anodic oxidation of several hydra-
zones has been examined, including the direct electrolytic oxi-
dation of benzophenone hydrazone to afford diphenyldiaz-
omethane,2 and the electrolytic oxidation of acylhydrazones of
aliphatic ketones and aliphatic aldehydes to afford nitriles3 and
oxadiazoles,4 respectively. In the present work, we found that
the indirect electrochemical oxidation of benzil hydrazone 1a
and 4,4ꢀ-dichlorobenzil hydrazone 1d using KI as a halide ion
source afforded corresponding diazo compounds, 2a5 and 2d,5
in good yield, respectively (Table 1). However, we were not
able to isolate the corresponding diazo compounds from 4,4ꢀ-
dimethylbenzil hydrazone 1b and 4,4ꢀ-dimethoxybenzil hydra-
zone 1c using KI under the same electrolytic conditions. In
these cases, mixtures of the corresponding 5 and 7 were
formed. Since continual evolution of nitrogen gas from the
anolyte was observed during these electrolyses, it can be rea-
soned that both Wolff rearrangement6 and further oxidation of
the corresponding diazo compounds occurred simultaneously.
Scheme 1.
In contrast, benzil dimethyl acetals 7a–d were obtained as the
result of replacing the electrolyte from KI to KBr under the
same electrolytic conditions (Table 2). Electrolysis in the pres-
ence of KBr, differing from KI, required approximately a two-
fold increase in the amount of electricity for the reactions to go
to completion. Interestingly, 7a7 was obtained in 80% yield
through an alternate route by means of the anodic oxidation of
2a using KBr. It can be suggested that 7a–d were additionally
oxidized products of the corresponding diazo compounds. Al-
so, when using NaI or NaBr instead of KI or KBr as the halide
ion source 2a, 2d, and 7a–b were obtained in comparable
yields, respectively. However, direct oxidation in the absence
of a halide ion source resulted in an almost complete recovery
of the starting material 1a–d. Moreover, as shown in Scheme
1, attempted to one-pot conversions of 1a to diphenylketene
3a, diphenylacetic acid 4a, methyl diphenylacetate 5a, and
diphenylacetamide 6a were carried out. In conclusion, we car-
ried out the electrochemical preparation of diazo compounds
and benzil dimethyl acetals from benzil hydrazones using KI
and KBr as electrolytes under mild conditions, respectively,
without the use of a metal oxidant.8–9
Table 1.
Indirect Electrochemical Oxidation of Benzil
Hydrazones in the Presence of KI
Hydrazone
X
Current passed
Product yielda)
%
Experimental
F mol−l
2.0
2.2
Hydrazones were prepared from the corresponding benzils ac-
cording to reported methods.10 The electrolyses were carried out
at a constant current of 0.3A in a 50 mL beaker cell, which was
separated from the cathode compartment by a glass filter. The an-
1a
1d
H
Cl
2a
2d
76
90
a) Isolated yields.

2060 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
© 2002 The Chemical Society of Japan
ode was a cylindrical platinum gauge (diameter, 33 mm; height,
40 mm) and the cathode was a nickel wire coil. The cell was
cooled to 15 °C, and the anolyte solution was magnetically stirred.
Typical procedures were as follows. Procedure A (Table 1): A
mixture of 1a (1.79 g, 8 mmol), KI (0.66 g, 4 mmol), and NaOAc
(0.33 g, 4 mmol) was placed in the cell, and MeOH (40 mL) was
added. 2.0 F/mol of electricity (0.3 A, 1.43 h) was passed through
the reaction mixture. After concentration of the anolytes in vacuo
at room temperature, the resulting precipitates were collected by
filtration, washed with cold MeOH, and dried to afford 2a (1.35 g,
76%). Procedure B (Table 2): A mixture of 1a (1.79 g, 8 mmol),
KBr (0.48 g, 4 mmol), and NaOAc (0.33 g, 4 mmol) was placed in
the cell, and MeOH (40 mL) was added. 4.2 F mol−1 of electricity
(0.3 A, 3.0 h) was passed through the reaction mixture. After
evaporation of MeOH, the residue was treated with water (30 mL),
and an oily layer was extracted with ether and dried. The residue
obtained after the removal of ether was purified by distillation
(169–170 °C/7 Torr) to afford 7a (1.58 g, 77%). One-pot conver-
sions of 1a into 3a–6a were achieved using a five-fold increase in
the amount of anolyte obtained by Procedure A as follows
(Scheme 1). 3a: After removing MeOH in vacuo at room temper-
ature, benzene (50 mL) was added, and the reaction mixture was
refluxed for 0.5 h. After removal of benzene, distillation in vacuo
afford 3a (4.66 g, 60%). 4a: After removing MeOH in vacuo at
room temperature, a mixture of NaOH (5 g) and water (40 mL)
was added, and the reaction mixture was refluxed for 0.5 h. The
reaction mixture was washed with ether, then acidified with conc.
HCl. The resulting precipitates were collected by filtration,
washed with water, and dried to afford 4a (5.15 g, 65%). 5a: After
concentrating the anolyte to one-fifth of its original volume, the
residual solution was refluxed for 2 h. To the solution, water (30
mL) was added, followed by extraction with ether. The residue
obtained after the removal of ether was recrystallized from MeOH
to afford 5a (5.42 g, 60%). 6a: After removing MeOH in vacuo at
room temperature, conc. NH3 solution (80 mL) was added, then
refluxed for 2 h. The precipitates were collected by filtration, then
washed with a small amount of water and ether to afford 6a (4.81
g, 57%). The melting and boiling points of the product were in
agreement with those reported in the literature.11
(C), 134.18 (C), 138.65 (C), 143.54 (C), 194.73 (CO). MS m/z 253
(M − OMe)+, 166, 165, 119, 91. HRMS m/z found: 253.1239
(M − OMe)+, calcd for C17H17O2: 253.1229.
4,4ꢀ-Dimethoxybenzil Dimethyl Acetal (7c):
bp 194–196
°C/3 mmHg. IR (KBr): 1124, 1173, 1252, 1510, 1601, 1692 cm−1
.
1H NMR (CDCl3) δ 3.20 (6H, s, MeO × 2), 3.72 (6H, s, MeO ×
2), 6.7–7.0 (4H, m, Ar), 7.53, 8.18 (4H, dd, J = 9 Hz, Ar). 13C
NMR (CDCl3) δ 49.84 (MeO), 55.10 (MeO), 55.22 (MeO),
103.59 (C), 113.37 (CH), 113.86 (CH), 127.21 (C), 128.23 (CH),
129.33 (C), 132.42 (CH), 159.95 (C), 163.21 (C), 193.51 (CO).
MS m/z 285 (M − OMe)+, 182, 181, 135, 77. HRMS m/z found:
285.1109 (M − OMe)+, calcd for C17H17O4: 285.1127.
4,4ꢀ-Dichlorobenzil Dimethyl Acetal (7d): bp 175–176 °C/3
1
mmHg. IR (KBr): 1015, 1070, 1094, 1126, 1587, 1701 cm−1. H
NMR (CDCl3) δ 3.22 (6H, s, MeO × 2), 7.29, 8.00 (4H, dd, J = 9
Hz, Ar), 7.32, 7.54 (4H, dd, J = 9 Hz, Ar). 13C NMR (CDCl3) δ
50.21 (OMe), 103.35 (C), 121.00 (C), 128.43 (CH), 128.68 (CH),
129.00 (CH), 131.49 (CH), 132.46 (C), 135.36 (C), 139.67 (C),
190.67 (CO). MS m/z 293 (M − OMe)+, 187, 185, 139. HRMS
m/z found: 293.0140 (M − OMe)+, calcd for C15H11Cl2O2:
293.0136.
References
1
For a review, see: a) R. F. Nelson, “Techniques of Electro-
organic Synthesis,” N. L. Weinberg, Wiley, New York (1974), Part
1, Chap. 5. b) S. Torii, “Electroorganic Synthesis,” Kodansha,
Tokyo (1985), Part 1.
2
T. Chiba, M. Okimoto, H. Nagai, and Y. Takata, J. Org.
Chem., 48, 2968 (1983).
3
M. Okimoto and T. Chiba, J. Org. Chem., 55, 1070 (1990).
4
5
T. Chiba and M. Okimoto, J. Org. Chem., 57, 1375 (1992).
T. Ibata and G. S. Singh, Tetrahedron Lett., 35, 2581
(1994).
6
O. P. Strausz and G. Frater, J. Am. Chem. Soc., 92, 6654
(1970).
7
8
R. Kuhn and H. Trischmaun, Chem. Ber., 94, 2258 (1961).
C. D. Nenitzescu and E. Solomonica, Org. Synth., Coll.
Vol. ꢁ, 496 (1943).
4,4ꢀ-Dimethylbenzil Dimethyl Acetal (7b): bp175–176°C/3
9
L. I. Smith and K. L. Howard, Org. Synth., Coll. Vol. ꢂ,
1
mmHg. IR (KBr): 1069, 1113, 1124, 1180, 1609, 1697 cm−1. H
351 (1955).
NMR(CDCl3) δ 2.27 (6H, s, Me × 2), 3.20 (6H, s, MeO × 2),
7.0–7.3 (4H, m, Ar), 7.50, 8.00 (4H, d, d, J = 8 Hz, Ar). 13C NMR
(CDCl3) δ 21.13 (Me), 21.54 (Me), 49.92 (MeO), 103.72 (C),
126.89 (CH), 128.84 (CH), 129.21 (CH), 130.18 (CH), 131.85
10 M. N. Gudi and M. V. George, Indian J. Chem., 10, 881
(1972).
11 J. R. A. Pollack, and R. Stevens, “Dictionary of Organic
Compounds,” 4th ed, Great Britain (1965).