Electrooxidative conversion of aldehyde and ketone phenylhydrazones into the methoxy(phenylazo) alkanes

Article abstract of DOI:10.1246/bcsj.76.1447

Several aldehyde and ketone phenylhydrazones were converted into the corresponding methoxy(phenylazo)alkane derivatives by electrochemical oxidation in MeOH.

Full text of DOI:10.1246/bcsj.76.1447

Short Articles
Bull. Chem. Soc. Jpn., 76, 1447–1448 (2003) 1447
Table 1. Electrooxidation of Aldehyde and Ketone Phenyl-
hydrazonesaÞ
Electrooxidative Conversion of Al-
dehyde and Ketone Phenylhydra-
zones into the Methoxy(phenylazo)
alkanes
Current
passed
YieldcÞ of 2
Supporting
electrolytebÞ
Compound R1
R2
F molꢁ1
%
ꢀ
and Yukio Takahashi
1a
1b
1c
1d
1e
1f
Me
n-Pr
Me Me
i-Bu Me
n-Pr n-Pr
-(CH2)5-
H
H
TsONEt4
2.4
3.2
2.1
2.1
2.0
2.0
2.1
3.2
(52)
(44)
67
74
72
75
70
60
ꢂ
Mitsuhiro Okimoto, Yuji Nagata,
TsONEt4
KI
KI
Department of Applied Chemistry and Circumstance
Engineering, Kitami Institute of Technology,
Koen-cyo 165, Kitami 090-8507
KI
KI
KI
KI
1g
1h
-(CH2)7-
-(CH2)8-
(Received January 9, 2003)
a) 1: 10 mmol, MeOH: 80 mL, reaction temp: ca. 15 C. b)
TsONEt4: 6 mmol, KI: 10 mmol. c) Isolated yield. Values in
parenthesis were determined by GLC analysis.
Several aldehyde and ketone phenylhydrazones were
converted into the corresponding methoxy(phenylazo)alkane
derivatives by electrochemical oxidation in MeOH.
2c–2h, respectively, in 60–75% isolated yields, after 2.0–3.2 F
of electricity had been passed. As a note, in the case of the
aldehyde phenylhydrazones, significant amounts of a tar-like
material was observed during ether extraction steps. Although
details of the reaction mechanism of the electrooxidation are
not clear, it can be assumed that, in the case of oxidation of
ketone phenylhydrazones, the iodide ions serve as the electron
carriers. In fact, the phenylazo compounds were hardly pre-
parable when using NaOMe, NaCN, NaOAc, or NaClO4 as the
electrolyte instead of iodide ion from KI or NaI. In conclu-
sion, we have found that methoxy(phenylazo)alkanes can be
obtained in reasonable to good yields by the electrooxidation
of phenylhydrazones in MeOH using p-TsONEt4 or KI as the
electrolyte without the use of special oxidizing agents under
very mild conditions.
The oxidative conversions of aldehyde and ketone phenyl-
hydrazones into the corresponding alkoxy(phenylazo)alkanes
or acetoxy(phenylazo)alkanes have been reported in several
studies. Investigations on the oxidation of acetone phenylhy-
drazones to the alkoxy(phenylazo) derivatives have been re-
ported by Schantl,1 using iodine as an oxidant in various alco-
hols with yields of 23–50%, and by Chiusoli and co-workers,2
using a palladium catalyst, thiourea, and a stream of carbon
monoxide, with yields of less than 20%. Harrison and co-
workers3 have reported on the oxidation of benzophenone
phenylhydrazones in dichloromethane, using lead tetraacetate
as the oxidant. However, this application was limited to only
benzophenone phenylhydrazones. Based on previous studies
on the electrooxidation of various hydrazones,4{9 we carried
out our investigations on the oxidation of several aldehyde and
ketone phenylhydrazones by means of an electrochemical
method.
Our investigations on the electrooxidation of phenylhydra-
zones began with the determination of the optimal reaction
conditions. Consequently, for the ketone phenylhydrazones
(Table 1, 1c–1h), the use of an equimolar amount of KI as
the electrolyte was favorable in affording the corresponding
methoxy(phenylazo) compounds (2c–2h). In contrast, for the
aldehyde phenylhydrazones (1a, 1b), the corresponding meth-
oxy(phenylazo) compounds were not obtained when using KI
as the electrolyte. Subsequently, it was found that the corre-
sponding phenylazo products can be produced from aldehyde
phenylhydrazones in nearly 50% yields by using the tetraeth-
ylammonium salt of p-toluenesulfonic acid (p-TsONEt4) as
the electrolyte, and also by employing carbon poles instead
of a platinum net as the anode. As shown in Table 1, aldehyde
phenylhydrazones 1a and 1b afforded methoxy(phenylazo)
compounds 2a and 2b in 52% and 44% yields, respectively,
whereas ketone phenylhydrazones 1c–1h afforded compounds
Experimental
All electrooxidation products were identified from their physi-
cal and spectral data. Substrates were synthesized through typical
condensation reactions10 using equimolar amounts of phenylhy-
drazine and the corresponding carbonyl compounds. The subse-
quent electrooxidations were carried out immediately, since all
phenylhydrazones prepared in this study were relatively unstable,
gradually giving a dark tar-like material within a few weeks even
under storage in a refrigerator. Other reagents were obtained from
commercial suppliers and used without further purifications. Pre-
parative-scale electrooxidations were carried out in a tall 100-mL
beaker, equipped with a fine frit cup as the cathode compartment,
with either four pieces of carbon poles (diameter, 5 mm; height,
80 mm) for electrooxidation of 1a and 1b, or a cylindrical plati-
num net (diameter, 33 mm; height, 40 mm) for electrooxidation of
1c–1h used as the anode, and a nickel coil cathode. Aldehyde
phenylhydrazones 1a and 1b and ketone phenylhydrazones 1c–1h
were oxidized under conditions as follows: a solution of substrate
1a–1h (10 mmol) and p-TsONEt4 (6 mmol) for 1a and 1b or KI
(10 mmol) for 1c–1h in MeOH (80 mL) was electrooxidized under
a constant current (0.3 A). During the course of the electrooxida-
tion, the anolyte was magnetically stirred while maintaining the
Published on the web July 15, 2003; DOI 10.1246/bcsj.76.1447

1448 Bull. Chem. Soc. Jpn., 76, No. 7 (2003)
Ó 2003 The Chemical Society of Japan
ꢂ
temperature of the cell at approximately 15 C. After completion
129.1327 (M ꢁ PhN2)þ, calcd for C8H17O: 129.1279. CIMS
m=z 235 (M + 1)þ.
of the electrooxidation, the reaction mixture was treated with a
threefold amount of the anolyte solution. After concentrating the
combined anolyte solution in vacuo at approximately 30 ꢂC to
roughly one-fifth of its original volume, the resulting residue
was treated with water (50 mL), and the oily layer was extracted
with ether (3 ꢃ 40 mL). The combined ether extracts were wash-
ed with aqueous sodium thiosulfate solution (20% w/w, 30 mL),
and dried over anhydrous magnesium sulfate. After removal of
the solvent, the residue was purified by distillation under reduced
pressures or by silicagel column chromatography (diameter, 3.5
cm; height, 30 cm) with ether as the eluent.
1-Methoxy-1-phenylazocyclohexane (2f): bp 110–112 ꢂC/
1.4 hPa. IR (neat): 691, 766, 1088, 1452, 2858, 2936 cmꢁ1
.
1H NMR (CDCl3) ꢀ 1.3–2.0 (10H, m, CH2 ꢃ 5), 3.45 (3H, s,
MeO), 7.3–7.8 (5H, m, Ar). 13C NMR (CDCl3) ꢀ 22.19 (CH2),
25.41 (CH2), 32.01 (CH2), 49.92 (MeO), 98.71 (C), 122.33 (CH),
128.96 (CH), 130.63 (CH), 152.05 (C). MS m=z (rel intensity %)
113 ((M ꢁ PhN2)þ, 100), 81 (78), 77 (31), 44 (32), 39 (18).
HRMS m=z found: 113.1020 (M ꢁ PhN2)þ, calcd for C7H13O:
113.0966. CIMS m=z 219 (M + 1)þ.
1-Methoxy-1-phenylazocyclooctane (2g): bp 132–134 ꢂC/
1.4 hPa. IR (neat): 691, 766, 1072, 1457, 2851, 2926 cmꢁ1
.
1-Methoxy-1-phenylazoethane (2a): bp 80–82 ꢂC/1.8 hPa.
IR (neat): 692, 768, 1132, 1607, 2934, 2988 cmꢁ1
.
1H NMR
1H NMR (CDCl3) ꢀ 1.3–2.1 (14H, m, CH2 ꢃ 7), 3.45 (3H, s,
MeO), 7.3–7.8 (5H, m, Ar). 13C NMR (CDCl3) ꢀ 21.01 (CH2),
24.43 (CH2), 28.10 (CH2), 30.58 (CH2), 50.25 (MeO), 101.03 (C),
122.24 (CH), 128.92 (CH), 130.47 (CH), 151.97 (C). MS m=z (rel
intensity %) 141 ((M ꢁ PhN2)þ, 100), 109 (22), 77 (34), 67 (48),
44 (20), 39 (24). HRMS m=z found: 141.1343 (M ꢁ PhN2)þ,
calcd for C9H17O: 141.1279. CIMS m=z 247 (M + 1)þ.
(CDCl3) ꢀ 1.44 (3H, d, J ¼ 6 Hz, Me), 3.43 (3H, s, MeO), 4.64
(1H, t, J ¼ 6 Hz, CH), 7.3–7.8 (5H, m, Ar). 13C NMR (CDCl3) ꢀ
19.22 (Me), 56.52 (MeO), 101.56 (CH), 122.57 (CH), 129.04
(CH), 131.12 (CH), 151.52 (C). MS m=z (rel intensity %) 164
(Mþ, 2), 77 (21), 58 ((M ꢁ PhN2)þ, 100), 50 (16), 26 (16).
HRMS m=z found: 164.1002 Mþ, calcd for C9H12N2O: 164.0950.
ꢂ
ꢂ
1-Methoxy-1-phenylazobutane (2b): bp 76–78 C/1.8 hPa.
1H NMR
1-Methoxy-1-phenylazocyclononane (2h): bp 156–158 C/
IR (neat): 691, 768, 1121, 1456, 2874, 2961 cmꢁ1
.
1.8 hPa. IR (neat): 691, 764, 1094, 1450, 2850, 2927 cmꢁ1
.
(CDCl3) ꢀ 0.94 (3H, t, J ¼ 7 Hz, Me), 1.2–2.0 (4H, m, CH2
ꢃ
1H NMR (CDCl3) ꢀ 1.3–2.2 (16H, m, CH2 ꢃ 8), 3.36 (3H, s,
MeO), 7.3–7.9 (5H, m, Ar). 13C NMR (CDCl3) ꢀ 18.94 (CH2),
22.44 (CH2), 27.57 (CH2), 28.59 (CH2), 50.53 (MeO), 102.17 (C),
122.28 (CH), 128.92 (CH), 130.51 (CH), 151.97 (C). MS m=z (rel
intensity %) 155 ((M ꢁ PhN2)þ, 100), 81 (60), 77 (34), 67 (27),
55 (26), 41 (24). HRMS m=z found: 155.1389 (M ꢁ PhN2)þ,
calcd for C10H19O: 155.1436. CIMS m=z 261 (M + 1)þ.
2), 3.40 (3H, s, MeO), 4.51 (1H, t, J ¼ 7 Hz, CH), 7.4–7.9 (5H, m,
Ar). 13C NMR (CDCl3) ꢀ 13.97 (Me), 17.59 (CH2), 35.67 (CH2),
56.72 (MeO), 105.02 (CH), 122.61 (CH), 129.49 (CH), 131.04
(CH), 151.60 (C). MS m=z (rel intensity %) 192 (Mþ, 3), 87
((M ꢁ PhN2)þ, 60), 77 (27), 54 (16), 44 (100). HRMS m=z found:
192.1327 Mþ, calcd for C11H16N2O: 192.1263.
2-Methoxy-4-methyl-2-phenylazopentane (2d): bp 88–90
ꢂC/1.4 hPa. IR (neat): 691, 764, 1090, 1454, 2870, 2955 cmꢁ1
.
1H NMR (CDCl3) ꢀ 0.90 (3H, d, J ¼ 6 Hz, Me), 0.94 (3H, d, J ¼
6 Hz, Me), 1.40 (3H, s, Me), 1.6–2.2 (3H, m, CHCH2), 3.48 (3H,
s, MeO), 7.3–7.8 (5H, m, Ar). 13C NMR (CDCl3) ꢀ 20.93 (Me),
23.82 (Me), 24.19 (Me), 24.60 (CH), 46.83 (CH2), 50.58 (MeO),
100.42 (C), 122.37 (CH), 128.96 (CH), 130.63 (CH), 151.93 (C).
MS m=z (rel intensity %) 115 ((M ꢁ PhN2)þ, 100), 77 (32), 73
(50), 58 (57), 41 (27), 39 (23). HRMS m=z found: 115.1208 (M ꢁ
PhN2)þ, calcd for C7H15O: 115.1123. CIMS m=z 221 (M + 1)þ.
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5
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9
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