Anodic oxidation of ketone allylhydrazones into the corresponding azines

Article abstract of DOI:10.1080/00397910802136656

Several ketone allylhydrazones were electrochemically oxidized in methanol in the presence of sodium methoxide and potassium iodide to afford the corresponding azines. The electro-oxidation involves formation of a new carbon-nitrogen double bond between an allylic carbon atom and the nitrogen atom of a hydrazone to afford a conjugated system. Optimal yields were obtained when 0.5 equivalents of sodium methoxide and a catalytic amount of potassium iodide were used as the supporting electrolyte at room temperature. Presumably, the electro-oxidation involves a two-electron oxidation process where the iodide ion functions as electron carrier. Copyright Taylor & Francis Group, LLC.

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Anodic Oxidation of Ketone
Allylhydrazones into the
Corresponding Azines
Mitsuhiro Okimoto a , Takashi Yoshida a , Masayuki
Hoshi a , Kazuyuki Hattori a , Masashi Komata a
Kenta Tomozawa a & Tomohito Chiba a
a Department of Applied and Environmental
,
Chemistry, Kitami Institute of Technology, Kitami,
Hokkaido, Japan
Version of record first published: 12 Sep 2008.
To cite this article: Mitsuhiro Okimoto , Takashi Yoshida , Masayuki Hoshi , Kazuyuki
Hattori , Masashi Komata , Kenta Tomozawa & Tomohito Chiba (2008): Anodic
Oxidation of Ketone Allylhydrazones into the Corresponding Azines, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 38:19, 3320-3328
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Synthetic Communications1, 38: 3320–3328, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802136656
Anodic Oxidation of Ketone Allylhydrazones into
the Corresponding Azines
Mitsuhiro Okimoto, Takashi Yoshida, Masayuki Hoshi,
Kazuyuki Hattori, Masashi Komata, Kenta Tomozawa, and
Tomohito Chiba
Department of Applied and Environmental Chemistry, Kitami Institute of
Technology, Kitami, Hokkaido, Japan
Abstract: Several ketone allylhydrazones were electrochemically oxidized in
methanol in the presence of sodium methoxide and potassium iodide to afford
the corresponding azines. The electro-oxidation involves formation of a new
carbon–nitrogen double bond between an allylic carbon atom and the nitrogen
atom of a hydrazone to afford a conjugated system. Optimal yields were obtained
when 0.5 equivalents of sodium methoxide and a catalytic amount of potassium
iodide were used as the supporting electrolyte at room temperature. Presumably,
the electro-oxidation involves a two-electron oxidation process where the iodide
ion functions as electron carrier.
Keywords: Allylhydrazones, azines, conjugated system, dehydrogenation, electro-
oxidation, ketone hydrazones
Hydrazones are often employed as mediators in organic synthesis, in
which Ag2O[1,2] and HgO[3,4] are frequently used as the oxidant in the oxi-
dations of hydrazones. Unfortunately, such metal-based reagents are
expensive and=or produce environmentally harmful wastes. In contrast,
we have demonstrated that electrochemical methods for organic syn-
thesis, especially those involving nitrogenous compounds such as
amines,[5,6] enamines,[7–9] and hydrazones[10,11] as substrates, offer inex-
pensive, environmentally-friendly alternatives. In particular, we have
Received January 13, 2008.
Address correspondence to Mitsuhiro Okimoto, Department of Applied and
Environmental Chemistry, Kitami Institute of Technology, Koen-cho 165,
Kitami, Hokkaido 090-8507, Japan. E-mail: oki@chem.kitami-it.ac.jp
3320

Oxidation of Ketone Allylhydrazones
3321
reported the highly effective electrochemical technique for the oxidative
conversion of hydrazones to several useful organic compounds.[12–14] In
this study, we have successfully carried out the electro-oxidation of
ketone allylhydrazones 1 to afford the corresponding dehydrogenation
products 2 (azines), as illustrated in Scheme 1. Although a variety of
methods are found in the literature for the formation of azines via hydra-
zones,[10,15–18] to the best of our knowledge, only a few reports have
described the chemical behavior of aldehyde or ketone allylhydrazones;
moreover, studies that describe the oxidation of ketone allylhydrazones,
including those that employ typical chemical oxidizing reagents, have
yet to be reported.[19–22] Consequently, we believe that our approach in
oxidizing ketone allylhydrazones into the corresponding azines using
electrochemical methods is novel.
To begin our study, three probable outcomes for the electro-
oxidation of ketone allylhydrazones 1, as shown in Scheme 1, were
proposed. Subsequent experimental results revealed that the ketone
allylhydrazones were oxidized by forming an unsaturated bond to yield
the corresponding azine 2, with an extended conjugated system within
the entire molecule. Contrary to our expectations, the formation of
alkadiene A and cyclic B, both possessing an azo group, was not
observed.[12] According to our results, the electro-oxidation products gen-
erally possess one carbon–carbon and two carbon–nitrogen double bonds
that are conjugated to each other. Accordingly, the electro-oxidation,
under similar reaction conditions, of acetophenone ethylhydrazone,
which has a structure analogous to acetophenone allylhydrazone (1a),
hardly gave the corresponding dehydrogenation product.
To optimize the reaction conditions for the electro-oxidation of the
allylhydrazones, we investigated the reaction of 1a (as a model substrate)
to yield the corresponding azine, acetophenone allylidenehydrazone (2a).
First, the electro-oxidations were examined in the presence of various
Scheme 1. Probable outcomes for the anodic oxidation of ketone allylhydrazones.

3322
M. Okimoto et al.
supporting electrolytes, as listed in Table 1. When using neutral salts
(runs 1–3), the yields of 2a were less than 10%— in such cases, consider-
able amounts of tar-like material remained after removal of the reaction
solvent, along with acetophenone (as detected by gas chromatography
[GC] analysis). The yield improved markedly by employing basic electro-
lytes such as NaCN (run 4), NaOH (run 5), and NaOMe (run 6). Further-
more, the addition of a catalytic amount of KI in the presence of NaOMe
resulted in the highest yield (88%, run 7). Other halide ion sources such
as KBr (run 8) and KCl (run 9), which can often act as electron carriers,
did not exhibit any advantageous effects on the yields.[23] Our studies also
showed that the amount of the added KI, from 2 mmol (88%) to 10 mmol
(75%), had a small effect on the yields.
Preliminary electro-oxidations revealed that the amount of the charge
passed (0.5, 1.0, and 1.5 F molꢀ1) increased the formation of 2a propor-
tionally (24%, 44%, and 60%, respectively). The maximum yield of
88% was obtained after passage of 2.2 F molꢀ1 (ca. 118 min). Furthermore,
excessive passage (3.0 F molꢀ1) caused a decrease in the yield (73%).
Using the optimal reaction conditions corresponding to the best
yields listed in Table 1, the electro-oxidation of ketone allylhydrazones
1b–h were carried out, and the results are shown in Table 2. During
the course of the electrooxidations, the composition of the reaction mix-
ture was monitored using GC analysis to determine the end point of the
Table 1. Electro-oxidation of acetophenone allylhydrazone 1aa
Runs
Supporting electolyte
Quantity (mmol)
Yields of 2a (%)b
1
2
3
4
5
6
7
8
9
p-TsON(Et)4
NaClO4
KI
NaCN
NaOH
NaOMe
NaOMe þ KI
NaOMe þ KBr
NaOMe þ KCl
5
5
5
5
5
9
4
4
48
63
69
88
31
49
5
5 þ 2
5 þ 2
5 þ 2
aSubstrate 1a: 10 mmol, MeOH: 40 mL, constant current: 0.3 A, current passed:
ꢁ
ꢀ1
2.2 F mol , rt: ca. 15 C.
bDetermined by GC analysis.

Oxidation of Ketone Allylhydrazones
3323
Table 2. Electro-oxidation of several ketone allylhydrazones 1a
Substrate
Current passed
(F molꢀ1
)
Yield of 2
(%)b
Runs
R1
R2
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
Ph
Me
Me
Me
Me
Et
2.2
2.5
2.4
2.4
2.4
2.2
2.3
2.5
78
78
83
77
72
56
61
60
4-MeꢀC6H4ꢀ
4-MeOꢀC6H4ꢀ
4-ClꢀC6H4ꢀ
Ph
ꢀ(CH2)4ꢀ
1g
1h
ꢀ(CH2)5ꢀ
n-C3H7
n-C3H7
aSubstrate 1: 10 mmol, KI: 2 mmol, NaOMe: 5 mmol, MeOH: 40 mL, constant
current: 0.3 A, rt: ca. 15 ꢁC.
bIsolated yield.
reaction. Although the electrolytic currents that were necessary to
consume the allylhydrazones (2.2–2.5 F molꢀ1) were not significantly
different, higher yields of the corresponding azines were observed for
the aromatic (1a–e) than for the aliphatic substrates (1f–h). For these
electro-oxidations, the products were obtained as a yellow viscous oily
liquid that gradually changed into a brownish liquid with increased vis-
cosity upon standing at room temperature within several days. Especially
products obtained from aliphatic substrates were considerably more
unstable than those that possessed an aromatic moiety. It is reasonable
to assume that the newly formed conjugated systems can extend to
include the aromatic moiety and stabilize the structure against decompo-
sition and=or polymerization. Interestingly, the reaction is analogous to
our previously reported dehydrogenation of benzyl-type amines,[6] where
a hydrogen leaves anodically from a benzylic carbon, whereas a hydrogen
leaves from an allylic carbon in this current study.
Although details of the electro-oxidation process remain unclear, a
reasonable reaction pathway for the formation of the conjugated system
in the presence of iodide ion is shown in Scheme 2. Initially, the iodide ion
ꢀ
(I, derived from KI) is oxidized on the anode to form an iodonium ion
(Iþ). The nitrogen atom of substrate 1 would be attacked by the iodo-
nium ion to form ammonium ion C, followed by deprotonation to form

3324
M. Okimoto et al.
Scheme 2. Proposed reaction pathway for the formation of the conjugated
system.
N-iodide D. Immediate elimination of HI from the N-iodide D would give
desired azine 2. The iodonium ion would continue to be reproduced on
the anode. Presumably, the reaction proceeds only via indirect electroox-
idation, in which the iodide ion plays an important role as an electron
carrier.[6,24,25] In such a case, NaOMe may serve as a suitable base to
facilitate both the deprotonation of ammonium ion C and elimination
of HI from N-iodide D. Essentially, substrate 1 loses two protons and
two electrons (two-electron oxidation) during the course of the reaction.
In conclusion, we have demonstrated that the electrochemical tech-
nique can be effectively utilized to oxidize ketone allylhydrazones 1 to
afford the novel conjugated compounds 2. The benefits of this electro-
chemical methodology include (1) absence of any oxidants and=or special
reagents, (2) mild reaction conditions (room temperature), (3) readily
available substrates, and (4) a simple, one-pot procedure.
EXPERIMENTAL
Ketone allylhydrazones were prepared via typical condensation reaction
by refluxing the ketone with slightly excess molar amounts (1.1–1.3 molar
equivalent to ketone used) of allylhydrazine in ethanol. Azines obtained
(2a–h) were characterized by IR, mass, and NMR spectroscopy, as listed
here. Preparative-scale electro-oxidations were carried out in a tall 50-mL
beaker equipped with a fine frit cup (as the cathode compartment), a
nickel coil (as the cathode), and a cylindrical platinum net (55 mesh)
(as the anode). Electro-oxidations of the substrates were carried out as
follows: a solution of the substrate (1a–h, 10 mmol) and KI (0.33 g,
2 mmol) in absolute MeOH (40 mL) containing NaOMe (5 mmol) was
electro-oxidized under a constant current (0.3 A). During the course of
the electro-oxidation, the anolyte was magnetically stirred while the
temperature of the cell was maintained at ca. 15 ꢁC. Composition of the
anolyte was frequently monitored using GC analysis (SE-30: 2.0 m).

Oxidation of Ketone Allylhydrazones
3325
Upon near-complete consumption of the substrate, the DC power supply
was switched off, and the reaction mixture was concentrated in vacuo to
about 15 mL at approximately 40 ꢁC. The residue was subsequently
extracted with n-hexane (6 ꢂ 30 mL). The hexane extracts were combined
and dried over anhydrous magnesium sulfate. After the removal of the
solvent, the residue was purified by silica-gel column chromatography
using a mixture of ether and hexane (1:2) as the elution solvent. The pro-
ducts obtained were stored in a refrigerator.
Data
Acetophenone Allylidenehydrazone 2a
Bp 97–99 ꢁC=2 mmHg. IR (neat): 2964, 1631, 1598, 1576, 1445, 1364,
1
1287, 998, 762, 692 cmꢀ1. H NMR (CDCl3): d ¼ 2.42 (s, 3H, CH3),
=
=
5.7–5.8 (m, 2H, CH2), 6.6–6.8 (m, 1H, CHꢀ), 7.3–7.5 (m, 3H, Arom),
7.8–7.9 (m, 2H, Arom), 8.04 (d, J ¼ 10 Hz, 1H, ꢀN CHꢀ). 13C NMR
=
(CDCl3): d ¼ 15.12, 126.84, 127.08, 128.38, 130.10, 134.74, 138.17,
159.83, 164.15. MS m=z (%): 172 (50) [Mþ], 145 (44) [Mþ-vinyl], 144
(22), 130 (100), 104 (57), 103 (28), 77 (76), 51 (34). HRMS: m=z calcd.
for C11H12N2: 172.1000; found: 172.0997 [Mþ].
4-Methylacetophenone Allylidenehydrazone 2b
Bp 104–106 ꢁC=3 mmHg. IR (neat): 2962, 2921, 1630, 1597, 1362, 1291,
1
1184, 1116, 997, 927 cmꢀ1. H NMR (CDCl3): d ¼ 2.39 (s, 3H, CH3),
=
2.41 (s, 3H, CH3), 5.7–5.8 (m, 2H, ¼ CH2), 6.6–6.8 (m, 1H, CHꢀ),
7.21 (d, J ¼ 8 Hz, 2H, Arom), 7.77 (d, J ¼ 8 Hz, 2H, Arom), 8.05 (d,
J ¼ 10 Hz, 1H, ꢀN CHꢀ). 13C NMR (CDCl3): d ¼ 15.04, 21.37,
=
126.80, 126.90, 129.10, 134.79, 135.38, 140.34, 159.72, 164.20. MS m=z
(%): 186 (49) [Mþ], 159 (24) [Mþ-vinyl], 144 (100), 132 (12), 118 (41),
117 (28), 91 (43), 65 (24). HRMS: m=z calcd. for C12H14N2: 186.1157;
found: 186.1132 [Mþ].
4-Methoxyacetophenone Allylidenehydrazone 2c
Bp 125–127 ꢁC=2 mmHg. IR (neat): 2960, 1628, 1606, 1511, 1363, 1307,
1
1252, 1175, 1030, 833 cmꢀ1. H NMR (CDCl3): d ¼ 2.41 (s, 3H, CH3),
=
=
3.85 (s, 3H, CH3O), 5.7–5.8 (m, 2H, CH2), 6.6–6.8 (m, 1H, CHꢀ),
6.93 (d, J ¼ 9 Hz, 2H, Arom), 7.85 (d, J ¼ 9 Hz, 2H, Arom), 8.06
(d, J ¼ 10 Hz, 1H, ꢀN CHꢀ). 13C NMR (CDCl3): d ¼ 14.88, 55.35,
=
113.71, 126.76, 128.43, 130.73, 134.85, 159.76, 161.30, 163.93. MS m=z

3326
M. Okimoto et al.
(%): 202 (70) [Mþ], 175 (22) [Mþ-vinyl], 160 (100), 148 (27), 134 (42), 133
(34), 119 (11), 92 (18). HRMS: m=z calcd. for C12H14ON2: 202.1106;
found: 202.1117 [Mþ].
4-Chloroacetophenone Allylidenehydrazone 2d
Bp 110–112 ꢁC=2 mmHg. IR (neat): 2926, 1630, 1596, 1489, 1398, 1363,
1
1289, 1094, 1012, 831 cmꢀ1. H NMR (CDCl3): d ¼ 2.41 (s, 3H, CH3),
=
5.7–5.8 (m, 2H, ¼ CH2), 6.6–6.8 (m, 1H, CHꢀ), 7.38 (d, J ¼ 9 Hz, 2H,
=
Arom), 7.81 (d, J ¼ 9 Hz, 2H, Arom), 8.03 (d, J ¼ 10 Hz, 1;H, ꢀN CHꢀ).
13C NMR (CDCl3): d ¼ 14.19, 127.39, 128.16, 128.42, 134.67, 136.17,
136.57, 160.13, 163.04. MS m=z (%): 206 (45) [Mþ], 179 (42) [Mþ-vinyl],
166 (31), 164 (100), 144 (43), 138 (49), 111 (40), 75 (34). HRMS: m=z calcd.
for C11H11N2Cl: 206.0611; found: 206.0600 [Mþ].
Propiophenone Allylidenehydrazone 2e
Bp 95–97 ꢁC=2 mmHg. IR (neat): 2973, 2936, 1629, 1595, 1576, 1446,
1
999, 924, 772, 693 cmꢀ1. H NMR (CDCl3): d ¼ 1.13 (t, J ¼ 8 Hz, 3H,
=
CH3), 2.97 (q, J ¼ 8 Hz, 2H, CH2), 5.7–5.8 (m, 2H, CH2), 6.6–6.8 (m,
=
1H, CHꢀ), 7.4–7.5 (m, 3H, Arom), 7.8–7.9 (m, 2H, Arom), 8.06 (d,
J ¼ 10Hz, 1H, ꢀN CHꢀ). 13C NMR (CDCl3): d ¼ 12.09, 21.99,
=
126.94, 127.09, 128.48, 130.01, 134.81, 136.98, 159.94, 169.22. MS m=z
(%): 186 (51) [Mþ], 159 (28) [Mþ-vinyl], 130 (100), 117 (19), 104 (66),
103 (19), 77 (51), 51 (21). HRMS: m=z calcd. for C12H14N2: 186.1157;
found: 186.1131 [Mþ].
Cyclopentanone Allylidenehydrazone 2f
Bp 98–101 ꢁC=17 mmHg. IR (neat): 2960, 2871, 2827, 1652, 1618, 1418,
1
1201, 1117, 1000, 930 cmꢀ1. H NMR (CD3OD): d ¼ 1.7–1.9 (m, 4H,
2CH2), 2.4–2.6 (m, 4H, 2CH2), 5.8–5.9 (m, 2H, ¼ CH2), 6.5–6.6 (m,
1H, CHꢀ), 7.90 (d, J ¼ 10 Hz, 1H, ꢀN CHꢀ). 13C NMR (CD3OD):
d ¼ 30.25, 30.85, 34.01, 34.38, 128.85, 135.34, 161.50, 183.57. MS m=z
(%): 136 (59) [Mþ], 135 (34), 109 (54) [Mþ-vinyl], 80 (100), 54 (51), 39
(37), 28 (36), 27 (35). HRMS: m=z calcd. for C8H12N2: 136.1000; found:
136.0994 [Mþ].
=
=
Cyclohexanone Allylidenehydrazone 2g
Bp 105–107 ꢁC=15 mmHg. IR (neat): 2933, 2858, 1641, 1610, 1448, 1312,
1
1102, 1059, 1000, 928 cmꢀ1. H NMR (CD3OD): d ¼ 1.6–1.8 (m, 6H,

Oxidation of Ketone Allylhydrazones
3327
3CH2), 2.3–2.4 (m, 2H, CH2), 2.6–2.7 (m, 2H, CH2), 5.8–5.9 (m, 2H,
=
=
=
CH2), 6.5–6.6 (m, 1H, CHꢀ), 7.89 (d, J ¼ 10Hz, 1H, ꢀN CHꢀ).
13C NMR (CD3OD): d ¼ 26.79, 27.76, 28.81, 29.57, 36.49, 128.55,
135.47, 161.47, 175.52. MS m=z (%): 150 (100) [Mþ], 148 (87), 123 (58)
[Mþ-vinyl], 80 (69), 54 (42), 41 (53), 39 (40), 27 (32). HRMS: m=z calcd.
for C9H14N2: 150.1157; found: 150.1147 [Mþ].
4-Heptanone Allylidenehydrazone 2h
Bp 86–88 ꢁC=17 mmHg. IR (neat): 2961, 2933, 2872, 1637, 1610, 1463,
1
1378, 1118, 998, 925 cmꢀ1. H NMR (CD3OD): d ¼ 0.92 (t, J ¼ 7 Hz,
3H, CH3), 0.97 (t, J ¼ 7 Hz, 3H, CH3), 1.51 (six, J ¼ 8 Hz, 2H, CH2),
1.63 (six, J ¼ 8 Hz, 2H, CH2), 2.3–2.4 (m, 2H, CH2), 2.4–2.5 (m, 2H,
=
=
CH2), 5.8–5.9 (m, 2H, CH2), 6.5–6.7 (m, 1H, CHꢀ), 7.86 (d, J ¼ 10 Hz,
1H, ꢀN CHꢀ). 13C NMR (CD3OD): d ¼ 14.26, 14.62, 21.06, 33.80,
=
39.64, 128.23, 135.56, 160.80, 175.51. MS m=z (%): 166 (11) [Mþ], 139
(7) [Mþ-vinyl], 123 (39), 97 (48), 82 (100), 68 (51), 54 (63), 41 (77).
HRMS: m=z calcd. for C10H18N2: 166.1470; found: 166.1457 [Mþ].
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