Electrochemical oxidations of 1,3'dihydro-1,3-diazaazulanones and effects of reaction temperature

Article abstract of DOI:10.3987/COM-02-S(M)19

Electrochemical oxidations of 1-tosyl-3-aryl-1,3-dihydro-1,3'diazaazulanones at 0°C afforded 3-aryl-6-tosyl-1,3-dihydro-1,3-diazaazulanones via migrations of the tosyl group. However, the same type of electrochemical oxidations at an elevated temperature

Full text of DOI:10.3987/COM-02-S(M)19

HETEROCYCLES, Vol. 58, 2002, pp. 63-70, Received, 7th March, 2002
ELECTROCHEMICAL OXIDATIONS OF 1,3-DIHYDRO-1,3-DIAZA-
AZULANONES AND EFFECTS OF REACTION TEMPERATURE
Katsuhiro Saito,* Yosuke Ueda, Ayako Kawamura, Akiyoshi Kajita,
Hiroyuki Ishiguro, Takayasu Ido, Katsuhiko Ono, and Yasutaka Awadu
Department of Applied Chemistry, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, 466-8555, Japan
Abstract ― Electrochemical oxidations of 1-tosyl-3-aryl-1,3-dihydro-
1,3-diazaazulanones at 0 ℃ afforded 3-aryl-6-tosyl-1,3-dihydro-1,3-
diazaazulanones via migrations of the tosyl group. However, the same
type of electrochemical oxidations at an elevated temperature (25 ℃)
gave another type of product, i.e., double bond isomers,
1-tosyl-3-aryl-1,3-dihydro-1,3-diazaazulanones
via
1,5-hydrogen
migration,
together
with
3-aryl-6-tosyl-1,3-dihydro-1,3-diaza-
azulanones.
2
On account of the sp hybridization of two nitrogen atoms and a carbonyl group in the
five-membered ring moiety, the 1,2,3,3a-tetrahydrocyclohepta[d]imidazol-2-one
(1,3-dihydro-1,3-diazaazulanone) system (1) is considered to have a 12π-electrons
1
electron-poor cross conjugated system. This system (1) also contains a cycloheptatriene
(tropilidene) system, which is known to form a well stabilized 6π-electrons aromatic cation
2
system, tropylium ion, leaving a suitable anion group. These facts suggest that the system
(1) has an ability to generate both cation and anion forms under the appropriate reaction
conditions.
As a precursor of a member of nonbenzenoid aromatic compounds, the thermochemical
behaviors of 1 have been researched extensively, but the number of the documents on the
3
electrochemistry of 1 seems to be small.
4
As a series of our research on the reactivities of 1, we investigated the reactivities of 1
under the electrochemical conditions. Here the results will be discussed.
Figure 1 demonstrates the cyclic voltammograms of 1-tosyl-3-p-substituted phenyl-1,3-
dihydro-1,3-diazaazulanones (1a-d), and Table 1 shows the redox potentials of 1. The
oxidation potentials of all of 1 are almost the same value, suggesting that the substituents
on the phenyl group have almost no influence on the oxidation potentials.

350
300
250
200
150
100
50
1a
1b
1c
1d
Table 1. Redox potentials of 1.
Ar
1a
1b
1c
Oxidation
1.24
Reduction
-0.37
1.23
-0.37
0
1.19
-0.39
-50
1d
1.21
-0.38
-100
-150
V vs. Ag/AgCl
-2
-1
0
1
2
Potential, E/V vs.Ag/AgCl(0.1M KCl)
Temperature：25℃
Scan Rate: 0.2V/s
Figure 1. Cyclic voltammograms of 1.
Ar
Ar
electrolysis
N
N
O
O
N
N
H
Ts
1
2
a : Ar=p-BrC6H4 b : Ar=p-ClC6H4
c : Ar=p-MeC6H4 d : Ar=p-OMeC6H4
Previously we have reported that the electrochemical reduction of 1 afforded
5
3-aryl-1,3-dihydro-1,3-diazaazulanone derivatives (2).
electrochemical oxidation of 1.
This time we examined the
The electrochemical oxidations of 1 were influenced by both the substituents on the phenyl
groups and the reaction temperatures. Preparative controlled-potential electrolysis of 1a
was performed in anhydrous acetonitrile containing tetra-n-butylammonium perchlorate
(TBAP) as a supporting electrolyte. A platinum gauze was employed as an anode, and a
platinum wire was employed as a cathode. Controlled-potential electrolysis was carried out
at + 1.3 V vs. Ag/AgCl in a divided cell under a nitrogen stream at 0℃. After evaporation of
the solvent, the reaction mixture was chromatographed on silica gel column to give the
products (3). As mentioned previously, the yields of 3a, 3b, 3c, and 3d were 20, 42, 50, and
6
37%, respectively.
It was found that the yield of 3a was improved by the reaction temperatures. Thus, the
yield of 3a increased to 37 % at 25 ℃, and further to 79 % at 50 ℃. However, at 70 ℃, the
yield slightly decreased to 64 %.

Ar
N
Ar
N
Ar
N
4
8
4
5
7
5
7
H
electrolysis
+1.3V
6
+
O
O
O
Ts
6
H
N
N
N
H
8
Ts
Ts
CH CN-TBAP
3
3
1
4
a : Ar= p-BrC H , b : Ar= p-ClC H
6 4
6
4
c : Ar= p-MeC H d : Ar= p-OMeC H
6
4,
6 4
Another remarkable feature of this reaction is that the new products (4) were formed at only
“medium” temperatures, i.e., 25 and 50 ℃. Thus the reaction of 1a at 25 ℃ afforded a
double bond isomer (4a) of 1a in 8 % yield together with 37 % yield of 3a. At higher
temperature, 4a was not obtained and only 3a was formed as an isolatable product. As
shown in Table 2, this aspect is observed in the reactions of all types of 1.
Table 2. Effect of temperature on the product yields.
Yields (%)
4*
Starting
Temp (℃)
materials
3*
Recovery of 1
0
20
37
79
64
0
8
0
0
56
19
9
25
50
70
1a
1b
0
0
42
51
37
0
9
7
59
26
5
25
50
0
50
32
24
35
37
41
0
30
0
67
35
18
15
20
3
25
50
70
25
50
1c
0
12
15
1d
* Product yields were calculated except the recovery of 1
Ar
N
electrolysis
Ts
O
Recovery of 3
N
H
3
Ar
N
Ar
N
electrolysis
H
H
Ts
O
O
N
H
N
Ts
3
4

The control experiments made it clear that 3 could be formed from 4, but the inverse was not
true. The electrochemical oxidations of 3 under the same reaction conditions as these of 1
resulted in the recoveries of the starting material (3). On the other hand, the oxidation of
4d afforded 3d in 14% yield.
7
The structure of 4 was determined using an X-Ray crystallographic analysis (Figure 2).
Figure 2. ORTEP drawing of 4.
In order to investigate the possible influence of the temperature on the oxidation potential of
1, the cyclic voltammetry of 1a was measured at 0 and 25 ℃. As clearly demonstrated in
Figures 3 and 4, no influence was detected on the oxidation potential of 1a.
300
100
1a (0℃)
Eox = 1.20
250
200
150
100
50
1a (25℃)
80
60
40
Eox = 1.19
20
0
0
-20
-40
-50
0
0.5
1
1.5
2
2.5
0
0.5
1
1.5
2
Potential, E/V vs. Ag/AgCl(0.1M KCl)
Scan Rate : 0.2 V/s
Potential, E/V vs. Ag/AgCl(0.1M KCl)
Scan Rate : 0.2 V/s
Figure 3.
Figure 4.
Cyclic voltammograms of 1a at 0 ℃.
Cyclic voltammograms of 1a at 25 ℃.

These facts suggest that the temperature responsibility of the reaction is not the result of
the electrochemical redox stage. The species generated by initial electron transfer from 1
should be always the same regardless of reaction temperature. The temperature should
influence on the chemical process, which followed the electrochemical step.
Ar
N
Ar
N
Ar
N
-e
O
O
O
N
N
N
H
H
H
9
O S O
O S O
S
O
O
Tol
8
Tol
Tol
1
+
-H
Ar
N
Ar
N
O
O
N
N
H
H
S
O
O
O S O
Tol
11
10
Tol
+e
Ar
+e
Ar
N
H
H
N
O
N
Ts
O
N
H
Ts
3
4
The electrochemical oxidation of 1 is considered to proceed as follows. As shown in Table 1,
the oxidation potential of 1 is independent of the substituents on the phenyl groups. This
fact suggests that the removal of the electron by the electrochemical oxidation process
should not occur at the conjugated framework containing the phenyl group.
As a consequence, the electron on the tosyl group is considered to be removed to form a
cation radical intermediate (8). The subsequent elimination of a tosyl cation generates a
radical intermediate (9), which then affords a full conjugated cyclic radical intermediate (10)
via a hydrogen migration. A recombination of the tosyl cation with 10 followed by an
appropriate electron abstraction from the solvent or the cathode can afford the final product
(3).

Ar
N
-0.138
Ar
N
1.274
-0.022
-0.067
-0.168
1.045
1.078
1.181
-0.151
1.238
O
O
N
H
N
H
-0.040
0.927
-0.085
1.035
Electron densities of 10
Free valences of 10
Figure 5.
Figure 5 demonstrates free valances and electron densities on the carbon atoms of 10. The
largest free valence (1.238) and the second largest electron density (-0.151) at the 6-position
explain the position of the recombination of 10 and the tosyl cation.
The formation of 4 from 1 is also considered to proceed through the same cation radical
intermediate (8) as the case of 3. A 1,5-hydrogen migration via a full conjugated cyclic
radical intermediate (11) can form the final product (4).
Ar
N
Ar
N
Ar
N
-e
H
H
H
H
H
H
O
O
O
N
N
N
Ts
Ts
S
13
4
O
O
12
Tol
Ar
N
Ar
N
1,5 H Shift
Ts
O
O
N
H
N
H
S
O
O
Tol
3
10
The electrochemical oxidation of 4 is thought to proceed as follows. One electron
elimination from the tosyl group of 4 forms a cation radical intermediate (12), which then
removes a tosyl cation to form a radical intermediate (13). A hydrogen migration on 13 to
generate a stable full conjugated cyclic radical species generates a radical intermediate (10).
The subsequent recombination between 10 and the tosyl cation followed by an appropriate
electron abstraction affords the final product (3).
The mechanism of the influence of the reaction temperature on the reaction of 1 is not
enough clear now. The authors suggest tentatively the following explanation.

A conceptual reaction coordinate is depicted in Figure 6. One electron oxidation of 1
generates a cation radical intermediate (8) via an appropriate transition state (T1). Two
reaction paths are possible for 8, i.e., one is a path to 3 through a transition state T2, and the
other is to 4 through T3. The activation energies for T2 (E2) and T3 (E3) are considered to
o
control the choice of the reaction paths. At low temperature (0 C) only the path to 3, which
requires a smaller activation energy is active. However, at elevated temperatures (25 or 50
o
C) the second path, i.e., path to 4 becomes also active. But at the still higher temperatures
o
(50 or 70 C), the reactions of 4 are considered to take place. One of those may be the
electrochemical oxidation to form 3, and the other may be the decomposition of 4.
T3
T1
T2
E3
E2
(8)
Intermediate
Electrooxidation
Electrooxidation
56.6 KJ
Ar
N
Ts
H
O
N
H
40.0 KJ
(3)
Ar
N
Ar
N
H
H
O
O
2.84 KJ
N
N
Ts
Ts
(1)
(4)
Reaction
Figure 6. Effects of reaction temperature.
REFERENCES AND NOTES
1. W. E. Stewart and T. H. Siddall, Chem. Rev., 1970, 70, 517; T. Saito, Y. Fukazawa, Y.
Fujise, and S. Ito, Heterocycles, 1977, 7, 807.
2. T. Shono, T. Nozoe, H. Maekawa, Y. Yamaguchi, S. Kanetaka, T. Okada, and S. Kashimura,
Tetrahedron, 1991, 47, 593; K. Conrow, J. Am. Chem. Soc., 1959, 81, 5461; W.
Tochtermann, C. Degel, H. O. Horstmann, and D. Krauss, Tetrahedron Lett., 1970, 4719;
W. D. Stohrer, Chem. Ber., 1973, 106, 970.

3. K. Takahashi, H. Yamamoto, and T. Nozoe, Bull. Chem. Soc. Jpn., 1970, 43, 2000; S. Kondo,
H. Suzuki, T. Hattori, T. Ido, and K. Saito, Heterocycles, 1998, 48, 1151; K. Saito, M.
Hattori, T. Sato, and K. Takahashi, ibid., 1992, 34, 129.
4. T. Doi, H. Ishiguro, M. Sato, and K. Saito, Heterocycles, 1999, 51, 1775; K. Saito, T. Ido, Y.
Awadu, T. Kanie, and S. Kondo, ibid., 2000, 53, 519.
5. K. Ito, K. Saito, and K. Takahashi, Bull. Chem. Soc. Jpn., 1992, 65, 812; M. Nitta, H.
Tomioka, A. Akaogi, K. Takahashi, K. Saito, and K. Ito, J. Chem. Soc., Perkin Trans. 1,
1994, 2625; K. Sanechika, S. Kajigaeshi, and S. Kanemasa, Chem. Lett., 1977, 861.
6. T. Ido, S. Kondo, and K. Saito, Heterocycles, 1999, 50, 63; K. Ito, K. Saito, and K.
Takahashi, ibid., 1991, 32, 1117.
7. The MS, NMR, single crystal X-Ray analytical data of 4d were as follows.
4d: Yellow crystals. mp 147 ℃. HRMS m/z: 408.1150. Calcd for C22H20N2O4S m/z:
+
408.1153. MS m/z (rel intensity): 408 (15, M ), 253 (100), 237 (5), 210 (8). IR (KBr):
-1
1
1728, 1514, 1375, 1251, 1167 cm . H-NMR (CDCl3),δ: 2.42 (3 H, s), 2.44 (2 H, t, H6, J
= 7.1 Hz), 3.84 (3 H, s), 5.26-5.42 (2 H, m, H5, H7), 6.05 (1 H, d, J = 9.3 Hz), 6.91 (2 H, d,
J = 9.1 Hz), 7.08 (1 H, d, H8, J = 9.8 Hz), 7.18 (2 H, d, J = 8.8 Hz), 7.31 (2 H, d, J = 8.5
Hz), 7.97 (2 H, d, J = 8.2 Hz). Crystal data: yellow prism, C20H20N2O4S, M = 408.47,
monoclinic, space group P21/a, a = 15.511(3)Å, b = 6.402(1)Å, c = 19.969(4)Å, β =
3
-1
91.672(5)°, V = 1982.0(6)Å3, Z = 4, Dcalcd = 1.369 g/cm , μ(MoKα) = 1.95 cm , F(000) =
856.00. A Total of 5992 reflections for 2θmax = 55°was collected with I > 2σ(I) using a
Rigaku / MSC Mercury CCD diffractometer (MoKα radiation, λ = 0.71070 Å) at 296
K. The structure was solved using direct method (SHELXS-97) and refined by
Full-matrix least squares analysis giving values of R = 0.102, Rw = 0.164, R1 = 0.078, S =
1.44, ⊿ρmax / ⊿ρmin = 0.48 / -0.32 eÅ-3.