An efficient synthesis of 2-arylimidazoles by oxidation of 2-arylimidazolines using activated carbon-O2 system and its application to palladium-catalyzed Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.tet.2007.01.014

Oxidative conversion of 2-substituted imidazoline (dihydroimidazole) to the corresponding imidazole was achieved by an activated carbon-O₂ system. Also, the 2-arylimidazolines and 2-arylimidazoles have been found to work as simple ligands in the palladium-catalyzed Mizoroki-Heck reaction.

Full text of DOI:10.1016/j.tet.2007.01.014

Tetrahedron 63 (2007) 2414–2417
An efficient synthesis of 2-arylimidazoles by oxidation
of 2-arylimidazolines using activated carbon–O2
system and its application to palladium-catalyzed
Mizoroki–Heck reaction
*
Satoshi Haneda, Ayaka Okui, Chigusa Ueba and Masahiko Hayashi
Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Hyogo, Japan
Received 18 November 2006; revised 28 December 2006; accepted 9 January 2007
Available online 12 January 2007
Abstract—Oxidative conversion of 2-substituted imidazoline (dihydroimidazole) to the corresponding imidazole was achieved by an
activated carbon–O system. Also, the 2-arylimidazolines and 2-arylimidazoles have been found to work as simple ligands in the
2
palladium-catalyzed Mizoroki–Heck reaction.
Ó 2007 Elsevier Ltd. All rights reserved.
1
1
1
. Introduction
acid (IBX) have been used as oxidizing agents. Very
recently, Togo and Ishihara reported an efficient method
using (diacetoxyiodo)benzene (DIB) in the presence of
We have recently reported an activated carbon–O oxidation
2
1
2
system. That is, a variety of 2-arylbenzoxazoles, 2-arylbenz-
imidazoles and 2-arylbenzothiazoles were directly synthe-
sized from the reaction of the substituted 2-aminophenols,
K CO . Encouraged by these reports, as an extension of
2 3
our activated carbon–O oxidation system, we examined
2
the oxidation of imidazolines to imidazoles.
1,2-phenylenediamines, or 2-aminobenzenethiols with alde-
hydes in the presence of activated carbon (Darco KB or
Shirasagi KL) in xylene under an oxygen atmosphere.
Ò
On the other hand, the Mizoroki–Heck reaction has been
widely used for construction of a carbon–carbon bond since
1
1
3
Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyr-
azolines were also oxidized to the corresponding aromatic
compounds by the aid of activated carbon in both acetic
the first report in 1971. As ligands, phosphine and related
compounds are often used. Recently, electron-rich and bulky
alkyl phosphines, such as P(t-Bu) , were developed as highly
3
2
14
acid and xylene. Furthermore, oxidative aromatization of
9,10-dihydroanthracenes to anthracenes using molecular
oxygen was also promoted by activated carbon.
effective ligands. Especially, in view of the advances made
in the field of palladacycles, N-heterocyclic carbenes
1
5
3
16,17
should be noteworthy.
We are interested in the use of
non-phosphine ligands, namely, nitrogen-based ligands
such as imidazole and imidazoline in palladium-catalyzed
coupling reaction. Among the nitrogen-based ligands, we
focused on 2-arylimidazole derivatives, because their struc-
tures are simple, they are easy to treat and inexpensive. Fur-
thermore, the introduction of a variety of substituents at the
2-position of imidazole is also easily possible. There are
some reports on the compounds containing imidazole moie-
ties used as ligands in the Mizoroki–Heck reaction; however,
most of them are palladium carbene complexes possessing
a carbon–palladium bond.
Substituted imidazoles are important moieties constituted in
pharmaceuticals, pesticides and bioactive compounds. Fur-
thermore, recently, imidazoles have been found to be attrac-
tive as a skeleton of ionic liquids in many fields such as
environmentally friendly solvents in organic synthesis, elec-
trolytes, liquid crystals and so on. Therefore, there have been
many reports for the synthesis of imidazole derivatives.
Among those, oxidative conversion of imidazoline to imid-
azole is a general and reliable method. Actually, KMnO ,
4
4
5
6
7
8
9
MnO , BaSO , Pd/C, Pd/C–DMSO, (COCl) –DMSO,
2
4
2
1
0
trichloroisocyanuric acid (TCCA) and o-iodoxybenzoic
Here, we report an efficient oxidative synthesis of
2
-substituted imidazoles from the corresponding imidazo-
lines and the Mizoroki–Heck reaction catalyzed by PdCl2–
*
0
Corresponding author. Tel.: +81 78 803 5687; fax: +81 78 803 5688;
e-mail: mhayashi@kobe-u.ac.jp
2-arylimidazoline and PdCl –2-arylimidazole complexes.
2
040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.01.014

S. Haneda et al. / Tetrahedron 63 (2007) 2414–2417
2415
2
. Results and discussion
Table 2. Oxidative aromatization of 2-arylimidazolines to the correspond-
ing imidazoles
a
2.1. Oxidative conversion of 2-arylimidazolines
to 2-arylimidazoles
The starting 2-arylimidazolines were prepared by the
reported methods. They are classified into two methods.
One is the reaction of nitrile with ethylenediamine in the
b
Yield /%
Entry
1
Imidazoline
Time/h
24
1
8
84
presence of a catalytic amount of S, and the other is the re-
action of aldehyde with ethylenediamine in the presence of
a stoichiometric amount of NBS, or molecular iodine in
1
9
2
3
4
5
6
7
8
9
24
24
24
24
24
25
24
25
24
80
72
77
80
84
78
84
82
70
1
2,20,21
the presence of K CO .
3
2-Arylimidazolines prepared
by the above methods were treated with 100 wt % of acti-
vated carbon (Shirasagi KL, Japan EnviroChemicals, Ltd.)
2
ꢀ
in xylene at 120 C under oxygen atmosphere. The results
in Table 1 clearly show that combination of activated carbon
and oxygen accelerated oxidative conversion of imidazo-
2
2
lines to the corresponding imidazoles. As shown in Table
, a variety of imidazolines having aryl groups at the 2-posi-
2
tion were smoothly converted to the corresponding imid-
azoles in good to high yield. Unfortunately, in the case of
an aliphatic substituent at the 2-position, only 2-tert-butyl
imidazoline was converted to the corresponding imidazole
2
3
in 62% yield. It should be noted that we confirmed the pos-
sibility of reuse of activated carbon.
2.2. Mizoroki–Heck reaction of 4-bromotoluene
with some olefins
We examined the reaction of 4-bromotoluene with methyl
acrylate and tert-butyl acrylate under the conventional con-
ditions using N,N-dimethylformamide (DMF) as a solvent
1
0
ꢀ
and K CO as a base at 120 C. As for the palladium source,
2
3
a
All reactions were carried out using 100 wt % of activated carbon (Shira-
ꢀ
PdCl was the choice in our reaction system, because PdCl2
2
showed higher reactivity than Pd(OAc) . We confirmed that
2
PdCl alone, without a ligand almost did not catalyze the
2
sagi KL) in xylene at 120 C.
Isolated yield.
b
reaction (only 9% yield). Therefore, it is clear that 2-aryl-
imidazoline and 2-arylimidazole ligands accelerated the
Mizoroki–Heck reaction as shown in Table 3. Generally,
was obtained in 62% yield, whereas, only 6% of the product
was obtained by the use of 2-phenylimidazole (Scheme 1).
2
-arylimidazoline–palladium complexes showed higher cat-
It is noteworthy that when the reaction proceeded smoothly,
the formation of a precipitate, Pd black, was not observed.
alytic activity than 2-arylimidazole–palladium complexes in
Mizoroki–Heck reaction only except for entry 1. This phe-
nomenon was remarkable in the reaction of 4-bromotoluene
with styrene (1.25 equiv) as shown in Scheme 1. That is,
when 2-phenylimidazoline was used as a ligand, the product
Table 3. Mizoroki–Heck reaction using 2-arylimidazolines or 2-arylimid-
azole–palladium complexes
a
Table 1. Oxidative conversion of 2-phenylimidazoline to 2-phenylimid-
azole
a
1
R
2
R
b
Yield /%
Entry
Imidazoline
Imidazole
b
Yield /%
Entry
Activated carbon
Atmosphere
1
2
3
4
5
6
Me
Me
Me
t-Bu
t-Bu
t-Bu
OMe
H
CN
OMe
H
54
86
92
89
95
64
87
85
81
77
70
62
1
2
3
4
None
O
O
Air
Ar
2
2
21 (79)
84 (0)
51 (15)
10 (74)
Shirasagi KL
Shirasagi KL
Shirasagi KL
CN
a
All reactions were carried out using 100 wt % of activated carbon (Shira-
ꢀ
a
sagi KL) in xylene at 120 C for 24 h.
Yield was determined by H NMR analysis. The values in parentheses
indicate the yield of recovered starting material.
All reactions were carried out in a ratio of 4-bromotoluene–acrylate–
ꢀ
b
1
PdCl
Isolated yield.
2
–ligand¼1:2:0.01:0.02 in DMF at 120 C.
b

2416
S. Haneda et al. / Tetrahedron 63 (2007) 2414–2417
+
(
M ). Anal. Calcd for C H N O: C, 68.94; H, 5.79; N,
1
0 10 2
1
6.08. Found: C, 68.78; H, 5.76; N, 15.82.
0
96–197 C. IR (KBr): n
4
1
1
.1.4. 2-(2 -Thienyl)imidazole. R 0.66 (ethyl acetate). Mp
f
ꢀ
651, 1558, 1520, 1471, 1102. H NMR (400 MHz,
ꢁ1
(cm ) 3435, 3107, 2913,
max
1
Scheme 1.
DMSO-d ): d 6.9 (br s, 1H), 7.10 (dd, J¼4.8 Hz, J¼
6
4
1
.0 Hz, 1H), 7.2 (br s, 1H), 7.48 (dd, J¼4.8 Hz, J¼1.6 Hz,
H), 7.49 (dd, J¼4.0 Hz, J¼1.6 Hz, 1H), 12.50 (s, 1H).
We assumed that two 2-arylimidazolines or two 2-arylimid-
azoles could coordinate to the palladium atom. It is also
noted that these complexes are stable and insensitive to air.
13
C NMR (100.6 MHz, DMSO-d ): d 123.3, 125.6, 127.8,
6
+
134.6, 141.5. MS m/z: 151.4 (M ). Anal. Calcd for
C H N S: C, 55.97; H, 4.03; N, 18.65. Found: C, 55.95;
7
6 2
H, 4.21; N, 18.37.
3. Conclusion
0
tate). Mp 219–221 C (lit. 223 C). IR (KBr): n
3
4
.1.5. 2-(4 -Methylphenyl)imidazole. R 0.53 (ethyl ace-
f
In this paper we have disclosed a facile conversion of
-arylimidazolines to 2-arylimidazoles by the aid of an
activated carbon–O system. Furthermore, simple PdCl –
ꢀ
687, 1578, 1517, 1443, 1104, 822, 731. H NMR
8
ꢀ
ꢁ1
(cm )
max
1
2
2
2
(400 MHz, DMSO-d ): d 2.32 (s, 3H), 7.0 (br s, 1H), 7.2
6
2
-arylimidazoline and PdCl –2-arylimidazole catalyst sys-
2
(
1
1
br s, 1H), 7.24 (d, J¼8.0 Hz, 2H), 7.81 (d, J¼8.0 Hz, 2H),
tems for the Mizoroki–Heck reaction have been developed.
Further investigation on application of the new catalyst to
other coupling reactions is now in progress.
1
3
2.39 (s, 1H). C NMR (100.6 MHz, DMSO-d ): d 20.8,
6
+
24.7, 128.2, 129.2, 137.2, 145.7. MS m/z: 159.5 (M ).
Anal. Calcd for C H N : C, 75.92; H, 6.37; N, 17.71.
1
0 10 2
Found: C, 76.03; H, 6.45; N, 17.39.
4
. Experimental
0
tate). Mp 264–266 C. IR (KBr): n
4
.1.6. 2-(4 -Cyanophenyl)imidazole. R 0.59 (ethyl ace-
f
ꢀ
2981, 2222, 1654, 1610, 1506, 1448, 1067, 837. H NMR
ꢁ1
4.1. General procedure for oxidation of 2-arylimid-
azolines to 2-arylimidazoles
(cm ) 3446, 3161,
max
1
(
400 MHz, DMSO-d ): d 7.1 (br s, 1H), 7.4 (br s, 1H),
6
1
3
2
-Arylimidazoline (1 mmol), activated carbon (100 wt %;
Shirasagi KL) and xylene (4 mL) were placed in a 100 mL
7.91 (d, J¼8.0 Hz, 2H), 8.10 (d, J¼8.0 Hz, 2H). C NMR
(100.6 MHz, DMSO-d ): d 109.9, 118.9, 125.1, 132.8,
6
ꢀ
+
three-necked flask and the mixture was heated at 120 C.
134.7, 143.9. MS m/z: 170.5 (M ). Anal. Calcd for
C H N : C, 70.99; H, 4.17; N, 24.83. Found: C, 71.17; H,
4.33; N, 24.55.
After decreasing pressure inside the flask, it was filled
with oxygen using 1 L balloon. After confirmation of the
completion of the reaction, the mixture was filtrated using
Celite and washed with methanol. The filtrate was concen-
trated to obtain the product in analytically pure form.
1
0 7 3
0
tate). Mp 250–251 C (lit. 248 C). IR (KBr): n
4.1.7. 2-(4 -Chlorophenyl)imidazole. R 0.61 (ethyl ace-
f
ꢀ
455, 1640, 1499, 1449, 1096, 826, 724. H NMR
8
ꢀ
ꢁ1
(cm )
max
1
3
(400 MHz, DMSO-d ): d 7.0 (br s, 1H), 7.30 (br s, 1H),
4
.1.1. 2-Phenylimidazole. R 0.53 (ethyl acetate). Mp 144–
f
6
ꢀ
4
ꢀ
053, 2989, 1670, 1560, 1506, 1461, 1106, 948, 705, 683.
ꢁ1
1
46 C (lit. 140–142 C). IR (KBr): n
(cm ) 3567,
7.50 (d, J¼8.8 Hz, 2H), 7.94 (d, J¼8.8 Hz, 2H), 12.58 (s,
max
1
3
3
1H). C NMR (100.6 MHz, DMSO-d ): d 126.4, 128.7,
6
1
+
H NMR (400 MHz, DMSO-d ): d 7.0 (br s, 1H), 7.2 (br s,
6
129.7, 132.3, 144.5. MS m/z: 179.3 (M ).
1
H), 7.33 (t, J¼8.0 Hz, 1H), 7.44 (dd, J¼8.0 Hz,
1
3
0
tate). Mp 134–135 C (lit. 133 C). IR (KBr): n
J¼8.0 Hz, 2H), 7.93 (d, J¼8.0 Hz, 2H), 12.49 (s, 1H). C
NMR (100.6 MHz, DMSO-d ): d 124.8, 128.0, 128.8,
4.1.8. 2-(3 -Chlorophenyl)imidazole. R 0.53 (ethyl ace-
f
ꢀ
3451, 3061, 2807, 1668, 1606, 1556, 1466, 1108, 768, 680.
8
ꢀ
ꢁ1
(cm )
6
max
+
1
30.8, 145.6. MS m/z: 145.4 (M ).
1
H NMR (400 MHz, DMSO-d ): d 7.1 (br s, 1H), 7.3 (br
6
0
tate). Mp 155–157 C (lit. 152–154 C). IR (KBr): n
4
.1.2. 2-(4 -Methoxyphenyl)imidazole. R 0.39 (ethyl ace-
f
s, 1H), 7.39 (d, J¼8.0 Hz, 1H), 7.47 (dd, J¼8.0 Hz, J¼
13
ꢀ
24
ꢀ
cm ) 3444, 3073, 2840, 1616, 1518, 1439, 1253, 1101,
8.0 Hz, 1H), 7.90 (d, J¼8.0 Hz, 1H), 7.98 (m, 1H), 12.64
max
ꢁ
1
(
(s, 1H). C NMR (100.6 MHz, DMSO-d ): d 123.3, 124.4,
127.7, 130.8, 132.8, 133.6, 144.2. MS m/z: 179.5 (M ).
6
1
+
1
7
2
029, 840. H NMR (400 MHz, DMSO-d ): d 3.79 (s, 3H),
6
.00 (d, J¼8.8 Hz, 2H), 7.1 (br s, 2H), 7.85 (d, J¼8.8 Hz,
1
3
0
tate). Mp 247–249 C. IR (KBr): n
H), 12.31 (s, 1H). C NMR (100.6 MHz, DMSO-d ):
6
4.1.9. 2-(4 -Bromophenyl)imidazole. R 0.57 (ethyl ace-
f
ꢀ
2873, 1605, 1560, 1495, 1446, 1107, 824, 724. H NMR
ꢁ1
d 55.3, 114.2, 123.7, 126.3, 145.7, 159.2. MS m/z: 175.4
(
(cm ) 3436, 2964,
max
+
1
M ).
(
400 MHz, DMSO-d ): d 7.0 (br s, 1H), 7.3 (br s, 1H),
6
0
tate). Mp 155–156 C. IR (KBr): n
3
4
.1.3. 2-(3 -Methoxyphenyl)imidazole. R 0.50 (ethyl ace-
f
7.64 (d, J¼8.8 Hz, 2H), 7.87 (d, J¼8.8 Hz, 2H), 12.59 (s,
ꢀ
017, 1618, 1587, 1231, 1109, 945, 773. H NMR
ꢁ1
13
(cm ) 3448, 3141,
1H). C NMR (100.6 MHz, DMSO-d ): d 120.9, 126.7,
6
max
1
+
130.0, 131.6, 144.5. MS m/z: 225.4 (M ). Anal. Calcd for
C H BrN : C, 48.46; H, 3.16; N, 12.56. Found: C, 48.17;
H, 3.16; N, 12.75.
(
400 MHz, DMSO-d ): d 3.79 (s, 3H), 6.90 (dd, J¼8.0 Hz,
6
9
7
2
J¼2.4 Hz, 1H), 7.0 (br s, 1H), 7.2 (br s, 1H), 7.34 (dd,
J¼8.0 Hz, J¼8.0 Hz, 1H), 7.50–7.52 (m, 2H), 12.48 (s,
1
3
0
tate). Mp 309–311 C (lit. 310–312 C). IR (KBr): n
1
H). C NMR (100.6 MHz, DMSO-d ): d 55.2, 110.1,
6
4.1.10. 2-(4 -Nitrophenyl)imidazole. R 0.68 (ethyl ace-
f
ꢀ
25
ꢀ
1
13.8, 117.2, 129.9, 132.1, 145.5, 159.6. MS m/z: 175.5
max

S. Haneda et al. / Tetrahedron 63 (2007) 2414–2417
2417
ꢁ
1
1
(
cm ) 3694, 1515, 1333, 1107, 856. H NMR (400 MHz,
7. Amemiya, Y.; Miller, D. D.; Hsu, F. L. Synth. Commun. 1990,
20, 2483.
8. Anastassiadou, M.;Baziard-Mouysset, G.; Payard, M. Synthesis
2000, 1814.
9. Huh, D. H.; Ryu, H.; Kim, Y. G. Tetrahedron 2004, 60, 9857.
DMSO-d ): d 7.2 (br s, 1H), 7.4 (br s, 1H), 8.16 (d, J¼
6
1
3
9
.2 Hz, 2H), 8.30 (d, J¼9.2 Hz, 2H), 12.97 (s, 1H).
C
NMR (100.6 MHz, DMSO-d ): d 124.3, 125.3, 136.6,
6
+
1
43.6, 146.4. MS m/z: 190.5 (M ).
1
0. Mohammadpoor-Baltork, I.; Zolfigol, M. A.; Abdollahi-
Alibeik, M. Synlett 2004, 2803.
4
methanol). Mp 151–152 C. IR (KBr): n
2
.1.11. 2-tert-Butylimidazole. R 0.50 (4:1 ethyl acetate–
(cm ) 3566,
f
ꢀ
960, 1661, 1568, 1430, 1372, 1103, 974, 735. H NMR
ꢁ1
1
11. Nicolaou, K. C.; Mathison, C. J. N.; Montagnon, T. Angew.
Chem., Int. Ed. 2003, 42, 4077.
12. Ishihara, M.; Togo, H. Synlett 2006, 227.
13. (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn.
1971, 44, 581; (b) Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem.
1972, 37, 2320.
max
(
(
400 MHz, DMSO-d ): d 1.27 (s, 9H), 6.7 (br s, 1H), 6.9
6
br s, 1H), 11.57 (s, 1H). C NMR (100.6 MHz, DMSO-
1
3
+
d ): d 29.6, 32.4, 155.0. MS m/z: 125.6 (M ). Anal. Calcd
6
for C H N : C, 67.70; H, 9.74; N, 22.56. Found: C,
7
12 2
6
7.99; H, 9.80; N, 22.23.
14. (a) Littke, A. F.; Fu, G. C. J. Org. Chem. 1999, 64, 10; (b)
Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989;
For N-donating ligand–palladium-catalyzed Heck reaction,
see: Done, M. C.; R €u ther, T.; Cavell, K. J.; Kilner, M.;
Peacock, E. J.; Braussaud, N.; Skelton, B. W.; White, A.
J. Organomet. Chem. 2000, 607, 78.
4
.2. General procedure for Mizoroki–Heck reaction
PdCl (0.02 mmol), 2-arylimidazoline (or 2-arylimidazole)
2
(
0.04 mmol) and K CO (2 equiv) were mixed in DMF
2 3
ꢀ
toluene (2 mmol) and methyl acrylate (or tert-butyl acrylate)
¨
15. (a) Herrmann, W. A.; Brossmer, C.; Ofele, K.; Reisinger, C.-P.;
(10 mL) and the mixture was stirred at 50 C for 1 h. 4-Bromo-
Priermeier, T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1844; (b) Herrmann, W. A.; B €o hm, V. P. W.;
Reisinger, C.-P. J. Organomet. Chem. 1999, 576, 23.
16. (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290; (b)
Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang, C.;
Nolan, C. S. P. J. Organomet. Chem. 2002, 653, 69.
17. See also for recent palladium-catalyzed coupling reactions: (a)
Review: Phan, N. T. S.; Sluys, M. V. D.; Jones, C. W. Adv.
Synth. Catal. 2006, 348, 609; (b) Reetz, M. T.; Lohmer, G.;
Schwickardi, R. Angew. Chem., Int. Ed. 1998, 37, 481; (c)
Beller, M.; Zapf, A. Synlett 1998, 792.
(
1
4 mmol) were added. The mixture was then stirred at
ꢀ
20 C for 24 h. The reaction mixture was cooled, and
then precipitates were removed by filtration and extracted
with diethyl ether. The combined organic layer was dried
over anhydrous magnesium sulfate and filtered. After evap-
oration, the obtained residue was purified by silica gel col-
umn chromatography to give the coupling products.
4
.2.1. (E)-Methyl 4-methylcinnamate. R ¼0.53 (5:1 hex-
ꢀ
ꢁ1
max
f
ane–ethyl acetate). Mp 54–55 C. IR (KBr): n
(cm )
3
1
401, 3028, 2948, 1712, 1634, 1606, 1515, 1436, 1332,
319, 1281, 1210, 1193, 1170, 999, 818, 521, 511, 493. H
18. Mohammadpoor-Baltork, I.; Abdollahi-Alibeik, M. Bull.
Korean Chem. Soc. 2003, 24, 1354.
1
NMR (400 MHz, CDCl ): d 7.67 (d, 1H, J¼16.4 Hz), 7.42
19. Fujioka, H.; Murai, K.; Ohba, Y.; Hiramatsu, A.; Kita, Y.
Tetrahedron Lett. 2005, 46, 2197.
3
(
d, 2H, J¼8.0 Hz), 7.19 (d, 2H, J¼7.6 Hz), 6.40 (d,
1
3
1
(
H, J¼16.0 Hz), 3.80 (s, 3H), 2.37 (s, 3H). C NMR
20. Other methods for imidazoline synthesis: (a) Neef, G.; Eder,
U.; Sauer, G. J. Org. Chem. 1981, 46, 2824; (b) Mitchell,
J. M.; Finney, N. S. Tetrahedron Lett. 2000, 41, 8431; (c)
Peddibhotla, S.; Tape, J. J. Synthesis 2003, 1433; (d) You, S.;
Kelly, J. W. Org. Lett. 2004, 6, 1681; (e) Gogoi, P.; Konwar,
D. Tetrahedron Lett. 2006, 47, 79; (f) Mirkhani, V.;
Moghadam, M.; Tangestaninejad, S.; Kargar, H. Tetrahedron
Lett. 2006, 47, 2129.
100.6 MHz, CDCl ): d 167.6, 144.8, 140.7, 131.6, 129.6,
3
1
7
28.0, 116.7, 51.6, 21.4. Anal. Calcd for C H O : C,
11 12 2
4.98; H, 6.86. Found: C, 74.83; H, 6.92.
Acknowledgements
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
21. In the case of 1,2-phenylenediamine and aldehydes, the corre-
sponding intermediate dihydro-cyclic benzimidazolines could
not be isolated, because successive oxidation step proceeded
very rapidly. On the other hand, in the present case, the product
of ethylenediamine and aldehydes, that is, 2-arylimidazolines
were stable and were isolated, and can be used as a substrate
of oxidation leading to the formation of 2-arylimidazoles.
22. As for the role of activated carbon in oxidative aromatization,
we found that surface area of micropore is one of the important
factors. The details will be published in a separate full paper.
23. We examined oxidation of a variety of imidazolines having
alkyl groups at the 2-position such as methyl, n-pentyl, dihy-
drocinnamyl, cyclohexyl; however, all of them gave only low
yield of the corresponding imidazole. Only 2-tert-butyl imid-
azoline was oxidized to the corresponding imidazole in 62%
yield.
References and notes
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