Microwave-assisted Cadogan reaction for the synthesis of 2-aryl-2H-indazoles, 2-aryl-1H-benzimidazoles, 2-carbonylindoles, carbazole, and phenazine

Article abstract of DOI:10.1002/jhet.267

(Chemical Equation Presented) The Cadogan reaction, a widely accepted route for the synthesis of nitrogen containing heterocycles, is modified by using microwave radiation as the source of heat instead of the conventional heating by reflux in a nitrogen atmosphere for several hours. Appropriate starting materials were mixed with triethyl phosphite or triphenylphosphine and irradiated with microwaves for several minutes at a specific power to give the desired products. The indazoles were prepared by irradiating N-(2-nitrobenzylidene) anilines with triethyl phosphite at 200 W for 12-14 min to give 85-92% product yields. Irradiation of the mixture of N-benzylidene-2-nitroanilines and triphenylphosphine at 200 W for 3-5 min yielded 93-96% of the benzimidazoles. The carbonylindoles were obtained in 61-68% yields by irradiating 2-nitrochalcone or alkyl 2-nitrocinnamates and triphenylphosphine with microwaves at 80-200 W for 8-11 min. The mixture of 2-nitrobiphenyl and triphenylphosphine yielded 96% of carbazole when irradiated with microwaves at 200 W for 2 min while 75% of phenazine was obtained by irradiating the mixture of 2-nitrodiphenylamine and triphenylphosphine with microwaves at 200 W for 3.5 min. These results show that microwave-assisted Cadogan reactions gave better product yields at shorter reaction times.

Full text of DOI:10.1002/jhet.267

November 2009
Microwave-Assisted Cadogan Reaction for the Synthesis of
2-Aryl-2H-indazoles, 2-Aryl-1H-benzimidazoles,
2-Carbonylindoles, Carbazole, and Phenazine
1309
Evelyn Cuevas Creencia,a* Masahiro Kosaka,b Toshikatsu Muramatsu,b
Masashi Kobayashi,b Tomohiro Iizuka,b and Takaaki Horaguchib*
aDepartment of Chemistry, College of Science and Mathematics,
MSU-Iligan Institute of Technology, Iligan City 9200, Philippines
*E-mail: ec.creencia@gmail.com
bDepartment of Chemistry, Faculty of Science, Niigata University,
Ikarashi, Niigata 950-2181, Japan
*E-mail: hora@chem.sc.niigata-u.ac.jp
Received June 8, 2009
DOI 10.1002/jhet.267
Published online 10 November 2009 in Wiley InterScience (www.interscience.wiley.com).
The Cadogan reaction, a widely accepted route for the synthesis of nitrogen containing heterocycles,
is modified by using microwave radiation as the source of heat instead of the conventional heating by
reflux in a nitrogen atmosphere for several hours. Appropriate starting materials were mixed with
triethyl phosphite or triphenylphosphine and irradiated with microwaves for several minutes at a specific
power to give the desired products. The indazoles were prepared by irradiating N-(2-nitrobenzylidene)
anilines with triethyl phosphite at 200 W for 12–14 min to give 85–92% product yields. Irradiation of
the mixture of N-benzylidene-2-nitroanilines and triphenylphosphine at 200 W for 3–5 min yielded 93–
96% of the benzimidazoles. The carbonylindoles were obtained in 61–68% yields by irradiating 2-nitro-
chalcone or alkyl 2-nitrocinnamates and triphenylphosphine with microwaves at 80–200 W for 8–11
min. The mixture of 2-nitrobiphenyl and triphenylphosphine yielded 96% of carbazole when irradiated
with microwaves at 200 W for 2 min while 75% of phenazine was obtained by irradiating the mixture
of 2-nitrodiphenylamine and triphenylphosphine with microwaves at 200 W for 3.5 min. These results
show that microwave-assisted Cadogan reactions gave better product yields at shorter reaction times.
J. Heterocyclic Chem., 46, 1309 (2009).
INTRODUCTION
the microwave oven for chemical syntheses [1]. Since
then, many researchers have been using the technique
for organic syntheses, thus contributing to the enormous
volume of literatures we now see in print. Microwave-
assisted heating under controlled conditions is a valuable
technology for chemical syntheses as it can increase the
rate of reaction, improve the product yield, and reduce
the formation of side products [2].
The search for better methods for the syntheses of N-
containing heterocyclic compounds has never ended as
evidenced by the increasing number of articles devoted
to this topic. This has led some researchers to look into
the possibility of improving the reaction by heating the
reaction mixture with microwave radiation instead of
the usual conventional heating procedure. The micro-
wave ovens have been with us for some time now but it
was not until 1986 when researchers started to utilize
Several methods for the syntheses of indazoles,
indoles and benzimidazoles have been modified by car-
rying out the reactions under microwave irradiation [3].
C
V
2009 HeteroCorporation

1310
E. Cuevas Creencia, M. Kosaka, T. Muramatsu, M. Kobayashi, T. Iizuka, and T. Horaguchi
Vol 46
Scheme 1
reported that rate enhancement in the Fischer indole
synthesis was observed when assisted by microwave
radiations, that the reaction goes to completion in a
short time furnishing good yields [3(n,o)]. Furthermore,
Varma described the microwave-enhanced solvent-free
synthetic approach to a variety of heterocyclic com-
pounds and observed that the method was simple, easy
to manipulate, uses minimal amounts of solvents, and
give good product yields [3(p)]. Thus, the prospect of
using microwave radiation for organic synthesis seems
to be limitless, offering routes of shorter reaction times,
minimal side products, and better product yields. It is
this idea that led our laboratory to venture into micro-
wave-assisted organic synthesis.
The reduction or deoxygenation of aromatic nitro-
compounds by triethyl phosphite and related reagents is
referred to as the Cadogan reaction [4]. The reaction is
carried out at high temperature under nitrogen atmos-
phere for several hours. This reaction has been widely
investigated as a synthetic route for N-containing hetero-
cycles [5] and since the discovery of the reaction in
1962 [4], the reduction of aromatic nitro-compounds by
triethyl phosphite and related reagents has been
exploited as a route to a wide variety of nitrogen
Dubey and Moorthy did a comparative study on conven-
tional and microwave assisted synthesis of benzimida-
zoles and their derivatives and concluded that the micro-
wave assisted reactions have reduced the reaction times
by 96–98% and increased the yields by about 10 to 50%
[3(d)]. Yu et al. and Navarrete-Vazquez et al. have
developed a simple and rapid synthesis of substituted
benzimidazoles under solvent-free condition using read-
ily available reagents and the microwave oven [3(l,m)].
Sridar as well as Abramovitch and Bulman have
Table 1
Microwave-assisted Cadogan reaction for the synthesis of 2-aryl-2H-indazoles.
Starting
materiala,b
P(OEt)3
(mmol)
Time
(minutes)
Power
(W)
Time
(minutes)
Entry
Power (W)
Product
Yieldc (%)
1
2
3
1a
1a
1a
1a
1a
1a
1b
4.0
4.0, 4.0d
4.0
600
600
200
200
200
200
200
200
200
200
200
200
200
80
4
4, 4
14
14
–
–
–
–
–
–
4a
4a
4a
4a
4a
4a
4b
4c
4d
4a
4b
4c
4d
4a
4b
4c
4d
33
67
77
76
86
89
92
85
89
17
16
47
32
45
38
63
55
4
8.0
–
–
5
6
4.0, 4.0e
4.0, 4.0, 4.0f
4.0
14, 14
14, 14, 14
–
–
–
–
7
8
13
12
14
9
–
–
–
–
1c
1d
4.0
4.0
4.0
4.0
9
–
–
–
–
10g
11g
12g
13g
14h
15h
16h
17h
2 þ 3a
2 þ 3b
2 þ 3c
2 þ 3d
2 þ 3a
2 þ 3b
2 þ 3c
2 þ 3d
10
12
12
2
–
–
4.0
4.0
–
–
–
–
4.0
4.0
200
200
200
200
8
80
80
80
2
12
12
11
4.0
4.0
1
2
a Starting material: 1.0 mmol.
b Reaction vessel: test tube.
c Isolated yield.
d The starting material was added with 4.0 mmol P(OEt)3 and irradiated for 4 min followed by the addition of another 4 mmol P(OEt)3 and irradi-
ated for 4 min more.
e The starting material was added with 4.0 mmol P(OEt)3 and irradiated for 14 min followed by the addition of another 4 mmol P(OEt)3 and irradi-
ated for 14 min more.
f Same as [e], after which another 4 mmol of P(OEt)3 was added and the mixture irradiated for 14 min more.
g One-pot-one-step reaction.
h One-pot-two-steps reaction.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2009 Microwave-Assisted Cadogan Reaction for the Synthesis of 2-Aryl-2H-indazoles,
2-Aryl-1H-benzimidazoles, 2-Carbonylindoles, Carbazole, and Phenazine
1311
Scheme 2
not give any significant change in the yield (76%, Table
1, Entry 4). The procedure was repeated using 4.0 mmol
of triethyl phosphite. After irradiation for 14 min, the
mixture was added with another 4.0 mmol of triethyl
phosphite and irradiated for another 14 min at 200 W.
The reaction afforded 2-phenyl-2H-indazole 4a in 86%
yield (Table 1, Entry 5). A third addition of 4.0 mmol
of triethyl phosphite and 14 min more of irradiation
gave only a slight increase in the yield of 2-phenyl-2H-
indazole 4a (89%, Table 1, Entry 6). These results show
that irradiation of the mixture of N-(2-nitrobenzyl-
idene)aniline 1a and triethyl phosphite at 200 W gave
better results than irradiation of the mixture at 600 W,
and a second addition of the triethyl phosphite can
increase the yield further. This method gave a better
yield of 2-phenyl-2H-indazole 4a (89%) compared to
that reported by Song and Yee in the palladium-cata-
lyzed intramolecular amination of N-phenyl-N-(o-bromo-
benzyl)hydrazine which yielded only 58% of 2-phenyl-
2H-indazole 4a after 15 h [10]. On the other hand,
Varughese et al. reported a 60–65% yields of 2-phenyl-
2H-indazole 4a by the microwave-assisted Cadogan
reaction of 2-nitrobenzaldehyde and aniline [9(b)] while
the classical Cadogan method yielded 60% of 2-phenyl-
2H-indazole 4a after 6 h [6,11].
containing heterocyclic compounds, including carbazoles
[4,6], indoles [4,7], indazoles [6], and other related com-
pounds [5(b),6,8].
Because of the versatility of Cadogan reaction, a
number of researchers have tried to modify the method
to shorten the reaction time and improve the yield by
using microwave radiations as the source of heat and
were successful [9].
In this article, we report the microwave-assisted
Cadogan reaction for the synthesis of indazoles, benz-
imidazoles, indoles, carbazole and phenazine.
For the other three imines, 1b-d, the reaction was car-
ried out at 200W and various reaction times. When 1.0
mmol of N-(2-nitrobenzylidene)-2-fluoroaniline 1b and
4.0 mmol triethyl phosphite were mixed and irradiated
for 13 min at 200 W, 92% of 2-(2-fluorophenyl)-2H-in-
dazole 4b was obtained (Table 1, Entry 7). Irradiation
of 1-(2-nitrobenzylidene)-2-phenylhydrazine 1c at 200
W for 12 min gave 85% of 2-phenylamino-2H-indazole
4c (Table 1, Entry 8). Dyablo et al. obtained 16% of 2-
phenylamino-2H-indazole 4c by mixing the correspond-
ing amine with cupric acetate, phenylboric acid and trie-
thylamine and stirring the mixture at 20ꢀC for 17 h
[12]. Irradiation of N-(2-nitrobenzylidene)-1-naphthyl-
amine 1d at 200 W for 14 min gave 89% of 2-(1-naph-
thyl)-2H-indazole 4d (Table 1, Entry 9). The classical
Cadogan reaction produced 51% of 2-a-naphthylamine
after 6 h [6]. Sequential addition of triethyl phosphite
RESULTS AND DISCUSSION
The starting materials used were either synthesized
according to literature or purchased from the manufac-
turer and used as received. The Cadogan reaction was
done by irradiating the starting materials with micro-
waves from a domestic microwave oven. For the synthe-
sis of indazoles, the imines 1a–d were mixed with
triethyl phosphite and irradiated with microwaves to
give the corresponding indazoles 4a–d (Scheme 1). The
results are tabulated in Table 1. Initially, 1.0 mmol of
N-(2-nitrobenzylidene)aniline 1a was added with 4.0
mmol triethyl phosphite in a Pyrex test tube and irradi-
ated for 4 min at 600 W. The reaction afforded 2-phe-
nyl-2H-indazole 4a in 33% yield (Table 1, Entry 1).
The procedure was repeated but this time after 4 min of
irradiation, another 4.0 mmol of triethyl phosphite was
added and the mixture irradiated for 4 min more. This
resulted to an increase in the yield (67%) of 2-phenyl-
2H-indazole 4a (Table 1, Entry 2). However, the reac-
tion mixture showed signs of decomposition, so the
reaction was further investigated by using lower power
rating. A mixture of 1.0 mmol of N-(2-nitrobenzyl-
idene)aniline 1a and 4.0 mmol of triethyl phosphite was
irradiated for 14 min at 200 W. The reaction afforded 2-
phenyl-2H-indazole 4a in 77% yield (Table 1, Entry 3).
Increasing the amount of triethyl phosphite to 8.0 mmol
and irradiating the mixture for 14 minutes at 200 W did
Scheme 3
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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E. Cuevas Creencia, M. Kosaka, T. Muramatsu, M. Kobayashi, T. Iizuka, and T. Horaguchi
Vol 46
Scheme 4
Scheme 5
Triethyl phosphite was replaced with triphenylphosphine
as this reagent can also deoxygenate aromatic nitro-com-
pounds and is easily handled, inexpensive and a stable
solid (Scheme 4) [6,13]. One millimole of N-benzyli-
dene-2-nitroaniline 5a was mixed with 4.0 mmol of tri-
phenylphosphine and irradiated with microwaves for 5
min at 200 W. The reaction gave 96% of 2-phenyl-1H-
benzimidazole 8a (Table 2, Entry 1). 2-(4-Chloro-
phenyl)-1H-benzimidazole 8b and 2-(4-methylphenyl)-
1H-benzimidazole 8c were also synthesized from the
corresponding imines, N-(4-chlorobenzylidene)-2-nitro-
aniline 5b and N-(4-methylbenzylidene)-2-nitroaniline
5c, respectively, by irradiating the mixture with micro-
waves for 3 min at 200 W. The reactions gave 94% of
2-(4-chlorophenyl)-1H-benzimidazole 8b and 93% of 2-
(4-methylphenyl)-1H-benzimidazole 8c (Table 2, Entries
2, 3). The group of Sharghi reported the synthesis of
benzimidazoles by the reaction of phenylenediamine and
benzaldehyde in the presence of phorphyrinatoiron(III)
complex as catalyst. They were able to synthesize 2-
phenyl-1H-benzimidazole 8a at 97% by carrying out the
reaction for 30 min, 2-(4-chlorophenyl)-1H-benzimida-
zole 8b at 94% by carrying out the reaction for 55 min
and 2-(4-methylphenyl)-1H-benzimidazole 8c at 95% by
carrying out the reaction for 55 min [14]. The results of
the two methods are comparable but the microwave-
assisted Cadogan reaction does not require a metal-com-
plex catalyst and was complete in 3 to 5 min only.
Other researchers also reported the synthesis of 2-
was no longer done because yields of the products were
already good.
In the interest of saving time, a one-pot-one-step and
one-pot-two-steps reaction procedures for the synthesis
of indazoles were developed. For the one-pot-one-step
procedure, 2-nitrobenzaldehyde 2 and aryl amines 3a-d
were mixed together in a test tube and added with
triethyl phosphite. This mixture was then irradiated at
200 W for several minutes (Scheme 2). The results in
Table 1 show that the procedure gave a fair yield of 2-
phenylamino-2H-indazole 4c when the mixture was irra-
diated for 12 min at 200 W (47%, Table 1, Entry 12).
For the one-pot-two-steps procedure, 2-nitrobenzalde-
hyde 2 and aryl amines 3a–d were mixed in a test tube
and irradiated at 80 W for 1–2 min. After this, triethyl
phosphite was added and the mixture irradiated again at
200 W for several minutes (Scheme 3). The results
show that this method gave higher yields compared to
the one-pot-one-step procedure (Table 1, Entries 14–17).
However, the synthesis of indazoles from the starting
imines is a better method as it gave better yield. These
imply that the formation of the imine is an important
step in the synthesis of indazoles.
For the synthesis of benzimidazoles, the imines 5a–c
were added with triethyl phosphite and irradiated with
microwaves for several minutes at a specific power.
However, the reactions gave poor product yields.
Table 2
Microwave-assisted Cadogan reaction for the synthesis of 2-aryl-1H-benzimidazoles.
Entry
Starting materiala,b
PPh3 (mmol)
Power (W)
Time (minutes)
Product
Yieldc (%)
1
5a
5b
5c
6 þ 7a
6 þ 7b
6 þ 7c
4.0
4.0
4.0
4.0
4.0
4.0
200
200
200
200
200
200
5
8a
8b
8c
8a
8b
8c
96
94
93
82
78
81
2
3
3
3
4
4d
5d
6d
2.5
4
a Starting material: 1.0 mmol.
b Reaction vessel: test tube.
c Isolated yield.
d One-pot-one-step reaction.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2009 Microwave-Assisted Cadogan Reaction for the Synthesis of 2-Aryl-2H-indazoles,
2-Aryl-1H-benzimidazoles, 2-Carbonylindoles, Carbazole, and Phenazine
1313
Scheme 6
Scheme 7
on the other hand, heated azidocinnamates in xylene
under reflux for 15 min to yield 67% of 2-methoxycar-
bonylindole 10c [18].
Deoxygenation of 2-nitrobiphenyl 11 to give carba-
zole 12 was done with triethyl phosphite and triphenyl-
phosphine (Scheme 7). The results show that 64% of
carbazole 12 was obtained when 2-nitrobiphenyl 11 and
triethyl phosphite were irradiated with microwaves for
7.5 min at 600 W while 96% of carbazole 12 was
obtained when triphenylphosphine was used instead and
irradiating the mixture for 2 min at 200 W (Table 4,
Entries 2, 3). With the classical Cadogan reaction,
82.5% of carbazole 12 was obtained by refluxing 2-
nitrobiphenyl 11 and triethyl phosphite, under nitrogen
atmosphere, for 9 hours, and 43% of carbazole 12 was
obtained when triphenylphosphine was used and the
mixture placed in a sealed tube and heated at 130ꢀC for
9 h [6]. On the other hand, Freeman et al. obtained 91%
of carbazole 12 by refluxing 2-nitrobiphenyl 11 in tri-
phenylphosphine for 21 h [13(a)]. When 2-nitrodiphe-
nylamine 13 was mixed with triethyl phosphite and irra-
diated with microwaves for 5 min at 600 W, 43% of
phenazine 14 was obtained (Table 4, Entry 4) (Scheme
8). When triphenylphosphine was used and the mixture
irradiated at 200 W for 3.5 min, 75% of phenazine 14
was obtained (Table 4, Entry 5).
phenylbenzimidazole by other methods but the yields
were relatively low and reaction times longer [15].
The one-pot-one-step synthesis of the benzimidazoles
(Scheme 5) gave relatively lower yields compared to the
synthesis from the corresponding imines (Table 2,
Entries 4, 5, 6). However, the one-pot-one-step synthesis
is a simple and convenient procedure.
The carbonylindoles were synthesized using 2-nitro-
chalcone 9a and alkyl 2-nitrocinnamates 9b-c. The
reaction with triethyl phosphite gave low product
yields so triphenylphosphine was used instead (Scheme
6). The reaction of 2-nitrochalcone 9a with triphenyl-
phosphine at 200 W and 8 min gave 68% of 2-benzoyl-
indole 10a (Table 3, Entry 1). Mahboobi et al.
obtained 73% 2-benzoylindole 10a from a reaction
which required heating the reagents under reflux for 12
h [16]. On the other hand, the reaction with ethyl 2-
nitrocinnamate 9b yielded 64% of 2-ethoxycarbonylin-
dole 10b while reaction with methyl 2-nitrocinnamate
9c yielded 61% of 2-methoxycarbonylindole 10c, with
the reactions being carried out at 80 W for 10 min and
11 min, respectively (Table 3, Entries 2, 3). At higher
power, decomposition products are formed. Csomos
et al. obtained 83% of 2-ethoxycarbonylindole 10b by
reacting indole-2-carboxylic acid, thionyl chloride and
dry ethanol, and carrying out the reaction at different
temperatures, requiring a total of 4.5 h for the reaction
to complete [17]. Cadogan et al. reacted o-nitrocin-
namic acid with triethyl phosphite for 24 h to give
7.5% of 2-ethoxycarbonylindole 10b [6]. Sechi et al.,
The results in this article indicate that the use of
microwave radiation greatly enhances the yield of the
products and reduces the reaction time from hours to
minutes. When decomposition is observed, triphenyl-
phosphine is a better alternative because the reaction
can be carried out at lower power and still gives good
yields. The procedure can be used to synthesize a vari-
ety of N-containing heterocyclic compounds once the
appropriate starting materials have been prepared.
Table 3
Microwave-assisted Cadogan reaction for the synthesis of 2-carbonylindoles from chalcone and alkyl 2-nitrocinnamates.
Entry
Starting materiala
PPh3 (mmol)
Power (W)
Time (minutes)
Product
Yieldb (%)
1c
2d
3d
9a
9b
9c
4.0
3.0
3.0
200
80
80
8
10
11
10a
10b
10c
68
64
61
a Starting material: 1.0 mmol.
b Isolated yield.
c reaction vessel: test tube.
d reaction vessel: 50-mL round bottom flask.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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E. Cuevas Creencia, M. Kosaka, T. Muramatsu, M. Kobayashi, T. Iizuka, and T. Horaguchi
Vol 46
Table 4
Microwave-assisted Cadogan reaction for the synthesis of carbazole and phenazine.
Entry
Starting material
(POEt)3 (mmol)
PPh3 (mmol)
Power (W)
Time (minutes)
Product
Yielda (%)
1b
2d
3d
4d
5d
11c
11e
11c
13e
13c
4.0
3.0
–
2.0
–
–
–
600
600
200
600
200
15
7.5
2
5
3.5
12
12
12
14
14
63
64
96
43
75
3.0
–
3.0
a Isolated yield.
b Reaction vessel: 50-mL Erlenmeyer flask.
c Starting material: 1.0 mmol.
d Reaction vessel: test tube.
c Starting material: 0.5 mmol.
Scheme 8
the mixture and recystallized from ethanol to give yellow nee-
dles of N-(2-nitrobenzylidene)-2-fluoroaniline 1b (4.099 g,
84%), mp 76–78ꢀC (ref. [19] mp 72–73ꢀC); IR (KBr): 1520
cmꢂ1 and 1348 cmꢂ1 (NO2); 1H NMR (CDCl3): d 7.15–7.26
(m, 4H, 4 Ar-H), 7.65 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H),
7.76 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H), 8.10 (d, J ¼ 8.0
Hz, 1H, Ar-H), 8.35 (d, J ¼ 8.0 Hz, 1H, Ar-H), 9.02 (s, 1H,
N¼¼CH); 13C NMR (CDCl3): d 116.3 (d), 121.8 (s), 124.5 (d),
124.6 (d), 127.7 (d), 129.9 (d), 130.9 (s), 131.5 (d), 133.7(d),
139.4 (d), 149.2 (s), 155.4 (s), 158.2 (d).
EXPERIMENTAL
The microwave oven used for the reactions was Model YD-
17 (W), Yoshii Electric Co., Ltd. The reaction vessel was ei-
ther a Pyrex test tube (15 mm i.d. ꢁ 19 mm o.d. ꢁ 129 mm
h.) placed in a 50-mL Erlenmeyer flask for support, a 50-mL
round bottom flask placed in a beaker for support, or a 50-mL
Erlenmeyer flask. The melting points are uncorrected. Column
chromatography was performed on silica gel (Wakogel C-200).
Unless otherwise stated, anhydrous sodium sulfate was used as
the drying agent. The IR spectra were measured on a Hitachi
Model 270-30 IR spectrometer. The 1H NMR and 13C NMR
spectra were measured at 500 MHz and 125 MHz, respec-
tively, on a Varian Unity plus-500W NMR spectrometer, using
tetramethylsilane as the internal standard. The starting materi-
als were synthesized according to literature while those which
were available commercially were used as received.
N-(2-nitrobenzylidene)aniline (1a). Compounds 1a–d were
prepared according to the procedure in literature [9(b)]. o-
Nitrobenzaldehyde (3.022 g, 20 mmol) and aniline (2.235 g,
24 mmol) were placed into a 100 mL round bottom flask. The
mixture was heated in a water bath at 70ꢀC for 15 min with
continuous stirring. The resulting product was separated from
the mixture and recrystallized from ethanol to give yellow
plates of N-(2-nitrobenzylidene)aniline 1a (4.203 g, 93%), mp
63–64ꢀC (ref. [9(b)] mp 64–65ꢀC); IR (KBr): 1518 cmꢂ1 and
1346 cmꢂ1 (NO2); 1H NMR (CDCl3): d 7.26–7.31 (m, 3H, 3
Ar-H), 7.43 (dd, J ¼ 7.5 Hz and 7.5 Hz, 2H, 2 Ar-H), 7.63
(dd, J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-H), 7.75 (dd, J ¼ 7.5Hz
and 7.5Hz, 1H, Ar-H), 8.08 (d, J ¼ 7.5Hz, 1H, Ar-H), 8.32 (d,
J ¼ 7.5Hz, 1H, Ar-H), 8.95 (s, 1H, N¼¼CH); 13C NMR
(CDCl3): d 121.2 (d), 124.5 (d), 126.9 (d), 129.2 (d), 129.7 (s),
131.1 (d), 131.2 (d), 133.5 (d), 149.3 (s), 151.0 (s), 155.8 (d).
N-(2-nitrobenzylidene)-2-fluoroaniline (1b). o-Nitrobenz-
aldehyde (3.022 g, 20 mmol) and 2-fluoroaniline (2.667 g, 24
mmol) were placed into a 100-mL round bottom flask. The
mixture was heated in a water bath at 70ꢀC for 15 min with
continuous stirring. The resulting product was separated from
1-(2-nitrobenzylidene)-2-phenylhydrazine (1c) [20]. o-
Nitrobenzaldehyde (3.022 g, 20 mmol) and phenylhydrazine
(2.595 g, 24 mmol) were placed into a 100-mL round bottom
flask. The mixture was heated in a water bath at 70ꢀC for 5
min with continuous stirring. The product was separated from
the mixture and recrystallized from ethanol to give red crystals
of 1-(2-nitrobenzylidene)-2-phenylhydrazine 1c (4.627 g,
96%), mp 118–119ꢀC (ref. [20] mp 117ꢀC); IR (KBr): 3292
cmꢂ1 (NH), 1532 cmꢂ1, and 1334 cmꢂ1 (NO2); 1H NMR
(CDCl3): d 6.93 (t, J ¼ 8.0 Hz, 1H, Ar-H), 7.14 (d, J ¼ 8.0
Hz, 2H, 2 Ar-H), 7.31 (dd, J ¼ 8.0 Hz and 8.0 Hz, 2H, 2 Ar-
H), 7.40 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H), 7.61 (dd, J
¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H), 7.99 (d, J ¼ 8.0 Hz, 1H, Ar-
H), 8.06 (br s, 1H, Ph-NH), 8.27 (d, J ¼ 8.0 Hz, 1H, Ar-H),
8.32 (s, 1H, N¼¼CH); 13C NMR (CDCl3): d 113.1 (d), 121.0
(d), 124.8 (d), 127.7 (s), 128.1 (d), 129.4 (d), 130.4 (d), 131.6
(d), 133.1 (d), 143.8 (s), 146.9 (s).
N-(2-nitrobenzylidene)-1-naphthylamine (1d) [21]. o-
Nitrobenzaldehyde (3.022 g, 20 mmol) and 1-naphthylamine
(3.437 g, 24 mmol) were placed into a 100-mL round bottom
flask. The mixture was heated in a water bath at 70ꢀC for 15
min with continuous stirring. The product was separated from
the mixture and recrystallized from ethanol to give yellow
plates of N-(2-nitrobenzylidene)-1-naphthylamine 1d (6.536
g, 95%), mp 110–111ꢀC; IR (KBr): 1514 cmꢂ1 and 1336
cmꢂ1 (NO2); 1H NMR (CDCl3): d 7.17 (d, J ¼ 8.0 Hz, 1H,
Ar-H), 7.50 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H), 7.52–
7.55 (m, 2H, 2 Ar-H), 7.66 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H,
Ar-H), 7.81 (d, J ¼ 8.0 Hz, 2H, 2 Ar-H), 7.87–7.88 (m, 1H,
Ar-H), 8.10 (d, J ¼ 8.0 Hz, 1H, Ar-H), 8.34-8.36 (m, 1H,
Ar-H), 8.50 (d, J ¼ 8.0 Hz, 1H, Ar-H), 9.04 (s, 1H, N¼¼CH);
13C NMR (CDCl3): d 113.2 (d), 123.6 (s), 124.5 (s), 126.0
(d), 126.0 (d), 126.5 (d), 126.8 (d), 127.7 (d), 128.8 (d),
130.0 (s), 131.1 (d), 131.2 (d), 133.5 (d), 133.9 (d), 148.2 (s),
155.7 (s), 155.8 (d).
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November 2009 Microwave-Assisted Cadogan Reaction for the Synthesis of 2-Aryl-2H-indazoles,
2-Aryl-1H-benzimidazoles, 2-Carbonylindoles, Carbazole, and Phenazine
1315
Hz, 2H, 2 Ar-H), 7.95 (d, J ¼ 7.5 Hz, 1H, Ar-H), 8.36 (s, 1H,
CH¼¼N); 13C NMR (CDCl3): d 21.6 (q), 121.1 (d), 124.5 (d),
125.0 (d), 129.2 (d), 132.1 (d), 133.8 (s), 135.4 (d), 142.9 (s),
142.9 (s), 146.8 (s), 161.8 (d).
N-benzylidene-2-nitroaniline (5a). Imine compounds 5a-c
were prepared according to literature [22]. o-Nitroaniline
(2.486g, 18 mmol), benzaldehyde (1.592 g, 15 mmol), sulfu-
ric acid (5 drops), and molecular sieves (15 g) were added to
benzene (30 mL) in a 100-mL round bottom flask. The mix-
ture was heated under reflux for 8.5 h using Soxhlet extractor
packed with molecular sieves. The resulting mixture was
extracted with benzene, filtered to remove the molecular
sieves, and the solvent evaporated under reduced pressure.
The extract was then chromatographed on a silica gel column
and eluted with hexane:EtOAc (85:15, 2% triethylamine) to
give N-benzylidene-2-nitroaniline 5a (2.787g, 82%). The
structure of N-benzylidene-2-nitroaniline 5a was determined
by comparison of mp and 1H NMR spectrum with those of
literature [22]. Recrystallization from hexane gave yellow
crystals, mp 70–71ꢀC (ref. [22] mp 71–72ꢀC); IR (KBr):
Ethyl 2-nitrocinnamate (9b). 2-Nitrobenzaldehyde (1.09 g,
7 mmol), triphenylphosphine (2.56 g, 9.8 mmol), ethyl bro-
moacetate (1.87 g, 11.2 mmol) and saturated aqueous solution
of NaHCO3 (15 mL) were placed in an Erlenmeyer flask and
stirred for 1.5 hours at 20ꢀC. The resulting mixture was
extracted with benzene, dried over anhydrous sodium sulfate,
and the solvent evaporated under reduced pressure. The extract
was chromatographed on a silica gel column and eluted with
hexane:EtOAc (3:1) to give ethyl 2-nitrocinnamate 9b (1.09 g,
68%) as colorless liquid. The structure of ethyl2-nitrocinna-
mate 9b was determined by comparison of IR, 1H NMR and
13C NMR spectra with those of literature [24]. IR (neat): 1700
cmꢂ1 (CO2), 1510 cmꢂ1, and 1272 cmꢂ1 (NO2); 1H NMR
(CDCl3): d 1.36 (t, J ¼ 7.5 Hz, 3H, CH3); 4.30 (q, J ¼ 7.5
Hz, 2H, CH2), 6.37 (d, J ¼ 17 Hz, 1H, C¼¼CH), 7.55 (dd, J ¼
7.5 Hz and 7.5 Hz, 1H, Ar-H), 7.64–7.65 (m, 2H, 2 Ar-H),
8.04 (d, J ¼ 7.5 Hz, 1H, Ar-H), 8.11 (d, J ¼ 17 Hz, 1H, Ph-
CH¼¼C); 13C NMR (CDCl3): d 14.7 (q), 60.9 (t), 123.9 (d),
125.5 (d), 129.8 (d), 130.9 (s), 132.8 (d), 134.1 (d), 139.1 (d),
140.4 (s), 165.7 (s).
1
1594 cmꢂ1 and 1334 cmꢂ1 (NO2); H NMR (CDCl3): d 7.06
(d, J ¼ 7.5 Hz, 1H, Ar-H), 7.30 (dd, J ¼ 7.5 Hz and 7.5 Hz,
1H, Ar-H), 7.47–7.55 (m, 3H, 3 Ar-H), 7.59 (dd, J ¼ 7.5 Hz
and 7.5 Hz, 1H, Ar-H), 7.91 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H),
7.96 (d, J ¼ 7.5 Hz, 1H, Ar-H), 8.41 (s, 1H, CH¼¼N); 13C
NMR (CDCl3): d 121.0 (d), 124.6 (d), 125.3 (d), 128.8 (d),
129.2 (d), 132.1 (d), 133.8 (d), 135.4 (s), 142.9 (s), 146.8 (s),
161.8 (d).
Methyl 2-nitrocinnamate (9c). 2-Nitrobenzaldehyde (1.09
g, 7 mmol), triphenylphosphine (2.56 g, 9.8 mmol), methyl
bromoacetate (1.71 g, 11.2 mmol) and saturated aqueous solu-
tion of NaHCO3 (15 mL) were placed in an Erlenmeyer and
stirred for 1.5 h at 20ꢀC. The resulting mixture was extracted
with benzene, dried over anhydrous sodium sulfate, and the
solvent evaporated under reduced pressure. The extract was
chromatographed on a silica gel column and eluted with hexa-
ne:EtOAc (3:1) to give methyl 2-nitrocinnamate 9c (0.98 g,
66%). The structure of methyl 2-nitrocinnamate 9c was deter-
mined by comparison of mp, 1H NMR and 13C NMR spectra
with those of literature [25]. Recrystallization from hexane
gave colorless plates, mp 70–73ꢀC (ref. [25(a)] mp 71–73ꢀC);
IR (KBr): 1712 cmꢂ1 (CO2), 1504 cmꢂ1, and 1332 cmꢂ1
(NO2); 1H NMR (CDCl3): d 3.84 (s, 3H, CH3), 6.37 (d, J ¼
17 Hz, 1H, C¼¼CH), 7.56 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-
H), 7.64–7.66 (m, 2H, 2 Ar-H), 8.05 (d, J ¼ 7.5 Hz, 1H, Ar-
H), 8.12 (d, J ¼ 17 Hz, 1H, Ph-CH¼¼C); 13C NMR (CDCl3): d
52.5 (q), 123.5 (d), 125.5 (d), 129.7 (d), 131.0 (s), 132.9 (d),
134.2 (d), 139.4 (d), 140.7 (s), 166.2 (s).
N-(4-chlorobenzylidene)-2-nitroaniline (5b). o-Nitroaniline
(2.486 g, 18 mmol), p-chlorobenzaldehyde (2.109 g, 15
mmol), sulfuric acid (5 drops), and molecular sieves (10 g)
were added to benzene (30 mL) in a 100-mL round bottom
flask. The mixture was heated under reflux for 10 h using
Soxhlet extractor packed with molecular sieves. The resulting
mixture was extracted with benzene, filtered to remove the
molecular sieves, and the solvent was evaporated under
reduced pressure. The extract was then chromatographed on a
silica gel column and eluted with hexane:EtOAc (85:15, 2%
triethylamine) to give N-(4-chlorobenzylidene)-2-nitroaniline
5b (2.226 g, 57%). Recrystallization from hexane gave yellow
crystals, mp 78–79ꢀC (ref. [23] mp 79–80ꢀC); IR (KBr): 1586
cmꢂ1 and 1348 cmꢂ1 (NO2); 1H NMR (CDCl3): d 7.05 (d, J
¼ 7.5 Hz, 1H, Ar-H), 7.32 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H,
Ar-H), 7.47 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 7.60 (dd, J ¼ 7.5 Hz
and 7.5 Hz, 1H, Ar-H), 7.85 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 7.97
(d, J ¼ 7.5 Hz, 1H, Ar-H), 8.37 (s, 1H, CH¼¼N); 13C NMR
(CDCl3): d 120.9 (d), 124.6 (d), 125.5 (d), 129.2 (d), 130.4
(d), 133.9 (d), 133.9 (s), 138.3 (s), 142.9 (s), 146.5 (s), 160.4
(d).
General procedure of the microwave-assisted Cadogan
reaction for the synthesis of indazoles, benzimidazoles,
indoles, carbazole, and phenazine. Microwave-assisted Cado-
gan reaction for the synthesis of the various N-containing het-
erocyclic compounds was performed using the appropriate
starting materials. Thus, for indazoles, the following starting
materials were used: N-(2-nitrobenzylidene)aniline, N-(2-nitro-
benzylidene)-2-fluoroaniline, 1-(2-nitrobenzylidene)-2-phenyl-
hydrazine and N-(2-nitrobenzylidene)-1-naphthylamine; for
benzimidazoles: N-benzylidene-2-nitroaniline, N-(4-chloroben-
zylidene)-2-nitroaniline and N-(4-methylbenzylidene)-2-nitro-
aniline; for indoles: chalcone, ethyl 2-nitrocinnamate and
methyl 2-nitrocinnamate; for carbazole: 2-nitrobiphenyl; and
for phenazine: 2-nitrodiphenylamine. One millimole of the
starting material was placed in the reaction vessel and added
with 4 mmol of triethyl phosphite or triphenylphosphine. The
reaction vessel containing the mixture was plugged with quartz
N-(4-methylbenzylidene)-2-nitroaniline (5c). o-Nitroaniline
(2.486 g, 18 mmol), p-methylbenzaldehyde (1.802 g, 15
mmol), sulfuric acid (5 drops), and molecular sieves (7.5 g)
were added to benzene (30 mL) in a 100-mL round bottom
flask. The mixture was heated under reflux for 9 h using Soxh-
let extractor packed with molecular sieves. The resulting mix-
ture was extracted with benzene, filtered to remove the molec-
ular sieves, and the solvent evaporated under reduced pressure.
The extract was then chromatographed on a silica gel column
and eluted with hexane:EtOAc (85:15, 2% triethylamine) to
give N-(4-methylbenzylidene)-2-nitroaniline 5c (1.783 g,
49%). Recrystallization from hexane gave yellow crystals, mp
71–72ꢀC (ref. [23] mp 72–74ꢀC); IR (KBr): 1592 cmꢂ1 and
1348 cmꢂ1 (NO2); 1H NMR (CDCl3): d 2.43 (s, 3H, CH3),
7.05 (d, J ¼ 7.5 Hz, 1H, Ar-H), 7.27–7.30 (m, 3H, 3 Ar-H),
7.58 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-H), 7.80 (d, J ¼ 7.5
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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E. Cuevas Creencia, M. Kosaka, T. Muramatsu, M. Kobayashi, T. Iizuka, and T. Horaguchi
Vol 46
wool and then placed inside the cavity of the microwave oven
(Model YD-17 (W), Yoshii Electric Co., Ltd.). The mixture
was irradiated at various power ratings and different reaction
times to get the best result. The resulting mixture was then
extracted with acetone, filtered, and the solvent evaporated
under reduced pressure. The extract was chromatographed on a
silica gel column and eluted with benzene, benzene:EtOAc,
hexane:EtOAc, or hexane:acetone to yield the different
products.
In the one-pot-one-step procedure, 2-nitrobenzaldehyde and
aryl amine or 2-nitroaniline and aryl aldehyde were mixed
with triethyl phosphite or triphenylphosphine in the reaction
vessel and irradiated with microwaves at different power rat-
ings and reaction times to get the best results. In the one-pot-
two-steps procedure, 2-nitrobenzaldehyde and aryl amine
were placed in the reaction vessel and irradiated with micro-
waves at certain power rating and reaction time, after which,
triethyl phosphite was added and the mixture irradiated again.
The products were isolated in the same manner as described
above.
(d), 127.4 (d), 127.9 (d), 128.8 (s), 129.5 (d), 134.0 (s), 137.5
(s), 149.5 (s).
2-Phenyl-1H-benzimidazole (8a). The structure of 2-phenyl-
1H-benzimidazole 8a was determined by comparison of mp,
IR, 1H NMR and 13C NMR spectra with those of literature
[14]. Colorless crystals from hexane-EtOAc, mp (hexane:
EtOAc) 289–290ꢀC (ref. [14] mp 290–292ꢀC); IR (KBr): 3436
1
cmꢂ1 (NH); H NMR ((CD3)2CO): d 7.21 (dd, J ¼ 7.5 Hz and
7.5 Hz, 2H, 2 Ar-H), 7.47–7.68 (m, 5H, 5 Ar-H), 8.24 (d, J ¼
7.5 Hz, 2H, 2 Ar-H), 11.97 (br s, 1H, NH); 13C NMR
(CD3OD): d 123.9 (d), 123.9 (d), 127.7 (d), 130.1 (d), 130.9
(d), 131.3 (s), 153.3 (s).
2-(4-Chlorophenyl)-1H-benzimidazole (8b). The structure of
2-(4-chlorophenyl)-1H-benzimidazole 8b was determined by
1
comparison of mp, IR and H NMR spectra with those of liter-
ature [14]. Colorless crystals from acetone, mp 291–292ꢀC
(ref. [14] mp 292–293ꢀC); IR (KBr): 3405 cmꢂ1 (NH); 1H
NMR ((CD3)2CO): d 7.22–7.24 (m, 2H, 2 Ar-H), 7.51–7.70
(m, 4H, 4 Ar-H), 8.23 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 11.98 (br
s, 1H, NH). No data for 13C NMR, not determined due to low
solubility.
2-Phenyl-2H-indazole (4a) [6]. Compound 4a was obtained
as white plates from hexane, mp 80–82ꢀC (ref. [26] mp 80–
2-(4-Methylphenyl)-1H-benzimidazole (8c). The structure of
2-(4-methylphenyl)-1H-benzimidazole 8c was determined by
1
comparison of mp, IR and H NMR spectra with those of liter-
1
82ꢀC); H NMR (CDCl3): d 7.12 (dd, J ¼ 7.5 Hz and 7.5 Hz,
1H, Ar-H), 7.32 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-H), 7.40
(t, J ¼ 7.5 Hz, 1H, Ar-H), 7.53, (dd, J ¼ 7.5 Hz and 7.5 Hz,
2H, 2 Ar-H), 7.71 (d, J ¼ 7.5 Hz, 1H, Ar-H), 7.80 (d, J ¼ 7.5
Hz, 1H, Ar-H), 7.91 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 8.41 (s, 1H,
NACH¼¼C); 13C NMR (CDCl3): d 117.9 (d), 120.4 (d), 120.4
(d), 121.0 (d), 122.1 (d), 122.7 (s), 126.8 (d), 127.9 (d), 129.5
(d), 140.5 (s), 149.8 (s).
ature [14]. Colorless crystals from acetone, mp (acetoꢂne1) 270–
271ꢀC (ref. [14] mp 270–272ꢀC); IR (KBr): 3476 cm (NH);
1H NMR ((CD3)2CO): d 2.40 (s, 3H, CH3), 7.17–7.21 (m, 2H,
2 Ar-H), 7.35 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 7.49–7.66 (m, 2H,
2 Ar-H), 8.11 (d, J ¼ 7.5 Hz, 2H. 2 Ar-H), 11.82 (br s, 1H,
NH). No data for 13C NMR, not determined due to low
solubility.
2-(2-Fluorophenyl)-2H-indazole (4b) [27]. Compound 4b
was obtained as yellow liquid, IR (neat): 1222 cmꢂ1 (Ar-F);
1H NMR (CDCl3): d 7.12 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H,
Ar-H), 7.28–7.40 (m, 4H, 4 Ar-H), 7.73 (d, J ¼ 8.0 Hz, 1H,
Ar-H), 7.79 (d, J ¼ 8.0 Hz, 1H, Ar-H), 8.09 (dd, J ¼ 8.0 Hz
and 8.0 Hz, 1H, Ar-H), 8.51 (s, 1H, NACH¼¼C); 13C NMR
(CDCl3): d 116.9 (d), 117.7 (d), 120.5 (d), 122.4 (d), 122.5 (s),
124.5 (d), 124.6 (d), 125.0 (d), 125.8 (d), 127.1 (d), 129.0 (s),
149.1 (s), 154.1 (s).
2-Phenylamino-2H-indazole (4c). Compound 4c was
obtained as white needles from benzene-hexane, mp 138–
139ꢀC (ref. [12] mp 140–142ꢀC); IR (KBr): 3168 cmꢂ1 (NH);
1H NMR (CDCl3): d 6.54 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 6.97
(t, J ¼ 7.5 Hz, 1H, Ar-H), 7.15 (dd, J ¼ 7.5 Hz and 7.5 Hz,
1H, Ar-H), 7.21 (dd, J ¼ 7.5 Hz and 7.5 Hz, 2H, 2 Ar-H),
7.34 (dd. J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-H), 7.65 (br s, 1H,
NH), 7.70 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 8.14 (s, 1H,
NACH¼¼); 13C NMR (CDCl3): d 114.1 (d), 117.6 (d), 120.4
(d), 122.3 (d), 122.5 (d), 124.1 (s), 126.7 (d), 128.3 (d), 129.2
(d), 146.8 (s), 147.3 (s).
2-Benzoylindole (10a). The structure of 2-benzoylindole
10a was determined by comparison of mp, IR and 1H NMR
spectra with those of literature [16]. Pale yellow needles from
hexane, mp 147–148ꢀC (ref. [16] mp 145–147ꢀC); IR (KBr):
1
3300 cmꢂ1 (NH), 1610 cmꢂ1 (CO); H NMR (CDCl3): d 7.17
(s, 1H, indole CH), 7.17 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-
H), 7.38 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H, Ar-H), 7.48 (d, J ¼
7.5 Hz, 1H, Ar-H), 7.54 (dd, J ¼ 7.5 Hz and 7.5 Hz, 2H, 2
Ar-H), 7.62 (t, J ¼ 7.5 Hz, 1H, Ar-H), 7.72 (d, J ¼ 7.5 Hz,
1H, ArH), 8.00 (d, J ¼ 7.5 Hz, 2H, 2 Ar-H), 9.31 (br s, 1H,
NH); 13C NMR (CDCl3): d 112.3 (d), 113.0 (d), 120.9 (d),
123.1 (d), 126.5 (d), 127.6 (s), 128.4 (d), 129.3 (d), 132.3 (d),
134.3 (s), 137.7 (s), 138.0 (s), 187.3 (s).
2-Ethoxycarbonylindole (10b). The structure of 2-ethoxy-
carbonylindole 10b was determined by comparison of mp, IR,
1H NMR and 13C NMR spectra with those of literature [17].
Colorless needles from hexane, mp 121–123ꢀC (ref. [17] mp
124–125ꢀC); IR (KBr): 3300 cmꢂ1 (NH), 1680 cmꢂ1 (CO2);
1H NMR (CDCl3): d 1.42 (t, J ¼ 7.5 Hz, 3H, CH3), 4.41 (q, J
¼ 7.5 Hz, 2H, CH2), 7.15 (dd, J ¼ 7.5 Hz and 7.5 Hz, 1H,
Ar-H), 7.23 (s, 1H, indole CH), 7.32 (dd, J ¼ 7.5 Hz and 7.5
Hz, 1H, Ar-H), 7.42 (d, J ¼ 7.5 Hz, 1 H, Ar-H), 7.69 (d, J ¼
7.5 Hz, 1H, Ar-H), 8.91 (br s, 1H, NH); 13C NMR (CDCl3): d
14.4 (q), 61.0 (t), 108.6 (d), 111.9 (d), 120.7 (d), 122.5 (d),
125.3 (d), 127.4 (s), 127.4 (s), 136.9 (s), 162.2 (s).
2-(1-Naphthyl)-2H-indazole (4d) [6]. Compound 4d was
obtained as yellow liquid, 1H NMR (CDCl3):d 7.18 (dd, J ¼
8.0 Hz and 8.0 Hz, 1H, Ar-H), 7.38 (dd, J ¼ 8.0 Hz and 8.0
Hz, 1H, Ar-H), 7.48 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H),
7.54 (dd, J ¼ 8.0 Hz and 8.0 Hz, 1H, Ar-H), 7.56 (dd, J ¼ 8.0
Hz and 8.0 Hz, 1H, Ar-H), 7.64 (d, J ¼ 8.0 Hz, 1H, Ar-H),
7.74 (d, J ¼ 8.0 Hz, 1H, Ar-H), 7.77 (d, J ¼ 8.0 Hz, 1H, Ar-
H), 7.86 (d, J ¼ 8.0 Hz, 1H, Ar-H), 7.94 (d, J ¼ 8.0 Hz, 1H,
Ar-H), 7.98 (d, J ¼ 8.0 Hz, 1H, Ar-H), 8.28 (s, 1H, NACH¼¼);
13C NMR (CDCl3): d 117.8 (d), 120.2 (d), 121.9 (d), 122.2
(d), 122.9 (d), 123.7 (d), 124.8 (d), 125.3 (s), 126.5 (d), 126.6
2-Methoxycarbonylindole (10c). The structure of 2-methox-
ycarbonylindole 10c was determined by comparison of mp, IR
and 1H NMR spectra with those of literature [18]. Colorless
crystal from hexane, mp 144–147ꢀC (ref. [18] mp 145–
147ꢀC); IR (KBr): 3308 cmꢂ1 (NH), 1680 cmꢂ1 (CO2); 1H
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

November 2009 Microwave-Assisted Cadogan Reaction for the Synthesis of 2-Aryl-2H-indazoles,
2-Aryl-1H-benzimidazoles, 2-Carbonylindoles, Carbazole, and Phenazine
1317
vitch, R. A.; Bulman, A. Synlett 1992, 795; (p) Varma, R. S. J Hetero-
cycl Chem 1999, 36, 1565.
NMR (CDCl3): d 3.95 (s, 3H, CH3), 7.16 (dd, J ¼ 7.5 Hz and
7.5 Hz, 1H, Ar-H), 7.23 (s, 1H, indole CH), 7.33 (dd, J ¼ 7.5
Hz and 7.5 Hz, 1H, Ar-H), 7.42 (d, J ¼ 7.5 Hz, 1H, Ar-H),
7.69 (d, J ¼ 7.5 Hz, 1H, Ar-H), 8.90 (br s, 1H, NH); 13C
NMR (CDCl3): d 51.0 (q), 108.8 (d), 111.9 (d), 120.8 (d),
122.6 (d), 125.4 (d), 127.0 (s), 127.4 (s), 137.0 (s), 162.6 (s).
[4] Cadogan, J. I. G.; Cameron-Wood, M. Proc Chem Soc
1962, 361.
[5] (a) Cadogan, J. I. G. Synthesis 1969, 1, 11; (b) Cadogan, J.
I. G. Quart Rev 1968, 22, 222; (c) Cadogan, J. I. G.; Mackie, R. K.
Chem Soc Rev 1974, 3, 87.
1
Carbazole (12). The mp, IR, H NMR and 13C NMR spec-
[6] Cadogan, J. I. G.; Cameron-Wood, M.; Mackie, R. K.;
Searle, R. J. G. J Chem Soc 1965, 4831.
tra of the compound were identical with those of commercially
available sample. Colorless plates from ethanol, mp 245-250oC
(ref. [6] mp 246–248oC); IR (KBr): 3420 cmꢂ1 (NH); 1H
NMR (CDCl3): d 7.24 (dd, J ¼ 7.5 Hz and 7.5 Hz, 2H, 2 Ar-
H), 7.42 (dd, J ¼ 7.5 Hz and 7.5 Hz, 2H, 2 Ar-H), 7.42 (d, J
¼ 7.5 Hz, 2H, 2 Ar-H), 8.04 (br s, 1H, NH), 8.09 (d, J ¼ 7.5
Hz, 2H, 2 Ar-H); 13C NMR (CDCl3): d 110.5 (d), 119.4 (d),
120.3 (d), 123.3 (s), 125.8 (d), 139.4 (s).
[7] Sundberg, R. J.; Yamazaki, T. J Org Chem 1967, 32, 290.
[8] (a) Cadogan, J. I. G.; Kulik, S.; Todd, M. J. Chem Commun
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1
Phenazine (14). The mp, IR, H NMR and 13C NMR spec-
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tra of the compound were identical with those of commercially
available sample. Pale yellow needles from ethanol, mp 169–
171ꢀC; 1H NMR (CDCl3): d 7.80 (ddd, J ¼ 7.5 Hz, 7.5 Hz
and 1.5 Hz, 4H, 4 Ar-H), 8.26 (dd, J ¼ 7.5 Hz and 1.5 Hz,
4H, 4 Ar-H); 13C NMR (CDCl3): d 129.6 (d), 130.3 (d), 143.3
(s).
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Acknowledgments. ECC would like to thank the Japan Society
for the Promotion of Science (JSPS) for the research grant at Nii-
gata University, Japan.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet