The reaction of [(1-azaazulen-2-yl)imino]phosphorane with arylaldehydes: Formation of bis[(1-azaazulen-2-yl)amino]arylmethanes

Article abstract of DOI:10.3987/COM-10-12050

Reaction of [(3-ethoxycarbonyl-1-azaazulen-2-yl)imino]triphenylphosphorane with arylaldehydes gave bis[(3-ethoxycarbonyl-1-azaazulen-2-yl)-amino] arylmethanes via corresponding azaazulenylimine intermediacy. The Japan Institute of Heterocyclic Chemistry.

Full text of DOI:10.3987/COM-10-12050

HETEROCYCLES, Vol. 81, No. 11, 2010
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HETEROCYCLES, Vol. 81, No. 11, 2010, pp. 2625 - 2633. © The Japan Institute of Heterocyclic Chemistry
Received, 21st August, 2010, Accepted, 28th September, 2010, Published online, 30th September, 2010
DOI: 10.3987/COM-10-12050
THE REACTION OF [(1-AZAAZULEN-2-YL)IMINO]PHOSPHORANE
WITH ARYLALDEHYDES: FORMATION OF BIS[(1-AZAAZULEN-2-
YL)AMINO]ARYLMETHANES
Hiroyuki Fujii,a Kentaro Nagamatsu,b Takahiro Gunji,c Toshihiro
Murafuji,d and Noritaka Abe*c,d
aScience Research Center, Yamaguchi University, Yamaguchi 753-8512, Japan;
bGraduate School of Science and Engineering, Yamaguchi University, Yoshida,
c
Yamaguchi 753-8512, Japan; Department of Pure and Applied Chemistry,
Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba
278-8510, Japan; dGraduate School of Medicine, Yamaguchi University, Yoshida,
Yamaguchi 753-8512, Japan: E-mail noriabe@rs.noda.tus.ac.jp
Abstract – Reaction of [(3-ethoxycarbonyl-1-azaazulen-2-yl)imino]triphenyl-
phosphorane with arylaldehydes gave bis[(3-ethoxycarbonyl-1-azaazulen-2-yl)-
amino]arylmethanes via corresponding azaazulenylimine intermediacy.
Aza-Wittig reaction is a useful method for the synthesis of imines and nitrogen-containing heterocycles,
where the intermolecular reaction of iminophosphoranes with aldehydes and ketones gives imine
derivatives and the tandem aza-Wittig reaction/intramolecular cyclization produces nitrogen-containing
heterocycles.1-10 Generally, a simple aza-Wittig reaction with aldehydes proceeds to give the
corresponding imines.
In the azaazulene chemistry, aza-Wittig reactions have been also used for synthesis of azaazulene fused
heterocycles.8,11 It is thought that azulenylimines and azaazulenylimines would be good precursors for
constructing azaazulene fused heterocycles, but the synthesis of azulenylimines and azaazulenylimines
from azulenylamines and azaazulenylamines have not been reported. Only few reports, where the reaction
of [(1-azaazulen-8-yl)imino]phosphorane (1) with arylaldehydes gave 212 (Scheme 1) and
[(azulen-2-yl)imino]phosphorane (3) with aldehydes (4) gave 513 (Scheme 2), showed the possibility of
the formations of azulenylimine and azaazulenylimines as intermediacy. Therefore, we investigated the
synthesis of (1-azaazulen-2-yl)imines by the reaction of [(1-azaazulen-2-yl)imino]phosphorane with

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HETEROCYCLES, Vol. 81, No. 11, 2010
arylaldehydes. In the reaction, expected (1-azaazulen-2-yl)imines were not isolated, but an interesting
reaction was observed and bis[(3-ethoxycarbonyl-1-azaazulen-2-yl)amino]arylmethanes were obtained.
Ph
Ph
ArCHO
N
N
N
N
PPh3
Ar
1
2
Scheme 1
H2N
H
H
O
H
O
N
N PR3
+
H
4
5
3
Scheme 2
For the synthesis of (1-azaazulen-2-yl)imine, we examined the condensation reaction of
2-amino-1-azaazulene with arylaldehyde at first. Treatment of 2-amino-1-azaazulene (6a) with
benzaldehyde (7a) in the presence of MS 4A under reflux in dry benzene for 24 h did not proceed.
Treatment of ethyl 2-amino-1-azaazulene-3-carboxylate (6b) with 7a in the presence of MS 4A under
reflux in dry xylene for 75 h gave bis[(3-ethoxyxcarbonyl-1-azaazulen-2-yl)amino]phenylmethane (8ba)
in 48% yield together with recovered 6b (33%) (Scheme 3). It is considered that the produced imine (9b)
in situ might react with 6b in the reaction to give 8ba. The structure of 8ba was deduced by spectroscopic
1
1
data as well as elemental analysis and mass spectrum. In the H NMR spectrum of 8ba, H triplet
assignable to a methine proton appeared at  7.79 (J 8.5), which was coupled with 2H broad doublet
assignable to NH appeared at  8.60 (J 8.5), together with seven-membered protons (10H) and two ethyl
ester protons (10H). In the 13C NMR spectrum, a methine carbon appeared at  64.22. From these results,
we assigned the structure. Although (1-azaazulen-2-y)imine was not isolated, the formation of reactive
(1-azaazulen-2-y)imine in the reaction was shown.
The results show that the amino group on the 1-azaazulene, being blocked of the reactive C-3, reacted
with the aldehyde. Ito reported about the reaction of 2-aminoazulene with aldehydes, where initial
reaction of the aldehyde occurred but not at the amino group at C-1 of azulene nuclei.14 The result
suggests that the amino group of the 1,3-position blocked 2-aminoazulenes would be possible to react
with aldehydes.

HETEROCYCLES, Vol. 81, No. 11, 2010
2627
CO2Et
Ph
G
6b
PhCHO / MS 4A
N C
NH2
reflux in xylene
G = CO2Et
N
H
N
9b
6a : G = H
6b
: G = CO2Et
CO2Et
CO2Et
NH
Ph
H
N C
N
N
H
9b
H
H
C
Ph
N
N
NH
EtO2C
N
CO2Et
8ba
6b
Scheme 3
As mentioned above, aza-Wittig reaction is an excellent method for the conversion of a P=N bond into a
C=N bond, being conducted under neutral conditions in the absence of catalysts and generally at mild
temperature,1-10 and a further reaction of the iminophosphoranes with the produced imines did not
occur.15,16 It is reported that the aza-Wittig reaction of [(1-azaazulen-2-yl)imino]phosphorane with
2-bromotropone
gave
6,7-diazaazuleno[1,2-a]azulene17
[(1,3-diazaazulen-2-yl)imino]phosphoranes with aryl
and
the
reactions
of
[(1-azaazulen-2-yl)imino]phosphoranes or
isocyanate proceeded via both aza-Wittig reaction and abnormal aza-Wittig reaction to give
corresponding triazine ring fused heterocycles.18-20 These results suggested that the possibility of the
synthesis of (1-azaazulen-2-yl)imines by aza-Wittig reaction. Therefore, we next investigated the reaction
of [(1-azaazulen-2-yl)imino]phosphoranes with arylaldehydes.
For affirmation of the formation of (arylbenzylidene)arylimines, we examined the reaction of simple
aryliminophosphoranes (10a,b) with 7a under slightly severe conditions. Thus, the reactions of 7a and
10b with 7a were performed in dry xylene at 125 ºC for 250 h and the corresponding imines (11a :65%
and 11b :55%) were isolated (Entry 1 and 2) (Scheme 4). In the reactions, the secondary reaction products,
such as the products from the reaction of 11 with 10, were not obtained.
N PPh3
+
OHC
N CH
X
X
7a
10a
10b
: X = CH
: X = N
11a
: X = CH
11b
: X = N
Scheme 4

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HETEROCYCLES, Vol. 81, No. 11, 2010
Then, we examined the reaction of [(1-azaazulen-2-yl)imino]phosphoranes with arylaldehydes. Reaction
of [(1-azaazulen-2-yl)imino]triphenylphosphorane (12a) with benzaldehyde (7a) in dry xylene at 60 ºC
for 94 h did not proceed and 12a was recovered (Entry 3). When the reaction was carried out in xylene at
125 ºC for 140 h, the reaction occurred but presented a complex feature; no distinct product was obtained
(Entry 4). We previously reported about the reaction of 1 with arylaldehydes, the existence of Pd(OAc)2
improved the reacton.12 So, the reaction of 12a with 7a was carried out in the presence of Pd(OAc)2, but
the reaction presented a complex feature again (Entry 5).
Table 1. Reactions of the iminophosphoranes (ArNPPh3) with arylaldehydes
Entry
ArNPPh3
Aldehyde
Conditions
Products (Yield / %)
Additive Temp/ ºC
Time/h
250
250
94
1
2
10b
10b
12a
12a
12a
12b
12b
12b
12b
12b
7a
7a
7a
7a
7a
7a
7a
7b
7c
7c
125
125
11a (65)
11b (55)
12a (89)
3
-
60
125
125
125
125
125
125
125
4
-
140
200
200
200
200
200
complex mixture
5
Pd(OAc)2
complex mixture
6
-
8ba (20) 12b (25) 6b (0.1)
complex mixture
7
Pd(OAc)2
8
-
8bb (24) 12b (11) 6b (19)
9
-
8bc (56) 12b (2)
12b (3)
6b (16)
6b (1)
10
6b, MS 4A
50+50 8bc (92)
When 12b was treated with 7a in dry xylene at 125 ºC for 200 h, 8ba was obtained in 20% yield together
with recovered 12b (25%) and 6b (0.1%) (Entry 6). In the reaction, the (1-azaazulen-2-yl)imine was not
isolated; even under the conditions where the recovery of [(1-azaazulen-2-yl)imino]phosphorane existed.
The result showed that the electron-withdrawing group accelerated the reaction. Addition of Pd(OAc)2 as
a catalyst gave a complex feature again (Entry 7). The reaction of 12b with p-tolualdehyde (7b) gave a
similar result as the case of the reaction with benzaldehyde (Entry 8). The reaction of 12b with
p-cyanobenzaldehyde (7c) gave 8bc in good yield (56%) together with 12b (2%) and 6b (16%) (Entry 9).
Interestingly, when 6b (about 10% to 12b) and MS 4A were added in the midway of the reaction, the
reaction was accelerated and 8bc was obtained in 92% yield (Entry 10).
Plausible mechanistic pathways are shown in Scheme 5. Two pathways could be considerable for the
reaction. Aza-Wittig reaction of the iminophosphorane (12b) with aldehydes gave the

HETEROCYCLES, Vol. 81, No. 11, 2010
2629
(1-azaazulen-2-yl)imine (9). Successive reaction of 9 with 2-amino-1-azaazulene (6b), which would be
produced by hydration of 12b, gives 8 (path A). The imine (9) have a resonance form 9A. When the
reaction underwent between 9A and 12b, it considers that the reaction proceed as follows: the nitrogen of
the iminophosphorane (12b) attacks to the benzylic cationic carbon of 9A by squeezing the electrons on
the oxide moiety to the nitrogen as shown with arrows, and the nitrogen of imine of 9A attacks to
phosphonium of 12b concurrently, and the diazaphosphetane (13) could be formed as an intermediate.
Hydrolysis of 13 by work-up with H2O furnishes 8 (path B).
It seems that the result, being addition of 6b enhanced the reaction, supports path A at a glance. Never the
less, the possibility of being under way on path B remains. The reaction of 6b with 13 could give 8 and
12b. This would come to cause the catalytic acceleration of the reaction of 6b. In fact, the amount of
added 6b is little in the reaction, but the enhancement of the yield is large, and the reaction time is
shortened. In addition, the reaction was performed under water-free conditions, and a considerable
amount of 12b was recovered after work-up, even after the chromatography on silica gel. These implied
that the hydrolysis of 12b under the conditions is implausible. Furthermore, the results that the reaction
underwent in the case of using 12b, having an ester group at C-3 of 1-azaazulene skeleton, and the
p-cyano group on phenyl group accelerated the reaction are suggestive. Regarding the reaction of
iminophosphorans, Nitta reported that an iminophosphorane can undergo a nucleophilic attack to the
methylene, and the reaction depends on the nature of the Michael acceptor.13 From these considerations,
we prefer path B to path A.
O
O
OEt
OEt
G
C
C
Ar
H
Ar
H
7 : ArCHO
N PPh3
N C
N
C
G = CO2Et
N
N
N
N
7a : Ar = Ph
7b
9A
9
12a
: G = H
12b : G = CO2Et
Ph3P
N
: Ar = p-C6H4Me
7c : Ar = p-C6H4CN
6b
C
O
EtO
12b
path A
CO2Et
N C
path B
Ar
H
CO2Et
NH
N
CO2Et
N
Ar
9
H
H
H2O
N
H
N
C
N
C
H
Ar
N
EtO2C
N
N
N
NH
Ph3P
CO2Et
8ba : Ar = Ph
EtO2C
6b
13
6b
8bb
: Ar = p-C6H4Me
8bc : Ar = p-C6H4CN
+
12b
8
Scheme 5

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HETEROCYCLES, Vol. 81, No. 11, 2010
CONCLUSION
Reaction of [(3-ethoxycarbonyl-1-azaazulen-2-yl)imino]triphenylphosphorane (12b) with arylaldehydes
was investigated. From the reaction bis[(3-ethoxycarbonyl-1-azaazulen-2-yl)amino]arylmethanes (8)
were obtained. The reaction is thought to undergo via forming (1-azaazulen-2-yl)imine (9), and a
successive reaction of 9 with 12b gave diazaphosphetane (13) as an intermediate, and hydrolysis of 13
furnished 8.
EXPERIMENTAL
Melting points were determined with a Yanagimoto micro-melting point MP JP-3 apparatus and are
1
uncorrected. H NMR spectra and 13C NMR spectra were recorded on a Bruker AVANCE 400S (400
1
MHz for H and 100.6 MHz for 13C) using CDCl3 as a solvent with tetramethylsilane as an internal
standard; J values are recorded in Hz. IR spectra were recorded for KBr pellets on a Nicolet FT-IR
AVTAR 370DTGS unless otherwise stated. MS spectra were taken with a LC-MS Waters Integrity
System. Merck Kieselgel 60 was used for column chromatography. Benzene and xylene were distilled on
CaH2.
Reaction of ethyl 2-amino-1-azaazulene-3-carboxylate (6b) with benzaldehyde
A mixture of ethyl 2-amino-1-azaazulene-3-carboxylate (6b) (0.300 g, 1.39 mmol) and benzaldehyde (7a)
(1 mL) in dry xylene (30 mL) in the presence of MS 4A (1.50 g) was refluxed for 75 h. The solvent was
evaporated
and
chromatography
of the residue with hexane-CHCl3
(1:1)
gave
bis[(3-ethoxycarbonyl-1-azaazulen-2-yl)amino]phenylmethane (8ba) (0.0172 g, 48%) and 6a (0.100 g,
33%), successively.
8ba: Yellow needles (MeCN), mp 177.0-179.0 ºC; 1H NMR  1.45 (6H, t, J 7.1, CH3), 4.44 (8H, q, J 7.1,
OCH2), 7.29 (1H, t, J 7.1, H-p-phenyl), 7.36 (2H, dd, J 7.6 and 7.1, H-m-phenyl), 7.48 (2H, t, J 9.6,
H-6,6’), 7.63 (4H, like-t, J 9.6, H-5,7,5’,7’), 7.66 (2H, d, J 7.6, H-o-phenyl), 7.79 (1H, t, J 8.5, N-CH-N),
8.17 (2H, d, J 9.8, H-8,8’), 8.60 (2H, br d, J 8.5, NH), and 8.86 (2H, d, J 10.3, H-4,4’); 13C NMR  15.07,
60.40, 64.22, 98.19, 126.74, 128.37, 129.07, 129.91, 130.82, 132.56, 133.52, 134.04, 140.96, 148.50,
161.79, 166.57, and 168.18; IR max / cm-1 3379 (NH), 1668 (C=O); MS m/z (rel intensity) 521 (M+ + 1,
4), 369 (21), 306 (74), 275 (63), 259 (100), 216 (36), 170 (36), 144 (23), 105 (37), 91 (34), and 77 (40).
Anal. Calcd for C31H28N4O4•1/2CH3CN: C, 71.03; H, 5.50; N. 11.65. Found: C, 71.22; H, 5.64; N, 11.53.
Reaction of (arylimino)triphenylphosphoranes with benzaldehyde
A mixture of (phenylimino)triphenylphosphorane (10a) (0.176 g, 0.50 mmol), benzaldehyde (7a) (0.052

HETEROCYCLES, Vol. 81, No. 11, 2010
2631
mL, 0.50 mmol) in dry xylene (3 mL) was stirred for 250 h under heating at 125 ºC. The solvent was
evaporated. Chromatography of the residue with hexane-ethyl acetate (3:1) gave benzylideneaniline21
(11a) (0.0586 g, 65%).
1
11a: Pale yellow solid, mp 49-51 ºC (lit.,21 49-51 ºC); H NMR  7.20 (2H, dd, J 7.3 and 0.9,
H-o-N-phenyl), 7.21 (1H, td, J 7.3 and 0.9, H-p-N-phenyl), 7.38 (2H, t, J 7.3, H-m-N-phenyl), 7.44-7.66
(3H, m, H-m,p-phenyl), 7.79 (2H, m, H-o-phenyl), and 8.49 (1H, s, CH=N); 13C NMR  120.85, 125.91,
128.74, 128.79, 129.12, 131.35, 136.19, 152.06, and 160.36; IR max / cm-1 1627 (C=N); MS m/z (rel
intensity) 181 (M+, 35), 180 (41), 105 (11), 104 (11), 85 (64), 83 (100), and 77 (32). Anal. Calcd for
C13H11N: C, 86.15; H, 6.12; N, 7.73. Found: C, 86.04; H, 6.22; N, 7.65.
In a similar manner, the reaction of 10b with 7a gave 11b21 (55%).
1
11b: Pale yellow oil; H NMR  7.15 (1H, dd, J 7.4, 7.3, and 1.0, Py-H-5), 7.32 (1H, d, J 7.9, Py-H-3),
7.44-7.50 (3H, m, H-m,p-phenyl), 7.72 (1H, ddd, J 7.9, 7.4, and 1.9, Py-H-4), 7.99 (2H, dd, J 7.3 and 2.3,
13
H-o-phenyl), 8.49 (1H, ddd, J 4.9, 1.9, and 1.0, Py-H-6), and 9.14 (1H, s, CH=N); C NMR  119.77,
121.84, 128.74, 129.46, 131.90, 135.85, 138.09, 148.83, 161.09, and 162.87; IR max / cm-1 (neat) 1625
(C=N); MS m/z (rel intensity) 183 (M++1, 9), 182 (M+, 11), 181 (26), 107 (11), 105 (17), 94 (11), 79
(100), 78 (25), and 77 (20). Anal. Calcd for C12H10N2: C, 79.10; H, 5.53; N, 15.37. Found: C, 79.36; H,
5.68; N, 15.18.
Reaction of (1-azaazulen-2-ylimino)triphenylphosphoranes with arylaldehyde
Typical procedure – a) Under argon atmosphere, a mixture of [(3-ethoxycarbonyl-1-azaazulen-2-
yl)imino]triphenylphosphorane (12b) (0.120 g, 0.25 mmol), benzaldehyde (7a) (0.063 mL, 0.60 mmol) in
dry xylene (3 mL) was stirred for 200 h under heating at 125 ºC. The solvent was evaporated and
chromatography of the residue with hexane-EtOAc (3:1) gave bis[(3-ethoxycarbonyl-1-
azaazulen-2-yl)amino]phenylmethane (8ba) (0.0129 g, 20%), 12b (0.030 g, 25%), and ethyl
2-amino-1-azaazulene-3-carboxylate (6b) (0.0001 g, 0.1%).
b) Under argon atmosphere, a mixture of 12b (0.2404 g, 0.50 mmol), p-cyanobenzaldehyde (7c) (0.1301
g, 1.00 mmol) in dry xylene (3 mL) was stirred for 50 h under heating at 125 ºC. Then, to the mixture
were added 6b (0.010 g, 0.046 mmol) and MS 4A (0.050 g), and the heating was continued for 50 h at
125 ºC. The solvent was evaporated and chromatography of the residue with hexane-EtOAc (3:1) gave
8bc (0.1254 g, 92%), 12b (0.0078 g, 3%), and 6b (0.002 g, 1%).
The results of the reactions of 12b with 7a-c were shown in Table 1.
8bb: Yellow needles (benzene-hexane), mp 152.5-154.5 ºC; 1H NMR  1.44 (6H, t, J 7.1, CH3), 2.32 (3H,
s, CH3), 4.44 (8H, q, J 7.1, OCH2), 7.15 (2H, d, J 8.1, H-2’’,6’’-phenyl), 7.46 (2H, dd, J 10.5 and 9.2,

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HETEROCYCLES, Vol. 81, No. 11, 2010
H-6,6’), 7.55 (2H, d, J 8.1, H-3’’,5’’-phenyl), 7.62 (2H, dd, J 10.2 and 9.2, H-5,5’), 7.64 (2H, dd, J 10.5,
and 9.9, H-7,7’), 7.74 (1H, t, J 8.5, N-CH-N), 8.16 (2H, d, J 9.9, H-8,8’), 8.54 (2H, br d, J 8.5, NH), and
8.85 (2H, d, J 10.2, H-4,4’); IR max 3350 (NH), 1665 (C=O). Anal. Calcd for C32H30N4O4•1/2C6H6: C,
73.28; H, 5.80; N. 9.77. Found: C, 73.60; H, 5.55; N, 10.08.
8bc: Yellow needles (benzene-hexane), mp 121.0-122.5 ºC; 1H NMR  1.44 (6H, t, J 7.1, CH3), 4.44 (8H,
q, J 7.1, OCH2), 7.52 (2H, t, J 9.7, H-6,6’), 7.64 (2H, dd, J 10.4 and 9.7, H-5,5’), 7.66 (2H, d, J 8.4,
H-3’’,5’’-phenyl), 7.68 (2H, dd, J 10.0, and 9.7, H-7,7’), 7.71 (1H, t, J 8.3, N-CH-N), 7.75 (2H, d, J 8.4,
H-2’’,6’’-phenyl), 8.15 (2H, d, J 10.0, H-8,8’), 8.79 (2H, br d, J 8.3, NH), and 8.89 (2H, d, J 10.4,
H-4,4’); 13C NMR  14.07, 60.11, 64.10, 98.00, 111.52, 118.90, 127.43, 130.03, 130.68, 132.38, 132.70,
133.29, 133.71, 141.92, 148.25, 153.80, 161.03, and 166.03; IR max 3342 (NH), 2227 (CN), 1663 (C=O);
MS m/z (rel intensity) 544 (M++1, 1), 331 (9), 301 (28), 285 (29), 217 (100), 170 (37), 144 (28), 86 (83),
and 77 (18). Anal. Calcd for C32H27N5O4•1/2C6H6: C, 71.90; H, 5.17; N. 11.98. Found: C, 71.59; H, 4.93;
N, 11.90.
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