Synthesis of 2-arylquinoxalines: Triarylstibane-catalyzed oxidative cyclization of α-hydroxy ketones with 1,2-diamines under aerobic conditions

Article abstract of DOI:10.3987/COM-15-S(T)3

The reaction of α-hydroxy ketones with 1,2-diamines in the presence of triphenylstibane (10 mol%) as catalyst led to the formation of 2-arylquinoxalines in moderate to good yield under aerobic conditions. This reaction is the first example of oxidative cy

Full text of DOI:10.3987/COM-15-S(T)3

HETEROCYCLES, Vol. 93, No. 1, 2016, pp. -. © 2016 The Japan Institute of Heterocyclic Chemistry
Received,10th June, 2015, Accepted, 16th July, 2015, Published online, 24th July, 2015
DOI: 10.3987/COM-15-S(T)3
SYNTHESIS OF 2-ARYLQUINOXALINES: TRIARYLSTIBANE-
CATALYZED OXIDATIVE CYCLIZATION OF -HYDROXY KETONES
WITH 1,2-DIAMINES UNDER AEROBIC CONDITIONS
Mio Matsumura,1 Rie Takada,1 Yuu Ukai,1 Mizuki Yamada,1 Yuki Murata,2
Naoki Kakusawa,2 and Shuji Yasuike1*
1
School of Pharmaceutical Sciences, Aichi Gakuin University, 1-100 Kusumoto-
2
cho, Chikusa-ku, Nagoya 464-8650, Japan. Faculty of Pharmaceutical Sciences,
Hokuriku University, Kanagawa-machi, Kanazawa 920-1181, Japan
Abstract – The reaction of -hydroxy ketones with 1,2-diamines in the presence
of triphenylstibane (10 mol%) as catalyst led to the formation of
2-arylquinoxalines in moderate to good yield under aerobic conditions. This
reaction is the first example of oxidative cyclization catalyzed by organoantimony
compounds.
INTRODUCTION
Quinoxaline is an important building block for the synthesis of functional organic materials,1,2 and
biologically active compounds.3 For examples, 2-arylquinoxalines are known for biological activities
such as kinase inhibitor4 and adenosine receptor antagonist5 (Figure 1). Consequently, many methods
have been developed for the synthesis of 2-arylquinoxalines. Among these methods, the oxidative
cyclization of α-hydroxy ketones with 1,2-diamines in the presence of a transition metal or inorganic
catalyst such as MnO2,6 Pd(OAc)2,7 HgI2,8 CuCl2,9 manganese oxide octahedral molecular sieves,10
NbCl5,11 Ga(ClO4)3,12 and CaO13 are known.
Figure 1. Examples of biologically active compound containing 2-arylquinoxaline skeleton.

On the other hand, pentavalent organoantimony compounds such as stibane oxide,14
bromodiphenylstibane,15 dibromotriphenylstiborane,16 and stibane imides17 are known to be effective for
achieving the oxidation of thiols, benzyl alcohols, benzopinacol, and diaryl--ketoalcohols. In these
reactions, however, it is inevitably necessary to add a stoichiometric or excess amount of the antimony
reagent and/or the base to ensure a smooth reaction. Recently, we also reported triarylstibane to be a
useful catalyst for the oxidation of benzoin to the corresponding benzyl derivative under mild
conditions.18 As a continuation of our studies on the promotion of oxidative catalytic reactions by
organoantimony reagents, we now report the one-pot tandem oxidative cyclization of -hydroxy ketones
with 1,2-diamines to afford the corresponding 2-arylquinoxalines under aerobic conditions.
RESULTS AND DISCUSSION
Previously, we reported that benzoin compounds were oxidized to the corresponding benzyl derivatives in
the presence of a catalytic amount of triphenylstibane (3a).18 The oxidation of 2-hydroxyacetophenone
(1a) by using triphenylstibane (3a) was performed before the oxidative cyclization of 1a with
o-phenylendiamine (2a). However, the oxidation of 1a with 3a resulted in a complex mixture and did not
afford phenylglyoxal in dichloromethane at room temperature under aerobic conditions. Therefore, we
first studied the effect of solvents, to search for suitable catalysts and reaction conditions for the oxidative
cyclization of 1a with 2a. The results are shown in Table 1. Initially, we performed the reaction between
1a and 2a using 10 mol% of triphenylstibane (3a) as catalyst at room temperature under aerobic
conditions to compare the effect of solvent (entries 1-5). The expected 2-phenylquinoxaline (8) was
obtained in an excellent yield when toluene and dichloromethane were employed as the solvent, whereas
tetrahydrofuran (THF), acetonitrile, and ethanol gave inferior results. Next, several antimony catalysts
were also screened in toluene under the same conditions (entries 2, 6-13). Trivalent organoantimony
catalysts with electron-donating and -withdrawing groups on the aromatic ring, p-methoxyphenyl 3b and
p-trifluoromethylphenyl 3c, produced 8 in satisfactory yields (entries 6, 7). On the other hand, the
2,6-dimethoxyphenyl and mesityl derivatives (3d, e), which are sterically hindered, and the inorganic
reagent (3f) were inactive with a low yield despite longer reaction times (entries 8-10). Moreover, the
pentavalent organoantimony catalysts (3g-i) also produced inferior results (entries 11-13). As a result,
triphenylstibane (3a) proved to be superior to the other antimony catalysts (3b-i) in terms of the reaction
time, yield of 8, and the fact that this catalyst is commercially available. A remarkable decrease in the
yield of the product 8 under an argon atmosphere indicated that the presence of oxygen in the reaction
medium was essential for the reaction to proceed (entry 14). The progressive addition of pure dioxygen
shortened the reaction time (entry 15).

Table 1. Screening of reaction conditionsa
Entry
1
2
3
4
5
6
7
8
Catalyst M
Ar
X
Solvent
CH2Cl2
toluene
THF
Atmosphere
air
Time (h)
5
Yield (%)b
80
3a
3a
3a
3a
3a
3b
3c
3d
3e
3f
3g
3h
3i
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
Sb
N
C6H5
---
C6H5
---
air
3
83
C6H5
---
---
---
---
air
24
24
24
2
52
C6H5
MeCN
MeOH
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
air
46
C6H5
air
36
4-MeOC6H4
4-CF3C6H4
2,6-(MeO)2C6H4
mesityl
Br
air
78
---
air
4
80
---
---
---
C6H5
OAc
Cl
air
24
24
24
24
12
12
24
0.5
24
24
24
24
24
43
9
air
41
10
11
12
13
14
15
16c
17
18
19
20
air
32
C6H5
air
54
C6H5
air
65
C6H5
air
64
3a
3a
3a
4
C6H5
---
argon
O2
31
C6H5
---
75
C6H5
---
---
---
---
air
48
C6H5
air
20
5
P
C6H5
air
36
6
As
Bi
C6H5
air
34
7
C6H5
---
air
51
aAll reaction were carried out using 1a (1 mmol), 2a (1.2 mmol), catalyst (10 mol%) and solvent (6 mL). bIsolated
yield. cPh3Sb (5 mol%) was used.
Reactions under aerobic conditions are simple and easy to operate. It is apparent that the reaction is
sensitive to catalyst loading, as decreasing the loading of triphenylstibane from 10 mol% to 5 mol% gave
a significantly reduced yield of 8 (48%, entry 16). Finally, the reaction of 1a with 2a was repeated to
compare the reactivity with 10 mol% of pnictogen reagents as the catalyst (entries 2, 17-20). The results
clearly showed that triphenylstibane (3a) is the most effective catalyst to prepare the expected product (8).
Thus, the most favorable reaction conditions were determined to be as follows: 2-hydroxyacetophenone
(1a) was treated with o-phenylendiamine (2a) with the use of triphenylstibane (3a) as catalyst in toluene
or dichloromethane at room temperature under aerobic conditions.

The sufficiency and generality of this protocol was evaluated by examining the reactions of various
α-hydroxy ketones (1a-h) and o-phenylendiamines (2a-h), and the results are summarized in Table 2. The
reaction solvent was selected based on the solubility of 1 and 2. -Hydroxy ketones (1a-f) bearing
electron-donating or –withdrawing group on the aromatic ring gave the corresponding 2-arylquinoxalines
(9-13) in moderate to good yields (entries 1-5). The reaction of the naphthyl and heteroaryl derivatives
(1g, h) also obtained the expected products 14 and 15, respectively (entries 6, 7). Unfortunately, the
reaction of hydroxyacetone and methyl glycolate resulted in a complex mixture and did not afford the
desired quinoxalines. The results indicate that this oxidative cyclization has high substrate specificity and
is effective for converting α-hydroxy ketones containing an aryl group into the corresponding
2-arylquinoxalines. Various o-phenylendiamines (2b-h) were reacted with 2-hydroxyacetophenone (1a)
under the same reaction conditions. The diamine (2b-e) with electron-donating and -withdrawing group
afforded the product in moderate to good yields (entries 8-11). The reaction of 2,3-diaminonaphthalene
(2f) with 1a was also gave the benzo[g]quinoxaline (entry 12). The reactions of 2b, e, f, all of which were
least soluble in the solvent required prolonged reaction times. The mono-substituted diamines (2g and 2h)
afforded mixtures of two regioisomers, of which the 6-substituted isomer (b) was the major product
(entries 13, 14). When the aliphatic, instead of aromatic, diamine was used the reaction of 1a with
(±)-trans-1,2-diaminocyclohexane (23) gave the corresponding hexahydroquinoxaline (24) in 24% yield
(Scheme 1). The product (24) was unstable for purification by chromatography despite using amino-
functionalized silica and aluminum oxide, and therefore isolated in a disappointing yield.
Table 2. Synthesis of 2-arylquinoxalinesa
R1
NH2
NH2
R1
R2
N
N
Ar
O
Ph3Sb (10 mol%)
rt, air
OH
+
Ar
R2
1a-h
2a-h
9-22
Entry
1
Ar
2
R1
R2
Solvent
toluene
toluene
CH2Cl2
CH2Cl2
toluene
toluene
toluene
CH2Cl2
toluene
toluene
Time (h) Product
Yield (%)b
1
2
3
4
5
6
7
8
9
10
1b
4-MeOC6H4
4-MeC6H4
4-BrC6H4
4-CNC6H4
4-CF3C6H4
2-naphthyl
2-thienyl
C6H5
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
H
H
3
3
9
74
70
72
60
68
71
74
50
72
75
1c
1d
1e
1f
10
11
12
13
14
15
16
17
18
5
5
3
1g
1h
1a
1a
1a
3
4
MeO
Me
Br
MeO
Me
Br
24
6
3

11
12
13
1a
1a
1a
2e
2f
CN
CN
H
CH2Cl2
CH2Cl2
CH2Cl2
48
24
5
19
59
80
13d
42d
27d
52d
naphthyl
20
21ac
21bc
22ac
22bc
2g MeO
14
1a
2h CF3
H
toluene
3
a All reaction were carried out using 1 (1 mmol), 2 (1.2 mmol), 3a (10 mol%) and solvent (6 mL). b Isolated yield.
c 21a: 7-methoxy-2-phenylquinoxaline, 21b: 6-methoxy-2-phenylquinoxaline, 22a: 2-phenyl-7-trifluoromethyl-
quinoxaline, 22b: 2-phenyl-6-trifluoromethylquinoxaline. d Determined by 1H NMR.
O
N
N
NH2
NH2
OH
Ph3Sb (10 mol%)
KOH
+
CH2Cl2
rt, air
MeOH
reflux, 20 h
N
N
1a
23
24
25
Scheme 1. Synthesis of hexahydro- and tetrahydroquinoxaline (24, 25)
Furthermore, we examined the one-pot two-step reaction for the preparation of tetrahydroquinoxaline (25)
from 1a and 23 by using a Ph3Sb-catalyzed cyclization and the aromatization19 that was previously
published by Raw et al. This method involved the addition of KOH and MeOH after the completion of
the cyclization step to the same reaction flask, after which the reaction mixture was heated under reflux
for 20 h to afford 25 in 45% yield.
Figure 2. Possible mechanism

The reaction mechanism of this oxidative cyclization is unclear at present. The catalytic cycle of this
reaction would be similar to that of the oxidation of diaryl--keto-alcohols reported by us.18 Furthermore,
it is known that organoantimony compounds formed intermolecular non-bonding interaction between the
antimony atom and the nitrogen (N) and/or oxygen (O) atoms in other molecules.20 Possible mechanisms
of the reaction between α-hydroxy arylketones and o-phenylendiamines in the formation of quinoxalines
are depicted in Figure 2. The initial step of this reaction involves the oxidation of 3a to the SbPh-oxo
species (oxide) (C) with aerobic oxygen.18 Actually, the reaction of 1a with 2a by using a catalytic
amount of tripehenylstibane oxide (Ph3SbO) brought about the same cyclization to afford 8 in high yield
(84%). On the other hand, 1 is transformed into -iminoalcohol D by diamine (2). The oxide (C) formed
as mentioned above may react with D to produce pentavalent antimony intermediates E and F which
undergo reductive antimony(III) elimination to provide -iminoketone G and regenerate 3a. Finally,
cyclodehydration occurs in the transformation from G to afford 2-arylquinoxalines. Another pathway
through 1 is also oxidized by antimony oxide C and generates arylglyoxal J via H and I. J readily react
with diamine (2) to give intermediates K and/or G, which undergo cyclodehydration to form the
2-arylquinoxalines. We probed the mechanism of this transformation by using NMR spectroscopy. The
spectra clearly showed an increase in the products as the starting materials decreased. However, the
proposed intermediate species A~K, including G and J, could not be observed on the NMR time scale.
CONCLUSION
In conclusion, we found triphenylstibane to be a new class of catalyst for the synthesis of
2-arylquinoxalines, and it is the first examine of oxidative cyclization catalyzed by organoantimony
compounds. The reaction procedure is an experimentally simple oxidative cyclization of α-hydroxy
ketones with 1,2-diamines that can be conducted either toluene or dichloromethane at room temperature
under aerobic conditions. The antimony reagents, such as triphenylstibane have low toxicity,21 therefore
hold great promise for organic synthetic reagents. The synthetic application of this reaction and a detailed
investigation of the reaction mechanisms are in progress.
EXPERIMENTAL
Unless otherwise indicated, chemicals and solvents were purchased from commercial suppliers and used
without further purification. All chromatographic separations were accomplished with Silica Gel 60N
(Kanto Chemical Co., Inc.) or NH Silica Chromatorex DM2035 (Fuji Silysia Chemcal, Ltd.). Thin-layer
chromatography (TLC) was performed with MERCK TLC Silicagel 60 F254 and MERCK TLC
aluminum oxide F254, neutral. Melting points were taken on a Yanagimoto micro melting point hot-stage
apparatus. 1H NMR (TMS: δ: 0.00 ppm as an internal standard) and 13C NMR (CDCl3: δ: 77.00 ppm as an

internal standard) spectra were recorded on a JEOL JNM- AL400 (400 MHz and 100 MHz) spectrometers
in CDCl3. Mass spectra (MS) were obtained on a JEOL JMS-SX-102A instrument.
General procedure for the synthesis of 2-arylquinoxaline derivatives (8-22 and 24). Triphenylstibane
(35.5 mg, 0.1 mmol, 10 mol%) and diamine 2 (1.2 mmol) were added to a solution of -hydroxy ketone 1
(1 mmol) in toluene (6 mL) under air. The solution was stirred at room temperature and monitored by
TLC. The reaction mixture was concentrated under reduced pressure and the residue was purified by
column chromatography (CH2Cl2) on silica gel. The products were confirmed by comparison of mp,
NMR data, and MS spectra with that in the literature. 822: Yellow needles (from MeOH), mp 74-76 ˚C (lit.
75-78 ˚C), 922: Colorless needles (from CH2Cl2-n-hexane), mp 100-102 ˚C (lit. 97-100 ˚C), 1022:
Colorless needles (from CH2Cl2-n-hexane), mp 93-96 ˚C (lit. 93-96 ˚C), 1122: Colorless needles (from
CH2Cl2-n-hexane), mp 135-137 ˚C (lit. 138 ˚C), 1222: Colorless needles (from CH2Cl2-n-hexane), mp
191-193 ˚C (lit. 193-195 ˚C), 1323: Colorless needles (from CH2Cl2-n-hexane), mp 145.5-146.5 ˚C (lit.
142-143 ˚C), 1424: Colorless needles (from CH2Cl2-n-hexane), mp 140-142.5 ˚C (lit. 141-142 ˚C), 1524:
Colorless needles (from CH2Cl2-n-hexane), mp 120-121.5 ˚C (lit. 118-119 ˚C), 1625: Colorless needles
(from CH2Cl2-n-hexane), mp 130-133 ˚C (lit. 134 ˚C), 1726: Colorless needles (from CH2Cl2-n-hexane),
mp 126-128 ˚C (lit. 127-129 ˚C), 2025: Colorless needles (from CH2Cl2-n-hexane), mp 160-162 ˚C (lit.
1
163 ˚C), 2419: Pale yellow oil. Yields of compounds 21a, b27 and 22a, b28 were determined by H NMR
because they could not be separate by column chromatography.
6,7-Dibromo-2-phenylquinoxaline (18): Colorless needle (from CH2Cl2-n-hexane), mp 173-176 ˚C;
1
LRMS (EI) m/z: 364 (M+2+, 100%), 362 (M+, 50%), 364 (M+4+, 50%); H NMR (400 MHz, CDCl3) δ
13
7.54-7.58 (m, 3H), 8.17 (dd, 2H, J = 7.5, 1.1 Hz), 8.40 (s, 1H), 8.44 (s, 1H), 9.30 (s, 1H); C NMR (100
MHz, CDCl3) δ 126.0, 126.9, 127.5, 129.2, 130.8, 133.2, 133.6, 135.9, 140.7, 141.5, 144.3, 152.6. Anal.
Calcd for C14H8Br2N2: 46.19; H, 2.22; N, 7.70. Found: C, 46.32; H, 2.26; N, 7.58.
2-Phenylquinoxaline-6,7-dicarbonitrile (19): Pale yellow plate (from CH2Cl2-n-hexane), mp 275-277
˚C; LRMS (EI) m/z: 256 (M+, 100%); 1H NMR (400 MHz, CDCl3) δ 7.61-7.65 (m, 3H), 8.25 (dd, 2H, J =
5.0, 1.3 Hz), 8.61 (s, 1H), 8.63 (s, 1H), 9.56 (s, 1H); 13C NMR (CDCl3, 100 MHz) δ 113.7, 114.9, 115.0,
128.1, 129.6, 132.1, 134.8, 137.0, 137.3, 142.0, 143.1, 147.2, 155.4. Anal. Calcd for C16H8N4: C, 74.99;
H, 3.15; N, 21.86. Found: C, 74.12; H, 3.09; N, 21.75.
One-pot two-step reaction for the synthesis of 2-phenyl-5,6,7,8-tetrahydroquinoxaline (25).
Triphenylstibane (35.5 mg, 0.1 mmol, 10 mol%) and trans-1,2-diaminocyclohexane (136 mg, 1.2 mmol)
were added to the solution of 2-hydroxyacetophenone (199 mg, 1 mmol) in CH2Cl2 (6 mL) under air, and
stirred at room temperature. After TLC showed the disappearance the 2-hydroxyacetophenone, a solution

of potassium hydroxide (112 mg, 2 mmol) in MeOH (5 mL) was added. The mixture was heated under
reflux for 20 h. The product was extracted by CH2Cl2 washed with water. The organic layer was dried
over MgSO4 and concentrated under reduced pressure. The residue was subjected to column
chromatography on NH-silica gel (CH2Cl2) to give 25 as colorless oil (94.5 mg, 0.45 mmol, 45%). The
product was confirmed by comparison of NMR data, and MS spectra with that in the literature.19
ACKNOWLEDGEMENTS
We acknowledge financial support from Institute of Pharmaceutical Life Sciences, Aichi Gakuin
University and the Special Research Found from Hokuriku University.
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