I2 catalyzed tandem protocol for synthesis of quinoxalines via sp3, sp2 and sp C-H functionalization

Article abstract of DOI:10.1039/c4ra08589b

One-pot, atom-economic synthesis of quinoxalines has been achieved through generation of arylglyoxal from easily available ethylarenes, ethylenearenes and ethynearenes, and subsequent condensation with o-phenylenediamines. Catalytic I₂ with TBHP as an oxidant in DMSO is the system of choice for this domino reaction involving C-H functionalization/oxidative cyclization. This metal-free, mechanistically distinct and functional group tolerant tandem approach could be a powerful complement to traditional approaches for the synthesis of quinoxalines. This journal is

Full text of DOI:10.1039/c4ra08589b

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I2 Catalyzed Tandem Protocol for Synthesis of Quinoxalines via sp3, sp2 and
sp C-H Functionalization
An efficient metal-free tandem protocol for the synthesis of quinoxalines via sp3, sp2 and sp C-H
functionalization has been developed.
Kamlesh S. Vadagaonkar,a Hanuman P. Kalmode,a Kaliyappan Muruganb and Atul C. Chaskarac*
aDepartment of Dyestuff Technology, Institute of Chemical Technology, Mumbai 400019, India
bNational Taiwan University, Taipei 10617, Taiwan
cNational Centre for Nanosciences and Nanotechnology, University of Mumbai, Mumbai
400098, India
Tel.: +91-22- 3361 1111/ 2706; Fax: +91-22- 3361 1020; E-mail: achaskar25@gmail.com

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I2 Catalyzed Tandem Protocol for Synthesis of Quinoxalines via sp3, sp2 and
sp C-H Functionalization
Kamlesh S. Vadagaonkar,a Hanuman P. Kalmode,a Kaliyappan Muruganb and Atul C. Chaskarac*
aDepartment of Dyestuff Technology, Institute of Chemical Technology, Mumbai 400019, India
bNational Taiwan University, Taipei 10617, Taiwan
cNational Centre for Nanosciences and Nanotechnology, University of Mumbai, Mumbai
400098, India
Tel.: +91ꢀ22ꢀ 3361 1111/ 2706; Fax: +91ꢀ22ꢀ 3361 1020; Eꢀmail: achaskar25@gmail.com
Abstract:
Oneꢀpot, atomꢀeconomic synthesis of quinoxalines has been achieved through generation of
arylglyoxal from easily available ethylarenes, ethylenearenes and ethynearenes, and subsequent
condensation with oꢀphenylenediamines. Use of catalytic I2 with TBHP as an oxidant in DMSO
is the system of choice for this domino reaction involving CꢀH functionalization/oxidative
cyclization. This metalꢀfree, mechanistically distinct and functional group tolerant tandem
approach could be a powerful complement to traditional approaches for the synthesis of
quinoxalines.
Keywords: CꢀH Functionalization, Metalꢀfree synthesis, Quinoxalines, Iodine, tertꢀButyl
hydroperoxide (TBHP), Kornblum oxidation
Introduction
Quinoxaline is a widely explored nitrogen heterocyclic compound owing to it’s presence in
numerous compounds and has promising applications in dyes,1,2 organic semiconductors,3
electroluminescent materials4 and anionic receptors.5 Furthermore, quinoxaline derivatives
exhibit a broad range of biological activities such as antiviral, antibacterial, antiꢀinflammatory,
antiꢀprotozoal and kinase inhibitors.6 In addition, these are used as anticancer, antifungal,
insecticidal and anthelmintic agents.7 Notably, quinoxaline is a key constituent of antibiotics
such as echinomycin, levomycin and actinomycin.8 Quinoxaline derivatives also have promising
applications in cavitands, chemically controllable switches and DNA cleaving agents.9

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In view of the wide range of applications, various methods have been developed for the
synthesis of quinoxaline derivatives. A highly explored method amongst these is the acid
catalyzed condensation of 1,2ꢀdiaminobenzenes with 1,2ꢀdicarbonyl compounds.10 Quinoxaline
derivatives are also prepared by reaction of 1,2ꢀdiaminobenzenes with αꢀhydroxy ketones,11
epoxides,12 αꢀhaloketones,13 αꢀtosyloxy ketones,14 αꢀketocarboxylic acids,15 oxalic acid,16
diazenyl butenes,17 hydroxy acetylenes,18 alkynes,19 vicinal diols20 or diazoketones.21 A wide
range of catalysts, e.g. molecular sieves,22 HClO4/SiO2,23 βꢀcyclodextrin,24 Me3SiCl,25
DABCO,26 transition metals (Mn, Ru, Pd and Cu),27 Ga(ClO4)3,28 ionꢀexchanged
molybdophosphoric acid,29 KFꢀalumina30 and microwave irradiation31 have been employed, for
these reactions.
However, many of these catalysts cannot be widely used owing to environmental and /or
economical issues, including requirement of heavy metal catalysts, use of stoichiometric
amounts, an excess amount of base, high temperature, incompatibility with functionalized
substrates, long reaction time, low yield of products and multi step synthesis.
Therefore, development of economically viable procedures for the synthesis of quinoxaline
derivatives from easily available multiform starting materials still continues to be a challenging
task. In this context, summarizing the practical advantages of domino strategy of synthesis and
continuation in our efforts towards development of facile, efficient, environment and ecoꢀ
friendly synthetic protocols,32 herein we describe the synthesis of quinoxalines using I2/TBHP
and DMSO in one step from ethylarenes (1), ethylenearenes (4) and ethynearenes (5) (Scheme
1). The method encompasses three primary processesꢀ CꢀH functionalization, oxidation and
condensation.
NH2
N
I2, TBHP
or
or
Ar
Ar
Ar
DMSO,120 oC, 3 h
NH2
N
Ar
R1
R1
4
5
3
2
1
Scheme 1. Tandem metalꢀfree synthesis of quinoxalines from ethylarenes or ethylenearenes or
ethynearenes and oꢀphenylenediamines
We first studied the action of I2/TBHP and DMSO on ethylbenzene (1a) for CꢀH
functionalization and generation of phenylglyoxal, followed by in situ condensation with oꢀ
phenylenediamine.

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Table 1. Optimization of reaction conditions for tandem metalꢀfree synthesis of quinoxaline
from ethylbenzene and oꢀphenylenediaminea
Entry Catalyst (equiv.) Oxidant (equiv.)
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
ꢀ
Time (h) Temp. (oC) Yield (%)g
1
2
3
4
5
6
7
8
I2 (1.0)
I2 (1.0)
I2 (1.0)
I2 (1.0)
I2 (0.5)
I2 (0.3)
I2 (0.2)
I2 (0.1)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
ꢀ
TBHP (2)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
DTBP (3)
IBX (3)
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
0.5
1.0
1.5
2.0
1.0
2.0
3.0
0.5
1.0
1.5
2.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
80
80
28
39
54
67
75
84
90
85
63
95
76
57
71
90
82
45
59
100
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
ꢀ
ꢀ
ꢀ
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
73
90
84
Trace
0
41
DMP (3)
DIB (3)
Trace
Trace
Trace
Trace
Trace
0
ꢀ
26
19
23
HTIB (3)
DDQ (3)
K2S2O8 (3)
O2 (1 atm)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
TBHP (3)
NIS (0.2)
KI (0.2)
CuI (0.2)
TBAI (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
I2 (0.2)
35
ꢀ
ꢀ
ꢀ
1,4ꢀdioxane
THF
Toluene
ꢀ

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aReaction conditions: 1a (1.0 mmol), catalyst and oxidant was heated for 1.0 h in sealed tube, 2a
b
(1.0 mmol) in solvent (3.0 mL) was added and heating continued for 2.0 h. Reaction conditions
(entries 9ꢀ12): 1a (1.0 mmol), I2 (0.2 mmol) and TBHP (3.0 mmol) was heated for 0.5ꢀ2.0 h in
c
sealed tube. Reaction conditions (entries 13ꢀ15): 1a (1.0 mmol), I2 (0.2 mmol), TBHP (3.0
d
mmol) and DMSO (3.0 mL) was heated for 1.0ꢀ3.0 h in sealed tube. Reaction conditions
(entries 16ꢀ20): 1a (1.0 mmol), I2 (0.2 mmol) and TBHP (3.0 mmol) was heated for 1.0 h in
sealed tube, 2a (1.0 mmol) in DMSO (3.0 mL) was added and heating continued for 0.5ꢀ3.0 h.
eReaction condition (entry 21): 1a (1.0 mmol), 2a (1.0 mmol), I2 (0.2 mmol) and TBHP (3.0
f
mmol) was heated in DMSO (3.0 mL) in sealed tube at 120 °C for 3.0 h. Reaction condition
(entry 30): 1a (1.0 mmol) and TBHP (3.0 mmol) was heated in sealed tube at 120 °C for 1.0 h,
g
2a (1.0 mmol) in DMSO (3.0 mL) was added and heating continued for 2.0 h. Isolated yield.
DTBP = Diꢀtertꢀbutyl peroxide, IBX = oꢀIodoxyꢀbenzoic acid, TBHP = tertꢀButyl
hydroperoxide, DMP = 1,1,1ꢀTriacetoxyꢀ1,1ꢀdihydroꢀ1,2ꢀbenziodoxolꢀ3(1H)ꢀone, DIB =
(Diacetoxyiodo)benzene, HTIB = [Hydro(tosyloxy)iodo]benzene, DDQ = 2,3ꢀDichloroꢀ5,6ꢀ
dicyanoꢀ1,4ꢀbenzoquinone, K2S2O8= Potassium persulfate
Initially, the reaction of ethylbenzene (1a) with 1.0 equiv. I2 and 2.0 equiv. TBHP as an
o
oxidant was carried out at 80 C temperature for 1.0 h. oꢀPhenylenediamine (1.0 equiv.) in
DMSO (3.0 mL) was added and heating was continued for 2.0 h to afford quinoxaline (3aa) in
28% yield (Table 1, entry 1). Use of 3.0 equiv. of TBHP resulted in slightly higher yield (39%,
o
Table 1, entry 2). The reaction was then carried out at 100 and 120 C, whereupon yields of 54
and 67% were obtained (Table 1, entries 3 and 4), thereby indicating a significant role played by
temperature in this reaction. We next varied the iodine concentration from 0.5 equiv. to 0.1
equiv. in DMSO (3.0 mL) as a solvent with 3.0 equiv. of TBHP at 120 oC. The reaction with 0.5
equiv. of I2 led to 75% yield of product (Table 1, entry 5), whereas a higher yield of 84% was
achieved with 0.3 equiv. of I2 (Table 1, entry 6). Use of 0.2 equiv. of I2 and 3.0 equiv. of TBHP
in DMSO (3.0 mL) at 120 oC was found to be optimal resulting in 90% yield (Table 1, entry 7).
A slight drop in the yield (85%) was observed when I2 concentration was reduced to 0.1 equiv.
(Table 1, entry 8). In order to optimize the time required for formation of αꢀiodo acetophenone
(B) from ethylbenzene (1a), few experiments (Table 1, entries 9ꢀ12) were carried out with
varying time by using 0.2 equiv. of I2 and 3.0 equiv. of TBHP. The reaction time of 1.0 h was

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found to be the optimal time for this conversion. Similarly, few reactions (Table 1, entries 13ꢀ15)
were performed to optimize the time needed for conversion of ethylbenzene (1a) to
phenylglyoxal (C) with DMSO as a solvent. It was observed that reaction took 2.0 h for
completion and formed phenylglyoxal (C) in 90% yield. Beside this we conducted some
experiments (Table 1, entry 16ꢀ20) for optimization of the time necessary to form quinoxaline
(3aa) after addition of oꢀphenylenediamine (2a) in DMSO. It was noticed that the reaction was
completed in 2.0 h to afford quinoxaline (3aa) in 90% yield. We also tried simultaneous addition
of all reaction components but it afforded the product only in trace amount thus proving the
importance of sequential addition of the reactants (Table 1, entry 21). The other common
oxidants namely DTBP, IBX, DMP, DIB, HTIB, DDQ, K2S2O8, molecular O2 were also
screened, but TBHP was found to be the most effective oxidant for this transformation (Table 1,
entries 22ꢀ29). The reaction in absence of I2 was also attempted but it failed to give the desired
product thereby highlighting the key role of I2 as a catalyst in this reaction (Table 1, entry 30).
We screened catalysts such as NIS, KI, CuI and TBAI but all of these attempts resulted in low
yields (19 to 35%, Table 1, entries 31ꢀ34). I2, therefore, is the most efficient catalyst for this
reaction. No product was obtained when we conducted the reaction by using solvents such as
DMF, 1,4ꢀdioxane, THF, and toluene. These results confirm the dual role of DMSO in this
reaction, as a solvent and a coꢀoxidant (Table 1, entries 35ꢀ38).
With the optimized reaction conditions in hand, we explored the scope and limitations of
this transformation by using substituted ethylarenes. To our delight, the reaction was found to be
robust and unaffected by the nature as well as the position of the substituent present in the
aromatic ring. Ethylarenes bearing halogen substituents such as ꢀF, ꢀCl and ꢀBr at the ortho and
para positions reacted smoothly with oꢀphenylenediamine to offer the corresponding
quinoxalines 3ba, 3ca and 3da in yields ranging from 85ꢀ92%. The reaction of ethylarenes with
electron donating substituents such as ꢀMe, ꢀOMe proceeded easily with oꢀphenylenediamine
furnishing the corresponding quinoxalines 3ea, 3fa and 3ga in good yields (82ꢀ86%). The
reaction of ethylarenes bearing an electron withdrawing substituent such as ꢀNO2 with oꢀ
phenylenediamine resulted in formation of quinoxaline 3ha in excellent yield (91%).
Table 2. I2/TBHP catalyzed synthesis of quinoxalines from ethylarenes and oꢀ
phenylenediaminesa,b

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aReaction conditions: 1 (1.0 mmol), I2 (0.2 mmol) and TBHP (3.0 mmol) was heated for 1.0 h at
120 °C in sealed tube, 2 (1.0 mmol) in DMSO (3.0 mL) was added and heating continued for 2.0
h. bIsolated yield.

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Similarly, 1ꢀethylnaphthalene and 2ꢀethylnaphthalene also offered the corresponding
quinoxalines 3ia and 3ja in 87 and 89% respectively. Notably, heteroarylethanes such as 2ꢀ
ethylfuran, 2ꢀethylthiophene and 3ꢀethylꢀ2Hꢀbenzopyranꢀ2ꢀone afforded the corresponding
quinoxalines 3ka, 3la and 3ma in quantitative yields. To investigate the effect of substituent on
reactivity and regioselectivity, different oꢀphenylenediamines such as 4ꢀchloro, 4ꢀmethyl, 4ꢀ
methoxy and 4ꢀnitroꢀoꢀphenylenediamine were reacted with ethylbenzene (1a). The reaction of
4ꢀchloroꢀoꢀphenylenediamine with ethylbenzene (1a) formed regioisomers 3ab and 3ac in 2:1
ratio with a combined yield of 87% while 4ꢀmethylꢀoꢀphenylenediamine produced inseperable
1
mixture of regioisomers 3ad and 3ae in 1:4 ratio which is confirmed by H NMR with 83%
overall yield. 4ꢀMethoxyꢀoꢀphenylenediamine reacted smoothly with ethylbenzene (1a) and
offered regioisomers 3af and 3ag in 1:1 ratio with a total yield of 89%. 4ꢀNitroꢀoꢀ
phenylenediamine on reaction with ethylbenzene (1a) resulted in formation of regioisomers 3ah
and 3ai in 3:2 ratio with 80% overall yield.
Having obtained encouraging results with ethylarenes, we turned our attention to CꢀH
functionalization of ethylenearenes followed by oxidative cyclization with oꢀphenylenediamines.
When we reacted styrene (4a) with oꢀphenylenediamine (2a) under optimized conditions, CꢀH
functionalization followed by oxidative cyclization occurred smoothly to offer quinoxaline 3aa
in 92% yield. Ethylenearenes bearing halogen substituents such as ꢀCl and ꢀBr at the meta and
para positions also reacted smoothly with oꢀphenylenediamine to form the corresponding
quinoxalines 3na and 3oa in excellent yields (90 and 93% respectively). The reaction of
ethylenearenes with electron donating substituents ꢀMe, ꢀOMe proceeded to furnish the
respective quinoxalines 3fa and 3pa in good yields (88 and 78% respectively). The reaction of
ethylenearenes bearing an electron withdrawing substituent (ꢀNO2) with oꢀphenylenediamine
afforded quinoxaline 3ha in excellent yield (92%).
Likewise, 1ꢀethylenenaphthalene also produced the corresponding quinoxaline 3ia with
89% yield whereas, the reaction of heteroarylethylenes such as 2ꢀvinylfuran and 3ꢀvinylꢀ2Hꢀ
benzopyranꢀ2ꢀone formed quinoxalines 3ka and 3ma in 84 and 80% yield respectively. Next, we
studied the effect of substituted oꢀphenylenediamines on reactivity and regioselectivity. 4ꢀ
Chloroꢀoꢀphenylenediamine reacted easily with styrene (4a) to form regioisomers 3ab and 3ac in
2:1 ratio with an overall yield of 91%. The reaction of 4ꢀmethoxyꢀoꢀphenylenediamine with
styrene (4a) produced regioisomers 3af and 3ag in 1:1 ratio with a combined yield of 93%. 4ꢀ

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Nitroꢀoꢀphenylenediamine reacted smoothly with styrene (1a) to offer regioisomers 3ah and 3ai
in 3:2 ratio with a total yield of 84%.
Table 3. I2/TBHP catalyzed synthesis of quinoxalines from ethylenearenes and oꢀ
phenylenediaminesa,b
N
N
N
Cl
N
N
N
(92%)
(90%)
(93%)
3aa
3fa
3na
3pa
3oa
3ha
Br
N
N
N
O
N
N
N
(78%)
(92%)
(88%)
O
NO2
N
N
N
O
N
N
N
(89%)
(84%)
(80%)
3ia
3ka
3ma
O
O
Cl
N
N
O
N
N
N
Cl
N
(60%)
(31%)
(45%)
3ab
3ag
3ac
3af
N
O2N
N
N
N
O
N
O2N
N
(48%)
(51%)
(33%)
3ai
3ah
aReaction conditions: 4 (1.0 mmol), I2 (0.2 mmol) and TBHP (3.0 mmol) was heated for 1.0 h at
120 °C in sealed tube, 2 (1.0 mmol) in DMSO (3.0 mL) was added and heating continued for 2.0
h. bIsolated yield.
With results of ethylarenes and ethylenearenes in hand, we conducted the reaction of
phenylacetylene (5a) with oꢀphenylenediamine (2a) under optimized reaction conditions and
obtained quinoxaline 3aa in excellent yield (89%). We also found that substituted (halogens, Me,

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OMe and NO2) ethynearenes reacted smoothly and afforded the corresponding quinoxalines in
good to excellent yields (81ꢀ90%). 2ꢀEthynylnaphthalene gave quinoxaline 3ja in 85% yield. To
study the reaction scope of oꢀphenylenediamines, we carried out the reaction of 4ꢀchloroꢀoꢀ
phenylenediamine with phenylacetylene (5a). Notably, a mixture of regioisomers 3ab and 3ac
in 2:1 ratio was obtained with an overall yield of 79%. The reaction of 4ꢀmethylꢀoꢀ
phenylenediamine with phenylacetylene (5a) showed higher regioselectivity and formed
regioisomers 3ad and 3ae in 1:4 ratio with a total yield of 85%. 4ꢀMethoxyꢀoꢀphenylenediamine
reacted with phenylacetylene (5a) to produce regioisomers 3af and 3ag in 1:1 ratio with a
combined yield of 91%. 4ꢀNitroꢀoꢀphenylenediamine reacted smoothly with phenylacetylene
(5a) to afford regioisomers 3ah and 3ai in 3:2 ratio with total yield of 81%.
Table 4. I2/TBHP catalyzed synthesis of quinoxalines from ethynearenes and oꢀ
phenylenediaminesa,b

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aReaction conditions: 5 (1.0 mmol), I2 (0.2 mmol) and TBHP (3.0 mmol) was heated for 1.0 h at
120°C in sealed tube, 2 (1.0 mmol) in DMSO (3.0 mL) was added and heating continued for 2.0
h. bIsolated yield.
A few controlled experiments (Scheme 2) were performed in order to support our proposed
mechanism. To demonstrate the involvement of TBHP in the formation of αꢀiodoacetophenone
(B), a key intermediate, ethylbenzene (1a) or styrene (4a) or phenylacetylene (5a) were reacted
o
with I2 and DMSO at 120 C (Scheme 2a). These reactions failed to produce the αꢀ
iodoacetophenone (B), proving that TBHP plays a crucial role in the formation of αꢀ
iodoacetophenone (B) through radical intermediates. When ethylbenzene (1a) or styrene (4a) or
phenylacetylene (5a) were treated with I2/TBHP in absence of DMSO (Scheme 2b), the reaction
solely formed αꢀiodoacetophenone (B). Compound 1a on reaction with I2/TBHP (Scheme 2c)
resulted in successive formation of acetophenone (A) and αꢀiodoacetophenone (B), an important
intermediate, which is also obtained from compound 4a and 5a (Scheme 2d). αꢀ
Iodoacetophenone (B) was finally converted into phenylglyoxal (C) in presence of DMSO. It

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ascertained the requirement of DMSO as a solvent and a coꢀoxidant. Phenylglyoxal (C) reacted
with oꢀphenylenediamine (2a) in presence of I2 and DMSO to form quinoxaline (3aa)
exclusively (Scheme 2e). The reaction of compound 1a, 4a and 5a with oꢀphenylenediamine (2a)
in presence of I2 and TBHP gave quinoxaline 3aa in low yield (scheme 2f). Ethylbenzene (1a),
styrene (4a) and phenylacetylene (5a) did not react with oꢀphenylenediamine (2a) in DMSO
thereby clearly highlighting the role of I2/TBHP in CꢀH functionalization (Scheme 2g).
Scheme 2. Control experiment
A plausible mechanism for this transformation is presented in Scheme 3. Ethylbenzene (1a) is
oxidized to acetophenone (A) by I2/TBHP via a radical mechanism with 1ꢀphenylethanol as an

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intermediate product. Formation of acetophenone was confirmed by TLC, GCꢀMS and 1HNMR.
Acetophenone (A) on iodination is converted into αꢀiodoacetophenone (B). This key
intermediate is also produced from styrene (4a) and phenylacetylene (5a) by oxidation/iodination
with I2/TBHP as depicted in scheme 3. αꢀIodoacetophenone (B) is further transformed via
Kornblum oxidation into phenylglyoxal (C) which on reaction with oꢀphenylenediamine (2a)
gives intermediate D. Cyclization of D followed by oxidation offer quinoxaline (3aa).
OH
t-Buoo
t-BuOOH
H2O
OH
I2
HI
CH3
1a
TBHP
t-Buo
t-BuOOH
OH
O
N
A
OH
t-Buoo
t-BuOOH
HI
H2O
N
3aa
I2
OH
I2
I2
4a
H2O
t-Buo
t-BuOOH
OH
[o]
I2, TBHP
HI
HI
OH
t-Buoo
t-BuOOH
H2O
O
OH
I2
HI
I
H
N
OH
B
5a
I2
t-Buo
t-BuOOH
OH
HI
N
E
DMSO
O
I2
O
O
NH2
DMS
H
H2O
N
D
C
NH2
NH2
2a
Scheme 3. Plausible reaction mechanism
Experimental section

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General information
Chemical reagents were obtained from commercial companies. All reactions were performed in a
sealed tube and monitored by TLC performed on aluminum plates (0.25 mm, E. Merck)
precoated with silica gel Merck 60 Fꢀ254. Developed TLC plates were visualized under a shortꢀ
wavelength UV lamp. Yields refer to spectroscopically (1H, 13C NMR) homogeneous material
obtained after column chromatography performed on silica gel (100–200 mesh size) supplied by
1
S. D. Fine Chemicals Limited, India. H and 13C NMR spectra were recorded in CDCl3 or
DMSOꢀd6 solution with Brüker 400, Agilent 500 and 600 MHz spectrometers. Coupling
constants (J) were measured in Hertz. Chemical shifts (δ) are quoted in ppm, relative to SiMe4 (δ
= 0) as an internal standard. The number of protons (n) for a given resonance is indicated by nH.
High resolution mass spectra (HRMS) were obtained by using positive electronspray ionization
(ESI) by Time of Flight (TOF) method. Melting points were recorded on a standard melting
point apparatus from Sunder Industrial Product, Mumbai and uncorrected.
General experimental procedure for synthesis of quinoxalines:
A sealed tube equipped with magnetic stirring bar was charged with ethylarene (1) or
ethylenearene (4) or ethynearene (5) (1.0 mmol), I2 (0.2 mmol) and tertꢀbutyl hydroperoxide
(TBHP, 3.0 mmol, 70% aq. solution) at room temperature. The resulting mixture was heated to
o
120 ℃ and maintained at 120 C for 1.0 h. After disappearance of the reactant (monitored by
TLC), substituted oꢀphenylenediamine 2 (1.0 mmol) in DMSO (3.0 mL) was added and heating
continued at 120 oC for 2.0 h. After completion of the reaction, 20 mL of water was added to the
reaction mixture and it was extracted with ethyl acetate (2 × 20 mL). The organic layer was
washed with 10% Na2S2O3 solution (20 mL) and brine solution (20 mL) successively. The
organic layer was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The
residue was purified by column chromatography on 100:200 mesh silica gel using nꢀhexane:ethyl
acetate (8:2) as eluent to obtain the corresponding quinoxaline 3.
Product characterization data
1
2-Phenylquinoxaline (3aa). White solid; MP 76ꢀ78 °C; H NMR (600 MHz, CDCl3): δ = 7.53
(t, J = 7.8, 7.2 Hz, 1H), 7.58 (t, J = 7.8, 7.2 Hz, 2H), 7.76 (t, J = 7.8, 6.6 Hz, 1H), 7.80 (t, J = 7.2
Hz, 1H), 8.13 (d, J = 7.8 Hz, 1H), 8.17 (d, J = 8.4 Hz, 1H), 8.20 (d, J = 7.2 Hz, 2H), 9.34 (s, 1H);

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13C NMR (150 MHz, CDCl3): δ = 127.53, 129.08, 129.14, 129.54, 129.59, 130.17, 130.29,
136.74, 141.52, 142.28, 143.33, 151.85; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H11N2:
207.0923; found: 207.0925.
1
2-(2-Bromophenyl)quinoxaline (3ba). White solid; MP 116ꢀ118 °C; H NMR (400 MHz,
CDCl3): δ = 7.38 (t, J = 7.2 Hz, 1H), 7.51 (t, J = 7.2 Hz, 1H), 7.68 (d, J = 7.2 Hz, 1H), 7.75 (d, J
13
= 7.2 Hz, 1H), 7.81ꢀ7.84 (q, J = 3.2 Hz, 2H), 8.17 (d, J = 3.2 Hz, 2H), 9.19 (s, 1H); C NMR
(100 MHz, CDCl3): δ = 122.05, 128.10, 129.32, 129.67, 130.24, 130.39, 130.99, 131.95, 133.51,
138.57, 141.41, 142.16, 146.21, 153.69; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10BrN2:
285.0028; found: 285.0030.
1
2-(4-Fluorophenyl)quinoxaline (3ca). White solid; MP 120ꢀ122 °C; H NMR (400 MHz,
CDCl3): δ = 7.22ꢀ7.28 (m, 2H), 7.74ꢀ7.82 (q, J = 9.6, 8.0, 6.8 Hz, 2H), 8.13 (t, J = 8.0, 6.8 Hz,
2H), 8.20ꢀ8.23 (distorted q, J = 6.4, 5.6, 2.0 Hz, 2H), 9.30 (s, 1H); 13C NMR (100 MHz, CDCl3):
δ = 116.15, 116.37, 129.18, 129.50, 129.58, 129.62, 130.43, 132.99, 133.02, 141.55, 142.25,
142.97, 150.81, 163.05, 165.54; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10FN2:
225.0829; found: 225.0833.
1
2-(2,4-Dichlorophenyl)quinoxaline (3da). White solid; MP 144ꢀ146 °C; H NMR (400 MHz,
13
CDCl3): δ = 7.46ꢀ7.83 (m, 5H), 8.17 (s, 2H), 9.20 (s, 1H); C NMR (100 MHz, CDCl3): δ =
128.00, 129.35, 129.66, 130.16, 130.45, 130.52, 132.96, 133.40, 135.11, 136.37, 141.49, 142.33,
145.90, 151.35; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H9Cl2N2: 275.0144; found:
275.0146.
2-(o-Tolyl)quinoxaline (3ea). White solid; MP 88ꢀ90 °C; 1H NMR (400 MHz, CDCl3): δ = 2.47
(s, 3H), 7.37 (s, 3H), 7.55 (d, J = 6.8 Hz, 1H), 7.80 (distorted t, J = 3.6, 3.2 Hz, 2H), 8.16 (d, J =
6.8 Hz, 2H), 9.02 (s, 1H); 13C NMR (100 MHz, CDCl3): δ = 20.43, 126.42, 129.22, 129.51,
129.63, 129.84, 130.08, 130.35, 131.30, 136.66, 137.20, 141.08, 142.11, 145.98, 155.09; HRMS
(ESIꢀMS): m/z [M + H]+ calcd for C15H13N2: 221.1079; found: 221.1083.

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2-(p-Tolyl)quinoxaline (3fa). White solid; MP 92ꢀ94 °C; 1H NMR (600 MHz, CDCl3): δ = 2.46
(s, 3H), 7.38 (d, J = 7.8 Hz, 2H), 7.74 (t, J = 7.8, 7.2 Hz, 1H), 7.78 (t, J = 7.8, 7.2 Hz, 1H), 8.10ꢀ
13
8.12 (m, 3H), 8.15 (d, J = 8.4 Hz, 1H), 9.32 (s, 1H); C NMR (125 MHz, CDCl3): δ = 21.32,
127.37, 129.03, 129.48, 129.54, 129.77, 129.98, 133.96, 140.47, 141.42, 142.31, 143.35, 151.81;
HRMS (ESIꢀMS): m/z [M + H]+ calcd for C15H13N2: 221.1079; found: 221.1081.
1
2-(4-Methoxyphenyl)quinoxaline (3ga). White solid; MP 98ꢀ100 °C; H NMR (400 MHz,
CDCl3): δ = 3.91 (s, 3H), 7.09 (d, J = 8.8 Hz, 2H), 7.70ꢀ7.78 (m, 2H), 8.11 (t, J = 8.8 Hz, 2H),
8.18 (d, J = 8.8 Hz, 2H), 9.30 (s, 1H); 13C NMR (100 MHz, CDCl3): δ = 55.51, 114.66, 129.04,
129.13, 129.45, 130.25, 141.27, 142.38, 143.15, 151.51, 161.52; HRMS (ESIꢀMS): m/z [M +
H]+ calcd for C15H13N2O: 237.1029; found: 237.1033.
1
2-(4-Nitrophenyl)quinoxaline (3ha). Yellow solid; MP 188ꢀ190 °C; H NMR (400 MHz,
CDCl3): δ = 7.83 (t, J = 3.6, 3.2 Hz, 2H), 8.17 (t, J = 7.6 Hz, 2H), 8.40 (s, 4H), 9.38 (s, 1H); 13C
NMR (100 MHz, CDCl3): δ = 124.38, 128.42, 129.35, 129.92, 130.78, 130.97, 142.17, 142.27,
142.62, 142.88, 148.87, 149.30; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10N3O2:
252.0774; found: 252.0778.
1
2-(Naphthalen-1-yl)quinoxaline (3ia). Off white solid; MP 134ꢀ136 °C; H NMR (400 MHz,
CDCl3): δ = 7.51ꢀ7.58 (q, J = 6.8, 6.4 Hz, 2H), 7.64 (t, J = 7.6 Hz, 1H), 7.78 (d, J = 7.2 Hz, 1H),
7.81ꢀ7.86 (distorted q, J = 6.8, 4.0 Hz, 2H), 7.96 (d, J = 7.6 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H),
8.17 (d, J = 8.0 Hz, 1H), 8.20ꢀ8.24 (m, 2H), 9.17 (s, 1H); 13C NMR (100 MHz, CDCl3): δ =
125.08, 125.41, 126.35, 127.18, 128.54, 128.64, 129.31, 129.69, 129.91, 130.13, 130.39, 131.18,
134.05, 135.15, 141.40, 142.24, 146.63, 154.28; HRMS (ESIꢀMS): m/z [M + H]+ calcd for
C18H13N2: 257.1079; found: 257.1081.
1
2-(Naphthalen-2-yl)quinoxaline (3ja). Yellow solid; MP 138ꢀ140 °C; H NMR (400 MHz,
CDCl3): δ = 7.57 (s, 2H), 7.76ꢀ7.83 (distorted q, J = 8.8, 7.6 Hz, 2H), 7.92 (s, 1H), 8.03
(distorted d, J = 8.4 Hz, 2H), 8.14ꢀ8.22 (dd, J = 7.6 Hz, 2H), 8.37 (d, J = 7.6 Hz, 1H), 8.66 (s,
1H), 9.49 (s, 1H); 13C NMR (100 MHz, CDCl3): δ = 124.57, 126.79, 127.40, 127.59, 127.90,

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128.99, 129.14, 129.24, 129.68, 129.71, 130.45, 134.22, 141.64, 143.61, 151.81; HRMS (ESIꢀ
MS): m/z [M + H]+ calcd for C18H13N2: 257.1079; found: 257.1083.
1
2-(Furan-2-yl)quinoxaline (3ka). Light brown solid; MP 98ꢀ100 °C; H NMR (400 MHz,
CDCl3): δ = 6.63 (s, 1H), 7.32 (d, J = 2.8 Hz, 1H), 7.68ꢀ7.77 (m, 3H), 8.05ꢀ8.10 (q, J = 8.4, 4.4
Hz, 2H), 9.24 (s, 1H); 13C NMR (100 MHz, CDCl3): δ = 111.91, 112.58, 129.28, 129.42,
130.57, 141.35, 142.11, 142.17, 143.92, 145.19, 151.64; HRMS (ESIꢀMS): m/z [M + H]+ calcd
for C12H9N2O: 197.0716; found: 197.0720.
1
2-(Thiophen-2-yl)quinoxaline (3la). Pale yellow solid; MP 120ꢀ122 °C; H NMR (400 MHz,
CDCl3): δ = 7.20 (t, J = 4.4 Hz, 1H), 7.55 (d, J = 4.8 Hz, 1H), 7.69ꢀ7.76 (q, J = 7.6, 6.8 Hz, 2H),
13
7.86 (d, J = 3.2 Hz, 1H), 8.05ꢀ8.08 (distorted q, J = 3.6, 2.4 Hz, 2H), 9.24 (s, 1H); C NMR
(100 MHz, CDCl3): δ = 127.05, 128.55, 129.19, 129.28, 129.91, 130.51, 141.40, 142.12, 142.20,
142.30, 147.45; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C12H9N2S: 213.0487; found:
213.0491.
1
3-(Quinoxalin-2-yl)-2H-chromen-2-one (3ma). Yellow solid; MP 198ꢀ200 °C; H NMR (400
MHz, DMSOꢀd6): δ = 7.47 (t, J = 7.6, 7.2 Hz, 1H), 7.55 (d, J = 8.0 Hz, 1H), 7.75 (t, J = 7.6, 7.2
Hz, 1H), 7.90ꢀ7.96 (m, 2H), 8.03 (d, J = 7.6 Hz, 1H), 8.16ꢀ8.21 (distorted q, J = 7.2, 2.4 Hz, 2H),
8.94 (s, 1H), 9.60 (s, 1H); 13C NMR (100 MHz, DMSOꢀd6): δ = 116.14, 119.10, 123.62, 125.00,
128.86, 129.07, 129.76, 130.65, 130.77, 133.26, 141.24, 141.42, 144.59, 145.38, 147.84, 153.83,
159.61; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C17H11N2O2: 275.0821; found: 275.0823.
1
6-Chloro-2-phenylquinoxaline (3ab). White solid; MP 146ꢀ148 °C; H NMR (400 MHz,
CDCl3): δ = 7.52ꢀ7.59 (m, 3H), 7.71ꢀ7.74 (dd, J = 2.4, 2.0 Hz, 1H), 8.09 (d, J = 9.2 Hz, 1H),
8.11 (d, J = 2.0 Hz, 1H), 8.19 (d, J = 6.8 Hz, 2H), 9.33 (s, 1H); 13C NMR (100 MHz, CDCl3): δ
= 127.54, 128.11, 129.24, 130.45, 130.86, 131.34, 135.28, 136.41, 140.88, 141.86, 144.17,
151.98; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10ClN2: 241.0533; found: 241.0535.
1
7-Chloro-2-phenylquinoxaline (3ac). White solid; MP 126ꢀ128 °C; H NMR (400 MHz,
CDCl3): δ = 7.52ꢀ7.60 (m, 3H), 7.67ꢀ7.69 (dd, J = 2.0 Hz, 1H), 8.05 (d, J = 8.8 Hz, 1H), 8.15 (d,

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J = 2.4 Hz, 1H), 8.18ꢀ8.20 (dd, J = 1.2 Hz, 2H), 9.31 (s, 1H); 13C NMR (100 MHz, CDCl3): δ =
127.62, 128.51, 129.22, 130.37, 130.51, 130.56, 136.11, 136.33, 140.12, 142.67, 143.43, 152.56;
HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10ClN2: 241.0533; found: 241.0537.
6-Methyl-2-phenylquinoxaline (3ad) and 7-Methyl-2-phenylquinoxaline (3ae) 3ad:3ae = 1:4.
Pale yellow solid; 1H NMR (500 MHz, CDCl3): δ = 2.61 (s, 3H), 7.46ꢀ7.62 (m, 4H), 7.89 and
7.94 (s, 1H), 8.01 and 8.05 (d, J = 8.5 Hz, 1H), 8.18 (d, J = 7.0 Hz, 2H), 9.26 and 9.28 (s, 1H);
13C NMR (125 MHz, CDCl3): δ = 151.78, 151.08, 143.28, 143.13, 142.49, 142.37, 141.58,
140.76, 140.13, 136.93, 132.71, 132.50, 130.06, 129.80, 129.21, 129.16, 128.61, 128.50, 127.99,
127.42, 127.39; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C15H13N2: 221.1079; found:
221.1083.
6-Methoxy-2-phenylquinoxaline (3af). Pale yellow solid; MP 94ꢀ96 °C; 1H NMR (500 MHz,
CDCl3): δ = 3.99 (s, 3H), 7.38ꢀ7.40 (dd, J = 9.0, 2.5 Hz, 1H), 7.44 (d, J = 2.0 Hz, 1H), 7.51 (t, J
= 7.5, 7.0 Hz, 1H), 7.56 (t, J = 8.0, 7.0 Hz, 2H), 7.99 (d, J = 9.0 Hz, 1H), 8.16 (d, J = 7.5 Hz,
2H), 9.16 (s, 1H); 13C NMR (125 MHz, CDCl3): δ = 55.90, 106.99, 122.99, 127.51, 128.97,
129.21, 130.07, 136.99, 137.76, 140.82, 143.95, 151.93, 161.05; HRMS (ESIꢀMS): m/z [M +
H]+ calcd for C15H13N2O: 237.1029; found: 237.1031.
1
7-Methoxy-2-phenylquinoxaline (3ag). Pale yellow solid; MP 92ꢀ94 °C; H NMR (500 MHz,
CDCl3): δ = 3.99 (s, 3H), 7.40 (d, J = 2.0 Hz, 1H), 7.42ꢀ7.45 (dd, J = 9.5, 2.5 Hz, 1H), 7.49 (t, J
= 7.5, 7.0 Hz, 1H), 7.55 (t, J = 7.5 Hz, 2H), 8.03 (d, J = 9.5 Hz, 1H), 8.15 (d, J = 7.5 Hz, 2H),
13
9.24 (s, 1H); C NMR (125 MHz, CDCl3): δ = 55.88, 106.41, 123.41, 127.20, 129.19, 129.80,
130.50, 136.99, 138.40, 143.10, 143.26, 149.62, 160.54; HRMS (ESIꢀMS): m/z [M + H]+ calcd
for C15H13N2O: 237.1029; found: 237.1033.
1
6-Nitro-2-phenylquinoxaline (3ah). Yellow solid; MP 210ꢀ212 °C; H NMR (500 MHz,
CDCl3): δ = 7.60ꢀ7.63 (m, 3H), 8.25ꢀ8.29 (m, 3H), 8.54ꢀ8.56 (dd, J = 9.0, 2.5 Hz, 1H), 9.02 (d, J
13
= 2.5 Hz, 1H), 9.49 (s, 1H); C NMR (125 MHz, CDCl3): δ = 123.82, 125.72, 127.92, 129.27,
129.51, 131.14, 135.60, 140.31, 144.89, 145.40, 147.43, 154.29; HRMS (ESIꢀMS): m/z [M +
H]+ calcd for C14H10N3O2: 252.0774; found: 252.0776.

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1
7-Nitro-2-phenylquinoxaline (3ai). Yellow solid; MP 208ꢀ210 °C; H NMR (500 MHz,
CDCl3): δ = 7.60ꢀ7.63 (m, 3H), 8.24ꢀ8.29 (m, 3H), 8.49ꢀ8.51 (dd, J = 9.5, 2.5 Hz, 1H), 9.05 (d, J
13
= 2.0 Hz, 1H), 9.47 (s, 1H); C NMR (125 MHz, CDCl3): δ = 122.83, 125.81, 127.71, 129.26,
129.49, 130.82, 135.52, 141.32, 143.88, 146.05, 148.26, 153.69; HRMS (ESIꢀMS): m/z [M +
H]+ calcd for C14H10N3O2: 252.0774; found: 252.0778.
1
2-(3-Chlorophenyl)quinoxaline (3na). Yellow solid; MP 130ꢀ132 °C; H NMR (400 MHz,
CDCl3): δ = 7.49 (d, J = 4.4 Hz, 2H), 7.75ꢀ7.82 (q, J = 7.6, 6.8 Hz, 2H), 8.06 (s, 1H), 8.14 (t, J =
8.8 Hz, 2H), 8.22 (s, 1H), 9.29 (s, 1H); 13C NMR (100 MHz, CDCl3): δ = 125.59, 127.74,
129.24, 129.74, 130.06, 130.27, 130.44, 130.62, 135.42, 138.58, 141.87, 142.26, 143.01, 150.40;
HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10ClN2: 241.0533; found: 241.0535.
1
2-(4-Bromophenyl)quinoxaline (3oa). Yellow solid; MP 128ꢀ130 °C; H NMR (400 MHz,
CDCl3): δ = 7.69ꢀ7.82 (m, 4H), 8.08ꢀ8.16 (m, 4H), 9.31 (s, 1H); 13C NMR (100 MHz, CDCl3): δ
= 125.01, 129.03, 129.20, 129.64, 129.83, 130.50, 132.38, 135.68, 141.73, 142.27, 142.83,
150.69; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C14H10BrN2: 285.0028; found: 285.0032.
2-(2,6-Dimethoxyphenyl)quinoxaline (3pa). Yellow solid; MP 68ꢀ70 °C; 1H NMR (400 MHz,
CDCl3): δ = 3.86 (s, 3H), 3.87 (s, 3H), 7.01 (s, 2H), 7.48 (s, 1H), 7.75 (t, J = 3.6 Hz, 2H), 8.11ꢀ
8.16 (q, J = 6.8, 6.4 Hz, 2H), 9.35 (s, 1H); 13C NMR (100 MHz, CDCl3): δ = 55.98, 56.37,
113.09, 116.11, 117.30, 127.08, 129.09, 129.52, 129.57, 129.86, 141.11, 142.67, 147.28, 151.79,
152.01, 154.30; HRMS (ESIꢀMS): m/z [M + H]+ calcd for C16H15N2O2: 267.1134; found:
267.1136.
1
2-(4-Chlorophenyl)quinoxaline (3qa). White solid; MP 136ꢀ138 °C; H NMR (400 MHz,
CDCl3): δ = 7.53ꢀ7.59 (distorted q, J = 8.0, 7.2 Hz, 2H), 7.75ꢀ7.82 (q, J = 7.6, 6.4 Hz, 2H), 8.12ꢀ
13
8.17 (m, 4H), 9.31 (s, 1H); C NMR (100 MHz, CDCl3): δ = 128.80, 129.18, 129.42, 129.63,
129.81, 130.50, 135.23, 136.62, 140.64, 142.89, 143.98, 150.64; HRMS (ESIꢀMS): m/z [M +
H]+ calcd for C14H10ClN2: 241.0533; found: 241.0537.

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1
2-(m-Tolyl)quinoxaline (3ra). White solid; MP 84ꢀ86 °C; H NMR (400 MHz, CDCl3): δ =
2.50 (s, 3H), 7.33ꢀ7.46 (distorted dd, J = 7.2, 6.0 Hz, 2H), 7.76 (t, J = 8.4, 8.0 Hz, 2H), 7.96 (d, J
= 6.8 Hz, 1H), 8.02 (s, 1H), 8.11ꢀ8.17 (q, J = 8.4, 7.6 Hz, 2H), 9.31 (s, 1H); 13C NMR (100
MHz, CDCl3): δ = 21.65, 124.76, 128.28, 129.12, 129.19, 129.54, 129.65, 130.33, 131.08,
136.81, 139.03, 141.62, 142.36, 143.61, 152.14; HRMS (ESIꢀMS): m/z [M + H]+ calcd for
C15H13N2: 221.1079; found: 221.1081.
Conclusion:
In conclusion, we have developed a direct and efficient method to prepare quinoxaline
derivatives from easily available multiform substrates. Use of simple unactivated ethylarenes,
ethylenearenes and ethynearenes, tolerance to broad range of functional groups and use of
inexpensive catalyst are some striking features of this protocol. We anticipate that this
operationally simple tandem process will be widely adopted for construction of quinoxaline
moieties needed for the synthesis of complex molecules.
Acknowledgements
This work was supported by CSIRꢀNew Delhi (sanction no. 01(2427)/10/EMRꢀII dated
28/12/2010, pool scheme no. 8644ꢀA). K.S.V. and H.P.K. thank UGCꢀSAP and CSIRꢀNew
Delhi respectively for award of Senior Research Fellowships. The authors thank Prof. Prakash
M. Bhate for his generous support.
†Author contributions
The authors contributed equally to this work.
Notes and References:
1.
2.
N. D. Sonawane and D. W. Rangnekar, J. Heterocycl. Chem., 2002, 39, 303–308.
T. Hirayama, S. Yamasaki, H. Ameku, T. Ishii, T. Thiemann, and S. Mataka, Dye.
Pigment., 2005, 67, 105–110.
3.
S. Dailey, W. J. Feast, R. J. Peace, I. C. Sage, S. Till, and E. L. Wood, J. Mater. Chem.,
2001, 11, 2238–2243.

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