Palladium catalyzed synthesis and physical properties of indolo[2,3-b]quinoxalines

Article abstract of DOI:10.1039/c4ob00841c

A series of indolo[2,3-b]quinoxaline derivatives were efficiently synthesized from 2,3-dibromoquinoxaline by two pathways. A one-pot approach using Pd-catalyzed two-fold C-N coupling and C-H activation reactions gave indolo[2,3-b]quinoxaline derivatives in good yields, but with limited substrate scope. In addition, a two-step approach to indolo[2,3-b]quinoxalines was developed which is based on Pd-catalyzed Suzuki coupling reactions and subsequent annulation by Pd-catalyzed two-fold C-N coupling with aromatic and aliphatic amines. The electrochemical and photochemical properties of indolo[2,3-b]quinoxaline derivatives were investigated. These studies show that 6-(4-methoxyphenyl)-6H-indolo[2,3-b]quinoxaline showed the highest HOMO energy level and lowest band gap. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00841c

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1
Palladium Catalyzed Synthesis and Physical Properties of
Indolo[2,3-b]quinoxalines
Tran Quang Hung,a Do Huy Hoang,a,b Ngo Ngoc Thang,a,b Tuan Thanh Dang,a* Khurshid
d
Ayub,c Alexander Villinger,a Aleksej Friedrich,d Stefan Lochbrunner
,
Gerd-Uwe Flechsig,a,e
Peter Langera,b*
a Institut für Chemie, Universität Rostock, Albert-Einstein-Str. 3a, 18059 Rostock, Germany.
Fax: +49 381 4986412; Tel: +49 381 4986410
b
Leibniz Institut für Katalyse an der Universität Rostock e. V. (LIKAT), Albert-Einstein-Str.
29a, 18059 Rostock, Germany.
c
Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad,
Pakistan
d Institut für Physik, Universität Rostock, Universitätsplatz 3, 18051 Rostock, Germany.
e
Division of Chemistry and Environmental Science, Manchester Metropolitan University,
Chester Street, Manchester, M1 5GD, United Kingdom.
E-mail: peter.langer@uni-rostock.de; Email: thanhtuandang@hotmail.com
R1
N
N
Br
Br
N
N
N
R2
[Pd]

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Abstract
A series of indolo[2,3-b]quinoxaline derivatives were efficiently synthesized from 2,3-
dibromoquinoxaline by two pathways. A one-pot approach using Pd-catalyzed two-fold C-N
coupling and C-H activation reactions gave indolo[2,3-b]quinoxaline derivatives in good yields,
but with limited substrate scope. In addition, a two-step approach to indolo[2,3-b]quinoxalines
was developed which is based on Pd-catalyzed Suzuki coupling reactions and subsequent
annulation by Pd-catalyzed two-fold C-N coupling with aromatic and aliphatic amines. The
electrochemical and photochemical properties of indolo[2,3-b]quinoxaline derivatives were
investigated. These studies show that 6-(4-methoxyphenyl)-6H-indolo[2,3-b]quinoxaline showed
the highest HOMO energy level and lowest band gap.
Introduction
Indolo[2,3-b]quinoxalines found many applications in organic light-emitting diodes (OLEDs)1,2,3
and excitonic solar cells.4 Their ability to harvest both singlet and triplet energy for emission
improved the device efficiency. In 2010, Thomas et al. found that indolo[2,3-b]quinoxalines 1
lead to red-shift in absorption and emission spectra as well as larger Stokes shifts.4 The
introduction of indolo[2,3-b]quinoxaline segments increased the thermal stability and resulted in
a higher glass transition temperature. To reduce π-π stacking interaction, which was assumed to
decrease luminescence and propensity for crystal forming in the solid state, bulky and nonplanar
structural segments were introduced instead of tertiary amine groups.2 These novel materials
displayed good quantum yields in solution and remarkable fluorescence in solid state. Thomas et
al. also prepared electronic devices containing compound 2b as electron transporting (ETL) and
emitting layers (EML). These devices showed a maximum luminescence of 3910 cd/m2 and
maximum external quantum efficiency of 0.46%. In 2011, Cheng et al. prepared the novel host
material BIQS 3 for deep-red PhOLEDs.3 The BIQS material displayed a relatively low LUMO
energy that facilitates electron injection resulting in a significantly lower operating voltage
requirement and higher current density. This material provided an efficient energy transfer to
deep-red emitting layers, due to singlet and triplet energies. Two years later, Cheng et al.

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prepared three new host materials BIQF, BIQTP, BIQMCz with two indoloquinoline moiety.1
Using these molecules as hosts in the deep-red devices, these materials exhibited EQEmax over
20%. The operational lifetimes were also promoted and were much longer than in the CBP-based
devices.
Ar
N
N
Si
N
N
N
N
N
N
N
N
N
R
N
N
1
3
Ar = phenyl, 1-naphthyl
1-pyrenyl,9-anthracyl
2a
2b
R = H
R = phenyl
BIQS
N
N
N
N
N
N
N
N
N
N
N
N
N
BIQF
BIQMCz
N
N
N
N
N
N
BIQTP
Figure 1.Some materials based on indolo[2,3-b]quinoxaline moieties.
Indolo[2,3-b]quinoxalines not only possess many important applications in material sciences, but
also in medicinal chemistry. Earlier research showed that indolo[2,3-b]quinoxaline derivatives
exhibit a wide range of interesting biological activities, such as antivirus,56 anticancer,7,8
antimicrobial,9 and antibacterial activities.10 Indolo[2,3-b]quinoxalines also represent antivirus
agents.5 A series of indolo[2,3-b]quinoxaline derivatives were examined for antiviral activity
against Herpes virus. These results showed that B-220 exhibited potent antiviral activity against
herpes simplex virus type 1 (HSV-1), varicella-zoster virus (VZV) and cytomegalovirus (CMV)

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(Figure 2).5,6 In 2010, Shibinskaya et al. reported the synthesis of 6-(2-aminoethyl)-6H-
indolo[2,3-b]quinoxalines 4 (Figure 2),11 which act as potent interferon inducers and antiviral
agents with low toxicity. One year later these authors modified the structure of 4 to 7H-
benzo[4,5]indolo[2,3-b]quinoxalines 5,12 which bind to DNA more strongly (lgKa = 6.23-6.87)
than 4 (lgKa = 5.57-5.89). The antiviral activity is significantly reduced by the presence of an
annulated benzene ring present in compound 5. In their research related to the antitumor activity
of tetracyclic quinoline and quinoxaline derivatives, Deady et al. showed that quinoxaline
derivatives exhibit a broad range of cytotoxic activities.8 In 2001, compounds NCA0424 and
NCA0465 were reported to possess antitumor activity toward various types of blood cancer
(leukemia), fibrosarcoma sand melanomas.7 Recently, Kanugula et al. synthesized 6{(1-aryl-1H-
1,2,3-triazol-4-yl)methyl}-6H-indolo[2,3-b]quinoxaline derivatives 6 and examined these
compounds against three human cancer cell lines, namely cervical, prostate and lung using an
MTT assay.13 The results showed that 9-fluoroindolo[2,3-b]quinoxalines containing CF3, Cl, H
substituents located at the 3-position of the arene attached to the triazole ring enhanced their
bioactivity.
Figure 2. Some bioactive compounds containing the indolo[2,3-b]quinoxaline moiety

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Due to the importance of indolo[2,3-b]quinoxalines in both material sciences and medicinal
chemistry, we were interested in developing a new and efficient strategy for the synthesis of
indolo[2,3-b]quinoxalines. Until now, synthetic approaches to these molecules are often
complicated, low yielding and require several steps. Most of the known syntheses of indolo[2,3-
b]quinoxalines rely on the cyclocondensation of isatin with o-phenylenediamine derivatives.
Marchlewski carried out the first synthesis of parent indolo[2,3-b]quinoxaline by condensation of
isatin with o-phenylenediamine in the presence of AcOH as acidic catalyst. In 1980, Reisenauer
and coworkers reported the cyclization of carbodiimide compounds by rearrangement of nitrenes
to give indolo[2,3-b]quinoxalines in good yields.14 Indolo[2,3-b]quinoxalines could also be
prepared by cyclization of o-phenylenediamine with 1-acetyl-2-bromo-3-indolinone.15 In
general, the synthesis of highly functionalized indolo[2,3-b]quinoxalines is difficult, because
starting materials are not readily available. Herein, we wish to report a practical and efficient
two-step synthesis of indolo[2,3-b]quinoxalines by Pd-catalyzed Suzuki-Miyaura reaction of 2,3-
dibromoquinoxaline and subsequent Pd-catalyzed two-fold C-N coupling annulation with
amines. In addition, a second approach was developed based on the one-pot Pd-catalyzed
domino reaction of 1,2-dibromoquinoxaline with secondary aromatic amines. These reactions
also gave indolo[2,3-b]quinoxaline derivatives in good yields, but with some limitations with
regard to the substrate scope.
Results and discussion
We envisaged to prepare the indolo[2,3-b]quinoxaline scaffold based on two retrosynthetic
strategies depicted in Scheme 1. Our first approach relies on Ackermann’s procedure for the one-
pot Pd-catalyzed domino synthesis of carbazole derivatives from aryl 1,2-dihalides.16, 17 This
approach provides a direct access to the indolo[2,3-b]quinoxaline core structure. The second
approach is based on a two-step synthesis using a Pd-catalyzed Suzuki reaction and subsequent
two-fold C-N coupling annulation.18

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R
N
N
Br
RNH2
N
N
N
two-fold C-N coupling
[Pd]
+
Br
Br
(HO)2B
NH
R
C-N coupling
C-H activation
N
N
Br
Br
Suzuki reaction
[Pd]
[Pd]
Scheme 1. Retrosynthetic analysis of the synthesis of indolo[2,3-b]quinoxalines
We first started to study the one-pot reaction of 2,3-dibromoquinoxaline (7) with secondary
aromatic amines in the presence of Pd(OAc)2/PCy3·HBF4 as catalyst applying Ackermann’s
protocol developed for other heterocyclic substrates.16,17 We were pleased to find that the
reaction of 7, prepared in two steps from 1,2-diaminobenzene using Li’s procedure,19 with
diphenylamine afforded indolo[2,3-b]quinoxalines 11a in 90% yield (Scheme 2). The preparative
scope was studied (Table 1). We observed that good yields were obtained for indolo[2,3-
b]quinoxalines derived from sterically less bulky amines. Product 8a derived from an
unsymmetrical diarylamine could be successfully prepared, albeit, in only moderate yield. In
contrast, the synthesis of 8e failed. The yields generally decreased or the reactions completely
failed in case of sterically encumbered anilines containing substituents located at the ortho-
position (formation of complex mixtures). In case of 2-(methoxy)aniline, the bis(adduct) 8d
instead of the desired cyclization product was isolated. In addition, the employment of N-
alkylanilines ArN(Alkyl)H or simple anilines ArNH2 as substrates failed. The failure in case of
N-alkylanilines was already reported before by Ackermann for cyclization reactions with other
aromatic dihalides.16 To optimize the yields we have tried to vary the conditions by changing the
palladium catalyst in combination with different ligands, but so far we were not able to isolate
the products in good yields.

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Scheme 2. Synthesis of indolo[2,3-b]quinoxaline 8a. Conditions: (i) 1.5 equiv. of 7, 1 equiv. of
secondary amine, 3 equiv. of NaOtBu, 5% mol of Pd(OAc)2, 10% of PCy3·HBF4, toluene, 105
ºC, 18h.
Table 1. Synthesis of products 8a-f and 11a following the domino C-N/C-H bond activation
pathway
Entry
1
Amine
Product
Yield (%)a
90
H
N
H
N
2
47
54
Me
H
N
3
4
OMe
MeO
Me
Me
H
0b
N

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8
MeO
OMe
H
N
5
30
H
N
6
7
0 b
MeO
H
N
0 b
a Isolated yields; b formation of a complex mixture
In order to optimize the yields and to develop a more efficient procedure for the synthesis of
indolo[2,3-b]quinoxalines, we studied a second approach based on a two-step synthesis. In the
first step, a Suzuki-Miyaura reaction is carried out followed by a twofold C-N coupling
annulation (Scheme 3). The Suzuki-Miyaura reaction of 7 with 2-bromophenylboronic acid in
the presence of catalytic amounts of Pd(PPh3)4 afforded intermediate 9 in 87% isolated yield.
The Pd-catalyzed twofold C-N coupling annulation of 9 with different amines 10a-t gave the
desired products 11a-t in good to excellent yields (Table 3).
HO
OH
B
Br
H2N R
N
N
Br
Br
N
N
Br
R
10a-t
N
N
N
i
ii
Br
9 (87%)
7
11a-t

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Scheme 3. Synthesis of indolo[2,3-b]quinoxalines 11a-t. Conditions: (i) 1.2 equiv. of 2-
bromophenylboronic acid, 2.5 % of Pd(Ph3)4 catalyst, 3 equiv. of NaOH, THF, H2O, 70 ºC, 4h.
(ii) 3 equiv. of 10, 3 equiv. of NaOtBu, 5% mol of Pd2(dba)3, ligand (method A: 10 mol% of
dppf, toluene, 100 ºC, 6 h; or method B: 10 mol% of dpePhos, toluene, 100 ºC, 6h).
The conditions of the annulation reaction of 9 with p-toluidine 10b were optimized (Table 2).
The ligand, palladium precursor, solvent and temperature were varied. The results showed that
bidentate ligands often gave higher yields than monodentate ligands. In fact, up to 92% yield of
11b was achieved by employment of dppf as ligand in combination with Pd2(dba)3 as the Pd
source (method A).
Table 2. Optimization for the synthesis of 11b
Entry Pd precursor
Ligand
BINAP
XantPhos
DPEPhos
Dppe
Solvent
Tol
Temperature (ºC)
Yield (%)a
67
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd(OAc)2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
110
80
1
Tol
84
76
62
92
52
61
36
40
24
15
5
2
Tol
3
Tol
4
Dppf
Tol
5
PCy3·HBF4
PBu3·HBF4
XPhos
Tol
6
Tol
7
Tol
8
XPhos·tBu2
SPhos
Tol
9
Tol
10
11
12
13
14
15
16
17
DavePhos
RuPhos
Dppf
Tol
Tol
Tol
52
85
14
83
75
Dppf
1,4-Dioxane
DMF
Tol
Dppf
Dppf
Dppf
Tol

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aYield calculated by 1H-NMR of the crude product using 4-nitroacetophenone as an internal standard
P(t-Bu)2
PCy2
PCy2
PCy2
OMe
PCy2
Ph2P
N
O
MeO
O
i-Pr
i-Pr
PPh2
Dppe
XPhos(t-Bu)2
XPhos
DavePhos
SPhos
RuPhos
PPh2
PPh2
PPh2
PPh2
P
BF
4
H
P
H
BF4
Fe
O
O
PPh2
PPh2
PPh2
PPh2
.
.
P(Cy)3 HBF4
P(t-Bu)3 HBF4
Dppf
Xantphos
DpePhos
BINAP
Figure 3.Monodentate and bidentate phosphine ligands.
With the optimized conditions in hand, we explored the scope of the twofold C-N annulation
reaction of 9 with different amines. The employment of various secondary aromatic amines
afforded the corresponding products 11a-i in good to excellent yields in only 6 hours reaction
time (Table 3). The results showed that the annulations gave high yields for substrates bearing
both electron-withdrawing or -donating substituents. In contrast, the reactions of 9 with alkyl
amines, using our optimized conditions (method A), resulted in the formation of side products
which were difficult to separate from the main product. Therefore, an additional optimization of
the reaction conditions was carried out for the synthesis of product 11n derived from benzyl
amine. It was found that employment of DPEPhos as ligand in combination with Pd2dba3
(method B) resulted in the formation of product 11n in up to 96% yield (Table 4). The
application of these conditions allowed for the synthesis of products 11j-t, derived from aliphatic
amines, in very good yields (Table 3).

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Table 3. Synthesis of 11a-t
R
Method
A
Yield (%)a
83
11
Ph
a
b
c
d
e
f
4-MeC6H4
4-FC6H4
A
86
A
80
3-(CF3)C6H4
4-(MeO)C6H4
3,5-(MeO)2C6H4
4-(MeS)C6H4
4-(Et2N)C6H4
(4-NC)C6H4
n-C3H7
A
90
A
98
A
95
A
94
g
h
i
A
75
A
83
B
96
j
n-C5H11
B
93
k
l
n-C7H15
B
85
Allyl
B
73 b
m
n
o
p
q
r
s
Bn
B
94
4-(MeO)C6H4CH2
(4-FC6H4)CH2
3-(CF3)C6H4CH2
PhCH2CH2
PhCH2CH2CH2
B
92
B
87
B
84
B
89
B
91
Cyclohexyl
B
74
t
a
b
Isolated yields; the product was 6-(prop-1-en-1-yl)-6H-indolo[2,3-b]quinoxaline formed by
isomerization of the allylic double bond.
Table 4. Optimization for the synthesis of 11n

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Entry Pd precursor
Ligand
Solvent
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Temperature (ºC) Yield (%)a
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
BINAP
100
100
100
100
100
100
100
100
100
100
100
100
51
63
96
14
73
-
1
XantPhos
DPEPhos
Dppe
2
3
4
Dppf
5
PCy3·HBF4
PBu3·HBF4
XPhos
6
15
61
59
25
34
39
7
8
XPhos·tBu2
SPhos
9
10
11
12
DavePhos
RuPhos
aYield calculated by 1HNMR of the crude product using 4-nitroacetophenone as an internal standard
The structures of products 11a-t were established by spectroscopic methods. The structures of
11e and 11r were independently confirmed by X-ray crystal structure analyses. The geometric
parameters of the X-ray structure for compound 11e were also compared with those derived from
the DFT calculations. The optimized geometry of compound 11e (from DFT calculations) shows
a good correlation with the X-ray structure. The quinoxaline scaffold is planar, whereas the
methoxy phenyl ring has a dihedral angle of 52.3 degrees from the quinoxaline plane. A few
important calculated bond lengths and bond angles are compared with the experimental values
(Table 5). The differences between theoretical and experimental bond lengths and bond angles
are in the range of 0.015Å and 2.1 degrees, respectively.
Table 5. Comparison of experimental bond lengths and bond angles with theoretical values,
calculated at B3LYP/6-31G*
Bond Length
Experimental
Theoretical
Bond Angle
Experimental
Theoretical

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1.433
1.380
1.373
1.314
1.306
1.401
1.369
1.433
1.379
1.425
1.370
1.367
1.314
1.308
1.407
1.363
1.419
1.389
C1-N1-C5
C2-N2-C10
C1-N3-C3
C1-N3-C15
C3-N3-C15
C18-O1-C21
N1-C1-N3
N1-C1-C2
N2-C2-C4
113.1
114.0
N3-C15
N1-C5
N2-C10
N2-C2
N1-C1
N3-C3
O-C18
O-C21
N3-C1
114.57
108.15
127.67
124.18
116.93
126.22
125.21
130.76
115.09
108.09
125.63
126.25
118.29
126.59
124.60
130.98
Figure 3.Ortep plot of 11e

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Figure 4.Ortep plot of 11r
Electrochemical properties
Electrochemical properties of some compounds were examined by Cyclic Voltammetry (CV)
and Differential Pulse Voltammetry (DPV) measurements with three different concentrations (1
x 10-3; 3 x 10-3; 6 x 10-3 mol·L-1) in DMF by means of a µAutolab III potentiostat (Ecochemie,
Utrecht, The Netherlands). A 0.01 mol·L-1 solution of tetrabutylammonium hexafluorophosphate
(TBAPF6) was used as the supporting electrolyte. All potentials were calibrated with the
ferrocene/ferrocenium couple (Fc/Fc+) as internal standard. Oxidation and reduction energy
levels were determined from the DPV measurement (Table 6). The formal potential of Fc/Fc+ vs.
vacuum was assumed to be -4.8 eV.

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Cyclic Voltammograms of 11
Differential Pulse Voltammograms of 11
Figure 5. Electrochemical properties of some quinoxaline derivatives.
Table 6. Electrochemical properties of some synthesized quinoxaline derivatives
ꢆ/ꢇ
ꢀꢆꢁꢂ/ꢇꢃꢄꢅ (V
vs Fc/Fc+)a
-1.395
Comp.
∆Eg (eV)e
∆Egcal. (eV)f
ꢀꢈꢉꢊꢉ ꢀꢋꢌꢊꢉ
(eV)c (eV)d
ꢀ
(V) (V
ꢄꢅ
vs Fc/Fc+)b
11a
11b
11c
1.872 -6.672
1.779 -6.579
1.910 -6.710
-3.405
3.267
3.707
-1.488
-3.312
-3.344
3.267
3.366
3.670
3.704
-1.456

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11e
11j
-1.456
-1.496
-1.508
-1.545
-1.512
-1.512
1.666 -6.466
1.819 -6.619
1.813 -6.613
1.787 -6.587
1.874 -6.674
1.813 -6.613
-5.91
-3.344
-3.304
-3.292
-3.255
-3.288
-3.288
-2.51
3.122
3.315
3.321
3.332
3.386
3.325
3.40
3.605
3.637
3.750
11l
11m
11n
11t
CBP
3.784
3.744
ꢆ/ꢇ
a
b
c
ꢀꢁꢂꢃꢄꢅ = Eredox + (Eampli /2). Eampli = 0.0501 (V). Eredox values were determined by DPV in DMF.
ꢀꢆꢄꢅ/ꢇ= Eox + (Eampli /2).Eox values were determined by DPV in DMF.V vs Fc/Fc+ in 0.1 M TBABF6.
ꢆ/ꢇ
d
The HOMO levels were estimated from ꢀꢈꢉꢊꢉ = -(
ꢀ
+4.8) (eV). The LUMO levels were
ꢄꢅ
ꢆ/ꢇ
ꢁꢂꢃꢄꢅ
e
estimated from ꢀꢋꢌꢊꢉ = - (
ꢀ
+4.8) (eV). Electrochemical band gaps ∆Eg were estimated from
∆Eg = ꢀꢋꢌꢊꢉ - ꢀꢈꢉꢊꢉ.
d The band gaps ∆Egcal.were estimated from computational DFT calculation method.
The CV diagram of compound 11a showed a reversible and well-defined redox peak (Figure 5).
However, an oxidation peak was not present, due to low electrochemical activity of quinoxaline
in the CV experiment. Thus, the DPV method was chosen for investigation of the
electrochemical properties. The experiments showed that the band gaps were independent from
the exact substitution pattern. It is assumed that the quinoxaline core play the key role.
Compared to 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP), which is commonly used as host
material, quinoxaline derivatives gave lower HOMO and LUMO levels and slightly smaller band
gaps. Among derivatives containing a phenyl substituent located at the nitrogen atom, compound
11c containing an electron withdrawing group had a HOMO energy level lower than compound
11a. In contrast, compound 11e, containing an electron donating group, resulted in a shift to a
higher HOMO level yielding a smaller band gap. The smallest band gap and highest HOMO
energy level was displayed by 11e containing a 4-methoxyphenyl substituent. It is noteworthy
that compounds 11c and 11n, containing a 4-fluorophenyl and a benzyl substituent located at the

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17
nitrogen atom, respectively, were found to exhibit the lowest HOMO as well as the biggest band
gap.
Density functional theory (DFT) calculations have also been performed for the determination of
HOMO-LUMO band gaps. A comparison of theoretical and experimental values is given in
Table 6. Although the absolute values of theoretical and experimental band gaps are different
however, the trend is very similar. A linear relationship is observed between the theoretical and
experimental band gaps. The difference between theoretical and experimental band gaps has
already been discussed in the literature by us20 and others21. The difference between theoretical
and experimental HOMO-LUMO gaps decreases with increase in the size of the hydrocarbon.22
The results shown in Table 6 indicate that both theoretical and experimental band gaps are not
much affected by structural modifications. N-aliphatic or N-benzylindolo[2,3-b]quinoxalineshave
higher HOMO-LUMO band gaps as compared to their N-phenyl analogues. The highest HOMO-
LUMO band gap was calculated for 11n (3.78 eV) which is consistent with the highest
experimental band gap for this compound (3.38 eV). The lower band gap for N-
phenylindolo[2,3-b]quinoxalines is probably due to extended conjugation which provides
enhanced delocalization, and therefore the band gap decreases. Among N-phenylindolo[2,3-
b]quinoxalines, 11a and 11c have comparable bands gaps which indicate that introduction of a
fluorine atom has a negligible effect. This may be attributed to the high electronegativity of
fluorine which prevents its lone pairs to delocalize over the organic π frame. Introduction of a
methyl group at the para position of the N-phenyl group decreases the band gap by 0.037 eV
whereas a methoxy group at the same position decreases the band gap by 0.1 eV.
Molecular orbitals and iso density plots of HOMO-2 and LUMO+2 for N-phenylindolo[2,3-
b]quinoxalinesareare shown in Figure 6 as a representative example. The HOMO-1 and HOMO-
2 lie at 0.5 and 1.0 eV, respectively,lower in energy relative to HOMO. The HOMO-1 and
HOMO-2 are mainly centered on the indolo[2,3-b]quinoxaline skeleton whereas the HOMO is
also extended to the N-phenyl substituent. LUMO+1 and LUMO+2 orbitals are situated 1.374 eV
and 1.548 eV higher in energy, respectively, than LUMO. HOMO and LUMO of quinoxalines
11a, 11l and 11t were also analyzed and are shown in Figure 7. Replacement of the phenyl ring
of 11a with an aliphatic heptyl chain in 11l and an alicyclic fragment (cyclohexyl) in 11t does
not affect the iso densities of the LUMOs, however, a small effect on the HOMO is observed. In

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11l and 11t, HOMOs are centered on the quinoxaline core whereas in 11a it has some density on
the N-phenyl ring as well.

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Figure 6. HOMO-2 to LUMO+2 molecular orbitals of quinoxaline 11a

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Figure 7. HOMO and LUMO of 11a, 11l and 11t calculated at B3LYP/6-31G*

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Absorption and Fluorescence Properties
UV-VIS and fluorescence spectra of some selected indolo[2,3-b]quinoxalines 11 dissolved in
acetonitrile are shown in Figure 8 and the corresponding spectral data are summarized in Table
7. The UV-VIS absorption spectra of the compounds exhibit below 230 nm three bands around
400 nm, 350 nm, and 270 nm with increasing absorption strength. The spectra of all derivatives
11 are quite similar suggesting that the substituent located at the nitrogen atom has only a weak
influence. In the case of compound 11i the band around 270 nm seems to be split into two well
separated contributions. The absorptions of compounds 11j, 11l, 11t, bearing an aliphatic group
located at the nitrogen atom, are slightly red-shifted due to the positive inductive effect. For
compounds 11d and 11i, containing electron withdrawing groups, a shift to shorter wavelenghths
is observed.
The fluorescence spectra were measured using again acetonitrile as solvent and an excitation
wavelength of 350 nm. The fluorescence quantum yields were determined by comparison to the
standard quinin hemisulfate salt monohydrate in 0.05M H2SO4 which has a fluorescence yield of
0.52 [Meech, S. R.; Phillips, D. Journal of Photochemistry 1983, 23, 193-217.]. The spectra
feature emission bands around 480 nm. Compound 11d containing a trifluormethyl group shows
the bluest emission with a maximum at 471 nm while 11e containing a methoxy group exhibits
an emission band at 538 nm which is strongly red shifted compared to all other compounds. The
Stokes shift is of medium size and in the range of 80 nm to 90 nm. It is similar for all compounds
except for 11e which exhibits a large Stokes shift of 140 nm. The quantum yields of the
indolo[2,3-b]quinoxalines 11 are in the order of a few percent with the highest yield of 8.6%
observed for 11l. It is noteworthy that compound 11e shows also an exceptionally low yield of
only 1.1% which seems to correlate with the large red shift of its fluorescence.
The week sensitivity of the absorption and fluorescence spectra on the substituent are inline with
the small variation of the electrochemical properties and the band gap of the compounds (see
above). Only 11e exhibits a significant higher HOMO level and smaller band gap than the other
compounds which fits to its red shifted fluorescence. The general behavior can be rationalized by
the involved orbitals. As shown in Figure 7 HOMO and LUMO, which determine the

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22
fluorescence and the first absorption band, are more or less completely restricted to the
indolo[2,3-b]quinoxaline core and therefore only little affected by substitution. Since the energy
differences between the HOMO and HOMO-1 and HOMO-2 are smaller than those between the
LUMO and LUMO+1 and LUMO+2 the next higher lying electronically excited states should
dominantly contain configurations with excitations from HOMO-1 and HOMO-2 to the LUMO.
Since the former two orbitals are again restricted to the indolo[2,3-b]quinoxaline core (see
Fig. 6) the corresponding absorption bands around 350 nm and 270 nm are also rather insensitive
to the substituent.
Figure 8. Normalized absorption and emission spectra of selected compounds 11 measured in
acetonitrile. Emission spectra were recorded with excitation at 350 nm.
Table 7. Spectroscopic data characterizing the absorption and emission properties of 11
ꢑꢎꢅ
ꢆꢎꢏꢐ
ꢑꢎꢅ
ꢇꢎꢏꢐ
ꢑꢎꢅ
ꢒꢎꢏꢐ
ꢑꢎꢅ
ꢍ
ꢂꢑ
Comp.
Lgε
Lgε
Lgε
Stokes shift
[nm]
Φfluo
ꢍ
ꢍ
ꢍ
ꢑꢎꢅ
ꢑꢎꢅ
ꢑꢎꢅ
Quantum
yield [%]
[nm]
ꢍ
[nm]
ꢍ
[nm]
ꢍ
ꢒꢎꢏꢐ
[nm]
ꢆꢎꢏꢐ
ꢇꢎꢏꢐ
395
396
3.616
3.738
351
4.353
4.491
4.547
270 4.696
269 4.848
268 4.903
484
490
482
89
94
5.9
11a
11b
11c
351
351
5.2
5.9
394
389
398
394
389
3.809
3.684
3.954
3.387
3.463
88
82
350
351
351
350
4.338
4.795
4.078
4.133
269 4.642
269 5.188
270 4.451
262 4.500
471
538
485
474
4.3
1.1
4.8
4.6
11d
11e
11f
11i
140
91
85

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403
404
398
396
404
2.827
3.952
4.220
3.615
4.144
352
352
351
351
352
3.728
4.939
4.934
4.278
5.005
269 4.126
269 5.334
269 5.322
269 4.672
270 5.402
484
483
477
475
486
81
79
79
79
82
6.7
8.6
7.1
7.4
7.7
11j
11l
11n
11p
11t
Computational Methods
DFT calculations were performed with the Gaussian 09Revision C.01.23 The visualization of
the results was performed with GaussView. The geometries of indolo[2,3-b]quinoxalines
were optimized using the hybrid functional B3LYP method, which consists of Becke’s three-
parameter24 (B3) hybrid exchange functional in conjunction with the correlation functional of
LeeYang and Parr (LYP)25 method using 6-31G* basis set.26 The B3LYP/6-31G* method of
DFT has been reliable for the prediction of geometric and electronic properties of neutral20
and charged species27 ranging from simple molecular to polymer structures.28 Frequency
calculations were performed at the same method in order to confirm these structures as true
minima (absence of an imaginary frequency). Molecular orbital calculations are also
performed at the B3LYP/6-31G* level of theory.
Conclusion
In conclusion, we reported two strategies for the preparation indolo[2,3-b]quinoxalines. The
one-pot approach, using Pd-catalyzed two-fold C-N coupling and C-H activation reactions, gave
indolo[2,3-b]quinoxalines in good yields, but the substrate scope was limited. A two-step
approach, based on Suzuki coupling reaction followed by an annulation by Pd-catalyzed two-
fold C-N coupling, afforded indolo[2,3-b]quinoxalines in very good yields. The physical
properties of indolo[2,3-b]quinoxalines, including electronic, electrochemical and optical
properties, were studied experimentally and by DFT calculations. It turns out that the
electronic and spectroscopic properties are quite insensitive to the substituent since the
relevant orbitals are restricted to the indolo[2,3-b]quinoxaline core. The substituent might

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24
therefore be used to control and optimize the solubility, the interaction with the environment, and
the crystallization behavior in the solid state without changing the electronic properties of the
core.
Experimental Section
Synthesis of 2,3-dibromoquinoxaline. 2,3-Dibromoquinoxaline was synthesized in 94% of
overall yield using Li’s procedure by reflux of 1,2-phenylenediamine with diethyl oxalate, to
give 1,4-dihydroquinoxaline-2,3-dione, and subsequent reaction with PBr5.19 M.p. 179-180 ºC.
13
1H NMR (300 MHz, CDCl3) δ 8.08 – 8.01 (m, 2H), 7.86 – 7.78 (m, 2H); C NMR (75 MHz,
CDCl3) δ 141.42, 140.97, 131.49, 128.57; IR (ATR, cm-1): ν =3097 (m), 3034 (m), 1564 (m),
1549 (s), 1514 (s), 1479 (m), 1254 (s), 1169 (s), 1126 (m), 1107 (s), 1072 (m), 1059 (m), 957
(vs), 901 (m), 883 (m), 868 (s), 769 (vs), 692 (m), 677 (m), 621 (m), 582 (s); GC-MS (EI, 70
eV): m/z (%) = 288 (96), 209 (95), 128 (61), 102 (100), 75 (98), 50 (59); HRMS (EI): calcd. for
C8H4N2Br2 ([M]+): 285.87357; found: 285.87325; calcd. for C8H4N2Br1 Br1 ([M]+): 287.87153;
81
81
found: 287.87137; calcd. for C8H4N2 Br2 ([M]+): 289.86948; found: 289.86935.
General procedure for the preparation of 2-bromo-3-(2-bromophenyl)quinoxaline (9). 2,3-
Dibromoquinoxaline 7 (1 g, 3.5 mmol), 2-bromophenyl boronic acid (837 mg, 4.2 mmol),
Pd(PPh3)4 (100 mg, 87 µmol) and sodium hydroxide (417 mg, 10.4 mmol) were added to a 500
mL Schlenk flask. The mixture was back-filled several times with Argon. To the mixture 70 mL
THF and 10 mL distilled water were added, then, back-filled several times. The reaction was
heated at 70 ºC for 4h. The solvent was evaporated in vacuo. The residue was extracted with
dichloromethane and water. The organic layer was dried over MgSO4, filtered and the solvent
was evaporated in vacuo. The yellow residue was purified by column chromatography (silica gel,
Heptane/ethylacetate 10:1) to yield 2-bromo-3-(2-bromophenyl)quinoxaline 9 (1.1 g, 87 %) as
1
white solid. M.p. 127-129 ºC; H NMR (250 MHz, CDCl3) δ 8.20 – 8.07 (m, 2H), 7.90 – 7.79
(m, 2H), 7.73 (dd, J = 7.9, 0.8 Hz, 1H), 7.55 – 7.35 (m, 3H); 13C NMR (63 MHz, CDCl3) δ

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154.95, 142.47, 140.67, 140.11, 139.38, 132.99, 131.46, 131.01, 130.84, 130.49, 129.58, 128.57,
127.76, 122.83; IR (ATR, cm-1): ν =3059 (w), 1610 (w), 1556 (m), 1535 (w), 1477 (m), 1433
(m), 1385 (w), 1333 (m), 1290 (m), 1273 (w), 1252 (m), 1236 (w), 1213 (w), 1167 (w), 1147
(m), 1132 (m), 1084, 1041, 1024 (m), 999 (w), 970, 955 (m), 943 (m), 885 (m), 870 (w), 862
(w), 752 (vs), 727, 715, 710, 690 (m), 652 (m), 638 (m), 613 (m), 588, 571 (m), 557 (m); GC-
MS (EI, 70 eV): m/z (%) = 364 (32), 285 (100), 102 (48), 75 (28), 50 (14); HRMS (EI): calcd.
81
for C14H8N2Br2 ([M]+): 361.90488; found: 361.90467;calcd. for C14H8N2Br1 Br1 ([M]+):
81
363.90283; found: 363.90277; calcd. for C14H8N2 Br2 ([M]+):365.90078; found: 365.90082.
General procedure A for double C-N coupling with aniline derivatives,exemplified by the
synthesis of6-phenyl-6H-indolo[2,3-b]quinoxaline (11a). Aniline 10a (75 µL, 0.82 mmol) was
added to a pressure tube charged with 9 (100 mg, 0.28 mmol), Pd2(dba)3 (12 mg, 14 µmol),
ligand dppf (15mg, 27 µmol) and sodium tert-butoxide (79 mg, 0.82 mmol) under Argon. The
mixture was back-filled with Argon several times. The mixture was dissolved in anhydrous
Toluene (10 mL) and heated at 100 ºC for 7 h. After cooling, the reaction mixture was diluted
with dichloromethane (20mL) and filtered through a celite pad, washing with dichloromethane
(40 mL). The filtrate was reduced in vacuo. The product was separated via flashchromatography
(silica gel, heptanes/ethylacetate 5:1) to yield 6-phenyl-6H-indolo[2,3-b]quinoxaline 11a (67 mg,
1
83%) as a yellow solid; m.p. 238-239 ºC; H NMR (300 MHz, CDCl3) δ 8.56 (d, J = 7.7 Hz,
13
1H), 8.40 – 8.29 (m, 1H), 8.14 – 8.06 (m, 1H), 7.84 – 7.59 (m, 7H), 7.59 – 7.38 (m, 3H); C
NMR (75 MHz, CDCl3) δ 146.00, 144.90, 140.72, 140.08, 139.69, 135.50, 131.25, 129.92,
129.24, 128.99, 128.38, 128.13, 127.27, 126.71, 122.94, 122.02, 119.83, 110.75; IR (ATR, cm-1):
ν =3053 (m), 1608 (m), 1597 (m), 1581 (m), 1500, 1483 (m), 1470 (m), 1458, 1402, 1390, 1354
(m), 1336 (m), 1317 (m), 1304 (m), 1252 (m), 1227 (m), 1205, 1174 (m), 1147 (m), 1132 (m),
1126 (m), 1099 (m), 1072 (m), 1041 (m), 1024 (m), 1014 (m), 1007 (m), 955 (m), 924 (m), 779
(m), 766 (m), 748 (vs), 719 (m), 694, 687, 648, 590, 567 (m); GC-MS (EI, 70 eV): m/z (%) =
295 (100), 147 (9), 90 (6), 77 (6); HRMS (ESI): calcd. for C20H14N3 ([M + H]+): 296.11822;
found: 296.11835.

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6-(p-Tolyl)-6H-indolo[2,3-b]quinoxaline (11b) was prepared following general procedure A
using compound 9 (100 mg, 0.28 mmol) and toluidine (88 mg, 0.82 mmol). The product was
purified by flash chromatography (silica gel, heptanes/ethylacetate 5:1) to yield 11b (73 mg,
1
86%) as a yellow solid; m.p. 216-217 ºC; H NMR (300 MHz, CDCl3) δ 8.50 – 8.43 (m, 1H),
8.29 – 8.22 (m, 1H), 8.05 – 7.99 (m, 1H), 7.68 – 7.57 (m, 2H), 7.57 – 7.49 (m, 3H), 7.45 – 7.31
(m, 4H), 2.43 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 146.01, 144.99, 140.66, 140.06, 139.62,
138.04, 132.68, 131.06, 130.45, 129.17, 128.79, 128.27, 127.02, 126.46, 122.72, 121.73, 119.68,
110.62, 21.35; IR (ATR, cm-1): ν =3057 (w), 3034 (w), 2918 (w), 1606 (m), 1585 (m), 1514,
1485 (m), 1470 (m), 1460, 1404, 1354 (m), 1335 (m), 1317, 1304 (m), 1255 (m), 1227 (m), 1221
(m), 1205, 1182 (m), 1169 (m), 1130 (m), 1122 (m), 1099 (m), 1043 (m), 1016 (m), 955 (m), 924
(m), 816 (m), 764, 750 (vs), 721 (m), 710 (m), 673 (w), 633 (m), 602, 579, 567 (m), 559 (m);
GC-MS (EI, 70 eV): m/z (%) = 309 (100), 293 (8), 154 (7), 90 (5); HRMS (EI): calcd. for
C21H15N3 ([M]+): 309.12605; found: 309.12523.
6-(4-Fluorophenyl)-6H-indolo[2,3-b]quinoxaline (11c) was prepared following general
procedure A using compound 9 (100 mg, 0.28 mmol) and 4-fluoroaniline (78 µL, 0.82 mmol).
The product was purified by flash chromatography (silica gel, heptanes/ethylacetate 5:1) to yield
1
11c (69 mg, 80 %) as a yellow solid; m.p. 219-220 ºC; H NMR (300 MHz, CDCl3) δ 8.55 (d, J
= 7.8 Hz, 1H), 8.37 – 8.30 (m, 1H), 8.12 – 8.04 (m, 1H), 7.78 – 7.61 (m, 5H), 7.50 – 7.41 (m,
2H), 7.41 – 7.30 (m, 2H); 19F NMR (282 MHz, CDCl3) δ -113.01; 13C NMR (75 MHz, CDCl3) δ
162.08 (d, J = 247.9 Hz), 146.07, 144.90, 140.70, 140.01 (d, J = 18.0 Hz), 131.43 (d, J = 3.2
Hz), 131.32, 129.38, 129.18 (d, J = 8.4 Hz), 129.12, 128.32, 126.81, 122.98, 122.15, 119.92,
116.99 (d, J = 22.9 Hz), 110.52; IR (ATR, cm-1): ν =3057 (m), 1608 (m), 1579 (m), 1574 (m),
1514, 1485, 1471 (m), 1460, 1402, 1356 (m), 1335 (m), 1313, 1292 (m), 1259 (m), 1223, 1203,
1171 (m), 1151 (m), 1130 (m), 1122, 1099, 1043 (m), 1012 (m), 1007 (m), 949 (m), 924 (m),
872 (m), 831, 812 (m), 800 (m), 764, 748 (vs), 723 (m), 710, 673 (m), 638 (m), 629 (m), 602,
579, 567 (m), 557 (m), 548 (m); GC-MS (EI, 70 eV): m/z (%) = 313 (100), 156 (12), 75 (7);
HRMS (EI): calcd. for C20H12N3F1 ([M]+): 313.10098; found: 313.10007.

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6-(3-(Trifluoromethyl)phenyl)-6H-indolo[2,3-b]quinoxaline (11d) was prepared following
general procedure A using compound 9 (100 mg, 0.28 mmol) and 3-(trifluoromethyl)aniline
(103 µL, 0.82 mmol). The product was purified by flash chromatography (silica gel,
1
heptanes/ethylacetate 5:1) to yield 11d (90 mg, 90 %) as a yellow solid; m.p. 201-202 ºC; H
NMR (300 MHz, CDCl3) δ 8.46 (ddd, J = 7.7, 1.2, 0.7 Hz, 1H), 8.29 – 8.19 (m, 1H), 8.03 – 7.95
(m, 2H), 7.92 (ddd, J = 3.7, 3.0, 1.9 Hz, 1H), 7.76 – 7.53 (m, 5H), 7.48 – 7.33 (m, 2H); 19F NMR
(282 MHz, CDCl3) δ -62.58; 13C NMR (75 MHz, CDCl3) δ 145.55, 144.00, 140.39, 140.07,
136.11, 132.31 (q, J = 33.0 Hz), 131.25, 130.40, 130.32, 130.31, 129.33, 129.11, 128.23, 126.90,
124.49 (q, J = 3.7 Hz), 123.85 (q, J = 3.9 Hz), 122.92, 123.74 (q, J = 272.5 Hz),122.41, 120.16,
110.31; IR (ATR, cm-1): ν =3051 (w), 3028 (w), 1608 (w), 1597 (w), 1579 (w), 1574 (w), 1495
(m), 1464 (m), 1446 (m), 1406, 1356 (m), 1329, 1308 (m), 1279 (w), 1250 (m), 1230 (m), 1205
(m), 1167, 1134 (m), 1126 (m), 1113, 1105, 1095, 1068, 1045 (m), 1011 (m), 987 (w), 976 (w),
958 (m), 943 (m), 924 (w), 904 (m), 874 (w), 860 (w), 854 (w), 802 (m), 795 (m), 768 (m), 748
(vs), 719 (m), 700, 671, 656 (m), 631 (w), 615 (w), 588 (m), 567 (w), 546 (w); GC-MS (EI, 70
eV): m/z (%) = 363 (100), 294 (9); HRMS (ESI): calcd. for C21H12F3N3 ([M + H]+): 364.10561;
found: 364.10566; calcd. for C37H37N5Na ([M + Na]+): 574.29412; found: 574.2944.
6-(4-Methoxyphenyl)-6H-indolo[2,3-b]quinoxaline (11e) was prepared following general
procedure A using compound 9 (100 mg, 0.28 mmol) and p-anisidine (101 mg, 0.82 mmol). The
product was purified by flash chromatography (silica gel, heptanes/ethylacetate 3:1) to yield 11e
(88 mg, 98 %) as a yellow solid; m.p. 226-228 ºC; 1H NMR (300 MHz, CDCl3) δ 8.57 (d, J = 7.6
Hz, 1H), 8.35 (d, J = 8.9 Hz, 1H), 8.15 – 8.05 (m, 1H), 7.80 – 7.55 (m, 5H), 7.44 (t, J = 8.3 Hz,
13
2H), 7.21 – 7.13 (m, 2H), 3.94 (s, 3H); C NMR (75 MHz, CDCl3) δ 159.42, 145.47, 140.85,
139.42, 131.35, 129.13, 129.00, 128.72, 128.38, 128.02, 126.67, 123.02, 121.87, 119.53, 115.27,
113.39, 110.68, 55.77; IR (ATR, cm-1): ν =3076 (w), 3053 (m), 3022 (m), 2956 (m), 2933 (m),
2912 (m), 2839 (m), 1606 (m), 1585 (m), 1578 (m), 1512, 1506, 1487 (m), 1464, 1446, 1406,
1356 (m), 1336 (m), 1313 (m), 1296, 1244, 1230, 1205, 1178, 1167, 1136, 1128, 1103, 1041
(m), 1026, 1009 (m), 968 (m), 955 (m), 939 (m), 924 (m), 870 (m), 852 (m), 829, 820, 804 (m),
795 (m), 768, 748 (vs), 723, 715, 669 (m), 642 (m), 629 (m), 602, 579, 569, 550; GC-MS (EI, 70

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eV): m/z (%) = 325 (100), 310 (39), 282 (18), 141 (8); HRMS (ESI): calcd. for C21H15N3O ([M +
H]+): 326.12879; found: 326.12858.
6-(3,5-Dimethoxyphenyl)-6H-indolo[2,3-b]quinoxaline (11f) was prepared following general
procedure A using compound 9 (100 mg, 0.28 mmol) and 3,5-dimethoxyaniline (126 mg, 0.82
mmol). The product was purified by flash chromatography (silica gel, heptanes/ethylacetate 2:1)
1
to yield 11f (93 mg, 95 %) as a yellow solid; m.p. 188-189 ºC; H NMR (300 MHz, CDCl3) δ
8.45 (d, J = 7.6 Hz, 1H), 8.28 – 8.20 (m, 1H), 8.07 – 8.00 (m, 1H), 7.71 – 7.47 (m, 4H), 7.35
(ddd, J = 8.1, 6.9, 1.3 Hz, 1H), 6.81 (d, J = 2.3 Hz, 2H), 6.54 (t, J = 2.3 Hz, 1H), 3.79 (s, 6H);
13C NMR (75 MHz, CDCl3) δ 161.59, 145.81, 144.67, 140.60, 140.16, 139.77, 136.91, 131.08,
129.25, 128.83, 128.35, 126.55, 122.66, 121.86, 119.84, 110.92, 105.51, 100.33, 55.66; IR
(ATR, cm-1): ν =2993 (w), 2956 (w), 2926 (w), 1606 (m), 1591 (m), 1508 (w), 1491 (m), 1458,
1427 (m), 1404 (m), 1363 (w), 1325 (m), 1298 (m), 1257 (m), 1242 (m), 1207 (m), 1194, 1153,
1134 (m), 1124 (m), 1107 (m), 1066 (m), 1051 (m), 1039 (m), 1014 (m), 1003 (m), 993 (m), 953
(m), 933 (m), 912 (m), 877 (m), 860 (m), 847, 818 (m), 791 (m), 768, 735 (vs), 721, 688, 667
(m), 640 (m), 631 (m), 617 (m), 607 (m), 600 (m), 584, 577 (m), 565 (m), 534 (m); GC-MS (EI,
70 eV): m/z (%) =355 (100), 325 (13), 268 (12); HRMS (EI): calcd. for C22H17O2N3 ([M]+):
355.13153; found: 355.13066.
6-(4-(Methylthio)phenyl)-6H-indolo[2,3-b]quinoxaline (11g) was prepared following general
procedure A using compound 9 (100 mg, 0.28 mmol) and 4-(methylthio)aniline (103 µL, 0.82
mmol). The product was purified by flash chromatography (silica gel, heptanes/ethylacetate 3:1)
to yield 11g (88 mg, 94 %) as a white solid; m.p. 249-250ºC; 1H NMR (300 MHz, CDCl3) δ 8.47
(d, J = 7.4 Hz, 1H), 8.28 – 8.20 (m, 1H), 8.06 – 7.98 (m, 1H), 7.72 – 7.53 (m, 5H), 7.51 – 7.30
(m, 4H), 2.53 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 145.88, 144.73, 140.58, 140.17, 139.82,
138.69, 132.30, 131.09, 129.28, 128.89, 128.23, 127.62, 127.52, 126.56, 122.73, 121.91, 119.85,
110.56, 15.92; IR (ATR, cm-1): ν =2955 (m), 2920, 2850 (m), 1608 (m), 1579 (m), 1498, 1483
(m), 1460, 1431 (m), 1402, 1352 (m), 1335 (m), 1311, 1296 (m), 1252 (m), 1230 (m), 1203,
1184 (m), 1132 (m), 1124 (m), 1115 (m), 1103, 1090, 1041 (m), 1012 (m), 1003 (m), 984 (m),

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970 (m), 955 (m), 937 (m), 922 (m), 904 (w), 870 (m), 854 (w), 833 (m), 816, 768 (vs), 748 (vs),
719, 702, 661 (m), 634 (m), 625 (m), 590, 567, 548 (m); GC-MS (EI, 70 eV): m/z (%) = 341
(100), 326 (36), 294 (20), 102 (6); HRMS (ESI): calcd. for C24H22N4 ([M + H]+): 367.19172;
found: 367.19173.
5,7-Bis(4-(N,N-diethylamino)phenyl)-6H-indolo[2,3-b]quinoxaline (11h) was prepared
following general procedure A using compound 9(100 mg, 0.28 mmol) and N1,N1-
diethylbenzene-1,4-diamine (137 µL, 0.82 mmol). The product was purified by flash
chromatography (silica gel, Heptane/ethylacetate 3:1) to yield 11h (76 mg, 75 %) as a yellow
1
solid; m.p. 228-229 ºC; H NMR (300 MHz, CDCl3) δ 8.50 – 8.39 (m, 1H), 8.26 – 8.19 (m, 1H),
8.05 – 8.00 (m, 1H), 7.70 – 7.50 (m, 3H), 7.45 – 7.27 (m, 4H), 6.84 – 6.73 (m, 2H), 3.37 (q, J =
7.1 Hz, 4H), 1.17 (t, J = 7.1 Hz, 6H); 13C NMR (75 MHz, CDCl3) δ 147.61, 146.39, 145.80,
140.82, 140.23, 139.58, 130.90, 129.22, 128.57, 128.39, 128.29, 126.09, 122.55, 122.47, 121.29,
119.45, 112.09, 110.74, 44.55, 12.69; IR (ATR, cm-1): ν =2970 (w), 2926 (w), 2866 (w), 1626
(w), 1608 (m), 1578 (w), 1522, 1489 (m), 1462 (m), 1446 (m), 1429 (w), 1404 (m), 1371 (m),
1352 (m), 1333 (m), 1315 (m), 1279 (m), 1259 (m), 1228 (m), 1203, 1194, 1169 (m), 1157 (m),
1149 (m), 1134 (m), 1122 (m), 1101 (m), 1080 (m), 1041 (m), 1014 (m), 1003 (m), 978 (m), 953
(m), 924 (m), 864 (m), 849 (m), 814, 798, 758, 735 (vs), 723, 712, 667 (m), 640 (m), 631 (m),
596, 575, 563 (m), 548 (m), 532 (m); GC-MS (EI, 70 eV): m/z (%) = 366 (67), 351 (100), 322
(28), 294 (14), 243 (35), 194 (13), 165 (22); HRMS (ESI): calcd. for C24H22N4 ([M + H]+):
367.18780; found: 367.19184.
5,7-Bis(4-cyanophenyl)-6H-indolo[2,3-b]quinoxaline (11i) was prepared following general
procedure A using compound 9 (100 mg, 0.28 mmol) and 4-aminobenzonitrile (97 mg, 0.82
mmol). The product was purified by flash chromatography (silica gel, heptanes/ethylacetate 3:1)
1
to yield 11i (73 mg, 83 %) as a yellow solid; m.p. 272-273 ºC; H NMR (300 MHz, CDCl3) δ
8.47 (d, J = 7.7 Hz, 1H), 8.28 – 8.21 (m, 1H), 8.03 – 7.97 (m, 1H), 7.96 – 7.85 (m, 4H), 7.74 –
7.51 (m, 4H), 7.46 – 7.38 (m, 1H); 13C NMR (75 MHz, CDCl3) δ 145.27, 143.33, 140.25,
140.17, 140.11, 139.68, 133.65, 131.31, 129.34, 129.32, 128.20, 127.20, 127.02, 123.06, 122.85,