A comparison between Suzuki cross-coupling reaction and direct arylation in the synthesis of new antibacterial imidazo-pyrazines/pyridazines

Article abstract of DOI:10.1002/jhet.3902

Two efficient synthetic routes to preparation of 2,3-diarylimidazo[1,2-a]pyrazines and 2,3-diarylimidazo[1,2-b]pyridazines are described. The procedures involve a Suzuki cross-coupling reaction and a palladium-catalyzed direct arylation at position 3 and

Full text of DOI:10.1002/jhet.3902

Received: 16 October 2019
DOI: 10.1002/jhet.3902
Revised: 13 December 2019
Accepted: 24 December 2019
A R T I C L E
A comparison between Suzuki cross-coupling reaction
and direct arylation in the synthesis of new antibacterial
imidazo-pyrazines/pyridazines
Siavash Salek Soltani1 | S. Morteza F. Farnia1
| Alireza Foroumadi2,3
1School of Chemistry, College of Science,
Abstract
University of Tehran, Tehran, Iran
2Drug Design and Development Research
Center, The Institute of Pharmaceutical
Sciences (TIPS), Tehran University of
Medical Sciences, Tehran, Iran
3Department of Medicinal Chemistry,
Faculty of Pharmacy, Tehran University
of Medical Sciences, Tehran, Iran
Two efficient synthetic routes to preparation of 2,3-diarylimidazo[1,2-a]
pyrazines and 2,3-diarylimidazo[1,2-b]pyridazines are described. The proce-
dures involve a Suzuki cross-coupling reaction and a palladium-catalyzed
direct arylation at position 3 and finally a comparative study is achieved. The
antibacterial activities of the synthesized compounds in vitro were measured
and the results showed that most products presented moderate to good
antibacterial activities. Compounds 3a, 3c, 3j, 3k, 3l, and 3n revealed obvious
potency against Staphylococcus aureus, Bacillus subtilis, and Escherichia coli
with minimum inhibitory concentration values of 32 μg/mL, which were bet-
ter compared with the others.
Correspondence
S. Morteza F. Farnia, School of Chemistry,
College of Science, University of Tehran,
Tehran, Iran.
Email: mfarnia@khayam.ut.ac.ir
Alireza Foroumadi, Drug Design and
Development Research Center, The
Institute of Pharmaceutical Sciences
(TIPS), Tehran University of Medical
Sciences, Tehran, Iran.
Email: aforoumadi@yahoo.com
Funding information
Research Council of Tehran University of
Medical Sciences; University of Tehran
1 | INTRODUCTION
pyridazine is a prominent scaffold which has shown
diverse properties like antitumor,[16] antileukemic,[17]
Imidazo[1,2-a]pyrazines are a class of nitrogen bridgehead
moieties that have received significant attention due to
their potential biological activities.[1] Especially,
2,3-diarylimidazo[1,2-a]pyrazines have exhibited bro-
nchodilatory, nucleotide phosphodiesterase inhibition,[2]
antiviral,[18,19] and anti-apicomplexan.[20]
Figure 1A-C illustrates diverse compounds containing
an imidazo-pyrazine/pyridazine scaffold with a variety of
biological activities and pharmaceutical properties.[21–23]
Several pathways have been described for the synthe-
sis of imidazo-pyrazines/pyridazines.[24–26] Suzuki cross-
couplings and palladium-catalyzed direct arylations are
the most common methods which have been used for
arylation of these scaffolds.[27,28]
hypoglycaemic,[3]
inotropic
compound,[4]
anti-
inflammatory,[5] hypertensive controlling,[6] cardiac stimu-
lation, uterine protector,[7] adjusting allergic reaction,[8]
anticancer[9] antiulcer,[10,11] antimalarial,[12] and anti-
bronchospastic.[13] Moreover, it also shows photo-physical
traits such as chemiluminescence[14] and biolumines-
cence.[15] On the other hand, 2,3-diarylimidazo[1,2-b]
To the best of our belief, the two significant routes to
highly functionalized 2,3-diarylimidazo[1,2-a]pyrazines
and 2,3-diarylimidazo[1,2-b]pyridazines has not yet been
J Heterocyclic Chem. 2020;1–11.
wileyonlinelibrary.com/journal/jhet
© 2020 Wiley Periodicals, Inc.
1

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SALEK SOLTANI ET AL.
FIGURE 1 Structures of some
biologic compounds containing imidazo-
pyrazine/pyridazine scaffolds
SCHEME 1 Synthesis of 2,3-diphenylimidazo[1,2-b]pyridazine. Reagents and conditions: (A) C6H5COCH2Br, NaHCO3, EtOH, reflux;
(B) NBS, CHCl3, r.t.; (C) C6H5B(OH)2, Na2CO3, Pd(PPh3)4, Dioxane/H2O 3:1, 80ꢀC; (D) PhBr, Pd(OAc)2, PPh3, K2CO3, DMA, 130ꢀC,
sealed tube
compared in a literature. This interesting class of com-
pounds motivated us to find the most efficient synthetic
way. Also, with the continuous effort to explore for
improved potential biological activity, the potential
antibacterial properties of the synthesized imidazo-
pyrazine/pyridazines are as well explained along with
some of the chemical properties.
The reactivity of 3-bromo-2-phenylimidazo[1,2-b]
pyridazine 2a with phenylboronic acid toward Suzuki
cross-coupling reaction was studied (Table 1). The com-
pound 2a was firstly reacted with phenylboronic acid
under standard Suzuki reaction conditions (phe-
nylboronic acid 1.1 equiv; Na2CO3 2.5 equiv; Pd(PPh3)4
5 mol%; dioxane/water; 80ꢀC). The reaction furnished
3a with a satisfactory yield of 51% (Table 1, entry 1).
Using 2 equiv of boronic acid was found to raise the
yield of 3a to 63% (entry 2). The use of stronger bases
gave lower yields (entries 3-5). Also the use of
Pd(OAc)2 and PdCl2 instead of Pd(PPh3)4 in the same
conditions that used in entry 1 proceeded the reaction
in lower yields (entries 6,7). Utilizing of Pd(PPh3)4 as
catalyst and Na2CO3 in N,N-Dimethylformamide (DMF)
gave low-moderate yield (42%, entry 8). However, in
the presence of CuI and stronger base such as Cs2CO3
raised the yield to 67% (entry 9). So we chose the
entries 2 and 9 as favorite conditions A and B.
2 | RESULTS AND DISCUSSION
Firstly, for the design of 2,3-diphenylimidazo[1,2-b]
pyridazine 3a, we were prompted in preparing
2-phenylimidazo[1,2-b]pyridazine 1a as intermediate
(Scheme 1). To this aim, a general pathway was used
which involved the condensation of 2-aminopyridazine
with α-bromoacetophenone.[29] The moderate yield was
observed for this cyclization step (53%). The subsequent
treatment with N-Bromosuccinimide led to 3-bromo-
2-phenylimidazo[1,2-b]pyridazine 2a as reported,[26] and
followed by Suzuki cross-coupling to give the desired
product 3a (Scheme 1).
The direct arylation reaction at the 3-position was
then
examined
utilizing
2-phenylimidazo[1,2-b]
pyridazine 1a and aryl halides applying various condi-
tions (Table 2). In the presence of Pd(OAc)2 (5 mol%),
PPh3 (0.5 equiv), and K2CO3 (3 equiv), the reaction
proceeded slowly in dioxane-EtOH at 130ꢀC to gain
2,3-diphenylimidazo[1,2-b]pyridazine 3a with a low yield
of 32% (Table 2, entry 1). Using DMA as solvent raised
the yield significantly (56%, entry 2). In the same solvent
and utilizing of PdCl2 and Cs2CO3, the reaction remained
This could be examined due to the competitive direct
arylation
reaction
at
the
3-position
using
2-phenylimidazo[1,2-b]pyridazine 1a and phenyl bro-
mide, applying diverse procedures.[30–32] Consideration of
the overall yields of the two approaches in the initial
study show that the effectiveness was close to each other.
However, more accurate studies are still needed.

SALEK SOLTANI ET AL.
3
TABLE 1 Cross-coupling reaction optimization of 2a with phenylboronic acid (path a)
Entry
Boronic acid (equiv)
1.1 equiv
2 equiv
Base
Catalyst (5 mol%)
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(OAc)2
PdCl2
Solvent
Yielda (%)
1
2
3
4
5
6
7
8
9
Na2CO3 (2.5 equiv)
Na2CO3 (2.5 equiv)
K2CO3 (2.5 equiv)
Cs2CO3 (2.5 equiv)
K3PO4 (2.5 equiv)
Na2CO3 (2.5 equiv)
Na2CO3 (2.5 equiv)
Na2CO3 (2.5 equiv)
Dioxane/H2O 3:1
Dioxane/H2O 3:1
Dioxane/H2O 3:1
Dioxane/H2O 3:1
Dioxane/H2O 3:1
Dioxane/H2O 3:1
Dioxane/H2O 3:1
DMF
51
63
39
43
48
21
33
42
67
1.1 equiv
1.1 equiv
1.1 equiv
1.1 equiv
1.1 equiv
2 equiv
Pd(PPh3)4
Pd(PPh3)4
2 equiv
Cs2CO3 (2.5 equiv)
CuI (0.1 equiv)
DMF
aIsolated yield.
TABLE 2 Conditions of direct arylation in the synthesis of 3a
(path b)
sequence initiating from imidazo-pyrazines/pyridazines
1 and 2 with a variety of aryl boronic acids and aryl
halides to gain desired products 3a-n (Table 3). The
direct arylation proceeded in general lower yields than
the Suzuki pathway. However, the yields may be very
close when substrates with an electron-deficient group
are used (Table 3, entries 6, 7, 12-14). When 1 reacted
with electron-donating aryl halides, direct arylations
were accomplished in low yields (entries 3-5, 10, 11).
In contrast, The Suzuki cross-coupling is compatible
with a variety of boronic acids and accomplished in vari-
ous yields (44%-71%). Especially, in the presence of the
sensitive group such as free amine, the Suzuki coupling
was the only appropriate procedure due to its mild condi-
tions (entry 5). The comparisons showed that there were
not any clear impacts in cross-coupling with electron-
rich/deficient aryl boronic acids. However, it can be prob-
ably understood from entries 10 and 11, iodo-substituted
compounds have negative influence on yields in both of
procedures to afford 3j and 3k.
Yielda
Entry Conditions
(%)
1
2
3
4
5
PhBr, Pd(OAc)2, PPh3, K2CO3, dioxane-
32
EtOH, 130ꢀC, 24 h, sealed tube
PhBr, Pd(OAc)2, PPh3, K2CO3, DMA,
130ꢀC, 24 h, sealed tube
56
51
44
29
PhBr, PdCl2, PPh3, Cs2CO3, DMA,
140ꢀC, 24 h, sealed tube
PhBr, PdCl2, PPh3, K2CO3, DMF,
120ꢀC, 24 h, sealed tube
PhI, PdCl2, PPh3, Cs2CO3, DMF, 140ꢀC,
24 h, sealed tube
aIsolated yield.
2.1 | Antibacterial activity
The new synthesized imidazo-pyrazines/pyridazines were
appraised for the antibacterial activity in vitro against
gram-positive bacteria: Staphylococcus aureus (S. aureus)
and Bacillus subtilis (B. subtilis); and gram-negative bacte-
ria: Escherichia coli (E. coli). The results of biological
screening of all the novel prepared compounds against
the mentioned strains are showed in Table 4 with the
in moderate yield (51%, entry 3). Moreover, the reaction
led to lower yields in DMF under different conditions
(entries 4,5). Consequently, the entries 2 and 3 were cho-
sen as the most efficient conditions C and D.
With optimal conditions in hand, we examined the
scope of the Suzuki cross-coupling and direct arylation

4
SALEK SOLTANI ET AL.
TABLE 3 Suzuki cross-coupling reactions or direct arylation with various boronic acids/halides
Suzuki conditions
(% yield)
Direct arylation
conditions (% yield)
Entry Ar-X/Ar-B(OH)2
Product
1
Bromobenzene (phenylboronic acid)
3a
3b
A (63)
B (67)
C (56)
D (51)
2
Bromobenzene (phenylboronic acid)
A (71)
A (66)
C (46)
C (30)
3
4
5
1-Bromo-4-methoxybenzene
(4-Methoxyphenylboronic acid)
3c
3d
3e
1-Bromo-4-methoxybenzene
(4-Methoxyphenylboronic acid)
A (57)
B (48)
C (32)
4-Bromoaniline
D (Traces)
(4-Aminophenylboronic acid)
6
7
1-Bromo-4-fluorobenzene
(4-Fluorophenylboronic acid)
3f
B (54)
A (52)
C (49)
1-Bromo-4-fluorobenzene
(4-Fluorophenylboronic acid)
3g
C (56)
D (51)
8
9
Bromobenzene (phenylboronic acid)
Bromobenzene (phenylboronic acid)
3h
3i
B (67)
A (64)
C (49)
C (44)
(Continues)

SALEK SOLTANI ET AL.
5
TABLE 3 (Continued)
Entry Ar-X/Ar-B(OH)2
Suzuki conditions
(% yield)
Direct arylation
conditions (% yield)
Product
10
1-Iodo-4-methoxybenzene
3j
B (46)
D (16)
(4-Methoxyphenylboronic acid)
11
1-Iodo-4-methoxybenzene
3k
A (44)
D (19)
(4-Methoxyphenylboronic acid)
12
13
14
ethyl 4-bromobenzoate
(4-[ethoxycarbonyl]phenylboronic
acid)
3l
A (54)
D (43)
D (58)
D (53)
1-Bromo-4-nitrobenzene
(4-nitrophenylboronic acid)
3m A (64)
4-Bromopyridine (4-Pyridylboronic
acid)
3n
A (59)
Note: Conditions A: boronic acid 2 equiv; compound 2, 1 equiv; Na2CO3 2.5 equiv; Pd(PPh3)4 5 mol%; dioxane/water 3:1; 80ꢀC; Conditions B: boronic acid
2 equiv; compound 2, 1 equiv; Cs2CO3 2.5 equiv; CuI 0.1 equiv; Pd(PPh3)4 5 mol%; DMF; 80ꢀC; Conditions C: aryl halide 1 equiv; compound 1, 1 equiv; K2CO3
3 equiv; PPh3 0.5 equiv; Pd(OAc)2 5 mol%; DMA; 130ꢀC; 24 hours in sealed tube; Conditions D: aryl halide 1 equiv; compound 1, 1 equiv; Cs2CO3 3 equiv; PPh3
0.5 equiv; PdCl2 5 mol%; DMA; 140ꢀC; 24 hours in sealed tube.
minimum inhibitory concentration (MIC)[33] values (μg/
mL) and zone of inhibition (ZOI) values (mm). The
results show that most compounds exhibited moderate to
good antibacterial activity for gram-positive bacteria
(S. aureus and B. subtilis) and gram-negative strains
(E. coli) except 3e and 3g. Compounds 3d, 3e and 3g did
not exhibit any clear activity to E. coli. Interestingly, com-
pounds 3a, 3c, 3j, 3k, 3l, and 3n showed a good potency
against S. aureus and B. subtilis, with MIC values of
32 and 64 μg/mL. These compounds are effective in
inhibiting the growth of the E. coli as well. The rest of the
structures have a lower degree of potency against all the
examined microbes.
exhibited less or even no inhibitory effects, and as it is
obvious, there is no clear difference of antibacterial activ-
ity when the electron-withdrawing substituents were
changed with electron-donating ones on the phenyl ring,
and these observations are totally consistent with the
related previous paper.[34] Consequently, it was resulted
that the electronic impact of the substituent on the phe-
nyl rings may not be the key factor impacting antimicro-
bial activity as much as we thought.
3 | CONCLUSION
With focusing on the biological screening experiment
results, it looked that with group as methoxy in the rings,
the novel synthesized compounds showed good inhibi-
tory effects against all of the examined microbes. While
substituent such as fluoride, the synthesized compounds
In summary, we have performed an efficient synthesis of
the 2,3-diarylimidazo[1,2-a]pyrazines and 2,3-diarylimidazo
[1,2-b]pyridazines. These compounds are stable in the
Suzuki cross-coupling reactions and proceeds in mild condi-
tions in the synthesis of diverse substituted compounds.

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SALEK SOLTANI ET AL.
TABLE 4 Antibacterial activity of
synthesized imidazo-pyrazines/
pyridazines 3a-n
Staphylococcus aureus
Bacillus subtilis
Escherichia coli
Compounds
ZOI
13
12
15
11
7
MIC
64
ZOI
15
14
11
9
MIC
32
ZOI
4
MIC
64
3a
3b
64
64
11
4
64
3c
32
64
128
>128
>128
128
>128
128
128
64
3d
64
128
128
128
128
128
64
5
3e
128
128
128
64
8
4
3f
8
7
6
3g
8
8
5
3h
10
11
14
10
15
11
12
22
9
8
3i
64
10
15
13
14
9
9
3j
32
32
10
7
3k
64
32
128
64
3l
32
32
11
8
3m
64
128
32
128
64
3n
64
15
21
11
20
Ciprofloxacina
0.25
0.125
0.125
aCiprofloxacin was used as a control.
Abbreviations: MIC, minimum inhibitory concentration; ZOI, zone of inhibition.
The comparison of the results of the Suzuki cross-coupling
reaction with the palladium-catalyzed direct arylation path-
ways shows that the former route must be preferred with
sensitive substrates due to its mild conditions. On the other
hand, the direct arylation is sometimes better in efficiency
and time as well as material consumption since it is a direct
route. Finally, these two procedures remain complemen-
tary. Moreover, the results of initial screening of 11 struc-
tures exhibited that most of them showed antibacterial
activities, and revealed moderate to good potency against
the chosen bacteria containing S. aureus, B. subtilis, and
E. coli. Compounds 3a, 3c, 3j, 3k, 3l, and 3n showed clear
potency against all of the microorganisms with MIC values
of 32 μg/mL and 64 μg/mL.
CDCl3 on a Bruker FT-500 MHz spectrometer with tetra-
methyl silane (TMS) as the internal standard. Coupling
constants were reported in Hertz (Hz) and chemical
shifts are given as δ value (ppm) relative to TMS. Elemen-
tal analysis for C, H, and N were analyzed utilizing a
Thermo Finnigan Flash EA1112 instrument.
4.2 | General procedure A for the Suzuki
cross-coupling reaction
To a mixture of phenylboronic acid derivative (2 mmol)
and compound 2 (1 mmol) in 20 mL of solvent (dioxane/
water, 3:1) under argon were added Na2CO3 (265 mg,
2.5 mmol) and Pd(PPh3)4 (5 mol%). The reaction mixture
was heated at 80ꢀC and the proceeding was checked by
TLC. Then, the reaction mixture was diluted with EtOAc
and filtered through a pad of Celite. The filtrate was
washed several times with water. The organic layer was
dried on Na2SO4, filtered, and evaporated under vacuum.
The crude product was purified utilizing column chroma-
tography on silica gel (hexane/EtOAc, 1:1).
4 | EXPERIMENTAL
4.1 | Chemistry and apparatus
All reagents were purchased from supplier and used
without further purification. Melting points were deter-
mined with a Kofler hot stage apparatus and are
uncorrected. Column chromatography was carried out on
silica gel (230-400 mesh). Thin-layer chromatography
(TLC) was conducted on silica gel 250 μm, F254 plates.
IR spectra were recorded on a Bruker EQUINOX 55 spec-
trophotometer. 1H NMR (500 MHz) and 13C NMR
(125 MHz) spectra were recorded in DMSO-d6 and
4.3 | General procedure B for the Suzuki
cross-coupling reaction
To a mixture of phenylboronic acid derivative (2 mmol)
and compound 2 (1 mmol) in DMF (20 mL) under argon

SALEK SOLTANI ET AL.
7
were added Cs2CO3 (814 mg, 2.5 mmol), CuI (19 mg,
0.1 mmol), and Pd(PPh3)4 (5 mol%). The reaction mixture
was heated at 80ꢀC and the proceeding was checked by
TLC. Then, the reaction mixture was diluted with EtOAc
and filtered through a pad of Celite. The filtrate was
washed several times with water. The organic layer was
dried on Na2SO4, filtered, and evaporated under vacuum.
The crude product was purified utilizing column chroma-
tography on silica gel (hexane/EtOAc, 1:1).
116.9, 113.3; Anal. calcd. For C18H13N3: C, 79.68; H,
4.83; N, 15.49; found: C, 79.76; H, 4.73; N, 15.55%.
4.5.2 | 2,3-diphenylimidazo[1,2-a]
pyrazine (3b)
Orange solid; m.p. 137ꢀC to 139ꢀC; IR (υ cm−1): 3011, 1652,
1
1601, 1555, 1472, 1172, 1071; H NMR (500 MHz, DMSO-
d6): δH 9.15 (d, J = 1 Hz, 1H), 8.08 (dd, J = 4.5 Hz, J = 1 Hz,
1H), 7.86 (d, J = 4.5, 1H), 7.50 to 7.61 (m, 8H), 7.30 to 7.36
(m, 2H); 13C NMR (125 MHz, DMSO-d6): δC 143.8, 143.0,
139.9, 133.7, 130.9, 130.5, 130.1, 129.9, 129.4, 128.9, 128.6,
127.1, 122.1, 117.9; Anal. calcd. For C18H13N3: C, 79.68; H,
4.83; N, 15.49; found: C, 79.79; H, 4.72; N, 15.58%.
4.4 | General procedure C for the C-H
direct arylation
Imidazo-pyrazine/pyridazine 1 (1 mmol), aryl halide
(1 mmol), K2CO3 (414 mg, 3 mmol), PPh3 (131 mg,
0.5 mmol) and Pd(OAc)2 (5 mol%) in dimethyl acetamide
(10 mL) were degassed and then heated in a sealed tube
for 24 hours at 130ꢀC. The dark suspension was then
diluted with EtOAc and filtered through a pad of Celite.
The filtrate was washed several times with water. The
organic layer then was dried on Na2SO4, filtered, and
evaporated under vacuum. The crude product was puri-
fied utilizing column chromatography on silica gel (hex-
ane/EtOAc, 1:1).
4.5.3 | 3-(4-methoxyphenyl)-
2-phenylimidazo[1,2-b]pyridazine (3c)
Brown solid; IR (υ cm−1): 3010, 2972, 1654, 1607, 1552,
1469, 1174, 1070; 1H NMR (500 MHz, DMSO-d6): δH 8.39
(d, J = 6 Hz, 1H), 7.86 to 7.93 (m, 3H), 7.46 to 7.53 (m,
3H), 7.36 to 7.39 (m, 2H), 7.13 to 7.21 (m, 3H), 3.81 (s,
3H); 13C NMR (125 MHz, DMSO-d6): δC 160.3, 145.1,
139.5, 138.4, 135.1, 132.7, 130.3, 129.5, 128.8, 127.9, 126.8,
123.0, 115.6, 113.4, 56.4; Anal. calcd. For C19H15N3O: C,
75.73; H, 5.02; N, 13.94; found: C, 75.70; H,
5.10; N, 14.01%.
4.5 | General procedure D for the C-H
direct arylation
Imidazo-pyrazine/pyridazine
1 (1 mmol), aryl halide
(1 mmol), Cs2CO3 (977 mg, 3 mmol), PPh3 (131 mg,
0.5 mmol) and PdCl2 (5 mol%) in dimethyl acetamide
(10 mL) were degassed and then heated in a sealed tube for
24 hours at 140ꢀC. The dark suspension was then diluted
with EtOAc and filtered through a pad of Celite. The filtrate
was washed several times with water. The organic layer
then was dried on Na2SO4, filtered, and evaporated under
vacuum. The crude product was purified utilizing column
chromatography on silica gel (hexane/EtOAc, 1:1).
4.5.4 | 3-(4-methoxyphenyl)-
2-phenylimidazo[1,2-a]pyrazine (3d)
Brown solid; m.p. 109ꢀC to 111ꢀC; IR (υ cm−1): 3032,
2974, 1657, 1607, 1573, 1470, 1094, 912; 1H NMR
(500 MHz, DMSO-d6): δH 9.14 (d, J = 1 Hz, 1H), 8.06
(dd, J = 4.5 Hz, J = 1 Hz, 1H), 7.87 (d, J = 4.5, 1H), 7.67
(dd, J = 8 Hz, J = 1 Hz, 2H), 7.47 (d, J = 8 Hz, 2H), 7.31
to 7.36 (m, 3H), 7.15 (d, J = 8 Hz, 2H), 3.84 (s, 3H); 13C
NMR (125 MHz, DMSO-d6): δC 159.9, 143.8, 143.0, 139.7,
133.8, 132.1, 130.0, 128.9, 128.1, 127.3, 122.8, 120.1, 117.7,
115.7, 55.9; Anal. calcd. For C19H15N3O: C, 75.73; H,
5.02; N, 13.94; found: C, 75.65; H, 4.97; N, 13.89%.
4.5.1 | 2,3-diphenylimidazo[1,2-b]
pyridazine (3a)
Reddish brown solid; m.p. 125ꢀC to 127ꢀC; IR (υ cm−1):
3080, 1651, 1614, 1551, 1474, 1170, 1072; 1H NMR
(500 MHz, DMSO-d6): δH 8.41 (d, J = 5.5 Hz, 1H), 7.89 to
7.97 (m, 3H), 7.49 to 7.54 (m, 3H), 7.14 to 7.39 (m, 6H);
13C NMR (125 MHz, DMSO-d6): δC 145.2, 139.7, 138.6,
134.4, 132.3, 129.2, 129.0, 128.6, 127.3, 126.3, 125.0, 122.5,
4.5.5 | 4-(2-phenylimidazo[1,2-b]
pyridazin-3-yl)aniline (3e)
Orange-brown solid; IR (υ cm−1): 3341, 3261, 3041, 2974,
1
1649, 1605, 1564, 1477, 1163, 1031; H NMR (500 MHz,

8
SALEK SOLTANI ET AL.
DMSO-d6): δH 8.40 (d, J = 1.4 Hz, 1H), 7.74 to 7.78 (m,
3H), 7.43 to 7.47 (m, 3H), 7.21 to 7.25 (m, 3H), 6.72 (d,
9.15 (d, J = 1 Hz, 1H), 8.09 (dd, J = 5 Hz, J = 1 Hz,
1H), 7.91 (d, J = 5, 1H), 7.14 to 7.41 (m, 9H), 2.30 (s,
3H); 13C NMR (125 MHz, DMSO-d6): δC 143.6, 142.9,
139.7, 133.0, 130.4, 130.2, 129.7, 129.3, 129.1, 128.3,
126.9, 126.4, 125.6, 119.3, 22.9; Anal. calcd. For
C19H15N3: C, 79.98; H, 5.30; N, 14.73; found: C,
79.87; H, 5.20; N, 14.65%.
J
= 8.5 Hz, 2H), 5.72 (s, 3H); Anal. calcd. For
C18H14N4: C, 75.50; H, 4.93; N, 19.57; found: C, 75.62; H,
4.99; N, 19.49%.
4.5.6 | 3-(4-fluorophenyl)-
2-phenylimidazo[1,2-b]pyridazine (3f)
4.5.10 | 3-(4-methoxyphenyl)-2-(p-tolyl)
imidazo[1,2-b]pyridazine (3j)
Brown solid; IR (υ cm−1): 3006, 1662, 1609, 1535, 1475,
1
1407, 1035; H NMR (500 MHz, DMSO-d6): δH 8.39 (d,
J = 6 Hz, 1H), 7.86 to 7.93 (m, 3H), 7.46 to 7.54 (m, 3H),
7.19 to 7.32 (m, 5H); 13C NMR (125 MHz, DMSO-d6): δC
162.3, 145.3, 139.9, 137.8, 135.2, 132.2, 130.1, 128.2, 127.1,
126.5, 125.6, 122.3, 116.0, 113.6; Anal. calcd. For
C18H12N3F: C, 74.73; H, 4.18; N, 14.52; found: C,
74.62; H, 4.10; N, 14.45%.
Brown solid; IR (υ cm−1): 2982, 1657, 1609, 1568, 1478,
1
1189, 1050; H NMR (500 MHz, DMSO-d6): δH 8.42 (d,
J = 6.5 Hz, 1H), 7.88 to 7.93 (m, 3H), 7.49 to 7.56 (m,
3H), 7.26 (d, J = 9.5 Hz, 2H), 7.13 (d, J = 9 Hz, 2H), 3.79
(s, 3H), 2.33 (s, 3H); 13C NMR (125 MHz, DMSO-d6): δC
160.4, 144.7, 140.0, 138.7, 135.8, 133.1, 131.0, 129.4, 128.2,
127.3, 126.6, 124.7, 118.2, 116.2, 57.0, 24.3; Anal. calcd.
For C20H17N3O: C, 76.17; H, 5.43; N, 13.32; found: C,
76.08; H, 5.34; N, 13.41%.
4.5.7 | 3-(4-fluorophenyl)-
2-phenylimidazo[1,2-a]pyrazine (3g)
Brown solid; IR (υ cm−1): 1647, 1608, 1561, 1470, 1408,
1217, 1083; H NMR (500 MHz, DMSO-d6): δH 9.16 (d,
4.5.11 | 3-(4-methoxyphenyl)-2-(p-tolyl)
imidazo[1,2-a]pyrazine (3k)
1
J = 1 Hz, 1H), 8.07 (dd, J = 5, J = 1 Hz, 1H), 7.86 (d,
J = 5 Hz, 1H), 7.65 to 7.74 (m, 3H), 7.41 to 7.51 (m, 3H),
7.22 to 7.29 (m, 3H); 13C NMR (125 MHz, DMSO-d6): δC
162.1, 144.0, 143.1, 139.2, 133.3, 131.9, 130.8, 129.1, 128.1,
127.2, 123.0, 121.0, 117.8, 116.5; Anal. calcd. For
C18H12N3F: C, 74.73; H, 4.18; N, 14.52; found: C,
74.66; H, 4.09; N, 14.41%.
Brown solid; IR (υ cm−1): 2984, 1657, 1604, 1578,
1478, 1147, 1054; 1H NMR (500 MHz, DMSO-d6): δH
9.15 (d, J = 1.5 Hz, 1H), 8.07 (dd, J = 5 Hz, J = 1 Hz,
1H), 7.89 (d, J = 5 Hz, 1H), 7.68 (dd, J = 10.5 Hz,
J = 1.5 Hz, 2H), 7.49 (d, J = 9.5 Hz, 2H), 7.17 to 7.24
(m, 4H), 3.86 (s, 3H), 2.31 (s, 3H); 13C NMR
(125 MHz, DMSO-d6): δC 160.2, 144.0, 143.2, 139.0,
134.1, 132.8, 130.5, 129.2, 129.1, 128.6, 126.4, 125.9,
4.5.8 | 3-phenyl-2-(p-tolyl)imidazo[1,2-b]
pyridazine (3h)
122.7,
115.8,
56.1,
23.7;
Anal.
calcd.
For
C20H17N3O: C, 76.17; H, 5.43; N, 13.32; found: C,
76.22; H, 5.32; N, 13.24%.
Brown solid; IR (υ cm−1): 3062, 2986, 1659, 1602, 1574,
1
1474, 1058; H NMR (500 MHz, DMSO-d6): δH 8.43 (d,
J = 6 Hz, 1H), 7.87 to 7.92 (m, 3H), 7.43 to 7.51 (m, 3H),
7.20 to 7.33 (m, 5H), 2.28 (s, 3H); 13C NMR (125 MHz,
DMSO-d6): δC 144.2, 139.3, 138.1, 133.9, 133.2, 129.7,
129.0, 127.8, 127.0, 126.2, 125.6, 123.7, 117.0, 114.8, 23.4;
Anal. calcd. For C19H15N3: C, 79.98; H, 5.30; N, 14.73;
found: C, 79.83; H, 5.21; N, 14.81%.
4.5.12 | ethyl 4-(2-phenylimidazo[1,2-a]
pyrazin-3-yl)benzoate (3l)
1
Brown solid; m.p. 121ꢀC to 123ꢀC; H NMR (500 MHz,
DMSO-d6): δH 9.18 (d, J = 1 Hz, 1H), 8.21 (d, J = 4 Hz,
1H), 8.15 (d, J = 9 Hz, 2H), 7.91 (d, J = 4 Hz, 1H), 7.72
(d, J = 8.5 Hz, 2H), 7.59 to 7.61 (m, 2H), 7.33 to 7.37 (m,
3H), 4.32 (q, J = 8 Hz, 2H), 1.34 (t, J = 8 Hz, 3H); 13C
NMR (125 MHz, DMSO-d6): δC 165.3, 143.9, 143.1, 139.8,
133.0, 132.7, 130.7, 130.5, 130.3, 129.9, 128.7, 128.3, 127.9,
121.1, 117.2, 61.1, 14.2; Anal. calcd. For C21H17N3O2: C,
73.45; H, 4.99; N, 12.24; found: C, 73.33; H,
5.10; N, 12.35%.
4.5.9 | 3-phenyl-2-(p-tolyl)imidazo[1,2-a]
pyrazine (3i)
Brown solid; IR (υ cm−1): 3029, 2980, 1643, 1602, 1576,
1480, 1119, 1033; 1H NMR (500 MHz, DMSO-d6): δH

SALEK SOLTANI ET AL.
9
4.5.13 | 3-(4-nitrophenyl)-
2-phenylimidazo[1,2-a]pyrazine (3m)
colonies of bacterial strains. The inoculated tubes were
incubated at 37ꢀC for 24 hours, for bacterial strains. Then,
250 μL of bacterial culture (106-108 bacteria every mL) was
added carefully to 20 mL of solid culture (a mixture of
Tryptone, Sabouraud dextrose agar and Yeast Extract) at
45ꢀC, stirred enough, and poured into a sterile Petri dish
(90 mm). A sterile cork-borer of 6 mm diameter was uti-
lized to make two wells on each of the Petri dish. One was
filled with 10 μL of the screened compounds. In order to
survey the influence of the solvent, a control test was per-
formed as well containing just dimethyl sulfoxide. Then,
the cultures were incubated at 37ꢀC for 24 hours for the
bacterial strains. After incubation, the ZOI on the plates
was recorded. The products with the diameter of the
growth-inhibition zones greater than 4 mm were consid-
ered positive, and could be screened for further evaluation
of the MIC values. The test was carried out three times for
each compound. All the novel products (3a-3n) were used
to preliminary biological screening.
1
Brown solid; m.p. 139ꢀC to 141ꢀC; H NMR (500 MHz,
DMSO-d6): δH 9.20 (d, J = 1.5 Hz, 1H), 8.43 (d, J = 7 Hz,
2H), 8.28 (dd, J = 4.5 Hz, J = 1.5 Hz, 1H), 7.95
(d, J = 5 Hz, 1H), 7.85 (d, J = 9.5 Hz, 2H), 7.56 to 7.59
(m, 2H), 7.37 to 7.40 (m, 3H); 13C NMR (125 MHz,
DMSO-d6): δC 144.8, 143.1, 140.3, 135.1, 132.6, 131.7,
130.0, 128.7, 128.6, 128.1, 124.7, 117.3, 115.3, 112.7; Anal.
calcd. For C18H12N4O2: C, 68.35; H, 3.82; N, 17.71;
found: C, 68.43; H, 3.91; N, 17.60%.
4.5.14 | 2-phenyl-3-(pyridin-4-yl)imidazo
[1,2-a]pyrazine (3n)
1
Brown solid; m.p. 124ꢀC to 126ꢀC; H NMR (500 MHz,
DMSO-d6): δH 9.20 (d, J = 1 Hz, 1H), 8.79 (d, J = 5 Hz,
2H), 8.31 (dd, J = 4.5 Hz, J = 1 Hz, 1H), 7.92 (d,
J = 4.5 Hz, 1H), 7.54 to 7.61 (m, 4H), 7.36 to 7.40 (m,
3H); 13C NMR (125 MHz, DMSO-d6): δC 150.9, 144.4,
143.2, 139.9, 136.1, 132.9, 130.0, 128.7, 128.6, 128.1, 124.7,
119.7, 117.2; Anal. calcd. For C17H12N4: C, 74.98; H,
4.44; N, 20.58; found: C, 75.09; H, 4.53; N, 20.44%.
4.8 | Determination of the MIC
A broth tube dilution method[37,38] was followed to
measure the MIC values for all the screened imidazo-
pyrazine/pyridazines against the abovementioned bacte-
ria, and for comparison ciprofloxacin was utilized as a
standard reference. In order to sample preparation, all of
the test compounds and reference were dissolved in
dimethyl sulfoxide at a concentration of 1280 μg/mL, and
further dilutions of the compounds and reference in cul-
ture medium MHB were prepared at the required quanti-
ties of 128, 64, etc., down to 0.125 μg/mL. Microbial
strains were kept on Muller-Hinton Agar (MHA)
medium at 37ꢀC for 24 hours. The bacterial inoculations
were prepared by suspension in 10 mL of culture
medium for colonies from culture on MHA. The cell den-
sity of all inoculum was put in culture medium of a 0.5
McFarland standard. The final amount used, was
105 CFU/mL for bacteria. The bacterial inocula were
added to the 2-fold diluted samples. MIC values were
read after incubation at 37ꢀC for bacterial strains. After
24 hours, the last tube with no growth of microorganisms
was recorded to represent the MIC value expressed in
mg/mL. All experiments were repeated three times.
4.6 | Pharmacology
The below micro-organisms were utilized for exploring
antibacterial activity: S. aureus ATCC 25923, B. subtilis
ATCC 6633 and E. coli ATCC 25922. They were prepared
from the Pasteur Institute of Iran. The preliminary
screening for antibacterial activity of the 11 tested
imidazo-pyrazine/pyridazines in DMSO (25 mg/mL) was
measured applying plate-hole diffusion assay.[35,36]
A
2-fold serial dilution method[37] was utilized to measure
the MIC values of the screened imidazo-pyrazine/
pyridazines. The MIC value was attended as quantitative
pathway and was used for measurement of the
antibacterial activity of the prepared compounds against
the mentioned micro-organisms.
4.7 | Preliminary screening for
antibacterial activity
ACKNOWLEDGMENTS
Suspensions of the above microbes were prepared by inoc-
ulating new stock cultures into apart broth tubes, each
including (8 mL) of Muller-Hinton broth (MHB) for
This work was supported by the University of Tehran
(UT) and the Research Council of Tehran University of
Medical Sciences (TUMS). Special thanks to Dr. Reza

10
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Committee on Clinical Laboratory Standards, Wayne, IL
2000, p. 2.
How to cite this article: Salek Soltani S, F.
Farnia SM, Foroumadi A. A comparison between
Suzuki cross-coupling reaction and direct arylation
in the synthesis of new antibacterial imidazo-
pyrazines/pyridazines. J Heterocyclic Chem. 2020;
1–11. https://doi.org/10.1002/jhet.3902
[38] Y. K. Qi, B. Jiao, X. D. Ma, W. P. Cui, S. T. Ma, Arch Pharm
Chem Life Sci 2010, 343, 458.
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