Full text of DOI:10.1002/ejoc.201800286

A Journal of
Accepted Article
Title: Base-Promoted Transition metal-free Arylation of Imidazo fused
Heterocycles with Diaryliodonium salts
Authors: Adimurthy Subbarayappa, Rahul Kumar, Chitrakar Ravi, and
Deepa Rawat
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800286
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800286
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10.1002/ejoc.201800286
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Base–Promoted Transition metal–free Arylation of Imidazo
fused Heterocycles with Diaryliodonium salts
Rahul Kumar, Chitrakar Ravi, Deepa Rawat and Subbarayappa Adimurthy*
Abstract: Potassium tert-butoxide promoted arylation of reactions proceed through radical pathway without
imidazo heterocycles with diaryliodonium salts under additives and oxidants with good to excellent yields and
transition metal-free conditions has been described. The functional group tolerance of arylated products.
impurities in the final products. Recently, some transition metal-
Introduction
free
methods
for
the
preparation
of
3-aryled
imidazo[1,2−a]pyridines have been reported,[10] but they requires
activated starting materials, such as bromoalkynes,
bromoketones, bromoaldehydes, and iodonium salts, keto
aldehydes.[10]
Pyridine fused heterocycles such as imidazo[1,2–a]pyridines,
imidazo[1, 5–a]pyridines, imidazo[1,2–a]pyrazines, indolizines,
imidazo[2,1–a]isoquinolines and pyrazolo[1, 5–a]pyridines are
important intermediate in both medicinal chemistry and drug
development.[1] Imidazo[1,2–a]pyridine scaffolds are widely
distributed in many natural products pharmacologically important
compounds.[2] Particularly, alpidem, zolpidem, olprinone (the
treatment of acute heart failure), minodronic acid,[3] zolimidine
(peptic ulcer), necopidem, saripidem (sedative and anxiolytic),[4]
optically active GSK812397 candidate (HIV infection) and as well
as CXC-chemokine receptor (CXCR4) antagonists are derived
from imidazo[1,2–a]pyridine cores.[5] Recently imidazo[1,2–
a]pyridines are recognized as NO synthases, GABA inhibitors, L-
Dopa and Dopamine pro-drugs.[6] In addition, they received
considerable attention as core ligands in the field of electronic
devices, and abnormal N-heterocyclic carbenes.[7]
To the best of our knowledge the direct C3-arylation of
imidazo[1,2–a]pyridines under metal-free conditions has not been
reported. In continuation of our research on the functionalization
of imidazo[1,2–a]pyridines,[11] we describe herein arylation of
imidazo[1,2–a]pyridines and imidazo thiazoles under transition
metal-free conditions using diphenyl iodonium salts as aryl source
(Scheme 1).
The functionalized heterocycles through C−C, C−hetero bond
formations exhibit their enhanced biological activities compared
to their basic moiety. Although, these methods are efficient, but
often required the expensive transition metal precursors such as
platinum, palladium, rhodium, gold, ruthenium, cobalt, copper,
etc.[8] In particular C3−aryl imidazo[1,2−ɑ]pyridine showed
diverse biological activities.[4-7] Different synthetic methods have
been developed for the preparation of C3−arylated
imidazo[1,2−a]pyridines using various transition metal catalysts
and different aryl sources such as haloarenes and phenylboronic
acids.[9] The development of metal-free conditions for the arylation
of imidazo[1,2−a]pyridines is desirable due to the milder reaction
Scheme 1. Arylation of imidazo[1,2−a]pyridine
Results and Discussion
The combination of diphenyl iodonium salts and base plays a
crucial role as alternate to transition metals for the arylation of
imidazoheterocycles in the present work. To optimize the reaction
conditions, we initiated the arylation of 2-phenylimidazo[1,2–
a]pyridine (0.25 mmol) 1a with diphenyliodonium chloride (0.25
mmol) 2a using Cs2CO3 (0.5 mmol) as a base at 60°C in DMF
as solvent for 24h, the desired product 2, 3-diphenylimidazo[1,
2–a]pyridine 3a was isolated in 18 % yield (Table 1, entry 1). With
the same conditions, reaction was performed in other solvents
like DCE, MeOH and ACN; in these solvents, the desired product
3a was obtained in 26%, 22% and 30% yield respectively (Table
1, entry 2–4). Under the same conditions with ACN as solvent,
the reaction was screened with different bases such as Na2CO3,
K2CO3, NaHCO3, NaOH, KOH, CsOH, t-BuONa, DBU and t-
BuOK (entries 5-13), among these bases the good yield (56%) of
Academy of Scientific & Innovative Research, CSIR–Central Salt & Marine
Chemicals Research Institute, G.B. Marg, Bhavnagar-364 002. Gujarat
(INDIA).
*E-mail: adimurthy@csmcri.res.in
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
conditions, less expensive reagents, and lack of residual metallic
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10.1002/ejoc.201800286
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3a was isolated with t-BuOK (Table 1, entry 13). Further to
improve the yield, diphenyliodonium chloride 2a was increased
from 0.25 mmol to 0.3 - 0.5 mmol with respect to 1a, (entries 14-
16) to our surprise, the desired product 3a yield was increased to
86% at 0.5 mmol of 2a (Table 1, entry 16). To see the effect
temperature, when the reaction was conducted at room
temperature and at 45° C, the yield of the product 3a was declined
(Table 1, entries 17 and 18). The yield of the desired product was
further dropped by decreasing the amount of base (Table 1,
entries 19–21). The reaction was failed in yield the desired
product in absence of base (Table 1, entry 22).
First, we examined C-3 arylation of para substituted 2-phenyl
imidazo[1, 2–a]pyridines with diaryliodonium chloride.
To our delight, the protocol was found to be broadly pertinent to
presence of electron rich and electron deficient groups (Me, Et,
Cl, Br, and CN) underwent smoothly to the optimized conditions
and gave good yields (60−86%) of selective C-3 arylated products
3a-f. Also the presence of cyanide and methyl-sulfanedione
groups at meta and para position of phenyl ring of 2-phenyl
imidazo[1, 2–a]pyridines gave the C-3 arylated products 3-(3-
phenylimidazo[1,2–a]pyridin-2-yl)benzonitrile
(3g)
and2-(4-
(methylsulfonyl)phenyl)-3-phenylimidazo[1,2–a]pyridine (3h) in
60% and 65% yields respectively.
Table 1. Optimization of the reaction conditions [a]
Table 2. Screening of the counter anions[a]
[a] Reaction conditions: 1a (0.25 mmol) and 2a (0.5 mmol) were heated in 1 mL
solvent at 60°C, 24h. Yields are of isolated products.
Further, the presence of electron-rich or electron-deficient groups
(Me, OMe, Cl, F and CN) at different positions on the phenyl as
well as pyridyl ring of 2-phenyl imidazo[1, 2–a]pyridines were
reacted smoothly with diaryliodonium chloride and afford the
corresponding C-3 arylated imidazo[1, 2-a]pyridines 3i-s in
moderate to good yields (48-88%). The present conditions also
Scheme 2. Scope of various imidazo[1,2−a]pyridines[a]
[a] Reaction conditions: 1a (0.25 mmol) and 2a (0.5 mmol) were heated in 1 mL
solvent at 60°C, 24 h. [b] Isolated products.
We then verified the reactivity of other diphenyl iodonium salts
2b–e (Table 2) as arylation sources for the arylation of 1a under
the optimized reaction conditions (Table 1, entry 16) in place of
2a. Among, these diaryliodonium salts (such as diphenyliodonium
nitrate 2b, diphenyliodonium hexafluorophosphates 2c,
Diphenyliodonium
Trifluoromethanesulfonate
2d,
and
diphenyliodonium tetrafluoroborates 2e), comparable yield (81%)
of 3a was obtained with 2d (Table 2, entries 1–5). However, the
best yield of 3a was observed with diphenyliodonium chloride 2a
under the present conditions. Hence the optimum conditions were
set as 0.25 mmol of 1a, 0.5 mmol of 2, and 0.5 mmol of t-BuOK
in 1.0 mL of acetonitrile at 60° C for 24 h.
With the optimized conditions in hand (Table 1, entry 16), we
examined the versatility of C-3 arylation of imidazo[1, 2–
a]pyridines and as well as diaryliodonium chlorides (Scheme 2).
[a]Reaction conditions: 1 (0.25 mmol) and 2 (0.5 mmol) were heated in 1 mL
ACN at 60°C, 24h, isolated yields.
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10.1002/ejoc.201800286
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amenable
to
the
2-(thiophen-2-yl)imidazo[1,2–a]pyridine
derivatives and successfully obtained the C–3 arylated products
3t-v in moderate to good yields. Similarly, the reaction of
substituted
imidazo[1,
2–a]pyridines
with
bis(4-(tert-
butyl)phenyl)iodonium chloride was examined under the
optimized conditions and the desired arylated products (3w-3aa)
were obtained in good to excellent yields (52-91%).
Subsequently, to ascertain the scope of the present methodology,
C-3 arylation of different heterocycles was tested (Scheme 3).
Various substituted 2-aryl benzo[d]imidazo[2,1–b]thiazoles,
including
2-(thiophen-2-yl)benzo[d]imidazo[2,1–b]thiazole
underwent to this arylation procedure and provided the
Scheme 5. Selectivity experiments
corresponding products 5a-h with moderate to good yields (50–
70%).
Further, the reaction of 1a and 4a with iodonium salt containing
different arene rings 10 was conducted under the optimized
conditions, the arylated product 11 and 13 were isolated in 67%
and 64% respectively. In both reactions, the selective electron
withdrawing group (CF3) containing arene substituted product
was observed instead of 12 and 14 (Scheme 5, eq.3 & 4).
Scheme 3. Scope of fused imidazo heterocycles[a]
[a]Reaction conditions: 1 (0.25 mmol) and 2 (0.5 mmol) were heated in 1 mL
ACN at 60°C, 24h, isolated yields.
Additionally, to validate the present protocol, the reaction of 1a
was carried out at gram scale using 5.4 mmol of 1a and 10.8 mmol
of 2a and 3a was obtained in 66% yield (Scheme 4). This study
indicates the feasibility of the method for industrial/commercial
preparation of such products.
Scheme 6. Control experiments
To gain insights into the reaction mechanism, we performed some
control experiments with radical scavengers (Scheme 6). Neither
desired product nor adduct formation was observed with radical
scavengers TEMPO and BHT (Scheme 6, eq.1).
Further, the reaction with benzoquinone, the adduct 15 was
isolated in 66% (Scheme 6, eq. 1). On the basis of these
experimental results, and previous reports,[12] the reaction may
proceed both ionic and radical pathway. Also the optimized
reaction conditions were applied to other aryl source like
iodobenzene, bromobenzene and phenyl boronic acid, in these
cases only trace to no product formation was observed (Scheme
6, eq.2).
Scheme 4. Gram scale reaction
Next, we checked the selectivity of present reaction (Scheme 5).
Reaction was performed with C-3 substituted imidazopyridine that
Based on the control experiments and the literature reports,[12]
a
is 3-methylimidazo[1,2–a]pyridine
6
under the optimized
plausible reaction mechanism has been proposed for the present
transformation (Scheme 7). Initially (path-a), the reaction of
diaryliodonium salt 2a with t-BuOK generates aryl radical B, via
the formation of benzene carbocation. The aryl radical B reacts
conditions (Scheme 5 eq.1). With this substrate no desired
product 7 formation was observed. In the same way reaction with
simple unsubstituted imidazo[1,2–a]pyridine 8, the major C3
arylated product 9 was isolated in 38% yield along with a mixture
of products (Scheme 5, eq.2). These selectivity studies indicates
that the reaction may proceed in both ionic as well as radical
pathway.
with imidazopyridine
1
and generates another radical
intermediate C. Subsequently, the oxidation of C through
intermediate D, yield the desired product 3. Alternately, the
[12f-h]
reaction may also proceed through aryne
intermediate A
(path-b).
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10.1002/ejoc.201800286
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under reduced pressure, the left out residue was purified through column
chromatography using silica gel (20% EtOAc/hexane) to obtain 2,3-
diphenylimidazo[1,2-a]pyridine 3a in 86 % yield (0.0584g).
Characterization data
2, 3-diphenylimidazo[1,2-a]pyridine 3a[13]
(Eluent: 20% EtOAc/hexane); 86% yield (58.1 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.95 (d, J =7.2 Hz, 1H), 7.68-7.64 (M 3H), 7.51 (t, J = 7.2
Hz, 2H), 7.84-7.43 (m, 3H), 7.28-7.23 (m,3H), 7.20 (t, J=7.8 Hz, 1H), 6.73
(t, J= 6.6 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ 144.7, 142.3, 134.0, 130.7,
129.8, 129.5, 128.8, 128.2, 128.0, 127.4, 124.6, 123.2, 117.5, 112.2.
3-phenyl-2-(p-tolyl)imidazo[1,2-a]pyridine 3b
(Eluent: 20% EtOAc/hexane); 83% yield (58.9 mg); white solid; 1H NMR
(600 MHz, CDCl3) δ 7.94 (d, J = 6.0 Hz, 1H), 7.68 (d, J = 9.0 Hz, 1H), 7.54-
7.49 (m, 4H), 7.47-7.43 (m, 3H), 7.19 (t, J = 7.2 1H), 7.08 (d, J= 8.4 Hz,
2H), 6.72 (t, J= 6.6Hz 1H), 2.31 (s, 3H). 13C NMR (150 MHz, CDCl3) δ
144.7, 142.4, 137.2, 131.1, 130.7, 129.9, 129.4, 128.9, 128.7, 127.9, 120.7,
117.4, 112.1, 21.2. HRMS(ESI-TOF)m/z: calcd for C20H16N2Na : 307.1206,
found:307.1206.
Scheme 7. Probable mechanism
Conclusions
(4-ethylphenyl)-3-phenylimidazo[1,2-a]pyridine 3c
In summary, we have developed a novel and efficient protocol for
the arylation of imidazo[1, 2–a]pyridine mediated by K-OtBu
under mild reaction conditions without transition metal catalyst.
The method also applicable to the imidazo thiazoles, which were
reacted smoothly and delivered desired products in good yields.
We also demonstrated the efficacy of the present strategy for the
gram-scale synthesis of arylated imidazo[1, 2–a]pyridine
including with good functional group tolerance in all cases studied.
(Eluent: 15% EtOAc/hexane); 77% yield (57.3 mg); white solid; 1H NMR
(600 MHz, CDCl3) δ 7.93 (d, J = 6.6 Hz, 1H), 7.68 (d, J = 9.0 Hz, 1H), 7.57
(d, J = 8.4 Hz, 2H), 7.53 (t, J = 7.2 Hz, 2H), 7.48-7.44 (m, 3H), 7.19 (t,
J=7.8Hz, 1H), 7.11(d, J=8.4 Hz, 2H), 6.72 (t, J=6.6 Hz, 1H), 2.63 (q, J =
6.6 Hz, 2H),1.22 (t, J = 7.2 Hz, 3H). 13C NMR (150 MHz, CDCl3) δ 143.5,
142.1, 130.7, 129.5, 128.8, 127.9, 127.7, 124.5, 123.1, 117.3, 112.1, 28.5,
15.2. HRMS (ESI-TOF)m/z: calcd for C21H19N2
found:299.1545.
:
299.1543,
2-(4-chlorophenyl)-3-phenylimidazo[1,2-a]pyridine 3d[13]
Experimental Section
(Eluent: 20% EtOAc/hexane); 74% yield (56.2 mg); solid; 1H NMR (600
MHz, CDCl3 ) δ 7.93 (d, J = 7.2 Hz, 1H), 7.68 (d, J = 9.6 Hz, 1H),7.58 (d, J
= 7.8 Hz, 2H), 7.53( t, J= 8.4Hz, 2H) 7.49-7.48 (M,1H), 7.42(d, J = 7.2
Hz,2H), 7.24-7.19 (m,3H), 6.74 (t, J=6.6Hz,1H), 13C NMR (150 MHz,
CDCl3) δ 144.8, 141.1, 133.3, 132.5, 130.6, 129.6, 129.4, 129.2, 129.0,
128.4,124.9, 123.2,121.2, 117.4, 112.4.
General: All commercially available chemicals and reagents were used
without any further purification unless otherwise indicated. 1H and 13 C
NMR spectra were recorded at 500, and 125 MHz, respectively. The
spectra were recorded in CDCl3 as solvent. Multiplicity was indicated as
follows: s (singlet); d (doublet); t (triplet); m (multiplet); dd (doublet of
doublets), etc. and coupling constants (J) were given in Hz. Chemical shifts
are reported in ppm relative to TMS as an internal standard. The peaks
around delta values of 1H NMR (7.2), and 13C NMR (77.0) are correspond
to deuterated solvent chloroform respectively. Mass spectra were obtained
using electron impact (EI) ionization method. Progress of the reactions was
monitored by thin layer chromatography (TLC). All products were purified
through column chromatography using silica gel 100-200 mesh size using
hexane/ethyl acetate as eluent unless otherwise indicated.
2-(4-bromophenyl)-3-phenylimidazo[1,2-a]pyridine 3e[13]
(Eluent: 20% EtOAc/hexane); 80% yield (69.6 mg); solid; 1H NMR (600
MHz, CDCl3 ) δ 7.93 (d, J = 7.2 Hz, 1H), 7.67 (d, J = 9.0 Hz, 1H), 7.54–
7.47 (m, 5H), 7.43 (d, J = 8.4 Hz, 2H), 7.40 – 7.38 (m, 2H), 7.21 (dd, J =
8.8, 7.2 Hz, 1H), 6.73 (t, J = 6.8 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ
131.51, 130.74, 129.76, 129.66, 129.20, 125.06, 123.41, 117.63, 112.55,
77.29, 77.08, 76.87
4-(3-phenylimidazo[1,2-a]pyridin-2-yl)benzonitrile 3f [14]
(Eluent: 20% EtOAc/hexane); 60% yield (44.2 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.91 (s, 1H), 7.76 (s, 2H), 7.67 (s, 1H), 7.54 (d, J = 5.4 Hz,
5H), 7.43 (s, 2H), 7.25 (s, 1H), 6.77 (s, 1H). 13C NMR (150 MHz, CDCl3) δ
145.10, 140.24, 138.90, 132.19, 130.72, 129.96, 129.62, 129.16, 128.34,
125.60, 123.58, 122.63, 119.16, 117.83, 112.96, 110.76.
General procedure for 3a
A clean washed boiling tube equipped with a magnetic stir bar was
charged with 2-phenylimidazo[1,2-a]pyridine 1a (0.0485 g, 0.25 mmol),
diphenyliodonium chloride 2a (0.157 g, 0.5 mmol), 0.5 mmol of t-BuOK and
acetonitrile (1mL), the above mixture was stirred for 24h at 60°C
temperature. After completion of the reaction, the mixture was poured into
10mL of NaHCO3 solution. The product was extracted with ethyl acetate
(10 mL × 3) and dried with anhydrous Na2SO4. Removal of the solvent
3-(3-phenylimidazo[1,2-a]pyridin-2-yl)benzonitrile 3g
1
(Eluent: 20% EtOAc/hexane); 60% yield (44.2 mg); solid; H NMR (600
MHz, CDCl3) δ 7.97 (s, 1H), 7.93 (d,J =6.6 1H), 7.86 (d,J =7.8 1H), 7.67
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10.1002/ejoc.201800286
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(d, J = 9.0 Hz, 1H), 7.55 – 7.48 (m, 4H), 7.42 (d, J = 7.2 Hz, 2H), 7.35 (t, J
= 7.2 Hz, 1H), 7.24 (t, J = 6.6 Hz, 1H),, 6.77 (t, J = 6.6 Hz, 1H). 13C NMR
(150 MHz, CDCl3) δ 132.10, 131.47, 130.88, 130.65, 129.97, 129.63,
129.14, 125.45, 123.58, 117.78, 112.85,102.23. HRMS (ESI-TOF)m/z:
calcd for C20H14N3: 296.1182 found:296.1179
(Eluent: 20% EtOAc/hexane); 88% yield (74.3 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.84 (d, J = 7.1 Hz, 1H), 7.64 (s, 1H), 7.56 – 7.50 (m, 5H),
7.41 (d, J = 7.4 Hz, 2H), 7.24 (d, J = 2.2 Hz, 2H), 6.72 (dd, J = 6.9, 2.2 Hz,
1H). 13C NMR (150 MHz, CDCl3) δ133.74, 131.87, 131.44, 130.65, 129.88,
129.48, 129.29, 128.65, 124.96, 123.73, 116.39, 114.15. HRMS (ESI-
TOF)m/z: calcd for C19H13N2Cl2: 339.0450 found:339.0445.
2-(4-(methylsulfonyl)phenyl)-3-phenylimidazo[1,2-a]pyridine 3h
1
(Eluent: 20% EtOAc/hexane); 65% yield (56.5 mg); solid; H NMR (600
2-(4-chlorophenyl)-7-methoxy-3-phenylimidazo[1,2-a]pyridine 3o
(Eluent: 20% EtOAc/hexane); 60% yield (50.1mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.75 (d, J = 7.8 Hz, 1H), 7.55 – 7.54 (m, 2H), 7.50 (t, J =
7.5 Hz, 2H), 7.46 (d, J = 6.6 Hz, 1H), 7.40 – 7.38 (m, 2H), 7.22 – 7.20 (m,
2H), 6.96 (d, J = 2.2 Hz, 1H), 6.46 (dd, J = 7.9, 2.3 Hz, 1H), 3.87 (s, 3H).
13C NMR (150 MHz, CDCl3) δ 158.35, 146.37, 142.53, 140.51, 133.16,
133.03, 132.81, 132.42, 130.77, 130.47, 129.69, 129.39, 129.12, 129.03,
128.71, 128.52, 127.73, 123.93, 107.63, 94.56, 55.67.HRMS(ESI-
TOF)m/z: calcd for C20H16N2ClO: 335.0946 found:335.0943.
MHz, CDCl3) δ 7.93 (d, J = 7.2 Hz, 1H), 7.84 (q, J = 7.8 Hz, 4H), 7.69 (d,
J = 9.6 Hz, 1H), 7.56 -7.52 (m, 3H), 7.43 (d, J = 7.2 Hz, 2H), 7.25 -7.22
(m, 1H), 6.78 (t, J = 7.2 Hz, 1H), 3.02 (s, 3H). 13C NMR (150 MHz, CDCl3)
δ
145.0, 140.0, 139.7, 138.7,130.6, 129.8, 129.5, 129.0, 128.5,
127.3,125.5, 123.5, 122.6, 117.7, 112.8, 44.4.HRMS(ESI-TOF)m/z: calcd
for C20H17N2O2S : 349.1005found:349.1005.
6-fluoro-2,3-diphenylimidazo[1,2-a]pyridine 3i[15]
1
(Eluent: 20% EtOAc/hexane); 62% yield (44.6 mg); solid; H NMR (600
MHz, CDCl3) δ 7.87 (d, J = 2.4 Hz, 1H), 7.66 – 7.61 (m, 4H), 7.54(t,
J=6.6Hz,2H), 7.50 (d, J=7.2Hz,1H) ,7.43 (t, J = 6.0 2H), 7.27 – 7.24 (m,
2H), 7.13-7.10 (m, 1H). 13C NMR (150 MHz, CDCl3) δ 143.71, 130.55,
130.11, 129.79, 129.63, 129.29, 129.10, 128.41, 128.07, 127.76, 125.87,
118.12,
4-(7-methyl-3-phenylimidazo[1,2-a]pyridin-2-yl)benzonitrile 3p
1
(Eluent: 20% EtOAc/hexane); 47% yield (36.3 mg); solid; H NMR (600
MHz, CDCl3) δ 7.80 (d, J = 7.1 Hz, 1H), 7.75 (d, J = 8.0 Hz, 2H), 7.58 –
7.50 (m, 5H), 7.41 (d, J = 8.1 Hz, 3H), 6.60 (d, J = 6.8 Hz, 1H), 2.41 (s,
3H). 13C NMR (150 MHz, CDCl3) δ 145.45, 139.80, 139.01, 136.53, 132.02,
130.59, 129.77, 129.34, 129.25, 128.14, 122.66, 122.01, 119.10, 115.94,
6-fluoro-3-phenyl-2-(p-tolyl)imidazo[1,2-a]pyridine 3j
(Eluent: 20% EtOAc/hexane); 60% yield (45.3mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.86 (d, J = 1.8 Hz, 1H), 7.65 (dd, J = 9.7, 5.1 Hz, 1H), 7.54
– 7.47 (m, 5H), 7.42 (d, J = 7.2 Hz, 2H), 7.11 – 7.07 (m, 3H), 2.31 (s, 3H).
13C NMR(150 MHz, CDCl3) δ 137.60, 130.56, 129.76, 129.22, 129.16,
127.95, 119.42, 117.97, 116.51, 115.65, 110.07, 77.30, 77.09, 76.88,
21.31. HRMS (ESI-TOF)m/z: calcd for C20H16N2F : 303.1292 found:
303.1293.
115.49, 110.43, , 21.34.HRMS(ESI-TOF)m/z: calcd for C21H16N3 :
310.1339 found: 310.1342.
4-(8-methyl-3-phenylimidazo[1,2-a]pyridin-2-yl)benzonitrile 3q
(Eluent: 20% EtOAc/hexane);48% yield (37.0 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.78 (d, J = 7.6 Hz, 3H), 7.59 – 7.47 (m, 5H), 7.41 (d, J =
6.4 Hz, 2H), 7.02 (d, J = 5.9 Hz, 1H), 6.68 (t, J = 6.4 Hz, 1H), 2.68 (s, 3H).
13C NMR (150 MHz, CDCl3) δ 145.62, 139.29, 132.16, 130.73, 129.85,
129.55, 129.44, 128.46, 127.88, 124.12, 122.97, 121.42, 119.28, 112.94,
110.53, 17.14. HRMS (ESI-TOF)m/z: calcd for C21H16N3 : 310.1339
found:310.1343.
2-(4-chlorophenyl)-6-methyl-3-phenylimidazo[1,2-a]pyridine 3k
(Eluent: 20% EtOAc/hexane); 62% yield (49.2 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.68 (s, 1H), 7.56-7.52 (m 5H), 7.50 (dd, J = 8.5, 6.4 Hz,
1H), 7.42 (d, J = 7.3 Hz, 2H), 7.22 (d, J = 8.1 Hz, 2H), 7.06 (d, J = 9.2 Hz,
1H), 2.25 (s, 3H). 13C NMR (150 MHz, CDCl3) δ 130.8, 129.7, 129.2, 129.0,
128.9, 128.5, 128.1, 123.8, 120.9, 120.0, 116.9, 110.1, 77.3, 77.0, 76.8,
18.4. HRMS (ESI-TOF)m/z: calcd for C20H15ClN2Na:341.0816
found:341.0812.
4-(6-methyl-3-phenylimidazo[1,2-a]pyridin-2-yl)benzonitrile 3r
(Eluent: 20% EtOAc/hexane); 43% yield (33.2 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.74 (d, J = 8.0 Hz, 2H), 7.66 (s, 1H), 7.59 – 7.51 (m, 6H),
7.42 (d, J = 7.0 Hz, 2H), 7.09 (d, J = 9.1 Hz, 1H), 2.26 (s, 3H). 13C NMR
(150 MHz, CDCl3) δ 144.21, 139.10, 132.15, 130.77, 129.93, 129.52,
128.79, 128.20, 122.69, 121.08, 119.21, 117.13, 110.56,18.42. HRMS
(ESI-TOF)m/z: calcd for C21H16N3 : 310.1339 found:310.1335.
7-methoxy-2,3-diphenylimidazo[1,2-a]pyridine 3s[15]
7-chloro-2,3-diphenylimidazo[1,2-a]pyridine 3l[15]
1
(Eluent: 20% EtOAc/hexane); 48% yield (36.4 mg); solid, H NMR (600
(Eluent: 20% EtOAc/hexane); 50% yield (37.5mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.75 (s, 1H), 7.62 (d, J = 7.8 Hz, 2H), 7.48 (d, J = 7.0 Hz,
2H), 7.41 (s, 3H), 7.27 – 7.23 (m, 2H), 7.23 – 7.16 (m, 1H), 6.97 (d, J = 2.2
Hz, 1H), 6.46 – 6.42 (m, 1H), 3.87 (s, 3H). 13C NMR (150 MHz, CDCl3) δ
158.19, 146.31, 141.63, 134.25, 130.84, 129.56, 128.80, 128.29, 127.94,
127.34, 123.89, 120.11, 107.41, 94.62, 55.64.
MHz, CDCl3) δ 7.86 (d, J = 6.6 Hz, 1H), 7.66 (s, 1H), 7.62 (d, J = 5.4 Hz,
2H), 7.52 – 7.49(m, 3H), 7.42 (s, 2H), 7.2 (d, J = 10.2 Hz, 4H). 13C NMR
(150 MHz, CDCl3) δ144.58, 143.23, 133.73, 131.20, 130.72, 129.75,
129.36, 129.27, 128.42, 128.12, 127.84, 123.70, 116.37, 113.99.
6-chloro-2-(4-chlorophenyl)-3-phenylimidazo[1,2-a]pyridine 3m[13]
(Eluent: 20% EtOAc/hexane); 55% yield (46.4mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.93 (s, 1H), 7.61 (d, J = 9.3 Hz, 1H), 7.56 (dd, J = 18.0,
6.9 Hz, 5H), 7.41 (d, J = 7.1 Hz, 2H), 7.24 (s, 2H), 7.17 (d, J = 9.4 Hz, 1H).
13C NMR (150 MHz, CDCl3) δ143.22, 140.57, 130.63, 129.96, 129.60,
129.29, 128.96, 128.66, 126.41, 121.24, 117.99.
3-phenyl-2-(thiophen-2-yl)imidazo[1,2-a]pyridine 3t
1
(Eluent: 20% EtOAc/hexane); 65% yield (44.8 mg); solid; H NMR (600
MHz, CDCl3) δ 7.83 (d, J = 6.9 Hz, 1H), 7.64 (d, J = 9.0 Hz, 1H), 7.56 (d,
J = 7.0 Hz, 2H), 7.54 – 7.48 (m, 3H), 7.21 (d, J = 5.3 Hz, 1H), 7.17 (d, J =
7.9 Hz, 1H), 7.08 (d, J = 3.4 Hz, 1H), 6.91 (dd, J = 5.2, 3.9 Hz, 1H), 6.70
7-chloro-2-(4-chlorophenyl)-3-phenylimidazo[1,2-a]pyridine 3n
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10.1002/ejoc.201800286
European Journal of Organic Chemistry
FULL PAPER
(s, 1H). 13C NMR (150 MHz, CDCl3) δ 144.76, 137.72, 131.13, 129.69,
7.1 Hz, 1H), 1.38 (s, 9H). 13C NMR (150 MHz, CDCl3) δ137.13, 130.30,
128.39, 128.11, 127.73, 127.65, 126.64, 126.52, 126.18, 123.90, 118.27,
116.29, 113.81, 29.78. HRMS (ESI-TOF)m/z: calcd for C24H24ClN2O:
391.1572 found:391.1575
129.48, 129.22, 127.50, 125.30, 124.95, 124.61, 123.33, 117.36, 112.46,
30.99.
HRMS(ESI-TOF)m/z:
calcd
for
C17H13N2S
:277.0794
found:277.0795.
6-methyl-3-phenyl-2-(thiophen-2-yl)imidazo[1,2-a]pyridine 3u
(Eluent: 20% EtOAc/hexane); 60% yield (43.5 mg); liquid, 1H NMR (600
MHz, CDCl3) δ 7.58-7.52 (m, 5H), 7.50 (d, J = 7.8 Hz, 2H), 7.19 (d, J =
4.2 Hz, 1H), 7.03 (d, J = 10.2 Hz, 2H), 6.90 (s, 1H), 2.23 (s, 3H). 13C NMR
(150 MHz, CDCl3) δ143.84, 137.94, 137.58, 131.19, 129.67, 129.39,
128.11, 127.46, 125.04, 124.33, 122.15, 120.95, 116.69, 18.35.
HRMS(ESI-TOF)m/z: calcd for C18H15N2S : 291.0950 found: 291.095.
7-chloro-3-phenyl-2-(thiophen-2-yl)imidazo[1,2-a]pyridine 3v
4-(3-(4-(tert-butyl)phenyl)imidazo[1,2-a]pyridin-2-yl)benzonitrile 3aa
(Eluent: 20% EtOAc/hexane); 65% yield (57.0 mg); solid, 1H NMR (600
MHz, CDCl3) δ 7.92 (t, J = 7.5 Hz, 1H), 7.78 (d, J = 8.2 Hz, 2H), 7.65 (s,
1H), 7.55 (t, J = 7.5 Hz, 4H), 7.34 (d, J = 8.0 Hz, 2H), 7.25 (s, 1H), 1.39 (s,
9H). 13C NMR δ 152.81, 132.17, 130.31, 128.31, 126.84, 125.44, 123.77,
117.77, 112.76, 31.37. HRMS(ESI-TOF)m/z: calcd for C24H22N3 :352.1808
found:352.1804.
2,3-diphenylbenzo[d]imidazo[2,1-b]thiazole 5a[16]
1
(Eluent: 20% EtOAc/hexane); 52% yield (40.3 mg); solid; H NMR (600
(Eluent: 20% EtOAc/hexane); 60% yield (48.9 mg); solid, 1H NMR (600
MHz, CDCl3) δ 7.66 (d, J = 7.6 Hz, 1H), 7.55 (s, 7H), 7.24 (t, J = 7.7 Hz,
3H), 7.19 (d, J = 6.6 Hz, 1H), 7.13 (t, J = 6.6 Hz, 1H), 6.82 (d, J = 7.9 Hz,
1H). 13C NMR (150 MHz, CDCl3) δ 148.21, 147.76, 133.88, 132.25, 130.39,
128.82, 127.62, 126.28, 125.28, 124.97, 124.49, 112.73, 106.94, 31.02.
3-phenyl-2-(p-tolyl)benzo[d]imidazo[2,1-b]thiazole 5b
MHz, CDCl3) δ 7.74 (d, J = 7.1 Hz, 1H), 7.63 (s, 1H), 7.61 – 7.51 (m, 3H),
7.49 (d, J = 6.9 Hz, 2H), 7.27 – 7.19 (m, 1H), 7.07 (s, 1H), 6.91 (s, 1H),
6.69 (d, J = 7.0 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ 144.47, 138.58,
137.18, 131.39, 131.03, 129.82, 129.77, 128.64, 127.58, 125.66, 124.90,
123.64, 120.17, 116.10, 114.07. HRMS (ESI-TOF)m/z: calcd for
C17H12ClN2S : 311.0404 found:311.0401.
(Eluent: 20% EtOAc/hexane); 62% yield (52.7 mg);solid; 1H NMR (600
MHz, CDCl3) δ 7.65 (d, J = 6.5 Hz, 1H), 7.54 (s 5H), 7.45 (d, J = 7.8 Hz,
2H), 7.24 (dd J = 6.6 Hz, 1H), 7.13 (t, J=7.2Hz,1H), 7.05 (d, J = 8.4 Hz,
2H), 6.82 (d, J = 8.4 Hz, 1H), 2.29 (s,3H). 13C NMR (150 MHz, CDCl3) δ
151.6, 147.1, 129.5, 129.4, 128.7, 128.4, 128.2, 126.7, 125.5, 124.2, 117.6,
3-(4-(tert-butyl)phenyl)-2-phenylimidazo[1,2-a]pyridine 3w[9e]
1
(Eluent: 20% EtOAc/hexane); 68% yield (55.4 mg); solid; H NMR (600
MHz, CDCl3) δ 7.96 (d, J = 6.8 Hz, 1H), 7.67 (t, J = 7.8 Hz, 3H), 7.52 (d, J
= 7.8 Hz, 2H), 7.36 (d, J = 7.7 Hz, 2H), 7.28 (t, J = 7.5 Hz, 2H), 7.23 (dd, J
= 12.6, 5.9 Hz, 1H), 7.19 – 7.15 (m, 1H), 6.70 (t, J = 6.6 Hz, 1H), 1.39 (s,
9H). 13C NMR (150 MHz, CDCl3) δ 151.91, 144.70, 142.19, 134.26, 130.27,
128.19, 128.06, 127.33, 126.66, 126.39, 124.53, 123.44, 121.15, 117.43,
112.08, 34.80, 31.29.
113.2,
113.6,
105.6.HRMS(ESI-TOF)m/z:
calcd
for
C22H16N2SNa :363.0926 found:363.0926.
2-(4-ethylphenyl)-3-phenylbenzo[d]imidazo[2,1-b]thiazole 5c
1
(Eluent: 20% EtOAc/hexane); 61% yield (53.9 mg); solid; H NMR (600
MHz, CDCl3) δ 8.21 (d, J = 6.5 Hz, 1H), 8.12 (d, J = 7.5 Hz, 2H), 8.06 (d,
J = 9.0 Hz, 2H), 7.79 (d, J = 9.0 Hz, 1H), 7.45-7.37 (m, 4H), 7.08 (d, J =
9.0 Hz, 2H), 6.94 (t, J = 6.5 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ 152.3,
147.6, 146.0, 144.9, 132.7, 129.0, 128.5, 128.2, 127.2, 125.1, 124.5, 124.0,
118.0 ,113.6, 103.5.HRMS(ESI-TOF)m/z: calcd for C23H19N2S :355.1263
found:355.1259.
3-(4-(tert-butyl)phenyl)-7-chloro-2-phenylimidazo[1,2-a]pyridine 3x
(Eluent: 20% EtOAc/hexane); 64% yield (57.6 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.88 (d, J = 6.6 Hz, 1H), 7.65 (s, 3H), 7.58 (d, J = 7.1 Hz,
2H), 7.34 (d, J = 6.9 Hz, 2H), 7.29 (dd, J = 15.8, 8.9 Hz, 3H), 6.70 (d, J =
7.1 Hz, 1H), 1.38 (s, 9H). 13C NMR (150 MHz, CDCl3) δ 137.13, 130.30,
128.39, 128.11, 127.73, 127.65, 126.64, 126.52, 126.18, 123.90, 118.27,
116.29, 113.81, , 29.78. HRMS (ESI-TOF)m/z: calcd for C23H22ClN2
361.1466 found:361.1466.
:
2-(4-methoxyphenyl)-3-phenylbenzo[d]imidazo[2,1-b]thiazole 5d
(Eluent: 20% EtOAc/hexane); 51% yield (45.3mg); solid; 1H NMR (600
MHz, CDCl3) δ 8.51 (d, J = 7.0 Hz, 1H), 8.28 (t, J = 7.0 Hz, 2H), 7.67 (d, J
= 9.0 Hz, 1H), 7.51 (t, J = 7.5 Hz, 2H), 7.43 (quin, J = 7.5 Hz, 1H), 7.33-
7.29 (m, 1H), 7.17 (q, J = 6.0 Hz, 1H), 7.04 (q, J = 6.0 Hz, 1H), 6.94 (q, J
= 7.0 Hz, 1H), 6.88 (q, J = 9.0 Hz, 1H). 13C NMR (150 MHz, CDCl3) δ 150.1,
146.5, 133.3, 129.9, 129.4, 128.5, 128.3, 127.8, 127.5, 126.5, 124.3, 117.6,
113.0, 109.1. HRMS (ESI-TOF)m/z: calcd for C22H17N2OS : 357.1056
found:357.1062.
3-(4-(tert-butyl)phenyl)-7-chloro-2-(4-chlorophenyl)imidazo[1,2-
a]pyridine 3y
1
(Eluent: 20% EtOAc/hexane); 58% yield (57.1 mg); solid; H NMR (600
MHz, CDCl3) δ 7.86 (d, J = 7.1 Hz, 1H), 7.64 (d, J = 1.9 Hz, 1H), 7.57 (d,
J = 8.9 Hz, 2H), 7.53 (d, J = 8.0 Hz, 2H), 7.32 (d, J = 8.7 Hz, 2H), 7.24 (d,
J = 8.1 Hz, 2H), 6.71 – 6.69 (m, 1H), 1.38 (s, 9H). 13C NMR (150 MHz,
CDCl3) δ152.56, 144.44, 141.84, 133.52, 132.34, 131.24, 130.14, 129.19,
128.52, 126.67, 126.31, 125.76, 123.82, 121.53, 116.17, 114.86, 113.90,
2-(4-chlorophenyl)-3-phenylbenzo[d]imidazo[2,1-b]thiazole 5e
1
34.88, 31.27. HRMS (ESI-TOF)m/z: calcd for C23H21Cl2N2
395.1076found:395.1078.
:
(Eluent: 10% EtOAc/hexane); 60% yield (54.0 mg); solid; H NMR (600
MHz, CDCl3) δ 7.66 (d, J = 8.4 Hz, 1H), 7.58-7.52 (m, 5H), 7.48 (d, J =
9.0 Hz, 2H), 7.25-7.22 (m 1H), 7.19 (d, J = 8.4 Hz, 2H), 7.14-7.11 (m, 1H),
6.28 (d, J = 7.8 Hz, 17.48 (d, J = 9.0 Hz, 2H), H), 13C NMR (150 MHz,
CDCl3) δ 139.6, 137.4, 135.0, 129.7, 128.6, 125.6, 124.6, 122.5, 120.4,
119.6, 109.6, 100.3, 33.0. HRMS (ESI-TOF)m/z: calcd for C21H14ClN2S :
361.0561, found:361.0557.
3-(4-(tert-butyl)phenyl)-2-(4-chlorophenyl)-7-methoxyimidazo[1,2-
a]pyridine 3z
1
(Eluent: 20% EtOAc/hexane); 58% yield (56.5 mg); solid; H NMR (600
MHz, CDCl3) δ 7.88 (d, J = 6.6 Hz, 1H), 7.65 (s, 3H), 7.52 (d, J = 7.1 Hz,
2H), 7.34 (d, J = 6.9 Hz, 2H), 7.27 (dd, J = 15.8, 8.9 Hz, 3H), 6.69 (d, J =
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201800286
European Journal of Organic Chemistry
FULL PAPER
2-(4-bromophenyl)-3-phenylbenzo[d]imidazo[2,1-b]thiazole 5f
(Eluent: 5% EtOAc/hexane); 50% yield (50.3 mg); liquid; 1H NMR (600
MHz, CDCl3) δ 7.66 (d, J = 7.8 Hz, 1H), 7.58-7.55 (M, 3H). 7.53-7.52 (M,
2H). 7.42 (d, J = 7.8 Hz, 2H), 7.34 (d, J = 7.2 Hz, 2H), 7.25-7.22 (M 1H),
7.14 (t, J = 8.4 Hz, 1H), 6.82 (d, J = 8.4 Hz, 1H). 13C NMR (150 MHz,
CDCl3) δ136.5, 134.4, 129.9, 128.8, 128.6, 127.9, 126.9, 125.6, 124.6,
122.7, 120.6, 119.8, 110.2, 101.3, 50.4. HRMS (ESI-TOF)m/z: calcd for
C21H14BrN2S : 405.0056; found:405.0053.
Acknowledgements
CSIR-CSMCRI communication No. 179/2017. R. K and C.R. are
thankful to AcSIR for their Ph.D. enrolment and the “Analytical
Discipline and Centralized Instrumental Facilities” for providing
instrumentation facilities. C.R also thankful to CSIR-New Delhi for
SRF. We thank DST, Government of India. EMR/2016/000010),
and CSIR-CSMCRI (OLP-087 and OLP-088) for financial support.
2-(3-methoxyphenyl)-3-phenylbenzo[d]imidazo[2,1-b]thiazole 5g
(Eluent: 10% EtOAc/hexane); 70% yield (62.3 mg); solid; 1H NMR (600
MHz, CDCl3) δ 7.66(d, J = 8.4 Hz, 1H), 7.56 (s, 5H), 7.24 (t, J = 8.4 Hz,
1H), 7.13 (t, J = 7.2 Hz, 4H), 6.82 (d, J = 8.4Hz, 1H), 6.74-7.73 (m, 1H),
3.64 (s 3H). 13C NMR (150 MHz, CDCl3) δ139.1, 136.4, 130.6, 129.0, 128.6,
125.7, 124.7, 123.0, 120.8, 119.6, 111.5, 102.7. HRMS (ESI-TOF)m/z:
calcd for C22H17N2OS: 357.1056; found:357.1051.
Keywords: • metal–free • potassium tert-butoxide • arylation •
regioselectivity.
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2-(3-methoxyphenyl)-3-phenylbenzo[d]imidazo[2,1-b]thiazole 5h
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1
(Eluent: 10% EtOAc/hexane); 65% yield (53.9 mg); solid; H NMR (600
MHz, CDCl3) δ 7.65(t, J=7.2Hz,1H), 7.60-7.59 (M,5H), 7.25-7.22 (M, 1H),
7.14-7.11 (M 2H), 6.95-6.94 (M 1H), 6.89-6.87 (M 1H), 6.82 (d, J = 8.4 Hz,
1H). 13C NMR (150 MHz, CDCl3) δ140.0, 139.3, 135.4, 130.2, 128.6, 125.4,
124.4, 122.1, 120.6, 118.9, 110.5, 93.3, 12.1. HRMS (ESI-TOF)m/z: calcd
for C19H13N2S2 : 333.0515, found:333.0523.
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3-phenylimidazo[1,2-a]pyridine 9
(Eluent: 15% EtOAc/hexane); 38% yield (18.2 mg); solid; 1H NMR (600
MHz, CDCl3) δ 8.34(d, J = 7.2 Hz, 1H), 7.69 (t, J = 9.0 Hz,2H), 7.56(d, J
= 7.8 Hz, 2H), 7.52 (t, J = 6.6 Hz, 2H), 7.42 (t, J = 7.8 Hz, 1H), 7.20 (t, J =
7.2 Hz, 1H), 6.81 (t, J = 6.6 Hz, 1H), 13C NMR (150 MHz, CDCl3) δ132.4,
129.3, 128.2, 128.1, 124.3, 123.4, 118.3, 112.6.
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2600.
2-phenyl-3-(4-(trifluoromethyl)phenyl)imidazo[1,2-a]pyridine 11
1
(Eluent: 15% EtOAc/hexane); 67% yield (56.4 mg); solid; H NMR (600
MHz, CDCl3) δ 7.99 (d, J = 7.8 Hz, 1H), 7.77 (d, J = 7.8 Hz, 2H), 7.70 (d,
J = 9.0 Hz, 1H), 7.60-7.57 (m, 4H), 7.31-7.26 (m,3H), 7.24-7.21 (m, 1H),
6.78-6.76 (m, 1H). 13C NMR (150 MHz, CDCl3) δ 145.2, 143.4, 133.6,
128.4, 127.8 126.4, 125.1,122.9, 199.4, 117.4, 112.7. HRMS (ESI-
TOF)m/z: calcd for C20H13F3N2Na : 361.0923, found:361.0916.
2-phenyl-3-(4-(trifluoromethyl)phenyl)benzo[d]imidazo[2,1-b]thiazole
13
[7]
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[1,1'-biphenyl]-2,5-dione 15
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C–H Arylation
Rahul Kumar, Chitrakar Ravi, Deepa
Rawat and Subbarayappa Adimurthy*
Page No. – Page No.
Base-Promoted Transition metal-free
Arylation of Imidazo fused
Heterocycles with Diaryliodonium
salts
The arylation of imidazo[1,2-a]pyridine and phenylbenzo[d]imidazo[2,1-b]thiazoles using Diaryliodonium salts as aryl source
under metal-free conditions were described.
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