Difluorinative-hydroxylation and C-3 functionalization (halogenation/SCN/NO) of imidazopyridine using Selectfluor as fluorine source or oxidant respectively

Article abstract of DOI:10.1016/j.tetlet.2021.153028

An efficient route for the difluorinative-hydroxylation through dearomative difunctionalization of imidazopyridine using Selectfluor as fluorine source has been developed in an aqueous medium. The reaction proceeds through ionic followed by radical pathwa

Full text of DOI:10.1016/j.tetlet.2021.153028

Tetrahedron Letters 71 (2021) 153028
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Difluorinative-hydroxylation and C-3 functionalization
(halogenation/SCN/NO) of imidazopyridine using Selectfluor
as fluorine source or oxidant respectively
Saradhi Kalari a,b, Sridhar Balasubramanian c, Haridas B. Rode a,b,
⇑
a Department of Organic Synthesis and Process Chemistry, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India
b Academy of Scientific and Innovative Research (AcSIR), CSIR-HRDC Campus, Kamla Nehru Nagar, Ghaziabad, Uttar Pradesh 201001, India
c Laboratory of X-ray Crystallography, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient route for the difluorinative-hydroxylation through dearomative difunctionalization of imida-
zopyridine using Selectfluor as fluorine source has been developed in an aqueous medium. The reaction
proceeds through ionic followed by radical pathway. The products are obtained in pure form without col-
umn purification. The method was successfully applied for a gram-scale production of 3,3-difluoro-2-(4-
fluorophenyl)-2,3-dihydroimidazo[1,2-a]pyridin-2-ol (3m). The Zolimidine drug successfully underwent
difluorinative-hydroxylation in high yield. Interestingly, addition of the tetra-n-butyl ammonium halide
or NaI or KSCN or NaNO2 in the reaction provided oxidative functionalization of imidazopyridine wherein
Selectfluor acted as an oxidant. The C-3 substituted (Cl, Br, I, SCN and NO) imidazo[1,2-a]pyridines were
prepared in good yields. These two significant reactions provided wide functional group tolerance, metal/
base-free, and green approach.
Received 12 January 2021
Revised 20 March 2021
Accepted 23 March 2021
Available online 26 March 2021
Keywords:
Difluorinative-hydroxylation
Fluorination
C-3 functionalization of imidazo[12-a]
pyridines
Ó 2021 Elsevier Ltd. All rights reserved.
Introduction
syn-dichlorination
of
alkenes
[7b],
halogenation
and
thiocyanation of 1-arylallenes [7c], thiocyanation of indoles, pyr-
role, and carbazole [7d]. Conversely, imidazopyridine scaffold is
extensively found in various natural products and drugs [8]. There-
fore, the manipulation of this omnipresent motif attracted much
attention of organic chemists.
Naturally occurring or synthesized fluorine containing com-
pounds have displayed wide range of significance in medicinal
chemistry, agrochemical, material science, polymer industries
and positron emission tomography (PET) imaging [1]. Incorpora-
tion of fluorine atom into organic molecule augments the biologi-
cal properties of parent counterparts [2]. Fluorine in bioactive
compounds significantly influences the conformation, membrane
permeability, pKa, metabolic stability and intrinsic potency. Hence,
fluorine is often used as bioisosteric replacement for hydrogen in
candidate molecule to address issues related to metabolism [3].
In past, geminal difluoro group has drawn great attention due to
its presence in numerous drugs (Fig. 1) [4,5]. Additionally, presence
of hydroxyl group in such difluoro-system would indeed potentiate
its properties such as bioavailability and catabolic stability. One
such an example is Lubiprostone (AmitizaÒ).
Along this line, many research groups have disclosed synthesis
of variety of C-3 functionalized complex imidazopyridines [9]. In
seminal line, Sun et al. [10] and Shinde et al. [11] reported C-3
monofluorination of imidazo[1,2-a] pyridines using Selectfluor as
fluorine source. The direct regioselective functionalization (F, Cl,
Br, I, SCN, NO) of heteroarenes using electrophile shows impor-
tance in organic synthesis [10,12]. In recent times, alternate route
has been explored to generate electrophiles from nucleophiles by
using oxidants. Recently, Adimurthy et al. [13] and Katrun et al.
[14] developed chlorination, bromination, iodination while Hajra
et al. [15] and Wang et al. [16,17] established thiocyanation on
2-aryl imidazo[1,2-a]pyridines. These approaches have the major
drawbacks like use of photo-catalyst, longer reaction time and
use of harsh condition. Noteworthy, to use these binary reactivities
(fluorine donor and oxidant) of Selectfluor on single target using
simple, safe, rapid, facile, sustainable and eco-friendly method is
highly desirable. Herein, we report a straightforward transforma-
tion for geminal-difluoro hydroxylation in aqueous medium
In general, Selectfluor is widely used as electrophilic fluorine
source in different organic transformations [6]. On the other hand,
the oxidative property of Selectfluor is used for electrophilic
substitution of arenes using various nucleophiles [7a],
⇑
Corresponding author.
E-mail address: haridas.rode@iict.res.in (H.B. Rode).
https://doi.org/10.1016/j.tetlet.2021.153028
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

S. Kalari, S. Balasubramanian and H.B. Rode
Tetrahedron Letters 71 (2021) 153028
in water afforded solely 3a in 90% yield (entry 9, Table 1). In the
crystal structure of 3a, two molecules of 3a exist in an asymmetric
unit (Fig. 2). This crystal packing arrangement is possible due to
the hydrogen bond between hydroxyl of one molecule with N1
of other molecule of 3a. Such interactions in crystal packing have
implications on solubility of these compounds and are important
in drug development. After having the optimized reaction condi-
tion in hand, the substrate scope of difluorinative-hydroxylation
reaction with respect to imidazo[1,2-a] pyridines were explored
(Scheme 1).
In some reactions, CH3CN:H2O (2:8) was employed due to lim-
ited solubility of substrates in water. Various 2-aryl imidazo[1,2-a]
pyridines containing both electron-donating and electron-with-
drawing groups on 2-aryl ring readily furnished the corresponding
products 3b-3h and 3i-3r in good to excellent yields. These sub-
stituents include alkyl (3b, 3c, 3d, 3e), methoxy (3f), 3,4-
methylenedioxy (3g), biphenyl (3h), cyano (3i), trifluoromethoxy
(3j), nitro (3k, 3l), fluoro (3m), 3,5-difluoro (3n), chloro (3o, 3p),
and bromo (3q, 3r). Furthermore, different substitution on pyridine
part of 2-aryl imidazo[1,2-a]pyridine scaffold smoothly underwent
difluorinative-hydroxylation affording 3s-3u in good to excellent
yields. Moreover, 2-sustituted polycyclic aromatic and heteroaro-
matic compounds such as 2-naphthyl, 2-bromofluorenyl, and 2-
thiophenyl, and 2-furyl were also compatible under the present
reaction condition and furnished the products (3v-3y) in moderate
yields. In addition, when the substrates with strong electron with-
drawing groups like CF3 and NO2 on imidazo[1,2-a]pyridine ring
were used, the reaction failed to give corresponding products. Of
note, Zolimidine drug underwent difluorohydroxylation to afford
3zb in 88% yield (Scheme 2). This protocol was successfully applied
to a gram scale synthesis of 3m (Scheme 3).
Interestingly, regioselective monofluorinated imidazo[1,2-a]
pyridines 2a-2f could be obtained when CHCl3:H2O (9:1) was used
in the reaction of 1a/1p/1r/1i/1y/1zb and Selectfluor (1 equivalent)
in moderate to good yields (Scheme 4). The substrates containing
4-cyanophenyl and furyl on imidazo[1,2-a]pyridine afforded
monofluorinated 2d and 2e respectively. Importantly, gastropro-
tective drug, Zolimidine (1zb), underwent monofluorination
affording 2f in 66% yield. Unfortunately, 3-pyridyl substituted imi-
dazo[1,2-a]pyridine failed to produce 2g. It is important to note
that this protocol is much better than the Sun et al. protocol
[10]. In Sun et al. protocol, the C-3 monofluorinated imidazo[1,2-
a]pyridines were obtained using DMAP as base with 2 equivalents
of Selectfluor in chloroform: water (3:1) for 14 h from imidazo[1,2-
a]pyridines. While, in current protocol, the products are obtained
in 45 min, require only 1 equivalent of Selectfluor and reaction
Fig. 1. Structre of the drugs containing difluoro substituent.
through dearomative difunctionalization and C-3 functionalization
of imidazo[1,2-a]pyridines using dual reactivity of Selectfluor
under simple and mild reaction condition.
Results and discussion
In order to identify the optimal reaction condition for difluoro-
hydroxylation, we employed 2-aryl imidazo[1,2-a]pyridine (1a)
and Selectfluor (1 equivalent) in dichloromethane at room temper-
ature (entry 1, Table 1). This condition furnished monofluorinated
compound 2a in 23% yield along with unreacted 1a. The use of
other chlorinating solvents i.e. CHCl3, DCE, TCE afforded 2a in
low yields and no traces of 3a were detected (entry 2, 3 and 4).
When the reaction was carried out in CHCl3:H2O (9:1, entry 5),
the trace amount of 3a was obtained along with 79% of 2a. The
similar observation was made when the reaction was performed
in TCE:H2O (9:1, entry 6). Further, we employed 2 equivalents of
Selectfluor in CH3NO2:H2O (9:1, entry 7) to afford 61% of required
3a with traces of 2a.
The reaction in CH3CN:H2O (9:1) resulted in 81% of 3a exclu-
sively. The increased yields for 2a or 3a in entry 5 to 8 could be
attributed to the presence of protic solvent as scarce solubility of
Selectfluor in aprotic solvent is reported [18]. Of note, the reaction
Table 1
Optimization of difluoro-hydroxylation of imidazo[1,2-a]pyridine.a
No.
Solvent
Selectfluor (equiv.)
Time (h)
Yield of 2ab
Yield of 3ab
1c
2c
3c
4c
5
6
7
8
9
CH2Cl2
CHCl3
DCE
TCE
CHCl3:H2O (9:1)
TEC:H2O (9:1)
CH3NO2:H2O (9:1)
CH3CN:H2O (9:1)
H2O
1
1
1
1
1
1
2
2
2
4
4
4
4
0.45
1.5
3.5
3
23
31
20
25
79
74
trace
–
–
–
–
–
trace
trace
61
81
90
0.45
–
a
b
c
Reaction condition: 1a (0.1 g), Selectfluor in 2 mL of solvent at room temperature. isolated yields. unreacted 1a isolated. DCE = 1,2-dichloroethane, TCE = 1,1,2,2-
tetrachloroethane.
2

S. Kalari, S. Balasubramanian and H.B. Rode
Tetrahedron Letters 71 (2021) 153028
Scheme 3. Gram scale synthesis of 3m.
F
CHCl3:H2O
(9:1)
N
N
SelectFluor
R
+
R
45 min
N
N
2a: Phenyl (79%)
R=
1a: Phenyl
R=
2b: 4-Cl Phenyl (65%)
2c: 4-Br Phenyl (52%)
2d: 4-CN Phenyl (58%)
2e: 2-Furyl (53%)
1p: 4-Cl Phenyl
1r: 4-Br Phenyl
1i: 4-CN Phenyl
1y: 2-Furyl
2f: 4-SO2Me Phenyl (66%)
2g: 3-Pyridyl (0%)
1zb: 4-SO2Me Phenyl
1zc: 3-Pyridyl
Scheme 4. Regioselective fluorination of imidazo[1,2-a]pyridines.
Reaction condition: 1 (0.5 mmol), Selectfluor (0.5 mmol), CHCl3:H2O (9:1, 1 mL).
Isolated yields are shown.
Fig. 2. A view of 3a, showing the atom-labeling Scheme. Displacement ellipsoids
are drawn at the 30% probability level and H atoms are represented by circles of
arbitrary radii. The asymmetric unit contains two crystallographically independent
molecules and forms a dimer by OAH. . .N hydrogen bonds (shown as dashed lines).
F
R1
R1
F
H2O
RT, 45 min
N
N
Scheme 5. Monofluorination of 1a.
SelectFluor
R2
R2
OH
3a-3za
+
N
N
1a-1za
F
3k: 3-NO2 Phenyl (66%)
3l: 4-NO2 Phenyl (82%)
3m: 4-F Phenyl (85%)
3n: 3,5-F Phenyl (82%)
3o: 3-Cl Phenyl (82%)
3p: 4-Cl Phenyl (81%)
3q: 3-Br Phenyl (79%)
3r: 4-Br Phenyl (83%)
3v: 2-Naphthyl (75%)
3x: 2-Thio Phenyl (61%)
3y: 2-Furyl (77%)
R2= 3a: Phenyl (90%)
likely via n-fluorobutan-1-aminium tetrafluoroborate (Scheme 5).
The n-fluorosubstiuted-1-aminium tetrafluoroborate intermediate
has been characterized by Chauhan et al. using NMR spectroscopy
[19]. Important to note, only 40% conversion of 1a to 2a was
observed when the reaction (Scheme 5) was conducted in CHCl3:
H2O (9:1) in the presence of n-butylamine for 8 h. Hence, absence
of nitrogen base DMAP is critical requirement for monofluorination
of imidazo[1,2-a]pyridines.
F
R2
OH
3b: 4-CH3 Phenyl (86%)
3c: 3,4-CH3 Phenyl (75%)
3d: 4-Et Phenyl (72%)
N
N
3e: 4-t-Bu Phenyl (82%)
3f: 4-OMe Phenyl (71%)
3g: 3,4-CH2(O)2 Phenyl (78%)
3h: 4-Ph Phenyl (75%)
3i: 4-CN Phenyl (83%)
3j: 3-OCF3 Phenyl (83%)
On the other hand, functionalization (Cl, Br, I, SCN, NO) reac-
tions on hetero arene have been scars and limited to thiocyanation
using Selectfluor as an oxidant [7d]. The use of tetra-n-
butylammonium chloride afforded 3-chloro derivatives 4a, 4b, 4c
with moderate yields whereas use of tetra-n-butylammonium
bromide and sodium iodide in this protocol produced
corresponding 3-bromo (4d, 4e, 4f) and 3-iodo (4g, 4h, 4i)
analogues, respectively (Scheme 6). Important to note, the use of
KSCN furnished 3-thiocyanate containing analogues 4j, 4k, 4l in
good yields. Next, the nitration with sodium nitrite in
acetonitrile was attempted. In this case, the stable nitroso-
compounds 4m, 4n, 4o were isolated.
Based on literature precedents [20,21] & control experiments,
the plausible mechanism for difluorinative-hydroxylation is pro-
posed (Figs. 3, 4). The reaction of 1a with 2 equivalents of Select-
fluor in H2O afforded 3a exclusively (Scheme 1). The treatment of
1a with 1 equivalent of Selectfluor in CHCl3:H2O (9:1) resulted in
monofluorinated 2a in 79% (Scheme 4). The presence of a radical
inhibitor, TEMPO, in this reaction could not suppress the formation
of monofluorinated compound 2a (equation 1). This indicate that
the monofluorination reaction proceeds through ionic mechanism.
On the other hand, when 1a was treated with Selectfluor in H2O in
the presence of TEMPO, the difluorinative-hydroxylation was
inhibited. The imidazopyridine radical was trapped and detected
in HRMS (equation 2). This indicate that the difluorinative-hydrox-
ylation likely goes through the ionic mechanism followed by radi-
cal pathway. Based on these observations, it can be concluded that
TEMPO is not oxidized by Selectfluor in difluorinative-hydroxyla-
F
F
F
Cl
F
F
Br
F
N
N
N
N
N
OH
3t (94%)
N
OH
OH
Me
3s (70%)
3u (95%)
F
F
F
F
F
O2N
F
Br
N
N
N
N
OH
N
OH
N
OH
3w (77%)
CF3
3za (0%)
3z (0%)
Scheme 1. Difluorinative-hydroxylation of various imidazo[1,2-a]pyridines*.
Reaction condition: 1 (0.5 mmol), Selectfluor (1 mmol), H2O (2 mL), RT, 45 min.
Shown are the isolated yields. * indicate CH3CN: H2O (2:8) was used in case of 3c,
3d, 3e, 3f, 3g, 3h, 3i, 3k, 3l, 3n, 3o, 3p, 3q, 3r and 3s instead of water.
Scheme 2. Synthesis of Zolimidine analogue.
does not need DMAP. This emphasizes the significance of current
protocol over Sun et al. protocol. The possible reason for this pro-
tocol being efficient is the absence of amine component in the
reaction. Compound 2a could be formed from 1a and Selectfluor
in the presence of n-butylamine, though with longer reaction time,
3

S. Kalari, S. Balasubramanian and H.B. Rode
Tetrahedron Letters 71 (2021) 153028
X
metal-free radical processes [22,23]. It is important to note that
Yang et al. described the amide-assisted fluorination of anilines
that does not need any metal or radical initiator [22]. The use of
Selectfluor in aqueous media was enough to successfully provide
the fluorination of anilines. In another example, Dan et al. reported
the selective ortho-fluorination of pyridin-2(1H)-ones and 2-
aminopyridines using Selectfluor in aqueous media in catalyst free
system [23].
SelectFluor
CH3CN, RT
30 min
N
N
R1
R1
+
R2X
N
4a-4o
N
R1= 1a: Phenyl
1p: 4-Cl Phenyl
1r: 4-Br Phenyl
X
N
R2
X
N
N
N
Br
Cl
N
N
Conclusion
4c: X = Cl (61%)
4f: X = Br (68%)
4i: X = I (65%)
4a: X= Cl (64%)
4d: X = Br (71%)
4g: X = I (77%)
4b: X = Cl (56%)
4e: X = Br (64%)
4h: X = I (62%)
In summary, we developed a difluorinative-hydroxylation of
imidazo[1,2-a]pyridines using Selectfluor in aqueous medium.
The reaction proceeds through ionic followed by radical pathway.
Importantly, the products are obtained in pure form without col-
umn purification. Importantly, gastroprotective drug Zolimidine
successfully underwent difluorinative-hydroxylation. Further,
oxidative nature of Selectfluor was explored in the presence of
charged nucleophiles to furnish C-3 functionalized (Cl, Br, I, SCN,
NO) imidazo[1,2-a]pyridines. These transformations tolerated
4l: X = SCN (76%)
4o: X = NO (85%)
4j: X = SCN (73%) 4k: X = SCN (81%)
4m:X = NO (69%) 4n: X = NO (80%)
Scheme 6. Functionalization of imidazo[1,2-a]pyridines with charged nucleophiles.
Reaction condition: 1 (0.5 mmol), Selectfluor (1 mmol), R2X = TBAC or TBAB or NaI
or KSCN or NaNO2 (1 mmol), CH3CN (1 mL). Isolated yields are shown. CH3CN:H2O
(2:8) was used in case of 4e, 4f, 4h and 4i instead of ACN. Water was used in case of
4d, 4g instead of ACN.
wide functional groups, and constitute
approach.
a metal/ base-free
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
The financial support through a research grant GAP0584 from
Department of Science and Technology, Government of India is
acknowledged. We thank Director, CSIR-IICT (Ms. No. IICT/
pubs./2019/241) for providing all the required facilities to carry
out this work.
Fig. 3. Control experiments for the mechanistic insights of difluorinative-
hydroxylation.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153028.
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