Multicomponent Synthesis of Imidazo[1,2- a]pyridines: Aerobic Oxidative Formation of C-N and C-S Bonds by Flavin-Iodine-Coupled Organocatalysis

Article abstract of DOI:10.1021/acs.orglett.0c02929

Herein, we report an aerobic oxidative C-N bond-forming process that enables the facile synthesis of imidazo[1,2-a]pyridines and takes advantage of a coupled organocatalytic system that uses flavin and iodine. Furthermore, the dual catalytic system can be applied to the one-pot, three-step synthesis of 3-thioimidazo[1,2-a]pyridines from aminopyridines, ketones, and thiols.

Full text of DOI:10.1021/acs.orglett.0c02929

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Letter
Multicomponent Synthesis of Imidazo[1,2‑a]pyridines: Aerobic
Oxidative Formation of C−N and C−S Bonds by Flavin−Iodine-
Coupled Organocatalysis
Hayaki Okai, Kazumasa Tanimoto, Ryoma Ohkado, and Hiroki Iida*
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ABSTRACT: Herein, we report an aerobic oxidative C−N bond-forming process that enables the facile synthesis of imidazo[1,2-
a]pyridines and takes advantage of a coupled organocatalytic system that uses flavin and iodine. Furthermore, the dual catalytic
system can be applied to the one-pot, three-step synthesis of 3-thioimidazo[1,2-a]pyridines from aminopyridines, ketones, and thiols.
1
4
he development of green and atom-economical aerobic
olprinone (a cardiotonic agent), and rifaximin (an anti-
15 16
T
oxidation systems for diverse organic transformations is a
biotic), as well as potential medicinal candidates contain
this scaffold (Figure 1). Due to their importance not only to
medicinal chemistry but also to materials science, a range of
methods for the synthesis of imidazo[1,2-a]pyridines has been
Among them, aerobic oxidative cyclizations
from simple and readily available 2-aminopyridines 1 and
ketones 2 is recognized to be an atom-economical and
1
central theme of modern organic chemistry. Molecular oxygen
is recognized to be an ideal terminal oxidant because of its
sustainable abundance, safety, cost-effectiveness, atom-econo-
my, and minimally polluting nature. However, aerobic
oxidation is generally kinetically unfavorable and often suffers
from narrow substrate scope and poor selectivity, which make
it difficult to develop multistep and multicomponent reactions
using an aerobic process, despite such reactions providing
atom- and step-economical syntheses that fulfill the strong
17
18,19
developed.
2
demands of green and sustainable chemistry. On the other
hand, living cells apply well-designed multiple catalytic systems
to synthesize diverse complex molecules through multistep
reactions that use molecular oxygen under mild conditions.
The construction of biomimetic multiple catalytic systems is
among the most promising approaches for designing efficient
3
multistep organic syntheses. Flavin catalysts have evolved
because they mimic the enzymatic function of flavin
monooxygenase, which promotes selective aerobic oxygen-
atom-transfer reactions in the presence of reductive coenzyme
4
NAD(P)H and, as a result, catalyzes a diverse range of aerobic
oxygenations that require reducing agents to activate molecular
Figure 1. Structures of biologically important imidazo[1,2-a]-
pyridines.
5
,6
oxygen.
As evidenced by the diverse range of flavin-
7
containing enzymatic systems in nature, flavin catalysis, with
its modest and versatile oxidizing ability, is expected to be
suitable for constructing multiple catalytic systems.
8
,9
Received: September 1, 2020
Imidazo[1,2-a]pyridine is a privileged heterocyclic structure
that is found in numerous biologically active pharmaceuticals
10
and natural products. Many commercially available drugs,
1
1
11
12
such as alpidem, zolpidem, nicopidem, saripidem
(
1
2
13
anxiolytic agents), zolimidine (a gastroprotective agent),
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c02929
A
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straightforward path that generates environmentally benign
water as the only byproduct, and copper-catalyzed systems that
use zinc, indium, and boron as a cocatalyst have been reported
Table 1. Flavin−Iodine-Catalyzed Synthesis of 3a from 1a
a
and 2a
2
0
(Figure 2A). However, the examples have been limited to the
entry
flavin (mol %)
I2 (mol %)
atmosphere
yield (%)
1
2
3
4
5
6
4·TfO (5)
none
4·TfO (5)
4·TfO (5)
4·TfO (5)
none
I2 (10)
I2 (10)
none
I2 (10)
I2 (10)
I2 (120)
O2
O2
O2
N2
air
N2
87
7
none
8
17
6
a
Conditions: 1a (1.0 M), 2a (1.5 M), flavin, I , and EtOAc under O ,
2
2
2
1
N , or air (1 atm, balloon) at 70 °C. Yield was determined by H
NMR or GC measurements with 1,1,2,2-tetrachloroethane or
triethylene glycol dimethyl ether as an internal standard.
reaction is promoted by aerobic oxidation catalyzed by the
flavin and iodine. The use of air afforded 17% yield, revealing
that the rate of the present reaction depended on the
concentration of molecular oxygen (entry 5). It is noteworthy
that the use of a stoichiometric amount of I (120 mol %)
2
afforded a poor yield under these conditions (entry 6)
+
probably due to the effect of the in situ generated H which
decreased the nucleophilic reactivity of 1 through the
protonation. This result highlights the apparent merit of the
present dual catalytic system.
Figure 2. (A, B) Methods for the synthesis of 3 by the aerobic
oxidative cyclization of 1 and 2. (C) One-pot synthesis of 6 through
the three-step oxidation of 1, 2, and 5.
With the optimized conditions in hand, we investigated the
substrate scope of the present imidazo[1,2-a]pyridine synthesis
method (Scheme 1). 2-Aminopyridines bearing both electron-
donating and electron-withdrawing substituents gave the
desired products 3a−c, although electron-deficient substrates
required somewhat longer reaction times, and 3- and 4-
substituted aminopyridines afforded the corresponding
imidazo[1,2-a]pyridines 3d and 3e without any loss of yield.
The reactions of variously substituted acetophenones pro-
ceeded smoothly to give 3f−i in good yields, with chloro and
ester functionalities tolerated, while thiazolyl and thiophenyl
ketones could be used in this reaction to produce 3j and 3k.
Furthermore, the present reaction is amenable to α,β-
unsaturated ketones, affording alkenyl imizadopyridines 3l
and 3m in yields of 59% and 45%, respectively. When
propiophenone was used instead of an acetophenone, the
corresponding 3-methylimidazo[1,2-a]pyridine 3n was ob-
tained in 66% yield. The reaction with aliphatic ketone was
relatively difficult, and the desired product 3o was given in 30%
yield.
copper-catalyzed method, and the development of other
approaches is required to increase options. In this paper, we
report the first metal-free aerobic system for the synthesis of
imidazo[1,2-a]pyridines 3 that uses a coupled flavin−iodine
(4·TfO/I ) catalyst (Figure 2B). The flavin−iodine-coupled
2
organocatalytic system was recently developed for the oxidative
formation of C−S bonds in which the biomimetic flavin
catalyst activates molecular oxygen through electron transfer
21
from the coupled iodine catalyst, thereby providing a green
oxidative transformation, with molecular oxygen (1 atm or air)
as the only sacrificial reagent and no other oxidizing or
22
reducing agent required. While successful examples have
22,23
been limited to the oxidative formation of C−S bonds,
here we demonstrate the first example of oxidative C−N bond
formation. Furthermore, we combined the present C−N bond-
formation chemistry with that previously reported for C−S
bond formation to synthesize 3-thioimidazo[1,2-a]pyridines 6
from 1, 2, and thiols 5 in a one-pot, three-component process
that involves three oxidation steps, namely, aerobic oxidative
C−N, S−S, and C−S bond formations (Figure 2C).
We performed control experiments to gain insight into the
reaction mechanism, as shown in Scheme 2. The pyridinium
adduct was formed through the reaction with 2a when pyridine
was used instead of 1a, suggesting that the formation of the
C−N bond plays an important role in the present imidazo[1,2-
a]pyridine synthesis (Scheme 2A). In the presence of
We began our study by examining the reaction of 2-amino-5-
methylpyridine (1a) and acetophenone (2a) in an atmosphere
of molecular oxygen (1 atm, balloon). We investigated the
effects of the flavin catalyst, iodine source, and solvent (Tables
S1 and S2), which revealed that the corresponding imidazo-
stoichiometric amount of I , the formation of phenacyl iodide
2
1
[
1,2-a]pyridine 3a was successfully obtained in 87% yield when
(7a) from 2a was displayed by the H NMR measurement in
the alloxazinium-type flavin catalyst 4·TfO (5 mol %), which
CD CN (Scheme 2B). Furthermore, the uncatalyzed reaction
3
was synthesized from commercially available riboflavin
of 1a and 7a under nitrogen provided 3a (Scheme 2C). Based
on these experimental results and previous reports, we propose
a plausible mechanism for the flavin−iodine-catalyzed reaction
2
4
(
%
vitamin B2), was used in the presence of iodine (10 mol
) in EtOAc at 70 °C (entry 1, Table 1). In sharp contrast, the
desired product was hardly produced in the absence of 4·TfO,
of 1 and 2 (Scheme 3). In this system, 1a reacts with I to
2
iodine, or molecular oxygen (entries 2−4), suggesting that the
produce the corresponding iodide 7, which undergoes
B
https://dx.doi.org/10.1021/acs.orglett.0c02929
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Scheme 1. Scope of the Flavin−Iodine-Catalyzed Synthesis
Scheme 3. Proposed Mechanism for the Catalytic Synthesis
of 3
a
of 3
6
(Figure 2C). Due to the pharmacological interest in
a
Conditions: 1 (1 M), 2 (1.5 M), 4·TfO (5 mol %), I (10 mol %),
2
sunfenylimidazo[1,2-a]pyridines bearing various thio function-
alities, various methods for the sulfenylation of 3 have been
developed. Recently, we reported that the flavin−iodine-
coupled catalysis promoted the aerobic oxidative sulfenylation
b
and EtOAc under O (1 atm, balloon) at 70 °C for 24 h. 3 equiv of 2
was used.
2
25
Scheme 2. Control Experiments
23a,b
of indole analogues including 3s with thiols.
Based on this
result, we anticipated that the catalytic sulfenylation of 3 with a
thiol would accompany the present imidazo[1,2-a]pyridine
synthesis catalyzed by flavin and iodine. To our delight, the 3-
sulfenylated imidazo[1,2-a]pyridine 6a was obtained in 78%
yield when p-toluenethiol was reacted with 1a and 2a in the
present reaction system; 6a was formed by sequential C−N
and C−S bond formation (Scheme 4). A series of 2-
aminopyridines, ketones, and thiols bearing a variety of
substituents were compatible with this one-pot synthesis
protocol to produce the desired products 6a−6e in good
yields. Although 4·TfO was not the best flavin catalyst for the
23b
simple sulfenylation of 6,
the sulfenylation in the present
three-component synthesis was almost quantitatively promoted
for 24 h. Thiazolyl and thiophenyl ketones were smoothly
transformed into the corresponding products 6g−6h. In
addition to the benzenethiols having electron-donating and
-withdrawing substituents, phenylmethanethiol and alkanethiol
were applied to this reaction to afford the corresponding 3-
thioimidazo[1,2-a]pyridines 6i and 6j in yields of 52%.
nucleophilic substitution by 2 to form adduct 8 and then
cyclodehydrates to form the desired imidazo[1,2-a]pyridine 3.
−
+
The generated I and H are converted to I and H O in this
2
2
flavin-catalyzed system in which the aerobic oxidation of HI
22
proceeds efficiently through oxidation and oxygenation steps.
The one-pot, three-component synthesis of 6 by flavin−
iodine-coupled catalysis proceeds through three aerobic
oxidative transformations, namely the formation of C−N, S−
+
The consumption of H is also the advantageous feature of the
present system, and it could be the reason why the catalytic
imidazopyridine formation smoothly occurred in contrast to
the result using a stoichiometric I2 (entry 6, Table 1).
Consequently, this imidazopyridine synthesis is driven by
molecular oxygen and generates environmentally benign water
as the sole byproduct.
S, and C−S bonds (Scheme 5). In addition to the oxidation of
−
I
to I , this dual catalysis promotes the aerobic oxidative
2
coupling of thiols 5 to form the S−S bonds in the
23a,b
corresponding disulfides 9.
In the presence of I , 9 is
2
then converted into the sufenyl iodide (RSI), which then
undergoes nucleophilic attack by 3 to form the C−S bond in 6.
These oxidative transformations are enabled by the flavin and
In a further set of experiments, we investigated the one-pot,
three-component synthesis of sulfenylimidazo[1,2-a]pyridines
C
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a
Scheme 4. One-Pot Three-Component Synthesis of 6
Experimental procedures and characterization data for
known and new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Hiroki Iida − Department of Chemistry, Graduate School of
Natural Science and Technology, Shimane University, Matsue
6
90-8504, Japan; orcid.org/0000-0002-7114-0364;
Email: iida@riko.shimane-u.ac.jp
Authors
Hayaki Okai − Department of Chemistry, Graduate School of
Natural Science and Technology, Shimane University, Matsue
6
90-8504, Japan
Kazumasa Tanimoto − Department of Chemistry, Graduate
School of Natural Science and Technology, Shimane University,
Matsue 690-8504, Japan
Ryoma Ohkado − Department of Chemistry, Graduate School
of Natural Science and Technology, Shimane University,
Matsue 690-8504, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02929
a
Notes
Conditions: 1 (1 M), 2 (1.5 M), 5 (1.2 M), 4·TfO (5 mol %), I (10
2
b
mol %), and EtOAc under O (1 atm, balloon) at 70 °C for 24 h. 2.5
equiv of 5 was used.
2
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported in part by JSPS/MEXT KAKENHI
Grant-in-Aid for Scientific Research (C), No. 19K05617).
■
(
Scheme 5. Sequential Three Aerobic Oxidative
Transformations Involved in the Catalytic Synthesis of 6
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