One-Pot Synthesis of C3-Alkylated Imidazopyridines from α-Bromocarbonyls under Photoredox Conditions

Article abstract of DOI:10.1002/ejoc.202100922

A convenient strategy is presented for the synthesis of C3-alkylated imidazopyridines through one pot condensation and alkylation of α-bromocarbonyl compounds with 2-aminopyridines. A series of C3-alkylated imidazopyridines were obtained in moderate to hi

Full text of DOI:10.1002/ejoc.202100922

Communications
doi.org/10.1002/ejoc.202100922
One-Pot Synthesis of C3-Alkylated Imidazopyridines from
α-Bromocarbonyls under Photoredox Conditions
Jinwen Tong,[a] Yanling Zhan,[a] Jingyu Li,[a] Ping Liu,*[a] and Peipei Sun*[a]
A convenient strategy is presented for the synthesis of C3-
alkylated imidazopyridines through one pot condensation and
alkylation of α-bromocarbonyl compounds with 2-aminopyr-
idines. A series of C3-alkylated imidazopyridines were obtained
in moderate to high yields. This strategy was also elaborated to
enable the synthesis of zolpidem in short steps, which
otherwise require multistep synthesis.
Imidazo[1,2-a]pyridine is considered as a “drug bias” hetero-
cyclic skeleton since compounds bearing this heterocycle
Figure 1. Representative pharmaceuticals containing imidazo[1,2-a]pyridine
core.
exhibit a wide range of pharmacological activities, such as anti-
inflammatory,[1] anti-ulcer,[2] antiviral activity,[3] anticancer,[4]
etc.[5] A series of marketed drugs containing imidazo[1,2-a]
pyridine core are listed in Figure 1, including zolpidem, alpidem,
saripidem, necopidem, minodronic acid, olprinone, etc, and
most of them are C3-alkylated imidazo[1,2-a]pyridines (Fig-
ure 1). Therefore, developing the efficient synthesis of C3-
alkylated imidazo[1,2-a]pyridines is of great important in
organic synthesis.
Traditional processes involved the construction[6] of imidazo
[1,2-a]pyridines and their C3-[7a–e] or C5-[7f] functionalization. For
example, a classic route to synthesize zolpidem involved seven
steps (Scheme 1). Bromination of 4-methyl acetophenone
followed by the condensation with 5-methyl-2-aminopyridine
gave 6-methyl-2-(p-tolyl)imidazo[1,2-a]pyridine (C). The inter-
mediate N,N-dimethyl amino imidazopyridine derivative D was
obtained from a Mannich reaction of imidazopyridine C,
formaldehyde and dimethylamine. The subsequent reaction of
D and methyl iodide generated quaternary ammonium salt E.
Compound E underwent nucleophilic substitution with sodium
cyanide to give the 3-cyanomethylated imidazopyridine F.[8] In
2009, Satyanarayana and co-workers modified this route to four
stages.[9] However, toxic reagents CH3I and NaCN were still used.
In 2017, to overcome this limitation, our group developed a
visible-light-induced regioselective cyanomethylation reaction
Scheme 1. Synthesis of zolpidem in different routes.
of imidazopyridines using bromoacetonitrile as the cyanometh-
[a] J. Tong, Y. Zhan, J. Li, Dr. P. Liu, Prof. P. Sun
yl source and applied this method to the synthesis of zolpidem
School of Chemistry and Materials Science,
Jiangsu Provincial Key Laboratory of Material Cycle Processes and Pollution
(Scheme 1).[10]
In order to improve the reaction efficiency to synthesize C3-
functionalized imidazo[1,2-a]pyridines, a variety of straightfor-
ward methods such as multicomponent reactions and “one-
pot” reactions were developed in the past decades. In 1998,
Groebke and co-workers made a seminal contribution to three
component reaction of 2-aminopyridines, aldehydes, and iso-
nitriles for the direct formation of 3-amino-substituted imidazo
[1,2-a]pyridines (Scheme 2a).[11a] In 2010, the groups of
Control,
Jiangsu Collaborative Innovation Center of Biomedical Functional Materials,
Nanjing Normal University,
Nanjing 210023, People’s Republic of China
E-mail: sunpeipei@njnu.edu.cn
pingliu@njnu.edu.cn
http://schools.njnu.edu.cn/chem/person/peipei-sun
http://schools.njnu.edu.cn/chem/person/ping-liu
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100922
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hydes and 2-aminopyridines to give 3-alkoxycarbonylated
Imidazopyridines was developed by Chuah et al.[16] Recently,
One pot reaction is also an operationally simple procedure. In
2015, a Cu(OAc)2À Et3N mediated one-pot strategy for the
synthesis of imidazo[1,2-a]pyridine derivatives was reported by
the group of Kamal and Maurya (Scheme 2e).[17] Thomas et al.
reported an one pot protocol for Imidazo[1,2-a]pyridine deriva-
tives formation from 2-aminopyridine and the in-situ generated
α-bromoketone.[18] Encouraged by the above excellent works
and based on our previous research on the synthesis of C3-
functionalized imidazo[1,2-a]pyridines,[10,19] we herein demon-
strated a visible light-induced one pot condensation of α-
bromocarbonyl compounds and 2-aminopyridines cascade the
subsequent C3-alkylation to generate C3-alkylated imidazopyr-
idines (Scheme 2f).
Initially, 2-aminopyridine (1a) and α-bromoacetophenone
(2a) were selected as model substrates to test the feasibility of
the condensation and alkylation reaction (Table 1). Gratifyingly,
the reaction indeed occurred in the presence of fac-Ir(ppy)3
(2 mol%) as the photocatalyst and Et3N (2 equiv.) as a base
under 5 W blue light-emitting diodes (LEDs) bulb irradiation in
DCM at room temperature and gave the C3-alkylated imidazo-
pyridine 3aa in 80% yield (entry 1). However, only a trace
amount of 3aa was observed when this reaction was carried
out in the absence of a base (entry 2). Several other commonly
used bases including NaHCO3 and K2CO3 were then inves-
tigated, and lower yields of 3aa were obtained (entries 1, 3 and
4). As for solvents, DCM was superior to other solvents such as
DCE, EtOAc and DMF (entries 1, 5–7). Then a variety of photo-
catalysts were screened, and the results showed that Ir-
(dtbbpy)(ppy)2(PF6) only resulted in the product 3aa in 43%
Table 1. Optimization of the reaction conditions.[a]
Scheme 2. Synthesis of C3-functionalized imidazo[1,2-a]pyridines via three
component reactions or one pot reactions.
Gevorgyan[12a] and Lin[12b] respectively developed copper-cata-
lyzed three component reaction of 2-aminopyridines, aldehydes
and alkynes to synthesize C3-alkylated imidazo[1,2-a]pyridines
(Scheme 2b). Benzyl cyanide is an operational safe cyanating
reagent. In 2015, Lu, Wang, et al. described a copper-mediated
three-component reaction to access 3-cyanoimidazo[1,2-a]
pyridines from 2-aminopyridines, acetophenones and benzyl
cyanide (Scheme 2c).[13] In 2018, Mohebat, Karimi-Jaberi and co-
worker reported a three component reaction of 2-amino-
pyridines, 2-bromo-1-arylethanones and aryl bromides cata-
lyzed by palladium nanoparticles for the synthesis of 2,3-
diarylimidazo[1,2-a]pyridine derivatives under microwave irradi-
ation (Scheme 2d).[14] In 2020, Singh and co-workers delivered a
visible-light-induced three component reaction of benzylamine,
2-aminopyridine and t-butyl isocyanide for the synthesis of 3-
aminoimidazo[1,2-a]pyridines.[15] Moreover, In(III)/Cu(I) co-cata-
lyzed three component reaction of malonic esters, benzalde-
Entry
Catalyst
Solvent
Base
Yield
[%]
1
2
3
4
5
6
7
8
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
Ir(dtbbpy)(ppy)2(PF6)
4CzIPN
DCM
DCM
DCM
DCM
DCE
EtOAc
DMF
DCM
DCM
DCM
DCM
DCM
DCM
DCM
Et3N
–
80
trace
41
42
55
51
33
43
74
70
0
NaHCO3
K2CO3
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
9
10
11[b]
12[c]
13[d]
14[e]
eosin Y
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
0
34
32
[a] Reaction conditions: 1a (0.2 mmol, 1.0 equiv.), 2a (0.6 mmol,
3.0 equiv.), photocatalyst (2 mol%), base (0.4 mmol, 2.0 equiv.), solvent
(2 mL), irradiated with 5 W blue LEDs (wavelength range 460–480 nm)
under Ar at room temperature for 12 h. [b] No light. [c] No photocatalyst.
[d] irradiated with 5 W white LEDs. [e] Under air.
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yield, while organic photocatalysts such as eosin Y and 4CzIPN
which might be due to the inhibition of oxygen to the
generation of the radical intermediate (entry 14).
displayed comparable catalytic activity in this reaction (en-
tries 8–10). In addition, no desired product was observed in the
absence of visible light irradiation or photocatalyst (entries 11
and 12). Irradiation with white LEDs led to a lower yield of 34%
(entry 13). The formation of product 3aa significantly reduced
when the reaction was carried out under air instead of argon,
With the optimized reaction conditions in hand, the scope
of the 2-aminopyridines were evaluated (Scheme 3). The results
showed that 2-aminopyridines with various substituents at the
C3-, C4- or C5- position reacted smoothly with α-bromoaceto-
phenone to give the corresponding products in moderate to
high yields (3ba–3da, 3fa–3ha). However, this reaction was
relatively sensitive to a substituent at the C6-position of
pyridine ring, which failed to enable the formation of 3ea,
presumably owing to the steric hindrance. In addition, when 2-
aminoquinoline was employed as the reactant, a mixture that
was hard to separate was obtained.
Next, we evaluated various α-bromoketones to examine the
generality and limitations of the reaction for the synthesis of
C3-alkylated imidazopyridines, and the results were summarized
in Scheme 4. Substrates with electron-donating groups (À Me,
À OMe) or electron-withdrawing groups (À F, À Cl, À Br, À CN) on
the para-position of benzene ring could be transferred to the
corresponding products in good to excellent yields (3ab–3ag).
Furthermore, substrates bearing -F or -OMe at the ortho-
position of benzene ring could also provide the desired
products 3ah and 3ai in 63% and 62% yields, respectively.
Moreover, 3,4-dimethoxy or β-naphthyl-substituted substrates
were all tolerated, providing the imidazo[1,2-a]pyridine deriva-
tives 3aj and 3ak in satisfactory yields. In addition, the product
3al bearing thienyl moiety was also obtained in 71% yield.
Interestingly, α-bromocarbonyl compounds 2 and alkyl bro-
mides 4 could be successively added to achieve one pot relay
processes for the facile synthesis of other C3-alkylated imidazopyr-
idines (Scheme 5). Alkyl bromides 4 with electron-withdrawing
Scheme 3. Scope of 2-aminopyridines. Reaction conditions: 1 (0.2 mmol,
1.0 equiv.), 2a (0.6 mmol, 3.0 equiv.), fac-Ir(ppy)3 (2 mol%), Et3N (0.4 mmol,
2.0 equiv.), DCM (2 mL), irradiated with 5 W blue LEDs under Ar at room
temperature for 12 h.
Scheme 5. Scope of various substituents on the alkyl bromides. Reagents
and conditions: 1) The solution of 1 (0.2 mmol, 1.0 equiv.), 2 (0.2 mmol,
1.0 equiv.) and Et3N (0.4 mmol, 2.0 equiv.) in DCM (2 mL) was stirred under
Ar at room temperature for 6 h. 2) fac-Ir(ppy)3 (2 mol%) and alkyl halides 4
(0.3 mmol, 1.5 equiv.) were added to the above mixture and the reaction
mixture was irradiated with 5 W blue LEDs under Ar at room temperature for
12 h.
Scheme 4. Scope of various α-bromoketones. Reaction conditions: 1a
(0.2 mmol, 1.0 equiv.), 2 (0.6 mmol, 3.0 equiv.), fac-Ir(ppy)3 (2 mol%), Et3N
(0.4 mmol, 2.0 equiv.), DCM (2 mL), irradiated with 5 W blue LEDs under Ar at
room temperature for 12 h.
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groups (À F, À Cl, À CN) or electron-donating groups (À Me, À OMe) on
me 6a). Notably, zolpidem could be easily obtained by the
hydrolysis of imidazopyridine ester 5m and subsequent amidation
according to the procedure of Namboothiri[20] and our previous
work[10] (Scheme 6b).
the para-position of aromatic ring could be transferred to the
corresponding products 5a–5e in good to high yields (63–78%).
Bromomethyl 2-naphthyl ketone (4f) were also tolerated well in this
one pot reaction and 62% yield of 5f was obtained. This protocol
was further extended to other alkyl bromides like bromoacetonitrile,
BrCF2COOEt and BrCH2COOEt to enlarge the generality of the
protocol. To our delight, A series of C3-alkylated imidazopyridines
5g–5m were obtained in satisfactory yields (65–78%).
The Stern-Volmer experiment showed that the luminescence of
fac-Ir(ppy)3 could be easily quenched with an increase in the
concentration of α-bromoacetophenone (2a) (Figure 2 and Figure 3,
for details see the Supporting Information). Several control experi-
ments were performed to study the mechanism of this visible-light-
induced condensation and alkylation for the synthesis of C3-
alkylated imidazopyridines (Scheme 7. For details see the Support-
ing Information). 2-Phenylimidazo[1,2-a]pyridine (7) was obtained
through the condensation of 2-aminopyridine and α-bromoaceto-
phenone in the absence of light and a photoredox catalyst
(Scheme 7-(1)). The direct C3-alkylation of 2-phenylimidazo[1,2-a]
pyridine with α-bromoacetophenone took place under the photo-
redox catalysis (Scheme 7-(2)). The above results shown that the
condensation and alkylation processes occurred independently.
Moreover, when a radical scavenge 2,2,6,6-tetramethyl-1-piperidiny-
loxy (TEMPO) was added to the reaction mixture of 2a and 2-
phenylimidazo[1,2-a]pyridine (7) under the same conditions, the C3-
alkylation reaction was completely suppressed, and the TEMPO-
adduct 6a was isolated in 68% yield (Scheme 7-(2)). Meanwhile,
when TEMPO was added to the reaction mixture of 1a and 2a
under the standard reaction conditions, the TEMPO-adduct 6a and
2-phenylimidazo[1,2-a]pyridine (7) could be isolated in 20% and
46% yields respectively (Scheme 7-(3)). These results suggested that
the photocatalytic reaction probably proceeded via a radical
pathway.
To demonstrate the practicality of this one pot protocol, the
reaction was performed on gram-scale with 2-aminopyridine (1a,
0.94 g, 10 mmol) and α-bromoacetophenone (2a, 5.97 g, 30 mmol),
providing the desired product 3aa in 62% yield (1.93 g) (Sche-
Scheme 6. Gram-scale reaction and further transformation of the product
5m to zolpidem.
Based on the above results and related reports, a possible
mechanism of the reaction is proposed in Scheme 8. Initially, the
displacement of the bromine atom of α-bromoacetophenone by
the nitrogen atom of pyridine ring led to the formation of
pyridinium salt H, which was subjected to a ring closure to
construct the imidazopyridine 7.[21] The fac-Ir(III)(ppy)3 was converted
to the excited state [fac-Ir(III)(ppy)3]* upon the irradiation of 5 W
blue LEDs. A single electron transfer (SET) from Ir*(III) to α-
bromoacetophenone resulted in the formation of Ir(IV) and radical
intermediate I.[22] The regioselective addition of the electron-
Figure 2. Emission spectra of fac-Ir(ppy)3 at different concentrations of 2a.
Figure 3. Stern-Volmer plot of fac-Ir(ppy)3 at different concentrations of 2a.
Scheme 7. Control experiments.
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deficient radical I to the electron-rich C3-position of imidazopyridine
7 furnished a new radical J, which was oxidized by the Ir(IV) to
produce the carbocation K and regenerate the Ir(III) photocatalyst
via the second SET process. Finally, the deprotonation of K with the
aid of the base Et3N produced the desired product 3aa.
In summary, we have presented an expedient one pot
condensation and alkylation of 2-aminopyridines with a wide variety
of α-bromocarbonyl compounds for the construction of C3-
alkylated imidazopyridines under visible-light photoredox catalysis.
This new protocol exhibits remarkable features, such as short
synthetic route, readily accessible substrates, operational simplicity,
and mild reaction conditions. This strategy provided an attractive
route for the rapid synthesis of zolpidem and can be extended
further to obtain other C3-functionalized imidazo[1,2-a]pyridine
derivatives.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (Project 21672104, 21502097) and the Priority Academic
Program Development of Jiangsu Higher Education Institutions.
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Conflict of Interest
The authors declare no conflict of interest.
Manuscript received: August 1, 2021
Revised manuscript received: August 5, 2021
Accepted manuscript online: August 6, 2021
Keywords: α-Bromocarbonyls · C3-Alkylated imidazopyridines ·
Photoredox catalysis · Visible light · Zolpidem
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