Micellar Catalysis: Visible-Light Mediated Imidazo[1,2-a]pyridine C—H Amination with N-Aminopyridinium Salt Accelerated by Surfactant in Water

Article abstract of DOI:10.1002/cjoc.202100489

A light-promoted metal-free protocol for the amination of imidazo[1,2-a]pyridines with N-aminopyridinium salt by the assistance of surfactants in water was reported, charactering mild and environmentally benign conditions, as well as great functional grou

Full text of DOI:10.1002/cjoc.202100489

Chinese Journal
of Chemistry
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中 国 化 学 - An International Journal
Accepted Article
Title: Micellar Catalysis: Visible-light mediated imidazo[1,2-a]pyridine C-H
amination with N-aminopyridinium salt accelerated by surfactant in water
Authors: Zhonglie Yang, Kun Cao, Xiaoyan Peng, Li Lin, Danchen Fan, Jun-Long Li,
Jingxia Wang, Xiaobin Zhang, Hezhong Jiang, Jiahong Li*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2021, 39, 10.1002/cjoc.202100489.
The final Version of Record (VoR) of it with formal page numbers will soon be
published online in Early View: http://dx.doi.org/10.1002/cjoc.202100489.
ISSN 1001-604X • CN 31-1547/O6
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Li Jiahong (Orcid ID: 0000-0003-2532-6480)
Cite this paper: Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
Micellar Catalysis: Visible-light mediated imidazo[1,2-a]pyridine C-H amination
with N-aminopyridinium salt accelerated by surfactant in water
Zhonglie Yang,a Kun Cao,a Xiaoyan Peng,a Li Lin,a Danchen Fan, a Jun-Long Li,b Jingxia Wang,c Xiaobin Zhang,c
Hezhong Jiang, a Jiahong Li*a
a
Sichuan Engineering Research Center for Biomimetic Synthesis of Natural Drugs, School of Life Science and Engineering, Southwest
Jiaotong University, Chengdu 610041, China.
b Antibiotics Research and Re-evaluation Key Laboratory of Sichuan Province, Sichuan Industrial Institute of Antibiotics, School of Phar-
macy, Chengdu University, Chengdu, 610106, China.
c Irradiation Preservation Technology Key Laboratory of Sichuan Province, Sichuan Institute of Atomic Energy, Chengdu 610101, China.
Keywords
Visible-light | Water | Micellar catalysis | N-aminopyridinium salt | Imidazo[1,2-a]pyridine
Main observation and conclusion
A light-promoted metal-free protocol for the amination of imidazo[1,2-a]pyridines with N-aminopyridinium salt by the assis-
tance of surfactants in water was reported, charactering mild and environmentally benign conditions, as well as great functional
group tolerance. Micelles with negative charge polar surface and hydrophobic core formed from sodium dodecyl sulfate serve
as an ideal medium for visible-light mediated radical reaction of cationic pyridine salt and midazo[1,2-a]pyridine in aqueous
phase. The electrostatic interaction between positively charged of N-aminopyridinium and negatively charged on the micelles is
of great significance in this method.
Comprehensive Graphic Content
*E-mail: jiahongljh@163.com.
View HTML Article
Supporting Information
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
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through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.202100489
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Running title
Chin. J. Chem.
Background and Originality Content
As
a high-frequent and key structural motify in
N-heteroarenes, imidazo[1,2-a]pyridine has extensively ex-
isted in many pharmaceutical, such as zolpidem, zolimidine,
olprinone, saripidem, alpidem, and minodronic acid, etc,[1]
showing anticancer, anti-inflammatory, antiviral, and other
biological activities.[2] Due to the importance of this promising
scaffold, great achievements for the building of various func-
tionalized imidazo[1,2-a]pyridines have been made in recent
years (Scheme 1, (1)).[1a, 2a-c, 3] However, the amination of
3i-k, 3m, 3p]
imidazo[1,2-a]pyridines is rare.[3g,
Despite these
Figure 1 Micellar free radical reactions involving ionic substrates
achievements, developing methodologies with mild and sus-
tainable conditions using new N-radical for the selective C−H
functionalization of imidazo[1,2-a]pyridines is still highly de-
sired.
Studies have shown that micelles formed by self-assembly
of ionic surfactants with polar hydrophilic surfaces and hy-
drophobic cores can be used as an ideal media for visible
light reactions.[6e, 6f, 7a, 8] In the hydrophilic area of micelles,
the charged surface can adsorb ionic substrates with oppo-
site charges, and the hydrophobic core can dissolve
fat-soluble catalysts and substrates, which greatly increase
both the ionic and hydrophobic compounds in micelles.
What’s more, the micelle effect may activate the substrates,
especially the hydrophobic interaction.[7c, 7d] Thus, with these
effects in micelle, the reaction between ionic and hydropho-
bic substrates could be processed well in water under visi-
ble-light conditions (Figure 1). Hence, a green protocol that
combines visible-light as energy and water as a solvent to
realize C-H amination of imidazole compounds has been
proposed (Scheme 1, (3)), accomplishing the reaction be-
tween the polar cationic N-aminopyridinium salts amino rad-
ical precursor and nonpolar imidazo[1,2-a]pyridines in water
for the first time.
In the meantime, as a novel radical precursor, pyridinium
salt has been widely used for regioselective functionalization
of substituted arenes and heteroarenes for the past few
years (Scheme 1, (2)).[4] Significantly, N-aminopyridinium salts
were used for introduce amino groups for compounds under
the irradiation of visible-light.[5] However, all of these reac-
tions in relate to aminopyridinium salts were conducted only
in organic phase, and there was no one reported in green
solvent water.
Though water as a solvent bears many merits, such as be-
ing safe, nontoxic, environmentally benign, and inexpensive,
etc, but the big problem is the insolubilization of substrates
and catalysts.[6] Our laboratory’s long-term enrichment of the
water phase reaction experience shows that the matter of
mass transfer from insoluble substrates and catalyst in water
can be settled by introducing amphiphilic surfactant into wa-
ter to form aggregates as micro reactors.[7]
Results and Discussion
Table 1 Optimization of the reaction conditionsa
Scheme 1 Selective C−H functionalization of imidazo[1,2-a]pyridines
Photocatalyst (2 mol%)
5 W Blue LEDs
N
Previous work
N
Surfactant
N
+
N
C-H Functionalization
-
N
N
N
N
(1)
(2)
BF4
N
H
, RT
2O, N2
N
R
N
Boc
Boc
R
2a
1a
H
3a
organic solvents
Diversity synthesis
R
hv
R
N
R
R
2
F
F
N
N
tBu
N
Ir
N
NC
N
CN
R
N
N
N
N
N
N
N
This work
F
3C
F
Ir
Ru
N
N
N
hv
N
N
N
N
N
+
(3)
N
R
N
N
R
tBu
-
PF6
water
2Cl-
F
CF3
N
R
R
Ru(bpy)3Cl2
fac-[Ir(ppy)3
4CzIPN
]
[Ir{dF(CF3)ppy}2(dtbpy)](PF6)
Yield
(%)b
Entry
Photocatalyst
Ir(ppy)3
Surfactant (equiv)
1
2
SDS
SDS
72
(Ir[dF(CF3)ppy]2
66
Chin. J. Chem. 2021, 39, XXX－XXX
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Table 2 Scope of imidazo[1,2-a]pyridinea, b
(dtbpy))PF6
Ru(bpy)3Cl2
4CzIPN
3
4
5
6
7
SDS
SDS
Trace
86
4CzIPN (2 mol%)
5 W Blue LEDs
N
N
4CzIPN
SDBS
72
Ar
+
N
Ar
-
N
N
SDS
N
BF4
R1
N
R1
Boc
Boc
H
, RT
2O, N2
4CzIPN
Sodium laurylsulfonate
Bromohexadecyl pyridine
56
1
2a
3
4CzIPN
NR
Boc
Boc
Boc
N
N
N
N
N
N
Dihexadecyldime-
thylammonium bromide
8
4CzIPN
NR
N
N
N
O
3a, 86%
3b, 82%
Boc
3c, 78%
Boc
9
4CzIPN
4CzIPN
CTAB
NR
32
O
N
N
Boc
N
Cl
O
N
N
N
10
Tween 80
N
N
N
3e, 86%
3d, 76%
3f, 80%
Boc
Polyoxyethylene lauryl
ether
11
4CzIPN
36
Boc
N
Boc
N
N
N
N
N
Ph
N
N
N
F3C
12
13
4CzIPN
None
None
SDS
trace
trace
NR
F
3i, 80%
3h, 81%
3g, 66%
Boc
N
Boc
N
Boc
N
N
HO
N
N
N
N
O
14c
15d
16
4CzIPN
4CzIPN
4CzIPN
4CzIPN
SDS
N
N
N
3l, 88%
Boc
3j, 82%
3k, 88%
SDS
trace
66
Boc
N
Boc
N
N
N
Br
CF3
SDS(0.25)
SDS(0.5)
CN
N
N
N
3n, 72%
3m, 88%
3o, 85%
17
48
Boc
N
Boc
N
Boc
N
Cl
N
F
a
Reaction conditions (unless otherwise stated): 0.2 mmol of 1a, 1.2
eq of 2a, 2 mol% of photocatalyst, 0.15 eq surfactant in water 4 mL,
5 W Blue LEDs, at N2 atmosphere and room temperature reaction for
24 h; NR = No reaction; CTAB = Cetyltrimethylammonium bromide;
SDS = Sodium dodecyl sulfate; SDBS = Sodium dodecylbenzene sul-
fonate. b Isolated yield. c Without light. d At air atmosphere.
N
N
N
N
N
3r, 73%
3q, 76%
3p, 73%
Boc
N
Boc
N
N
Boc
N
S
N
N
N
N
S
N
3u, 77%
3t, 87%
3s, 77%
Boc
N
Boc
N
To test our hypothesis, we initiated our exploration by employ-
ing 2-phenylimidazo[1,2-a]pyridine 1a and N-aminopyridinium 2a
as the model substrates under the atmosphere of N2 and the irra-
diation of 5 W blue LEDs at room temperature (Table 1). Firstly,
N
N
S
N
S
N
3w, 86%
3v, 88%
several
photocatalysts
including
4CzIPN,
Ir(ppy)3,
a Reaction conditions (unless otherwise stated): 0.2 mmol of
1
, 1.2 eq
(Ir[dF(CF3)ppy]2(dtbpy))PF6, and Ru(bpy)3Cl2 were explored in the
presence of SDS (entries 1−4). We were pleased to find that
4CzIPN was the best photocatalyst, delivering the desired product
3a in excellent isolated yield (86%, entry 4). Then, several surfac-
tants including both nonionic and ionic surfactants were screened
(entries 4-11). All the anionic surfactants have achieved good re-
sults (entries 4-6), and the SDS gave the best results (entriy 4). It is
worth noting that cationic surfactants worked poorly due to the
electrostatic repulsion between positive charge surface and
N-aminopyridinium salt (entries 7-9), which lowered the concen-
tration of substrates 2a in micelles. Lower yield was given when
nonionic surfactants were used (entries 10, 11). The yield was not
increased when the concentration of SDS was up to 0.25 or 0.5 eq
(entries 16-17). Expectedly, desired product was not given in the
absence of photocatalyst, surfactant, light, or N2 (entries 12-15).
of 2a, 2 mol% of 4CzIPN, 0.15 eq SDS in water 4 mL, 5 W Blue LEDs,
at N2 atmosphere and room temperature reaction for 24 h. b Isolated
yield.
With optimized conditions in hand, we next explored the sub-
strate scope of this protocol. As shown in Table 2, substituents 1
(−CH3, −OMe, −CH2CH3, −COOCH3 −F, −Cl, −CF3) on the pyridyl ring
could proceeded smoothly with N-aminopyridinium 2a, leading to
the target products 3a−3h in moderate to good yields (66%−86%).
Furthermore, 2-aryl imidazo[1,2a]pyridines
1 bearing elec-
tron-donating groups (−CH3, −OMe, −OH) or electron-withdrawing
groups (−CN, −CF3, −Br, −F, −Cl) all reacted smoothly to give the
target amination imidazo[1,2-a]pyridines 3i−3q with good yields
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Running title
Chin. J. Chem.
(72−88%) in phenyl moiety. It should be noted that imid-
azo[1,2-a]pyridines substituted by heterocyclic groups could still
proceed well with the optimized conditions (3r-3t). To our delight,
the corresponding thiomethylated products (3u-3w) could be ob-
tained in satisfactory yields.
N
4CzIPN (2 mol%)
5 W Blue LEDs
N
N
-
N
N
(1)
+
N
BF4
SDS, H
, RT
2O, N2
N
Boc
3a (86%)
Boc
1a
standard conditions
2a
without light
without SDS
+
+
1a
1a
2a
2a
(2)
(3)
3a (0%)
Table 3 Scope of N-aminopyridinium salt.a, b
3a (trace)
without 4CzIPN
+
1a
2a
(4)
(5)
3a (trace)
R1
+
+
without N2
1a
1a
2a
2a
3a (0%)
4CzIPN (2 mol%)
5 W Blue LEDs
SDS
N
N
+
additional BHT(2.0 eq)
R1
R3
N
N
R1
-
BF4
R2
N
N
(6)
(7)
3a (16%)
R2
N
R3
4
H
, RT
2O, N2
additional TEMPO(2.0 eq)
1a
+
2
1a
2a
3a (0%)
O
-
2c R = H
2d R = CH3
2e R = CF3
52%
82%
64%
BF4
N
N
-
N
-
N
BF4
BF4
N
HN
Boc
In order to explore the mechanism of the reaction, several con-
trol experiments were accomplished (Scheme 2). The results of (1)
and (3) indicated that micellar catalysis was involved. These re-
sults showed that photocatalyst (4), surfactant (3), light (2), and N2
(5) were the necessary conditions. Moreover, this transformation
could be terminated completely by introducing TEMPO
(2,2,6,6-tetramethylpiperidin-1-oxyl) (7), suggesting the reaction
was proceeded via radical pathway. And also the addition of BHT
(butylated hydroxytoluene) gave rise to a sharp decrease of the
yield (16%) (6).
Boc
R
2a 86%
2b 56%
-
2f R = H
86%
90%
92%
BF4
O
S
-N
N
BF4
R
O
S
-N
2g R = CH3
2h R = C6H13
N
N
BF4
O
S
S
N
O
O
O
R
2j R = Et 82%
2k R = Bn 58%
2i 82%
unsuccessful substrates
Ph
N
Ph
Ph
-
-
-
N
N
CO2CF3
CF3
N
S
SO2CF3
O
BF4
N
-
Based on the above investigations and previous reports,[3a, 4, 7a,
BF4
Ts
N
O
O
HN
Ts
CF3
7d]
O
a possible mechanism was provided (Figure 2). As shown in
Figure 2a, as an excellent precursor of N-radical, water-soluble
N-aminopyridinium salt with cationic polar structure could be
embedded on the polar surface of micelles with negative charge
due to electrostatic attraction, which greatly increased the con-
centration of pyridinium salt in micelles. Simultaneously, the con-
centration of all nonpolar reactants, including imid-
azo[1,2-a]pyridine and photocatalyst, in the hydrophobic core of
micelles significantly increased due to the hydrophobic interaction
in the core of micelle.
a
Reaction conditions (unless otherwise stated): 0.2 mmol of 1a, 1.2
eq of 2, 2 mol% of 4CzIPN, 0.15 eq SDS in water 4 mL, 5 W Blue LEDs,
at N2 atmosphere and room temperature reaction for 24 h. b Isolated
yield.
Next, we commenced exploring the scope of pyridinium salts 2.
As it is seen in Table 3, the amidyl radicals (RSO2N•R1, RCON•Me,
tBuOCON•H)
stemmed
from
the
corresponding
N-aminopyridinium salts involved in the amination to give the
desired imidazo[1,2-a]pyridines (2a-2k) in moderate to excellent
yields. Interestingly, compared with pyridinium salts 2a, the sub-
strate 2b without methyl substitution gave a lower yield, indicating
that the methyl substitution on nitrogen may have the function of
stabilizing N-radicals. It is worth mentioning that there are some
pyridinium salts showing no activity with this method, as shown in
Table 3.
Thus, with this micelle effect, N-radical could be generated eas-
ily under the visible-light irradiation in water, followed by a pho-
toredox process in micelle to finish the follow-up reaction. The
Figure 2b showed the detailed process. Firstly, 4CzIPN is excited by
blue light to form 4CzIPN*. Pyridinium salt 2a is reduced by excit-
ed species 4CzIPN*, giving N-radical A and pyridine B. Then,
N-radical A selectively attacks imidazo[1,2-a]pyridine to afford
radical C. After SET oxidation of C, cationic species D was given,
followed by deprotonating to yield the final product 3a.
Scheme 2 Mechanism study.
Chin. J. Chem. 2021, 39, XXX－XXX
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First author et al.
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tracted with EtOAc and dried over anhydrous Na2SO4, after re-
moval of EtOAc under vacuum, crude mixture left out was purified
by column chromatography (silica gel 300-400 mesh size) using
petroleum ether and ethyl acetate, the product 3a was isolated.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2021xxxxx.
Acknowledgement
We thank National Natural Science Foundation of China (Grant
No. 21901216 and 21871031), the Fund of Science and Technology
Agency of Chengdu (2019-YF09-00049-SN), the Open Project Pro-
gram of Irradiation Preservation Technology Key Laboratory of
Sichuan Province, Sichuan Institute of Atomic Energy (No.
FZBC2020002), and the Sichuan Science and Technology Program
(2020YFH0061, 2020YJ0222, 2021JDRC0018, 2021YFS0092,
2021YFSY0041, 2021YFH0064).
(a) Micellar catalysis
+
N
Boc
N
B
N
A
N
N
Boc
2a
N
1a
4CzIPN*
4CzIPN
N
N
N
Boc
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Conclusions
In conclusion, we have firstly reported a versatile and environ-
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Experimental
In a reaction tube equipped with a magnetic stir bar was
charged
with
40
mg
(0.20
mmol)
of
2-phenylimidazo[1,2-a]pyridine 1a, 83.5 mg (0.24 mmol) of
N-aminopyridinium 2a, 9.2 mg of sodium dodecyl sulfate (SDS)
and 3.2 mg (2 mol %) 4CzIPN were added in 4 mL of water at room
temperature, N2 atmosphere, under irradiation with 5W blue LED
for 24 h. After completion of the reaction, the mixture was ex-
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Running title
Chin. J. Chem.
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2021
Manuscript revised: XXXX, 2021
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Manuscript accepted: XXXX, 2021
Accepted manuscript online: XXXX, 2021
Version of record online: XXXX, 2021
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Chin. J. Chem. 2021, 39, XXX－XXX

Entry for the Table of Contents
Micellar Catalysis: Visible-light mediated imidazo[1,2-a]pyridine C-H amination with N-aminopyridinium salt accelerated by surfactant in water
Zhonglie Yang,a Kun Cao,a Xiaoyan Peng,a Li Lin,a Danchen Fan,a Jun-Long Li,b Jingxia Wang,c Xiaobin Zhang,c Hezhong Jiang,a Jiahong Li*a
Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
a Department, Institution, Address 1
E-mail:
c Department, Institution, Address 3
E-mail:
b Department, Institution, Address 2
E-mail:
Chin. J. Chem. 2018, template
© 2018 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim