Chemodivergent synthesis of: N -(pyridin-2-yl)amides and 3-bromoimidazo[1,2- a] pyridines from α-bromoketones and 2-aminopyridines

Article abstract of DOI:10.1039/c9ra06724h

N-(Pyridin-2-yl)amides and 3-bromoimidazo[1,2-a]pyridines were synthesized respectively from α-bromoketones and 2-aminopyridine under different reaction conditions. N-(Pyridin-2-yl)amides were formed in toluene via C-C bond cleavage promoted by I₂

Full text of DOI:10.1039/c9ra06724h

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Chemodivergent synthesis of N-(pyridin-2-yl)
amides and 3-bromoimidazo[1,2-a]pyridines from
a-bromoketones and 2-aminopyridines†
Cite this: RSC Adv., 2019, 9, 34671
a
Yanpeng Liu,ab Lixue Lu,a Haipin Zhou,c Feijie Xu,a Cong Ma, d Zhangjian Huang,
a
a
*
*
Jinyi Xu
and Shengtao Xu
N-(Pyridin-2-yl)amides and 3-bromoimidazo[1,2-a]pyridines were synthesized respectively from a-
bromoketones and 2-aminopyridine under different reaction conditions. N-(Pyridin-2-yl)amides were
formed in toluene via C–C bond cleavage promoted by I2 and TBHP and the reaction conditions were
mild and metal-free. Whereas 3-bromoimidazopyridines were obtained in ethyl acetate via one-pot
tandem cyclization/bromination when only TBHP was added, the cyclization to form imidazopyridines
was promoted by the further bromination, no base was needed, and the versatile 3-
bromoimidazopyridines could be further transferred to other skeletons.
Received 26th August 2019
Accepted 21st October 2019
DOI: 10.1039/c9ra06724h
rsc.li/rsc-advances
another developing approach to synthesize amides, but the
excellent stability of C–C bond makes this subject develop
Introduction
slowly, exploring new methods for constructing amides directly
by C–C bond cleavage is attractive. N-(Pyridin-2-yl)amide was
previously synthesized via a Cu-catalyzed dehydrogenative
reaction between aldehyde and aminopyridine in the presence
of I2 (Scheme 1a).7 Adimurthy's group synthesized N-(pyridin-2-
yl)amides via oxidative amidation of methylarenes using TBHP
in decane.8 Recently, two elegant works on Cu-catalyzed aerobic
oxidative C–C bond cleavage for the preparation of N-(pyridin-2-
yl)amide had been developed. Hwang et al. reported a visible-
light-promoted aerobic oxidative C–N coupling between 2-ami-
nopyridine and terminal alkynes through C^C triple bond
cleavage (Scheme 1b).9 Kaliappan disclosed a biomimetic
oxidation of methyl ketones for preparation of N-heterocyclic
amides (Scheme 1c).10 However, these methodologies suffer
from several drawbacks, such as harsh reaction conditions, long
reaction time and the use of transition metal complexes.
Recently, we reported a metal-free method for synthesizing N-
(pyridine-2-yl)amides from ketones via oxidative cleavage of C–C
Aminopyridines serve as pharmacophores for many molecules
with signicant biological and therapeutic value.1 Particularly,
N-(pyridin-2-yl)amides2 and imidazo[1,2-a]pyridines3 have
received great attention in recent years due to their varied
medicinal applications. Some examples of pharmaceutical
molecules containing N-(pyridin-2-yl)amides and imidazo[1,2-a]
pyridines are shown in Fig. 1. Owing to the attractive biological
properties of N-(pyridin-2-yl)amides and imidazo[1,2-a]pyri-
dines, developing convenient synthetic methods, especially by
which we can synthesize these two kinds of structures respec-
tively from the same starting materials, is very meaningful.
Amides are ubiquitous in organic compounds, polymers,
natural products, and pharmaceuticals.4 Synthesis of amides is
one of the most executed reactions in organic chemistry.5 Pre-
activated acyl halides or anhydrides are used as starting mate-
rials in traditional methods for constructing amide bond.
Recently, many alternative substrates, such as aldehydes, alco-
hols, azides, aldoximes, nitriles and halides compounds had
been employed for amide synthesis.6 C–C bond cleavage is
aState Key Laboratory of Natural Medicines and Department of Medicinal Chemistry,
China Pharmaceutical University, 24 Tong Jia Xiang, Nanjing 210009, China. E-mail:
jinyixu@china.com
bDepartment of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Hong
Kong SAR, P. R. China
cCollege of Materials & Chemical Engineering, Chuzhou University, Chuzhou 239000,
China
dState Key Laboratory of Chemical Biology and Drug Discovery, Department of Applied
Biology and Chemical Technology, The Hong Kong Polytechnic University, Kowloon,
Hong Kong SAR, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06724h
Fig. 1 Some pharmaceutical molecules based on pyridyl-amide and
imidazopyridine.
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Scheme 1 Recent methods for the construction of pyridyl-amides and halosubstituted imidazopyridines.
bond in water.11 Herein, we will continue to report the selective developed a method to synthesize N-(pyridin-2-yl)amides and
synthesis of N-(pyridine-2-yl)amides from a-bromoketones and imidazo[1,2-a]pyridines respectively from a-bromoketones and
2-aminopyridine via controlling reaction conditions.
2-aminopyridine by controlling reaction conditions (Scheme 1).
Intensive researches have been conducted to develop new
and efficient strategies for constructing imidazo[1,2-a]pyri-
dines.12 Halo-substituted imidazo[1,2-a]pyridines are important
building blocks and versatile synthons for preparing more
complex imidazopyridines.13 Classically, halo-substituted imi-
dazo[1,2-a]pyri-dines are synthesized by the halogenation of
imidazo[1,2-a]pyridines, such as Adimurthy's report.14 for the
synthesis of the intermediate imidazo[1,2-a]pyridines, base is
generally needed.15 While methods for direct synthesis of halo-
substituted imidazo[1,2-a]pyridines in one-pot reaction without
base were rarely reported. Jiang and co-workers reported
Results and discussion
Traditionally, C–C bonds cleavage has been accomplished in
the presence of transition-metal catalyst.20 Initially, various
metal catalysts were screened for the preparation of amide 3aa
from a-bromoacetophenone 1a and 2-aminopyridine 2a using
70% aqueous tert-butyl hydroperoxide (TBHP) as oxidant in
toluene, but the yield of 3aa was not pleasurable and the highest
yield was only 17% in the presence of CuI (Table 1, entry 1).
Surprisingly, aer extensive screening, the desired amide 3aa
was obtained solely with yields of 83% and 79% respectively
when I2 or KI were used as the catalyst (Table 1, entries 4, 5).
This transformation was sensitive to the types of the solvents
and oxidants (Table 1, entries 6–8). Finally, the optimized
reaction conditions for the synthesis of amides were obtained
as follows: TBHP (4 equivalents) and iodine (20 mol%) in
toluene at 100 ꢀC for 2 hours (protocol A, Table 1, entry 9).
Surprisingly, the 3-bromoimidazopyridine 4aa was isolated with
good yield when just using TBHP as oxidant in toluene (Table 1,
entry 12). To further improve the yield of 4aa, several solvents
were screened, and the results showed that ethyl acetate was the
most effective solvent for the preparation of 4aa (Table 1, entry
14). Other oxidants such as H2O2 and oxygen (balloon) were also
evaluated, however, the results were not pleasurable (entries 15–
16). Aer extensive screening, 4aa was successfully obtained in
yield of 93% ꢀusing 2 equivalents of TBHP as oxidant in ethyl
acetate at 90 C for 3 hours (protocol B, Table 1, entry 17).
With the optimized reaction conditions in hand, rst, the
scope of the oxidative amidation approach was examined with
a range of a-bromoketones and aminopyridines. As shown in
a
copper-catalyzed method for the synthesis of 2-halo-
substituted imidazopyridines from haloalkynes and amino-
pyridines (Scheme 1d).16 3-Iodoimidazopyridines were synthe-
sized by Zhao and co-workers in the presence of copper
supported on manganese oxide-based octahedral molecular
sieves OMS-2 (CuOx/OMS-2) via tandem cyclization/iodination
(Scheme 1e).17 Yan and Huang's group reported copper
bromide-mediated aerobic oxidative synthesis of 3-bromoimi-
dazo[1,2-a]pyridines with pyridines and enamides (Scheme
1f).18 Besides, metal-free tandem chlorocyclization of 2-amino-
pyridines with carboxylic acids to synthesize chloro-substituted
imidazopyridines was also described by Zeng's group.19
However, these transformations over-relied on metal catalysts
or stoichiometric excess of halogen sources. From the viewpoint
of green chemistry, developing an atom-economical and metal-
free approach to construct halo-substituted imidazo[1,2-a]pyri-
dine is highly desirable.
Given the importance of N-(pyridin-2-yl)amides and imidazo
[1,2-a]pyridines, chemodivergent synthesis of these two skele-
tons from same starting materials is very appealing. Herein, we
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Table 1 Optimization of the reaction conditionsa
Yieldc (%)
Entry
Solvent
Oxidantb (4 eq.)
Catalyst (20 mol%)
Temp (ꢀC)
t (h)
3aa
4aa
1
2
3
4
5
6
7
8
Toluene
Toluene
Toluene
Toluene
Toluene
DMF
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
TBHP
—
—
TBHP
TBHP
TBHP
H2O2
O2 (ballon)
TBHP (2 eq.)
CuI
CuBr
ZnBr2
I2
KI
I2
I2
I2
I2
—
I2
—
—
—
—
—
—
120
120
120
120
120
120
100
120
100
120
120
120
100
80
36
36
36
2
2
2
12
2
2
2
2
3
8
3
5
3
3
17
3
—
83
79
Trace
49
—
84
—
—
2
11
Trace
—
—
11
73
5%
Trace
Trace
41
3
28
Trace
—
—
83
—
83
36
—
93
H2O
Toluene
Toluene
Toluene
Toluene
Toluene
H2O
Ethyl acetate
Ethyl acetate
Ethyl acetate
Ethyl acetate
9
10
11
12
13
14
15
16
17
80
80
90
Trace
a
Reaction conditions: 1a (0.3 mmol), 2a (0.45 mmol), oxidant (1.2 mmol), catalyst (0.06 mmol), solvent (2 mL). b 70% TBHP in water and 30% H2O2
in water were used. c Isolated yields.
Scheme 2, both the electron-rich (–Me, –OMe) and electron- bromoacetone and ethyl bromopyruvate, could afford the
withdrawing (–CF3) groups on a-haloketones were well toler- desired products in good (4qa) or moderate (4ra) yield respec-
ated and produced the corresponding amides in good yields. tively. Next, we investigated the suitability of 2-aminopyridines
Halo-substituted a-bromoketones can also afford desired and their heteroaromatic analogues. The results showed that 2-
amides in moderate to good yields (3ca, 3da, 3ka, 3la). The steric aminopyridines with substituents at the C-3, C-4, C-5 position
factor has a slight effect on this transformation, as the meta- reacted smoothly to give their products in moderate yields (4ab–
substituted a-bromoketones gave their corresponding prod- 4ad, 4af, 4ag). This reaction is highly sensitive to the presence of
ucts in moderate yields (3ga, 3ha). Besides, heterocycle- and a substituent at the C-6 position of pyridine ring attributed to
alkyl-substituted a-haloketones were also tested and gave cor- steric hindrance (4ae).21 Besides, other ortho-amino-nitrogen-
responding amides in 70–87% yields (3ma, 3na). Subsequently, containing heteroaromatic derivatives were also investigated,
the substrate scope of various 2-aminopyridines was investi- and the results demonstrated that isoquinolin-1-amine was
gated and the results showed that most of the substituted 2- tolerated in this cascade transformation and could be trans-
aminopyridines could be transferred to their corresponding ferred to products with good to excellent yields, while
products in moderate to good yields.
pyrimidin-2-amine (4ai) and 2-aminothiazole (4aj) were not suit
Then, we evaluated various a-bromoketones 1 to examine the well for this reaction.
generality and limitations of the reaction for the synthesis of 3-
3-Iodoimidazo[1,2-a]pyridine could also be prepared using
bromoimidazopyridines, and the results was summarized in this methology in moderate yield (Scheme 4a). Moreover, the
Scheme 3. Substrates with electron-withdrawing groups (–CF3), synthetic utility of 3-bromoimidazo[1,2-a]pyridines were explored,
electron-donating groups (–Me, –OMe) or halogen groups (–F, C3-arylation and alkynylation reactions processed smoothly
–Cl, –Br) on the para position of aromatic ring could be trans- through a classical Pd-catalyzed cross-coupling strategy (Schemes
ferred to the corresponding products in good to excellent yields 4b and c). Overall, this simple, metal-free protocol could be
(4ba–4ga). Furthermore, substituents at the meta or ortho extended as an efficient and practical method to construct more
positions could also provide the corresponding products with complex C-3-substituted imidazopyridines.
good yields (4ha–4ma). Disubstituted or trisubstituted a-bro-
To understand the mechanism of this tandem reaction,
moacetophenones were all tolerated well (4ha, 4oa). The control experiments were carried out (Scheme 5). 5aa was isolated
product 4pa bearing heteroaryl moiety was also obtained in 97% in both reactions (Scheme 5, 1). In protocol A, the reaction was
yield.
Some
alkyl-substituted
a-bromoketones,
like suppressed remarkably when 2 equivalents of TEMPO, a radical
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Scheme
3
Constructing 3-bromoimidazo[1,2-a]pyridines directly
Scheme 2 Constructing amides 3 directly from a-haloketones 1 and
2-aminopyridines 2. aReaction conditions: 1 (0.3 mmol), 2 (0.45 mmol),
TBHP (1.2 mmol), I2 (0.06 mmol), toluene (2 mL), 100 ꢀC, 2 h.
from a-haloketones 1 and 2-aminopyridines 2. aReaction conditions: 1
(0.3 mmol), 2 (0.45 mmol), TBHP (0.6 mmol), EA (2 mL), 90 ꢀC, 3 h.
reaction.22 As shown in Scheme 6, the reaction proceeded via
the initial displacement of the halogen atom by the pyridine
ring nitrogen to afford pyridinium salt (8), which subjected
a facile closure and provided the shared intermediate 5aa. In
protocol A, t-BuOc, which was generated in I2/TBHP system,23
attacked the double bond of 5aa to afford 9, 9 subsequently
transferred to 11. Then, oxygen radical 12 was formed under t-
trapping reagent, was added into the reaction system, indicating
that the reaction might proceed via a free radical process
(Scheme 5, 1). We speculated that 5aa was the key intermediate
for these two reactions. Thus, some control experiments based on
5aa were conducted. 4aa was obtained in 42% yield when 5aa was
subjected to HBr (1 equivalent) and TBHP (2 equivalents)
(Scheme 5, 4) and 5aa reacted with Br2 could also form 4aa in
88% yield (Scheme 5, 5), these results showed that the bromine
was formed during the synthesis of 3-bromoimidazo[1,2-a]pyri-
dines. Furthermore, when 1-cyclohexen-1-ylbenzene was added
to consume the in situ generated bromine, no desired 3-bromo-2-
phenylimidazo[1,2-a]pyridine was observed (Scheme 5, 6). On the
other hand, the amide product 3aa could be obtained from 5aa in
I2/TBHP system (Scheme 5, 2) or only TBHP for longer time
(Scheme 5, 3). Besides, no amide product (3aa) was obtained
from 3-bromo-imidazopyridine (4aa) or 3-iodoimidazopyridine
(4sa), which illustrated that 4aa could not be further transferred
to 3aa. These results collectively indicated that there was a erce
competition between amidation and bromination, the iodine
played an extremely important role in this C–C bond cleavage
reaction.
Based on the above results and previous knowledge, we
proposed a plausible mechanism for this chemodivergent
Scheme 4 Investigation of the versatility of this strategy.
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bromoketones and 2-aminopyridine. Various N-(pyridin-2-yl)
amides or 3-bromo-imidazopyridines were obtained in
moderate to good yields with high selectivity by tuning the
catalyst. The one-pot tandem cyclization/bromination of a-
bromoketones with 2-aminopyridine in the presence of TBHP
provided versatile 3-bromo-imidazopyridines with excellent
functional group tolerance and scalability. The a-haloketone
acted as both the substrate and the bromo source, and the
avoidance of transition-metal, fewer synthetic steps and high
atom-economy make this protocol attractive. Whereas, the
addition of I2 promote the oxidative C–C bond cleavage ami-
dation afforded N-(pyridin-2-yl)amides in a metal-free condi-
tion. Less activity demonstrated by other metal catalysts
suggested that I2 played a vital role in this reaction. C–C bond
cleavage amidation is still a difficult and signicant subject
especially in modication and semisynthesis of natural medi-
cine, which can simplify the synthetic procedure and make the
semisynthesis more available. Iodine as a catalyst in C–C bond
cleavage amidation is rarely reported. We believe our work will
be useful for completing the C–C bond cleavage amidation eld.
Scheme 5 Control experiments.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work is supported by the National Natural Science Founda-
tion of China (No. 81673306, 81703348, 81874289), “Double First-
Class” University project (No. CPU2018GY04, CPU2018GY35),
China Pharmaceutical University, and the Early Career Scheme of
the Research Grants Council of Hong Kong (No. 25100017).
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In conclusion, we have developed a divergent synthesis of
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34676 | RSC Adv., 2019, 9, 34671–34676
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