Manganese-catalyzed dehydrogenative Csp3-Csp2coupling of imidazo[1,2-a]pyridines with methyl ketones

Article abstract of DOI:10.1039/d1ob00169h

A Mn(ii)-catalyzed efficient C-H alkylation of imidazoheterocycles with methyl ketones has been developedviadehydrogenative C(sp³)-C(sp²) coupling which can serve as a novel approach toward hydrocarboxylated imidazolopyridines. The merit of this strategy is illustrated by excellent functional group tolerance and the use of cheap and readily available starting materials.

Full text of DOI:10.1039/d1ob00169h

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Manganese-catalyzed dehydrogenative Csp3–Csp2
coupling of imidazo[1,2-a]pyridines with methyl
ketones†
Cite this: Org. Biomol. Chem., 2021,
19, 3479
Hua Yao, Xiaoyang Zhong, Bingqing Wang, Sen Lin, * Lichi Liu and Zhaohua Yan*
Received 29th January 2021,
Accepted 19th March 2021
A Mn(II)-catalyzed efficient C–H alkylation of imidazoheterocycles with methyl ketones has been
developed via dehydrogenative C(sp3)–C(sp2) coupling which can serve as a novel approach toward
hydrocarboxylated imidazolopyridines. The merit of this strategy is illustrated by excellent functional
group tolerance and the use of cheap and readily available starting materials.
DOI: 10.1039/d1ob00169h
rsc.li/obc
pre-established functional groups or harsh conditions.
Consequently, in view of green and sustainable chemistry,
Introduction
Imidazo[1,2-a]pyridine and its derivatives are important nitro- dehydrogenative Csp3–Csp2 coupling,10 via the activation of
gen-containing heterocycles and have attracted great attention the intrinsically less reactive C(sp3)–H bond in aliphatic mole-
for their wide range of biological activities such as anticancer, cules, should be appealing enough for hydrocarboxylated imi-
antifungal, antiulcer, anti-inflammatory, antiprotozoal, anti- dazolopyridine constructions.11 There have been
a few
bacterial, analgesic, and anxioselective activities.1 Moreover, methods reported to achieve direct coupling between imidazo
some commercially available drug formulas contain the [1,2-a]pyridines and the C(sp3)–H bond among common ali-
imidazo[1,2-a]pyridine motif such as zolpidem, alpidem, zoli- phatic substances, such as acetonitriles,12 alcohols and
midine, necopidem, and saripidem.2 Due to the important ethers.13 However, the use of aliphatic ketones for alkylation
values of imidazo[1,2-a]pyridine derivatives, a variety of syn- remains a challenge. In 2015, Cao and co-workers reported a
thetic strategies including condensation,3 oxidative coupling4 regioselective dicarbonylation of imidazo[1,2-a]pyridines by
and decarboxylative coupling5 have been developed for the syn- reacting with methyl ketones in toluene under an oxygen atmo-
thesis and functionalization of imidazoheterocycles over the sphere (Scheme 1b);14 in this approach, α-methyl was further
last decade.
oxidized to a carbonyl group to form 1,2-dicarbonylated pro-
Recently, C3–H alkylation via a radical process has become ducts. In continuation of our research project on the synthesis
a viable method for the preparation of C3-alkylated imidazo- and functionalization of imidazoheterocycles, herein, we
pyridines.6 In 2016, Patel and co-workers reported the methyl- report a novel and effective method to synthesize C3-alkylated
enation of imidazo[1,2-a]pyridines using H2O2 as a mild imidazo[1,2-a]pyridines via direct cross-dehydrogenative coup-
oxidant and DMSO (dimethyl sulfoxide) as the carbon synthon
leading to the formation of 3,3′-bis(imidazopyridinyl)-
methanes (Scheme 1a).7 Li et al. and Miranda et al. indepen-
dently developed a xanthate-based C3-alkylation of imidazo-
pyridines using DLP (dilauroyl peroxide) as the initiator and
oxidant in DCE.8 Lu et al. described a FeCl3-catalyzed tosyl-
methylation of imidazo[1,2-a]pyridines using TosMIC (p-tolue-
nesulfonylmethyl isocyanides) in a mixture of H2O and
PEG400 (7 : 3).9 These strategies are effective for assembling
C3-alkylated imidazopyridines, although they sometimes
suffer from poor reaction atom-economy and the requisition of
College of Chemistry, Nanchang University, No. 999, Xuefu Rd., Nanchang, 330031,
P. R. China. E-mail: senlin@ncu.edu.cn, yanzh@ncu.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00169h
Scheme 1 Synthesis of hydrocarbylated imidazo[1,2-a]pyridines.
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conditions (Table 1). The reaction of 1a and 2a in the presence
of DTBP (di-tert-butyl peroxide) (3.0 equiv.) was performed
under air at 120 °C for 24h, and the desired product 1-(2-phe-
nylimidazo[1,2-a]pyridine-3-yl)propan-2-one (3a) was obtained
in 42% isolated yield (Table 1, entry 1). In light of the above
results, various peroxides including CHP (cumene hydroperox-
ide), DCP (dicumyl peroxide), TBHP (tert-butyl hydroperoxide)
(70% in water), TBPB (tert-butyl perbenzoate), and DLP (dilaur-
oyl peroxide) were used under identical reaction conditions
(Table 1, entries 2–6); DCP showed a higher efficiency for this
reaction than other peroxides. The amount of oxidant was
screened, and the results showed that 3.0 equiv. of DCP pro-
vided 3a in better yield (Table 1, entries 7 and 8). To further
improve the reaction efficiency, several metal catalysts such as
CuCl2, CuCl, FeCl3, Mn(OAc)2·4H2O, Ni(OAc)2·4H2O, Mn(OAc)2
and MnCl2 were tested in this reaction (Table 1, entries 9–16),
and Mn(OAc)2·4H2O seemed to be the most appropriate for
this transformation. The effect of the solvent on the reaction
was also taken into account. Different solvents such as DMSO,
DMF, EtOH, and chlorobenzene were investigated, and all gave
inferior results (Table 1, entries 17–20). On increasing the reac-
tion temperature to 130 °C, there was no significant change in
the yield of 3a. A lower yield of 62% was obtained when the
temperature was decreased to 100 °C (Table 1, entries 21 and
Fig. 1 Imidazo[1,2-a]pyridine-containing drugs.
ling of imidazoheterocycles with methyl ketones. This alkyl-
ation reaction is catalyzed by Mn(OAc)2·4H2O, using DCP as
the oxidant and without additional solvent (Fig. 1).
Results and discussion
Initially, 2-phenylimidazo[1,2-a]pyridine (1a) and acetone (2a)
were chosen as the model substrates to optimize the reaction
Table 1 Optimization of the reaction conditionsa,b
Entry
Oxidant (equiv.)
Catalyst
Solvent
Temp. (°C)
Yieldb (%)
1
2
3
4
5
6
7
8
DTBP (3.0)
H2O2 (30%) (3.0)
DCP (3.0)
CHP (3.0)
TBHP (70%) (3.0)
DLP (3.0)
DCP (2.0)
DCP (4.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
DCP (3.0)
—
—
—
—
—
—
—
—
CuCl2
CuCl
FeCl3
MnCl2
Mn(OAc)2
Mn(OAc)2·4H2O
Mn(OAc)3·2H2O
Ni(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
DMSO
DMF
EtOH
C6H5Cl
—
—
—
—
—
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
130
100
120
120
120
120
120
42
Trace
55
37
26
48
47
54
35
34
Trace
67
74
78
73
70
18
Trace
35
29
76
62
69
76
70
72
78
9
10
11
12
13
14
15
16
17c
18c
19c
20c
21
22
23d
24e
25f
26g
27h
—
—
a Reaction conditions: 1a (0.2 mmol), 2a (2 mL), oxidant (3 equiv.), catalyst (0.02 mmol) and solvent (2.0 mL), 120 °C, 24 h, under air. b Isolated
yield. c 1a (0.2 mmol), 2a (0.6 mmol), and solvent (2.0 mL). d Catalyst (0.01 mmol). e Catalyst (0.04 mmol). f 18 h. g 30 h. h Under N2.
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22). The most appropriate amount of Mn(OAc)2·4H2O was substrates having other substituents (8-Me, 6-Cl, and 6-NO2)
10 mol% (Table 1, entries 14, 23 and 24). When the reaction on the pyridine ring of imidazo[1,2-a]pyridines furnished the
was carried out for 18 h or 30 h, the yield of 3a decreased desired products (3h–3j) in 42–56% yields. Moreover, when the
(Table 1, entries 25 and 26). In addition, the result had no phenyl ring was replaced with tert-butyl, the reactions pro-
obvious difference when the reaction was performed in N2 ceeded to give the product 3l in 37% yield. Imidazo[1,2-a]pyri-
(Table 1, entry 27). Finally, the optimized reaction conditions dine with C-2 thienyl performed well in the reaction, and gave
were achieved using 10 mol% of Mn(OAc)2·4H2O and 3.0 3m in 58% yield. However, the desired product 3n was not
equiv. of DCP at 120 °C for 24 h under air.
observed when the thiophene ring was replaced with electron-
After optimizing the reaction conditions, we turned our deficient pyridine.
attention toward the substrate scope of imidazo[1,2-a]pyri-
Subsequently, the scope of ketones was explored, and the
dines; the results are summarized in Scheme 2. Imidazo[1,2-a] results are summarized in Scheme 3. 7-Chloro-2-phenylimi-
pyridine having an electron donating group (p-OMe) on the dazo[1,2-a]pyridine was used to react with butanone, and two
C-2 phenyl ring reacted smoothly with acetone, and afforded different products were separated by column chromatography.
the product 3b in 69% yield. The substrate with an electron The C-3 butanone cross-coupling product 3o was obtained in
withdrawing group (p-F) on the C-2 phenyl ring of imidazo 57% yield, and the C-1 product 3p was separated only in 7%
[1,2-a]pyridine also delivered the corresponding product 3c in yield. Butanone reacted with 1a and gave the C-3 product 3q in
59% yield. 2-(2,4-Dimethylphenyl)imidazo[1,2-a]pyridine was 53% yield, but the C-1 product 3r was not observed in the reac-
also tolerated in this reaction, affording 3d in 56% yield. tion mixture. Acetophenone performed well in this optimized
Subsequently, the effect of the substituents on the pyridine reaction and gave the desired product 3s in 62% yield.
ring was explored. The performance of the C-7 substituents on Cyclohexanone was tolerated in this reaction and provided the
imidazo[1,2-a]pyridines was better than those at other posi- product 3t in 39% yield. However, the desired products 3u and
tions in this reaction. The substrate bearing an electron-with- 3v were not detected when the reactions were carried out using
drawing substituent such as –Cl at the C-7 position of imidazo ethyl acetate and N,N-dimethylacetamide as partners with 1a.
[1,2-a]pyridine reacted with 2a to produce 3e in 82% yield, and
In order to gain further insight into the mechanistic
the substrates with electron-donating substituents such as pathway of the reaction, control experiments were carried out
–OMe and –Me at the C-7 position of imidazo[1,2-a]pyridines (Scheme 4). When 3 equiv. of butylated hydroxytoluene (BHT),
proceeded well in the reaction, giving the corresponding pro- ethene-1,1-diyldibenzene and 2,2,6,6-tetramethylpiperidine-1-
ducts 3f and 3g in yields of 75% and 76%, respectively. The oxyl (TEMPO) were added to the reaction mixture under the
standard conditions, the desired product 3a was not detected
in these reaction mixtures. Notably, the radical scavenger sup-
pressed the reaction, and the acetone radical was captured by
BHT and ethene-1,1-diyldibenzene offered products 4 and 5
Scheme 2 Substrate scope of imidazo[1,2-a]pyridines. Conditions: 1
(0.20 mmol), 2a (2.0 mL), catalyst (0.02 mmol), and DCP (0.60 mmol),
120 °C, 24 h, under air. Isolated yield.
Scheme 3 Substrate scope of imidazo[1,2-a]pyridines and methyl
ketones. Conditions: 1 (0.20 mmol), 2 (2.0 mL), catalyst (0.02 mmol),
and DCP (0.60 mmol), 120 °C, 24 h, under air. Isolated yield.
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tron-donating and electron-withdrawing groups at different
positions of imidazo[1,2-a]pyridines are tolerated in the reac-
tion, and they provide the target alkylation products in moder-
ate to good yields. This method provides a potential route for
highly regioselective functionalization of imidazopyridines
that may find wide applications in pharmaceutical synthesis.
Conflicts of interest
The authors declare no competing financial interest.
Scheme 4 Control experiments for the investigation of the
mechanism.
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21362022) and the Natural Science Foundation of Jiangxi
Province (No. 20192BAB203006) for financial support.
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