Regioselective Copper-Catalyzed Oxidative Cross-Coupling of Imidazo[1,2-a]pyridines with Methyl Ketones: An Efficient Route for Synthesis of 1,2-Diketones

Article abstract of DOI:10.1002/adsc.201500803

An efficient copper-catalyzed oxidative coupling of imidazo[1,2-a]pyridines with methyl ketones to directly generate structurally sophisticated 1,2-dicarbonyl imidazo[1,2-a]pyridine derivatives under oxidative conditions is described. The reaction proceeds in good yields using the environmental friendly molecular oxygen as the oxidant. ¹⁸O-Labelling experiments unambiguously established that the oxygen of the dicarbonyl products originated from oxygen rather than from water.

Full text of DOI:10.1002/adsc.201500803

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DOI: 10.1002/adsc.201500803
Regioselective Copper-Catalyzed Oxidative Cross-Coupling of
Imidazo[1,2-a]pyridines with Methyl Ketones: An Efficient Route
for Synthesis of 1,2-Diketones
a
b
b
b
b
b
Sai Lei, GuiJun Chen, Yingying Mai, Longbin Chen, Huiyin Cai, Jingwen Tan,
a,
and Hua Cao *
a
College of Pharmacy, Guangdong Pharmaceutical University, Guangzhou 510006, Peopleꢀs Republic of China
Fax: (+86)-760-8820-7939; e-mail: caohua@gdpu.edu.cn
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan 528458, Peopleꢀs
b
Republic of China
Received: August 28, 2015; Revised: October 26, 2015; Published online: December 17, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500803.
Abstract: An efficient copper-catalyzed oxidative
coupling of imidazo[1,2-a]pyridines with methyl ke-
tones to directly generate structurally sophisticated
heterocyclic compounds for the synthesis of 1,2-dicar-
[5]
bonyl compounds which are important starting ma-
terials and intermediates in the preparation of fine
[6]
1,2-dicarbonyl imidazo[1,2-a]pyridine derivatives
chemicals. Recently, Li et al. reported the copper-
catalyzed oxidative cross-coupling of indoles to pre-
pare 1,2-dicarbonyl compounds(Scheme 1a); In 2013,
under oxidative conditions is described. The reac-
tion proceeds in good yields using the environmen-
1
8
[7]
tal friendly molecular oxygen as the oxidant. O-
Labelling experiments unambiguously established
that the oxygen of the dicarbonyl products originat-
ed from oxygen rather than from water.
they also described a palladium-catalyzed synthesis
of 3-acylated indoles involving the oxidative cross-
[8]
coupling of indoles (Scheme 1b); Wu et al. devel-
oped the I -mediated direct regioselective oxidative
2
cross-coupling of indoles with methyl ketones to real-
ize the C-3 oxidative coupling of indoles (Scheme 1c);
Such results are of particular interest because the 1,2-
dicarbonyl groups are robust yet readily converted to
many other functional groups.
Keywords: copper catalysts; cross-coupling; 1,2-di-
ketones; imidazo[1,2-a]pyridines; methyl ketones
Very recently, we have reported facile approaches
[9]
for the synthesis of imidazo[1,2-a]pyridines which
Transition metal-catalyzed oxidative cross-coupling exhibit a broad range of biological activities such as
[
1]
reactions are powerful tools for the formation of
carbon-carbon bonds to prepare complex and diverse
heterocyclic compounds, which remains a significant
challenge in organic synthesis. Such methods repre-
sent an atom-economical and environmentally benign
[2]
alternative to traditional coupling transformations,
which require the use of two prefunctionalized start-
ing materials. Several recent successful and elegant
[
3]
transformations have been developed for the prepa-
ration of heterocyclic compounds via transition metal-
catalyzed oxidative cross-coupling. Among those
transformations, Pd-, Cu-, Ag-, Rh-, and Ru-catalyzed
[
4]
direct oxidative cross-coupling reactions have been
extensively studied due to their high efficiency. Al-
though continuous efforts have been devoted to de-
velop efficient transformations for the synthesis of
heterocyclic compounds, oxidative coupling of hetero-
cyclic compounds is still highly favorable. In this
paper, we describe a direct oxidative cross-coupling of Scheme 1. Synthesis of dicarbonylated indoles.
Adv. Synth. Catal. 2016, 358, 67 – 73
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67

Sai Lei et al.
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[a]
Table 1. Optimization of the reaction conditions.
[b]
Entry
Catalyst
Oxidant
Additive
Solvent
Temp. [8C]
Yield [%]
[
c]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
Cu(OAc)2
Cu(OTf)2
CuBr2
CuCl2
CuI
O2
O2
O2
O2
O2
O2
O2
air
TBHP
AgOAc
O2
O2
O2
O2
O2
O2
O2
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
TFA
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMSO
DMF
NMP
dioxane
DCE
toluene
toluene
toluene
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
120
80
14
10
trace
N.P.
N.P.
trace
CuBr
[
[
d]
d]
[e]
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
85 (81)
20
trace
trace
82
N.P.
46
41
38
76
49
0
1
2
3
4
5
6
7
8
9
0
[
[
[
[
[
[
[
[
[
[
d]
d]
d]
d]
d]
d]
d]
d]
d]
d]
K CO
2
3
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
O2
O2
O2
84
62
N.P.
r.t.
[
a]
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), catalyst (5 mol%), oxidant (1.0 mmol), additive (0.1 mmol), solvent
(3.0 mL), room temperature to 1408C, 12 h.
b]
[
[
[
[
GC yield.
c]
d]
e]
O or air (500 mL).
2
Carried out in a sealed tube (25 mL).
Isolated yield.
[
10]
[11]
[12]
antiviral,
antiprotozoal,
antiherpes,
and anti- tube (Table 1, entry 7). But other oxidants, such as
[
13]
apoptotic action. Due to their application value and air, TBHP and AgOAc, disfavored the reaction to
prevalence, the synthesis of imidazo[1,2-a]pyridines varying degrees (Table 1, entries 8–10). A similar
[14]
has attracted critical attention of organic chemists.
yield of 3a was maintained with the use of 2 mol%
In continuation of our interest in preparing imida- TFA as the additive (Table 1, entry 11). However, no
zo[1,2-a]pyridine derivatives by oxidative cross-cou- product was formed using the same equivalent of
pling of imidazo[1,2-a]pyridines, we report a novel K CO as the additive (Table 1, entry 12). The results
2
3
and efficient copper-catalyzed C-3 oxidative coupling indicated that the acid system was essential for this
of imidazo[1,2-a]pyridines with methyl ketones utiliz- transformation. The effects of solvents and tempera-
ing molecular oxygen as the terminal oxidant.
ture were next investigated in the following tests
In an initial attempt, imidazo[1,2-a]pyridine (1a) (Table 1, entries 13–20). Among them, toluene was
was chosen as a test substrate to react with acetophe- demonstrated to be the best choice. Nonetheless, the
none using a copper catalyst under various conditions. yield could not be improved by decreasing the tem-
The results are outlined in Table 1. The desired prod- perature to 1208C (entry 23). The desired product 3a
uct 3a was formed in 14% or 10% yield when the re- was only formed in 62% yield at 808C in the presence
action carried out using Cu(OAc) or Cu(OTf) as of Cu(OAc) . And decreasing the temperature to
2
2
2
a catalyst, respectively, molecular oxygen as an oxi- room temperature did affect the reaction efficiency
dant (from an oxygen balloon) in toluene at 1408C and no product 3a was obtained.
for 12 h (Table 1, entries 1 and 2), whereas poor re-
On the basis of the screening of the above oxidative
sults were obtained in the presence of other catalysts, cross-coupling reaction conditions, it can be conclud-
such as CuBr , CuCl , CuI, CuBr (Table 1, entries 3– ed that the optimized reaction should be performed
2
2
6
). Gratifyingly, the product 3a was obtained in 85% in the presence of 5 mol% Cu(OAc) , 2 mol% AcOH,
2
yield when the reaction was carried out in a pressure and O in a pressure tube at 1408C for 12 h. With the
2
6
8
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Regioselective Copper-Catalyzed Oxidative Cross-Coupling
Table 2. Oxidative cross-coupling of imidazo[1,2-a]pyridines
with acetophenone.
Table 3. Oxidative coupling of imidazo[1,2-a]pyridine with
methyl ketones
[
a]
[a]
[a]
Isolated yields.
optimized reaction conditions established, various
substrates were subjected to cross-coupling reactions,
and representative results are described in Table 2
and Table 3. First, 2a was fixed as the substrate to test
various substituted imidazo[1,2-a]pyridine derivatives.
Imidazo[1,2-a]pyridine 1a, 8-methylimidazo[1,2-a]pyr-
idine 1b, 7-methylimidazo[1,2-a]pyridine 1c, 6-methyl-
imidazo[1,2-a]pyridine 1d and 5-methylimidazo[1,2-
a]pyridine 1e were examined. It was found that differ-
ent 2-unsubstituted imidazo[1,2-a]pyridine derivatives
could undergo the cross-coupling reaction smoothly
to afford the corresponding 1,2-dicarbonyl imida-
zo[1,2-a]pyridines in good yields. Excitingly, the re-
[
a]
Isolated yields.
sults indicated that Cl, Br, and I substituted on the tion possible. Different substituents on the imidazo
pyridine ring were also well-tolerated under opti- ring, such as CH , C(CH ) and Ph, also led to a bene-
3
3 3
mized conditions and led to a beneficial effect on the ficial effect on the reaction outcome. However, the
reaction outcome, which made further functionaliza- desired products 3v or 3w were not formed when the
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69

Sai Lei et al.
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reaction was carried out using strongly electron-with-
drawing substituted substrates, such as 2-(trifluorome-
thyl)imidazo[1,2-a]pyridine or ethyl 3-(2-oxo-2-phe-
nylacetyl)imidazo[1,2-a] pyridine-2-carboxylate. In ad-
dition, when other functional groups (CF ,OH,
3
NHCH , and CN) on the on the pyridine ring were
3
employed in the reaction the desired products were
not formed.
Scheme 2. Synthesis of 1,2-diketones.
Subsequently, various methyl ketones were also
tested under the standard reaction conditions. And
the results are summarized in Table 3. Selective C-3
oxidative coupling was observed using the para-substi-
tuted electron-donating acetophenone derivatives,
1
-p-tolylethanone, 1-(4-ethylphenyl)ethanone, 1-(4-
butylphenyl)ethanone, 1-(biphenyl-4-yl)ethanone, 1-
4-(benzyloxy)phenyl]ethanone, 1-[4-(methylthio)-
[
phenyl]ethanone, or 1-(4-hydroxyphenyl)ethanone, re- Scheme 3. Oxidative coupling of 8-methylimidazo[1,2-a]pyri-
dine with acetone.
sulting in 51–93% yields of the desired products (4a,
4
b, 4d, 4e, 4h, 4i, 4t). In most cases, meta- or ortho-
substituted acetophenone derivatives with electron-
rich groups (3-OCH , 2-OCH , 3-CH CONH) at the
3
3
3
benzene ring reacted smoothly and afforded the cor-
responding products in 57–82% yields (4f, 4g, 4s). To
our delight, 3,4-dimethyl substituents on the benzene
ring also led to a beneficial effect on the reaction out-
come (4c). However, only a trace of product (4u) or
no product (4x) was formed when the reaction was
carried out using 1-(2-hydroxyphenyl)ethanone or
1-(2-aminophenyl)ethanone, respectively, as the sub-
strate. Moreover, electron-deficient substituent
groups (F, Cl, Br, I, CO CH ) on the benzene ring re-
2
3
mained unaffected under these reaction conditions
and afforded the corresponding products (4j, 4k, 4l,
4
m, 4n, 4o, 4p, 4q, 4r) in good yields. Notably, selec-
tive C-3 oxidative coupling products were also ob-
tained in good yields, when the substrates 1-(naphtha-
len-2-yl)ethanone (4v) and 1-(furan-2-yl)ethanone
(4w) were employed in the reaction. The selective
C-3 cross-coupling products were obtained because
the electron density distribution of C-3 (À0.125) is
[
15]
higher than that of C2 (À0.054).
Similar results
[14j]
have also been reported by other groups.
Then, the Cu-catalyzed oxidative cross-coupling of
-methylimidazo[1,2-a]pyridine with methyl 4-acetyl-
2
benzoate was also tested and the corresponding prod-
uct 5a was formed in 83% yield (Scheme 2). Notably,
commercially available acetone was employed to ex- Scheme 4. Control experiments for investigation of the
plore the scope of this transformation, the target mechanism.
product 5b was also obtained in good yield
(
Scheme 3). The results indicated that the reaction
could be successfully extended to acetone under the performed. Interestingly, the results of the reaction
1
8
optimum conditions.
with O-labelled O2 and H O indicated that the
2
To define the possible mechanism of this reaction, oxygen in the generated CO group originated com-
a series of control experiments were carried out pletely from O rather than from H O (Scheme 1a,
2
2
(
Scheme 4). First, in order to confirm of the oxygen 1b). Furthermore, it was also found that when
source of this oxidative coupling procedure, several TEMPO, a radical inhibitor (Scheme 1c), was added
labelling experiments involving the essential O were into the reaction system, no coupling product 3a was
2
7
0
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Regioselective Copper-Catalyzed Oxidative Cross-Coupling
Scheme 5. Possible mechanism.
observed under the optimal conditions. This result in- tophenone 2a (0.6 mmol, 1.2 equiv.), HOAc (0.1 mmol,
(5 mmol), and toluene (3 mL) were
dicated that a possible radical pathway was involved 0.2 equiv.), Cu(OAc)
2
added. The reaction mixture was stirred at 1408C under an
in the mechanism.
O atmosphere for 12 h. The reaction mixture was then di-
luted with EtOAc and water, extracted with EtOAc. The or-
ganic layers were washed with brine and dried over Mg SO ,
evaporated under reduced pressure. The crude mixture was
purified by thin layer chromatography in a silica gel plate to
afford a yellow solid; mp 154–1568C. H NMR (400 MHz,
2
On the basis of above results and literature prece-
[
9e,16]
dents,
a plausible mechanism for this reaction was
2
4
proposed in Scheme 5. The radical intermediates A
and B have been formed via a single electron-transfer
(
SET) oxidation in the presence of the Cu(II) and O2.
1
And then the radical intermediates A and B undergo
radical coupling to give intermediate C’ which under- J=7.6, 2H), 7.85 (d, J=9.2, 1H), 7.65 (q, J=8.4, 2H), 7.52
CDCl ): d=9.73 (d, J=6.8 Hz, 1H), 8.41 (s, 1H), 8.10 (d,
3
1
3
goes a single electron-transfer oxidation to generate (t, J=7.6, 2H), 7.24 (t, J=7.2, 1H); C NMR (101 MHz,
intermediate D. This intermediate is trapped by mo- CDCl ): d=191.5, 181.2, 149.9, 147.4, 134.7, 133.0, 130.6,
3
+
1
30.3, 129.1, 128.8, 121.8, 118.1, 116.0; HR-MS (ESI ): m/
lecular oxygen to deliver peroxyl radical E which
then is converted into the desired product 3a. Un-
doubtly, AcOH has played a key role in this transfor-
mation. It aids the formation of 2aꢀ and Cꢀ and also
promotes the transformation of product from E.
In conclusion, we have reported an efficient and
+
z=251.0815, calcd. for C H N O [M+H] : 251.0821.
1
5
11
2
2
Acknowledgements
novel Cu-catalyzed oxidative coupling process for the The work was financially supported by National Natural Sci-
ence Foundation of China (21302023) and Department of
synthesis of 1,2-diketones via oxidative cross-coupling
of imidazo[1,2-a]pyridines with methyl ketones by
Education of Guangdong, China (2013 KJCX0111 and
2014 KTSCX120).
using O as oxidant. This approach offers a unique
2
strategy and alternative route for the preparation of
1
,2-diketones in good to excellent yields which are
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