Nickel-catalyzed decarboxylative acylation of heteroarenes by sp 2 C-H functionalization

Article abstract of DOI:10.1002/chem.201402516

Nickel-catalyzed ligand-free decarboxylative cross-coupling of azole derivatives with α-oxoglyoxylic acids has been developed. This work represents the first example of decarboxylative cross-coupling reactions, in a C-H bond functionalization manner, thro

Full text of DOI:10.1002/chem.201402516

DOI: 10.1002/chem.201402516
Communication
&
Nickel Catalysis
Nickel-Catalyzed Decarboxylative Acylation of Heteroarenes by
sp2 CÀH Functionalization
Ke Yang,[a] Cheng Zhang,[a] Peng Wang,[a] Yan Zhang,*[a] and Haibo Ge*[a, b]
(hetero)arenes with a-oxocarboxylic acids was developed in
Abstract: Nickel-catalyzed ligand-free decarboxylative
our laboratory, providing an efficient approach to access a vari-
cross-coupling of azole derivatives with a-oxoglyoxylic
ety of synthetically and medically important (hetero)aryl ke-
acids has been developed. This work represents the first
tones.[5] However, under many circumstances, a directing
example of decarboxylative cross-coupling reactions, in
group is required in this process, limiting the synthetic applica-
a CÀH bond functionalization manner, through nickel cat-
alysis, and tolerates various functional groups. Additional-
ly, this approach provides an efficient access to azole ke-
tones, an important structural motif in many medicinal
compounds with a broad range of biological activities.
tion of this method.[6]
Azole ketones belong to an important structural family and
are present in many medicinal compounds with a broad range
of biological activities, including anti-cancer, anti-diabetic, anti-
fibrotic, anti-infective, anti-inflammatory, anti-obesity, and anti-
ulcer activities.[7] The current direct synthesis of these com-
pounds relies primarily on a sequential process of deprotona-
tion/metalation, followed by copper-mediated coupling with
an acyl chloride, which suffers severely from poor functional
group tolerance.[7,8] Recently, Beller and co-workers reported
an efficient alternative approach for the direct synthesis of
azoles by a Pd-catalyzed sp2 CÀH functionalization process.[9]
However, this process requires the use of excess azoles and rel-
atively toxic CO under high pressure. Very recently, a Pd-cata-
lyzed decarboxylative process was developed with limited ex-
amples.[6k] In our continuing efforts to develop novel transi-
tion-metal-catalyzed cross-coupling reactions, herein we dis-
close the Ni-catalyzed direct acylation of azoles with a-oxocar-
boxylic acids. It is noteworthy that this process represents the
first example of decarboxylative cross-coupling reactions by
a sp2 CÀH functionalization process involving Ni catalysis.
Our investigation began with the decarboxylative cross-cou-
pling of benzoxazole (1a) and 2-oxo-2-phenylacetic acid (2a)
with catalytic [NiCl2(PCy3)2], in the presence of Ag2CO3 as an
external oxidant at 1708C (Table 1). After an extensive solvent
screening, benzene was found to be optimal and desired prod-
uct 3a was obtained in 58% yield (entry 2). Interestingly, it was
then found that a ligand is not required for this reaction, and
actually an improved yield was observed with a catalytic
amount of NiCl2 in the absence of a ligand (entry 5). Next, we
carried out a screening of the Ni catalysts. It turned out that
this reaction could be effectively catalyzed by a variety of Ni
species, among which Ni(ClO4)2 was optimal, providing the de-
sired product in 86% yield (entry 10). Additionally, this reaction
is not sensitive to water, and a comparable yield was observed
with Ni(ClO4)2·6H2O (entry 11). It is also noteworthy that re-
placement of Ag2CO3 with several other oxidants resulted in
either extremely low yield of product or no reaction (en-
tries 12–15). Further optimization showed that the amount of
Ni catalyst could be reduced to 7.5% without an apparent
effect on the reaction (entry 16). However, the reaction yield
Transition-metal-catalyzed cross-coupling reactions of arenes
and heteroarenes by means of sp2 CÀH functionalization re-
mains to be one of the most powerful methods for selective
carbon–carbon (CÀC) bond construction, and has found broad
applications in synthetic organic and medicinal chemistry re-
search.[1] Compared with the traditional approaches, these
methods are more economically favorable due to the avoid-
ance of the prefunctionalization of (hetero)arenes. Moreover,
this methodology can avoid the use of stoichiometric amounts
of expensive and/or moisture-sensitive organometallic cou-
pling partners and, thus, prevents the generation of stoichio-
metric amounts of toxic metal waste. Among these methods,
transition-metal-catalyzed decarboxylative processes have
been of great interest in recent years due to the low cost,
ready availability, and environmentally benign properties of
carboxylic acids.[2] In 2008, Crabtree and co-workers reported
the first example of direct decarboxylative cross-coupling reac-
tions of (hetero)arenes with benzoic acid derivatives through
a Pd-catalyzed sp2 CÀH functionalization process.[3] Since then,
extensive efforts have been devoted to extend the substrate
scope, and it was found that heteroaromatic acids are also ef-
fective substrates.[4] Very recently, the direct acylation of
[a] K. Yang, C. Zhang, P. Wang, Prof. Dr. Y. Zhang, Prof. Dr. H. Ge
Institute of Chemistry & BioMedical Sciences
School of Chemistry and Chemical Engineering
State Key Laboratory of Analytical Chemistry for Life Science
Nanjing University, Nanjing 210093 (P. R. China)
E-mail: njuzy@netra.nju.edu.cn
[b] Prof. Dr. H. Ge
Department of Chemistry and Chemical Biology
Indiana University Purdue University Indianapolis
Indianapolis, Indiana 46202 (USA)
Fax: (+1)317-2744701
E-mail: geh@iupui.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402516.
Chem. Eur. J. 2014, 20, 1 – 5
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Table 1. Optimization of reaction conditions.[a]
Entry Ni source [(mol%)]
Oxidant [(equiv)] Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
[NiCl2(PCy3)2] (10)
[NiCl2(PCy3)2] (10)
[NiCl2(PCy3)2] (10)
[NiCl2(PCy3)2] (10)
NiCl2 (10)
NiBr2 (10)
Ni(OAc)2 (10)
Ni(OTf)2 (10)
[Ni(cod)2] (10)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
AgOAc (3)
Ag2O (3)
toluene
39
58
5
benzene
o-xylene
p-xylene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
6
69
55
59
62
11
86
88
6
9
10
11
12
13
14
15
16
17
18[d]
19[d]
Ni(ClO4)2 (10)
Ni(ClO4)2·6H2O (10)
Ni(ClO4)2·6H2O (10)
Ni(ClO4)2·6H2O (10)
Ni(ClO4)2·6H2O (10)
Ni(ClO4)2·6H2O (10)
Ni(ClO4)2·6H2O (7.5) Ag2CO3 (3)
Ni(ClO4)2·6H2O (10) Ag2CO3 (2)
Ni(ClO4)2·6H2O (7.5) Ag2CO3 (3)
Ni(ClO4)2·6H2O (10) Ag2CO3 (3)
13
0
AgClO4 (3)
K2S2O8 (3)
benzene <5
benzene
benzene
benzene
benzene
87 (85)[c]
24
25
27
[a] Conditions: 1a (0.2 mmol), 2a (0.6 mmol), Ni source, oxidant, solvent
(2 mL), 1708C, 24 h. PCy=tricyclohexylphosphine, OTf=triflate, cod=1,5-
cyclooctadiene. [b] Yields and conversions are based on 1a, determined
by crude 1H NMR spectrum with dibromomethane as the internal stan-
dard. [c] Isolated yield. [d] At 1508C.
Figure 1. Scope of azole derivatives. Reaction conditions: 1 (0.2 mmol), 2a
(0.6 mmol), Ni(ClO4)2·6H2O (7.5 mol%), Ag2CO3 (3.0 equiv), benzene (2 mL),
1708C, 24 h. Isolated yields.
significantly decreased with reduced amounts of the oxidant,
or under lower reaction temperature (entries 17–19).
With the optimized conditions in hand, we then carried out
a substrate-scope study with respect to the azole moiety
(Figure 1). As expected, both electron-donating and electron-
withdrawing groups on the phenyl ring were compatible
under the current catalytic system (3b–g). In addition, halogen
atoms (F, Cl, and Br) were also tolerated, allowing for further
transformation of the initial products. Furthermore, this reac-
tion was not limited to benzoxazoles; benzothioazole was also
an effective substrate and provided desired product 3h in
60% yield. Additionally, oxazoles were effective coupling part-
ners, and good to high yields were obtained with both mono-
and disubstituted oxazoles (3i–q).
Next, a substrate-scope study with respect to the a-oxocar-
boxylic acids was carried out. As shown in Figure 2, phenyl-
glyoxylic acids, with either an electron-donating or electron-
withdrawing group at the para- or meta-position of the ring,
were well tolerated under the current reaction conditions (3s–
ac). However, there is an apparent steric effect of this ring, and
neither ortho-methyl- nor ortho-chloro-substituted phenyl-
glyoxylic acid generated the desired product. As expected, hal-
ogen atoms (F, Cl, and Br) were well tolerated under the cur-
rent catalytic system. Furthermore, a good yield of desired
product was also obtained with 3,4-dichlorosubstituted sub-
strate 3ad. It was further observed that the coupling of ben-
zoxazole with 2-oxo-2-(thiophen-3-yl)acetic acid could provide
the desired diheteroaryl ketone product 3ae, albeit with a low
yield.
On the basis of previous reports,[10] a plausible catalytic cycle
is proposed (Scheme 1). It is believed that the reaction is initi-
ated by nickelation of azole 1 in the presence of Ag2CO3 to
give the NiII intermediate A. Subsequent transmetalation of
this intermediate with the acyl Ag species B, generated by Ag-
mediated decarboxylation of a-oxocarboxylic acid 2, produces
the Ni species C. Reductive elimination of C provides ketone
product 3, and the Ni0 species, which could be re-oxidized to
the divalent NiII species by a AgI species.
In summary, we have developed an efficient Ni-catalyzed
ligand-free direct decarboxylative acylation of azole derivatives
by means of sp2 CÀH bond functionalization. This reaction tol-
erates a variety of functional groups, with good to excellent
yields. This transformation is the first example of decarboxyla-
tive cross-coupling reactions through Ni catalysis by a CÀH
bond functionalization pathway. Additionally, this novel
method provides an efficient complementary approach to
access synthetically and medically important heteroaryl ketone
derivatives.
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Communication
chromatography on silica gel (gradient eluent of 2% EtOAc in hex-
anes, v/v) to give the desired product.
Acknowledgements
We gratefully acknowledge the financial support from the Na-
tional Basic Research Program of China (2011CB935800), the
National Natural Science Foundation of China (21372115), the
Natural Science Foundation of Jiangsu Province (BK2012012).
We would also like to acknowledge Indiana University Purdue
University Indianapolis for financial support.
Keywords: acylation · decarboxylation · heteroarenes · nickel ·
sp2 CÀH bond functionalization
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Figure 2. Scope of a-oxocarboxylic acids. Reaction conditions: 1a
(0.2 mmol), 2 (0.6 mmol), Ni(ClO4)2·6H2O (7.5 mol%), Ag2CO3 (3.0 equiv), ben-
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Scheme 1. A plausible catalytic cycle.
Experimental Section
A 35 mL Schlenk tube was charged with azoles (1, 0.2 mmol), a-
oxocarboxylic acids (2, 0.6 mmol), Ni(ClO4)2·6H2O (5.5 mg,
0.015 mmol), Ag2CO3 (165.5 mg, 0.6 mmol), and benzene (2.0 mL).
The vial was sealed, and stirred rigorously at 1708C for 24 h. After
cooling to room temperature, the reaction mixture was diluted
with EtOAc (15 mL) and passed over a pad of Celite. The solvent
was removed under vacuum. The residue was purified by flash
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Received: March 7, 2014
Published online on && &&, 0000
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Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
4
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
&
Nickel Catalysis
K. Yang, C. Zhang, P. Wang, Y. Zhang,*
H. Ge*
&& – &&
Nickel catalysis is now possible:
functionalization process. This transfor-
mation is the first example of a decar-
boxylative cross-coupling reaction
Nickel-Catalyzed Decarboxylative
Acylation of Heteroarenes by sp2 CÀH
Functionalization
Nickel-catalyzed ligand-free decarboxyl-
ative cross-coupling of azole derivatives
with a-oxoglyoxylic acids was devel-
oped by means of an sp2 CÀH bond
through nickel catalysis by a CÀH bond
functionalization pathway (see scheme).
Chem. Eur. J. 2014, 20, 1 – 5
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ