Nickel-catalyzed decarboxylative arylation of heteroarenes through sp2 C-H functionalization

Article abstract of DOI:10.1002/ejoc.201403234

The direct decarboxylative arylation of hetereoarenes with benzoic acids through a nickel-catalyzed sp² C-H functionalization process was developed. This process provides the first examples of decarboxylative cross-coupling reactions with aroma

Full text of DOI:10.1002/ejoc.201403234

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403234
Nickel-Catalyzed Decarboxylative Arylation of Heteroarenes through sp2 C–H
Functionalization
Ke Yang,[a] Peng Wang,[a] Cheng Zhang,[a] Adnan A. Kadi,[b] Hoong-Kun Fun,[b]
Yan Zhang,*[a,b] and Hongjian Lu*[a]
Keywords: C–H activation / Decarboxylation / Arylation / Heteroarenes / Nickel
The direct decarboxylative arylation of hetereoarenes with
benzoic acids through a nickel-catalyzed sp2 C–H function-
alization process was developed. This process provides the
first examples of decarboxylative cross-coupling reactions
with aromatic acids through nickel catalysis and tolerates a
variety of functional groups. Moreover, this method provides
efficient access to 2-aryl-substituted azoles, an important
structural unit in natural products, medicinal compounds,
and functional materials.
carboxylates,[4f] and oxalates[4g] are all effective substrates
that allow the installation of a variety of functional groups
on the (hetero)aromatic rings. Moreover, copper-,[5] iron-,[6]
rhodium-,[7] and ruthenium-catalyzed[8] decarboxylative
cross-couplings have also been demonstrated. Recently, our
group introduced the decarboxylative acylation of (hereto)-
arenes with α-oxocarboxylic acids through nickel cataly-
sis.[9] 2-Aryl-substituted azoles represent important hetero-
cyclic moieties of biologically active natural products, me-
dicinal compounds, and functional materials.[10] The cur-
rent direct synthesis of these compounds relies primarily on
the transition-metal-catalyzed direct arylation of az-
oles.[3g,11] In our efforts to develop efficient approaches for
preparing 2-aryl-substituted oxazoles, herein we report the
nickel-catalyzed direct arylation of azoles with aromatic ac-
ids.
Introduction
Transition-metal-catalyzed cross-coupling reactions are
one of the most commonly used methods for the construc-
tion of selective carbon–carbon bonds.[1] However, in many
cases the methodology suffers from intrinsic disadvantages,
namely, the requirement for the use of stoichiometric
amounts of expensive and/or moisture-sensitive organome-
tallic coupling partners and, thus, the generation of stoi-
chiometric amounts of often-toxic metal waste. Therefore,
over the past couple of decades considerable effort has been
devoted to the development of alternative approaches.
The introduction of carbon-based leaving groups, espe-
cially carboxylic acids, constitutes an appealing alternative
method owing to the low cost, good air and moisture sta-
bility, and ready availability of these compounds. Thus, ex-
tensive studies have been performed with regard to the de-
velopment of transition-metal-catalyzed decarboxylative
cross-coupling reactions in the last decade, and significant
progress has been achieved with palladium catalysis in re-
cent years.[2] The direct decarboxylative arylation[3] of (het-
Results and Discussion
We began our investigation on the decarboxylative cross-
ero)arenes through a PdII-catalyzed sp2 C–H functionaliza- coupling of benzoxazole (1a) with 2-nitrobenzoic acid (2a)
tion process has been developed, and it has attracted con- in the presence of catalytic amounts of NiCl2 and PCy3 (Cy
siderable attention because prefunctionalization of the reac- = cyclohexyl) with Ag2CO3 as an external oxidant and the
tion substrates can be avoided. It was also demonstrated decarboxylation reagent at 170 °C (Table 1). Initial solvent
that alkyl,[4a,4b] alkenyl[4c] and alkynyl acids,[4d,4e] α-oxo- screening showed that the optimal result could be obtained
with (trifluoromethyl)benzene (Table 1, entry 1). It was then
[a] Institute of Chemistry & BioMedical Sciences, School of
Chemistry and Chemical Engineering, State Key Laboratory of
Analytical Chemistry for Life Science,
noticed that the reaction yield significantly decreased in the
absence of a ligand (Table 1, entry 3). On the basis of this
observation, extensive ligand screening was undertaken, in-
cluding evaluation of PPh3, tri-ortho-tolylphosphine
(TPO), 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride
(IPr·HCl), and 1,3-bis(2,4,6-trimethylphenyl)imidazolium
chloride (IMes·HCl), IPr·HCl was found to be optimal; it
provided the desired product in 62% yield (Table 1, en-
try 7). Next, screening of the nickel catalysts was under-
Nanjing University, Nanjing 210093, P. R. China
E-mail: njuzy@netra.nju.edu.cn
hongjianlu@nju.edu.cn
http://chem.nju.edu.cn/
[b] Department of Pharmaceutical Chemistry, College of
Pharmacy,
King Saud University, Kingdom of Saudi Arabia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403234.
Eur. J. Org. Chem. 0000, 0–0
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Y. Zhang, H. Lu et al.
SHORT COMMUNICATION
Table 1. Optimization of the reaction conditions.[a]
Entry
Ni source (mol-%)
Ligand (mol-%)[b]
Oxidant (equiv.)
Yield [%][c]
1
2
3
4
5
6
7
8
NiCl2 (10)
NiCl2(PCy3)2 (10)
NiCl2 (10)
NiCl2 (10)
NiCl2 (10)
NiCl2 (10)
NiCl2 (10)
Ni(ClO4)2 (10)
Ni(OTf)2 (10)
NiCl2 (10)
NiCl2 (10)
NiCl2 (10)
NiCl2(PCy3)2 (10)
NiCl2(PCy3)2 (10)
NiCl2(PCy3)2 (10)
NiCl2(PCy3)2 (5)
PCy3 (20)
–
–
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2O (3)
AgOAc (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (3)
Ag2CO3 (2.5)
Ag2CO3 (2.5)
53
52
14
41
46
56
62
47
60
PPh3 (20)
IMes·HCl (20)
TPO (20)
IPr·HCl (20)
IPr·HCl (20)
IPr·HCl (20)
IPr·HCl (20)
IPr·HCl (20)
9
10
11
12
13
14[e]
15[e,f]
16[e,f]
61
0[d]
68
IPr·HCl (20)/PCy3 (20)
IPr·HCl (20)
70
75
IPr·HCl (20)
IPr·HCl (20)
83 (81)[g]
53
IPr·HCl (20)
[a] Conditions: 1a (0.2 mmol), 2a (0.6 mmol), Ni source, ligand, Ag2CO3 (0.6 mmol), solvent (2 mL), 170 °C, 24 h. [b] IMes = 1,3-
Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene. IPr = 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene. TPO = Tri-ortho-tolylphosphine.
1
[c] Yields are based on 1a and were determined by analysis of the crude material by H NMR spectroscopy by using dibromomethane
as the internal standard; 1a was totally consumed under these reaction conditions. [d] Most of 1a remained. [e] Solvent (1.5 mL).
[f] Ag2CO3 (2.5 equiv.) with 1,4-benzoquinone (BQ) (0.5 equiv.). [g] Yield of isolated product.
taken, and it turned out that although this reaction could nickel-catalyzed sp2 C–H functionalization process with
be effectively catalyzed by a variety of nickel species, none good functional group tolerance. This method provides a
of the catalysts provided better results than those obtained complementary approach to access important 2-aryl-substi-
with NiCl2 (Table 1, entries 8 and 9). Further screening of tuted oxazoles and thiazoles derivatives.
the oxidants showed that this reaction could also be per-
formed with Ag2O (Table 1, entry 10). It was then noticed
that the yield of the reaction could be improved by using Experimental Section
NiCl2(PCy3)2 as the catalyst in a higher concentration
General Procedure: A 35 mL oven-dried pressure tube was charged
(1.5 mL of (trifluoromethyl)benzene). Interestingly, the re-
with the azole
1 (0.2 mmol), benzoic acid 2 (0.6 mmol),
action yield was further increased by the addition of 1,4-
benzoquinone as a co-oxidant (Table 1, entry 15). Notably,
the reaction yield significantly decreased with a lower cata-
lyst loading (Table 1, entry 16).
NiCl2(PCy3)2 (13.8 mg, 0.02 mmol), IPr·HCl (17 mg, 0.04 mmol),
Ag2CO3 (138 mg, 0.5 mmol), BQ (10.8 mg, 0.1 mmol), and (tri-
fluoromethyl)benzene (1.5 mL). The tube was then sealed and
stirred vigorously at 170 °C for 24 h. After cooling to room tem-
perature, the mixture was diluted with CH2Cl2 (15 mL), filtered
through a pad of Celite, and then the filtrate was concentrated
in vacuo. The residue was purified by flash chromatography on sil-
ica gel (gradient eluent of 5% CH2Cl2 in hexanes, v/v) to give the
desired product.
With the optimal conditions in hand, the substrate scope
of the azole moiety was subsequently investigated. As
shown in Table 2, a variety of functional groups such as
methoxy, methyl, halo (fluoro, chloro, and bromo), ester,
nitro, and trifluoromethyl were tolerated under the reaction
conditions. Furthermore, benzoxazoles 3a–h, oxazoles 3j–r,
and benzothioazole (3i) were all effective substrates.
Next, the generality of the reaction for aromatic acids
was examined. As shown in Table 3, 2-nitrobenzoic acids
with electron-donating and electron-withdrawing groups in
the C4, C5, or C6 position were compatible with the reac-
tion conditions (see products 3s–ae). Furthermore, mono-
and multifluorinated benzoic acids also produced the corre-
sponding biaryl products in moderate yields (see products
3af–aj).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and NMR spectroscopic data of all new
compounds.
Acknowledgments
The authors would like to acknowledge financial support from the
National Basic Research Program of China (grant number
2011CB935800), the National Natural Science Foundation of
China (NSFC) (grant numbers 21332005, 21372115, 21472085),
and the Natural Science Foundation of Jiangsu Province (grant
numbers BK2012012, BK20131267).
Conclusions
In summary, we developed the decarboxylative cross-
coupling of azole derivatives and benzoic acids through a
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Nickel-Catalyzed Decarboxylative Arylation of Heteroarenes
Table 2. Scope of the azole derivatives.[a]
Table 3. Scope of the benzoic acids.[a]
[a] Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), Ni(PCy3)2Cl2
(10 mol-%), IPr·HCl (20 mol-%), Ag2CO3 (2.5 equiv.), BQ
(0.5 equiv.), (trifluoromethyl)benzene (1.5 mL), 170 °C, 24 h. Yields
of isolated products are given. [b] Ag2CO3 (3.0 equiv.) without BQ.
[c] (Trifluoromethyl)benzene (2.0 mL).
[a] Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol), Ni(PCy3)2Cl2
(10 mol-%), IPr·HCl (20 mol-%), Ag2CO3 (2.5 equiv.), BQ
(0.5 equiv.), (trifluoromethyl)benzene (1.5 mL), 170 °C, 24 h. Yields
of isolated products are given.
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SHORT COMMUNICATION
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Received: September 19, 2014
Published Online: ᭿
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Nickel-Catalyzed Decarboxylative Arylation of Heteroarenes
C–H Functionalization
K. Yang, P. Wang, C. Zhang, A. A. Kadi,
H.-K. Fun, Y. Zhang,* H. Lu* ......... 1–5
Nickel-Catalyzed Decarboxylative Aryl-
ation of Heteroarenes through sp2 C–H
Functionalization
The nickel-catalyzed direct decarboxylative
arylation of hetereoarenes with benzoic
acids through an sp2 C–H functionalization
process is reported. This transformation
provides the first examples of decarb-
oxylative cross-coupling reactions with aro-
matic acids through nickel catalysis. IPr =
1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-
ylidene, BQ = 1,4-benzoquinone.
Keywords: C–H activation
/
Decarb-
oxylation
Nickel
/
Arylation
/
Heteroarenes
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Eur. J. Org. Chem. 0000, 0–0
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