Cobalt-Catalyzed Decarboxylative C-H (Hetero)Arylation for the Synthesis of Arylheteroarenes and Unsymmetrical Biheteroaryls

Article abstract of DOI:10.1021/acs.orglett.7b02730

A cobalt-catalyzed decarboxylative cross-coupling reaction of (hetero)aryl carboxylic acids with benzothiazoles or benzoxazoles is reported. This represents a first example of metal-catalyzed decarboxylative C-H heteroarylation of benzo-fused heterocycles. The transformation provides a convenient route, with good yields and functional group tolerance, to various important arylheteroaryl and unsymmetrical biheteroaryl structural motifs.

Full text of DOI:10.1021/acs.orglett.7b02730

Letter
pubs.acs.org/OrgLett
Cobalt-Catalyzed Decarboxylative C−H (Hetero)Arylation for the
Synthesis of Arylheteroarenes and Unsymmetrical Biheteroaryls
Yanrong Li,† Fen Qian,† Mengshi Wang,† Hongjian Lu,*,† and Guigen Li*,†,‡
†Institute of Chemistry and BioMedical Sciences, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210093, China
‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
S
* Supporting Information
ABSTRACT: A cobalt-catalyzed decarboxylative cross-coupling reaction
of (hetero)aryl carboxylic acids with benzothiazoles or benzoxazoles is
reported. This represents a first example of metal-catalyzed decarboxylative
C−H heteroarylation of benzo-fused heterocycles. The transformation
provides a convenient route, with good yields and functional group
tolerance, to various important arylheteroaryl and unsymmetrical
biheteroaryl structural motifs.
iheteroaryl compounds, such as those in Figure 1, are a class
of privileged scaffolds with interesting biological and
Scheme 1. Transition-Metal-Catalyzed Decarboxylative C−H
Heteroarylation of Heteroarenes
B
Figure 1. Selected biheteroaryl structures.
Pd(OAc)2/CuCO3 system.8a In 2012, Su et al. reported a Pd-
catalyzed decarboxylative C−H heteroarylation reaction of
thiophenes, providing various 2-heteroarylthiophenes with
Ag2CO3 as the oxidant.8b Currently, catalytic decarboxylative
C−H heteroarylation systems for the synthesis of unsymmetrical
biheteroaryls employ noble metal−palladium catalysts and have a
narrow substrate scope with nonbenzo-fused heterocycles and
limited heteroaryl carboxylic acids. In view of the importance of
biheteroaryls,1 the development of environmentally friendly
decarboxylative C−H heteroarylation of benzo-fused hetero-
cycles will be of prime synthetic value.
Recently, due to its inexpensiveness, low toxicity, and unique
catalytic modes, Co has been extensively explored in the catalysis
of C−H bond functionalization.10 Inspired by our recent work on
Co-catalyzed C−H bond functionalization,5c,11,12 we report here
the first example of Co-catalyzed decarboxylative C−H
heteroarylation of benzo-fused heterocycles, providing various
typesofbiheteroaryl structuralmotifs(Scheme1B). Thiscatalytic
system could be used in arylation reactions for the synthesis of
arylheteroarenes.
physical properties and are often found in organic functional
materials, pharmaceuticals, ligands, and natural products.1
Transition-metal-catalyzed cross-coupling reactions of two
functionalized heteroaromatic units are the traditional methods
with which to access biheteroaryl moieties.2 Recently, transition-
metal-catalyzed C−H heteroarylation has emerged as an efficient
and straightforward approach to the synthesis of biheteroaryls.3,4
Decarboxylative C−H functionalization,5 which merges the
modern C−H functionalization processes with state-of-the-art
decarboxylation protocols, uses widely available carboxylic acids
as attractive alternatives to traditional aryl (pseudo)halides and
organometallic counterparts to minimize waste products by loss
of CO2 and opens novel perspectives regarding functional and
space diversity.6,7 In the carboxylate substrate, the position of the
C−C bond formation is predefined, and in the hydrocarbon
reactant, dimerization is minimized. In this way, it becomes an
ideal strategy for C−C bond formation. In fact, construction of
biheteroaryl compounds by transition-metal-catalyzed decarbox-
ylative C−H heteroarylation by a few systems has been reported
(Scheme 1A).8,9 In 2010, Greaney et al. reported a decarbox-
ylative C−H heteroarylation reaction of oxazoles catalyzed by a
Received: September 1, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02730
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Organic Letters
Letter
a
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Synthesis of Unsymmetric Biheteroaryls
b
entry
[Co]
ligand
solvent
C6H5F
yield (%)
1
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
CoCl2
−
PCy3
25
2
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5F
C6H5CF3
C6H5Cl
C6H5Br
C6H5CH3
15
3
PPh3
5
4
TPO
29
5
IMe·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
IPr·HCl
41
6
65
7
18
8
Co(acac)2
Co(OAc)2
CoCO3
15
9
trace
41
10
11
12
13
14
15
16
17
18
19
20
21
Co(NO3)2·6H2O
CoClO4·6H2O
Co(acac)3
CoBr2
10
trace
trace
66
CoBr2
46
CoBr2
34
CoBr2
41
a
c
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), and 2-
CoBr2
2-F-C6H4CF3
3-F-C6H4CF3
4-F-C6H4CF3
C6H5F
82 (81 )
b
fluorobenzotrifluoride (0.8 mL), isolated yield. With 1.0 mmol scale.
CoBr2
72
62
0
CoBr2
and 2-fluorobenzotrifluoride proved to be an optimal solvent
providing 3a in 81% isolated yield (entry 18). Control
experiments showed that no desired product was formed in the
absence of a cobalt salt (entry 21). When the reaction was carried
out under thereduced catalyst loading, ligandor 2a, thedecreased
yield of 3a was observed (entries 22−26). At lower reaction
temperatures, the yield of 3a was greatly decreased (Table S1 in
SI).
d
22
CoBr2
CoBr2
CoBr2
CoBr2
CoBr2
2-F-C6H4CF3
2-F-C6H4CF3
2-F-C6H4CF3
2-F-C6H4CF3
2-F-C6H4CF3
36
18
29
11
59
e
23
f
24
g
25
h
26
a
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), [Co] (0.04
mmol), ligand (0.04 mmol), Ag2CO3 (0.5 mmol), and solvent (0.8
b
1
mL), 160 °C, 24 h. Crude H NMR yields determined by using
dibromomethane as an internal standard. Isolated yield. 10 mol %
CoBr2 was used. 5 mol % CoBr2 was used. 10 mol % IPr·HCl was
used. 5 mol % IPr·HCl was used. 2 equiv of 2a was used. PCy3 =
Tricyclohexyl phosphine, TPO = Triortho-tolylphosphine, IMes = 1,3-
Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, IPr = 1,3-Bis(2,6-diiso-
propylphenyl)-imidazol-2-ylidene.
With the optimum reaction conditions, a wide range of benzo-
fused heterocycle substrates were screened (Scheme 2). The
reaction of benzothiazoles bearing Cl or Br atoms in different
positions of the phenyl ring proceeded smoothly, providing the
corresponding halogenated biheteroaryl products in good to
excellent yields (3b, 3g−j). This is useful because the halogen
atoms in the products could provide more options for further
functional transformations. Aromatic rings substituted with
electron-donating groups such as methoxyl (3c−d, 3k, 3q) and
electron-withdrawing groups (such as Cl and Br) were tolerated.
The scope of benzo-fused heterocycles is not limited to
benzothiazoles. Benzoxazoles are also efficient substrates and
provided the desired products in good yields (3n−o). Without
further optimizing reaction conditions, nonfused heterocycles
such as 5-phenyloxazole and methyl 5-phenyloxazole-4carbox-
ylate could react with 2a to give the cross-coupling products,
despite in low yields (3s (37%), 3t (33%) in SI). No desired
product was observed when 4,5-dimethylthiazole was used as a
coupling component. The scope of heteroaryl carboxylic acids
was then examined under the optimal reaction conditions.
Substituted and unsubstituted benzothiophene-2-carboxylic
acids at the 3-position reacted with various benzothiazoles or
benzoxazoles to produce the corresponding decarboxylative
cross-coupling products 3a−e, 3n in good yields. In addition,
benzofuran-2-carboxylic acids with substituents in different
positons of the aromatic ring also reacted smoothly (3f−m,
3o). More interestingly, other heteroaryl carboxylic acids, such as
1-methyl-indole-2-carboxylic acid, benzothiazole-2-carboxylic
acid, and thiazole-5-carboxylic acid, were tolerated inthiscatalytic
c
d
e
f
g
h
Initially, we examined the decarboxylative cross-coupling of
benzothiazole (1a) with 3-methylbenzothiophene-2-carboxylic
acid (2a) in the presence of catalytic amounts of CoBr2 with
Ag2CO3 as an oxidant (Table 1, entry 1). The desired product, 2-
(3-methylbenzothiophen-2-yl)benzothiazole (3a), was obtained
in 25% yield, with the symmetric biheteroaryl 2,2′-bibenzothia-
zole (3a′),13 generated in 19% yield from the oxidative
homocoupling reaction of benzothiazole (1a) (Scheme S1 in
SupportingInformation(SI)). Tominimizetheformationofsuch
homocoupling products, anextra ligand was added to thereaction
system to control both the reaction rate of the Ag-mediated
decarboxylation and rate of the Co-promoted process (entries 2−
6). While phosphine ligands give lower yields, N-heterocyclic
carbene ligands promote the decarboxylative C−H functionaliza-
tion and inhabit the formation of bibenzothiazole 3a′. When 20
mol % of IPr·HCl was added, 3a was formed in 65% yield (entry
6). Several different types of commercially available cobalt salts
were screened (entries 6−13), and it was found that Co
complexes, including Co(acac)2, CoCO3, and Co(NO3)2·
6H2O, catalyzed the reaction, producing low yields of 3a.
Subsequently, different solvents were examined (entries 14−20)
B
DOI: 10.1021/acs.orglett.7b02730
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Letter
system and provided the desired products (3p−r) with different
biheteroaryl skeleton structures. When indole-2-carboxylic acid
was used to couple with benzothiazole (1a), no desired product
was obtained.
Scheme 4. Mechanistic Studies
Although Pd14 and first row transition metals such as Ni15 and
Cu16 can be utilized in the synthesis of arylheteroarenes by
decarboxylative C−H arylation reactions, the carboxylic acid
substratescopeofthesereactions islimitedtoafewarylcarboxylic
acids bearing strong electron-withdrawing groups in the ortho
position of the phenyl ring. To expand the utility of the reaction
system, the scope of aryl carboxylic acids was explored (Scheme
3). The reaction of 2-nitrobenzoic acid derivatives with electron-
a
Scheme 3. Synthesis of Arylheteroarenes
H/Dexchangeinthereactionofdeuteratedsubstrate D-1nand3-
methylbenzothiophene-2-carboxylic acid (2a) was essentially
absent (Scheme 4B). Primary kinetic isotope effects were
observed in parallel experiments (Scheme 4C), suggesting that
the C−H bond cleavage of benzoxazoles may be the turnover-
limiting step in the reaction. Two intermolecular competition
experiments were carried out (Scheme 4D), suggesting that
electron density in aromatic rings of benzoxazoles does not affect
the reaction rate.
On the basis of these results and previous reports,10,11 we
propose a possible Co(III/IV/II) catalytic cycle (Path 1, Scheme
5). First, the Co(II) catalyst is oxidized by Ag2CO3 to afford the
Scheme 5. Proposed Catalytic Cycle
a
Reaction conditions: 1 (0.2 mmol), 4 (0.6 mmol), and 2-
fluorobenzotrifluoride (0.8 mL), isolated yield.
donating or -withdrawing groups in the aromatic ring proceeded
smoothly providing the expected products in good to high yields
(5a−f). Various halogen substituents on the aromatic ring of
carboxylic acids were tolerated. The substrate scope of the
reaction was not limited to 2-nitrobenzoic acid derivatives.
Benzoic acids with a fluorine atom in ortho position of the phenyl
ring were also efficient substrates, affording the fluorinated
arylheteroarene products (5g−i) in moderate yields. Finally, aryl
carboxylic acid was used to couple with different benzo-fused
heterocycles, providing arylheteroarenes (5j−r) in good yields.
However, benzoicacidderivativeswithoutafluorineatomorNO2
groupintheorthopositionofthephenylring, suchasbenzoicacid,
5-bromo-2-chlorobenzoic acid, 2-chloro-5-nitrobenzoic acid, and
2,6-dimethoxybenzoic acid, could not react with benzothiazole
(1a) under standard conditions.
In order to further understand the mechanism of this catalytic
decarboxylative cross-coupling reaction, the control experiments
described in Scheme 4 were conducted. When the reaction was
performed under standard conditions in the presence of 1.0 equiv
of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), a free
radical scavenger, the yield of 3a was greatly reduced (Scheme
4A), suggesting that a radical mechanism may be involved in the
reaction. The H/D experiment showed that the intermolecular
Co(III) species CoIIIX3. With the assistance of Ag2CO3, this
reacts with benzothiazole (1) to generate the Co(III)
intermediate A through a cooperative deprotonation and
metallization process. The aryl radical species, which is formed
from an aryl carboxylic acid in the presence of Ag2CO3,17 reacts
with intermediate A to afford a Co(IV) species B. Subsequent
elimination of cobalt from B leads to the desired product 3 and
regenerates theCo(II)catalyst. Sincesilversaltsarealsoknownto
promote the decarboxylation of aromatic acids to organometallic
silver-aryl intermediates,18 a catalytic cycle via Co(II/III/I) (Path
2, Scheme 5) cannot be excluded for this reaction.
In summary, we have developed a first decarboxylative C−H
(hetero)arylation reaction of benzo-fused heterocycles with a
CoBr2/IPr·HCl based system. The key to this reaction is the
addition of extra N-heterocyclic carbene ligands which establishes
a balance between the Ag-mediated decarboxylation and the Co-
promoted C−H functionalization process. Compared to the
C
DOI: 10.1021/acs.orglett.7b02730
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ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b02730.
Experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hongjianlu@nju.edu.cn.
*E-mail: guigenli@nju.edu.cn.
ORCID
Hongjian Lu: 0000-0001-7132-3905
Guigen Li: 0000-0002-9312-412X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to acknowledge financial support from the
National Natural Science Foundation of China (Nos. 21332005,
21472085, 21672100), the Fundamental Research Funds for the
Central Universities (No. 020514380114), and the Robert A.
Welch Foundation (D-1361, USA).
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