Silver-catalyzed arylation of (hetero)arenes by oxidative decarboxylation of aromatic carboxylic acids

Article abstract of DOI:10.1002/anie.201408630

A long-standing challenge in Minisci reactions is achieving the arylation of heteroarenes by oxidative decarboxylation of aromatic carboxylic acids. To address this challenge, the silver-catalyzed intermolecular Minisci reaction of aromatic carboxylic acids was developed. With an inexpensive silver salt as a catalyst, this new reaction enables a variety of aromatic carboxylic acids to undergo decarboxylative coupling with electron-deficient arenes or heteroarenes regardless of the position of the substituents on the aromatic carboxylic acid, thus eliminating the need for ortho-substituted aromatic carboxylic acids, which were a limitation of previously reported methods.

Full text of DOI:10.1002/anie.201408630

Angewandte
Chemie
DOI: 10.1002/anie.201408630
Cross-Coupling
Silver-Catalyzed Arylation of (Hetero)arenes by Oxidative
Decarboxylation of Aromatic Carboxylic Acids**
Jian Kan, Shijun Huang, Jin Lin, Min Zhang, and Weiping Su*
Abstract: A long-standing challenge in Minisci reactions is
achieving the arylation of heteroarenes by oxidative decarbox-
ylation of aromatic carboxylic acids. To address this challenge,
the silver-catalyzed intermolecular Minisci reaction of aro-
matic carboxylic acids was developed. With an inexpensive
silver salt as a catalyst, this new reaction enables a variety of
aromatic carboxylic acids to undergo decarboxylative coupling
with electron-deficient arenes or heteroarenes regardless of the
position of the substituents on the aromatic carboxylic acid,
thus eliminating the need for ortho-substituted aromatic
carboxylic acids, which were a limitation of previously
reported methods.
copper-promoted decarboxylation to generate in situ aryl–
silver or aryl–copper intermediates, aromatic carboxylic acids
can cross-couple with aryl halides,[8] electron-rich heteroar-
enes,[9] polyfluorobenzenes,[10] and even other aromatic car-
boxylic acids[11] by using palladium catalysts. However, such
decarboxylative cross-coupling reactions are usually limited
to ortho-substituted aromatic carboxylic acids[12] because
ortho substituents are necessary for silver- or copper-pro-
moted decarboxylation to occur.[13] Besides, these reactions
À
are not amenable to the C H arylation of electron-deficient
(hetero)arenes. To expand the substrate scope of decarbox-
ylative cross-coupling reactions,
a new decarboxylation
approach needs to be identified. As a powerful synthesis
À
T
he importance of aryl–aryl scaffolds in materials science
tool, Minisci reactions effect the C H alkylation of electron-
and medicinal chemistry is a great impetus to the develop-
ment of catalytic methods for constructing such structures.
Among these established methods, the catalytic direct
deficient (hetro)arenes by silver-catalyzed oxidative decar-
boxylation of alkyl carboxylic acids to generate alkyl radi-
cals.[14] Unfortunately, previous studies illustrated that the
Minisci conditions failed to work for aromatic carboxylic
acids,[3a,14e] probably because it was either difficult for
aromatic carboxylic acids to undergo oxidative decarboxyla-
tion to generate aryl radicals, or the aryl radicals generated
were too reactive to be captured by electron-deficient
(hetero)arenes. Very recently, silver-catalyzed oxidative pro-
todecarboxylaton of aromatic carboxylic acids via aryl radical
generation and the related silver-catalyzed intramolecular
decarboxylative arylation of benzoylbenzoic acid have been
successfully achieved,[15] and encouraged us to reexamine the
Minisci reaction of aromatic carboxylic acids. Herein, we
report the first example of silver-catalyzed[16] intermolecular
decarboxylative arylation of electron-deficient (hetero)ar-
enes with aromatic carboxylic acids, thus providing a solution
to the long-standing challenge of Minisci reactions
(Scheme 1). This catalytic protocol enables a variety of
aromatic carboxylic acids to act as arylating reagents without
the need for ortho substituents, and is compatible with
a broad range of functional groups.
À
arylation of aromatic C H bonds is one of the most attractive
approaches to biaryl compounds because such a reaction
proceeds in a step- or atom-economical fashion.[1,2] In spite of
À
the progress in this area, the catalytic C H arylation of
electron-deficient arenes or heteroarenes, such as pyridines,
has remained a challenging goal. Only recently have pioneer-
ing studies on the arylation of pyridines been reported,[3–6]
including silver-catalyzed coupling with arylboronic acids
through aryl radical formation,[3a] nickel-catalyzed coupling
with arylzinc reagents by 1,2-addition of a nucleophile,[4]
palladium-[5a–c] or rhodium-catalyzed[5d] coupling with aryl
halides, and palladium-[6a–c] or rhodium-catalyzed[6d] C H/C
H cross-coupling with electron-rich heteroarenes. However,
significant room still exists for improvement of these reac-
tions with regard to generality, catalytic efficiency, and
reaction conditions.
In contrast, the palladium/silver- or palladium/copper-
catalyzed decarboxylative cross-coupling of aromatic carbox-
ylic acids to synthesize biaryl compounds has captured
considerable interest because aromatic carboxylic acids are
readily available, stable, and low-cost.[7] Based on sivler- or
À
À
[*] J. Kan, S. Huang, J. Lin, M. Zhang, Prof. W. Su
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Yangqiao West Road 155, Fuzhou, Fujian 350002 (China)
E-mail: wpsu@fjirsm.ac.cn
[**] Financial support from the 973 Program (Nos. 2011CB932404,
2011CBA00501), NSFC (Nos. 21332001, 21431008, 21301173) and
the CAS/SAFEA International Partnership Program for Creative
Research Teams is greatly appreciated.
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408630.
Scheme 1. Silver-catalyzed C H functionalization with (hetero)arenes
by oxidative decarboxylation to generate radicals.
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Table 1: Selected results for the screening of reaction conditions.[a]
reaction system as compared to the Minisci reaction of
boronic acids reported by Baran and co-workers.[3a]
With the optimized reaction conditions in hand, we next
evaluated the substrate scope with respect to aromatic
carboxylic acids (Scheme 2). Various ortho-, meta-, para-
Entry
AgX (mol%)
2a (mL)
MeCN (mL)
Yield [%][b]
1
AgOAc (20)
AgOAc (20)
AgOAc (20)
AgOAc (100)
AgOAc (5)
2 (90 equiv)
2
2
2
2
2
2
2
34
21
<5
<5
43
2[c]
3[d]
4
5
2
2
6
7
8
9
10
11
12[i]
Ag2SO4 (2.5)
Ag2SO4 (2.5)
Ag2SO4 (2.5)
AgNO3 (5)
AgTFA (5)
Ag2SO4 (10)
Ag2SO4 (2.5)
2
2
1
58[e]
71[f]
72[g]
76
1 (45 equiv)
0.5 (22.5 equiv)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
80
81 (77)[h]
<5
[a] Reaction conditions: 1a (0.25 mmol), 2a, silver(I) salt, K2S2O8
(0.75 mmol), 1208C, 24 h, N2. [b] Yield determined by GC.
[c] (NH4)2S2O8 (0.75 mmol) instead of K2S2O8. [d] Na2S2O8 (0.75 mmol)
instead of K2S2O8. [e] 36% of 1a was recovered and 1.6% cyanobenzene
and trace biphenyl were detected by GC. [f] 21% of 1a was recovered and
3.8% cyanobenzene and trace biphenyl were detected by GC analysis.
[g] 16% of 1a was recovered and 3.5% cyanobenzene and trace biphenyl
were detected by GC. [h] Yield of isolated product. [i] 20 mL H2O was
added. TFA=trifluoroacetate.
The decarboxylative cross-coupling between 4-cyanoben-
zoic acid (1a) with benzene (2a) was selected as a model
reaction for optimization studies (Table 1). At the very
beginning, the reaction of 0.25 mmol of 1a with a large
excess of 2a (2 mL), conducted in 2 mL MeCN at 1208C in
the presence of AgOAc (0.2 equiv) and K2S2O8 (3 equiv),
gave the arylated product 3a in 34% yield after 24 hours
(entry 1). When using Na2S2O8 or (NH4)2S2O8 as the oxidant
instead of K2S2O8, the yields of 3a dropped to 21% (entry 2)
and less than 5% (entry 3), respectively. To our surprise,
increasing the AgOAc loading to 1.0 equivalent give only
a trace amount of 3a along with a lot of the protodecarbox-
ylation side-product and toluene (in 8% yield relative to
AgOAc; entry 4), whereas reducing the AgOAc loading to
0.05 equivalents increased the yield to 43% (entry 5). After
that, the efficacy of this arylation reaction in the presence of
other sliver sources was examined. Several silver slats, such as
AgNO3, AgTFA and Ag2SO4 proved to be effective catalysts
and led to better conversion (see the Supporting Informa-
tion). Reducing the volumes of both benzene and MeCN
significantly improved the yields (entries 7 and 8). The
positive effect observed in entries 7 and 8 may result from
the increase in the concentration of the silver(II) complex
generated from oxidation of Ag2SO4 by K2S2O8, a step which
should accelerate decarboxylation of aryl carboxylic acid and
improve conversion of aryl carboxylic acid.[17] By using 0.5 mL
benzene and 0.5 mL MeCN, the yield increased to 80% when
using 5 mol% AgTFA and to 81% when using 10 mol%
Ag2SO4 (entries 9–11). Additionally, the addition of a minute
amount of water to the reaction system sharply diminished
the yield of 3a (entry 12). The success of this method is to
a large extent attributed to removal of water from the
Scheme 2. Scope of aromatic carboxylic acids. Reaction conditions:
aromatic carboxylic acid (0.25 mmol), benzene (0.5 mL), silver(I) salt
(5–10 mol%), K2S2O8 (0.75 mmol), MeCN (0.5 mL), 1208C, 24 h, N2.
Yields are those of isolated products. [a] 10 mol% AgSO4
(0.025 mmol). [b] 5 mol% AgNO3 (0.0125 mmol). [c] 5 mol% AgTFA
(0.0125 mmol).
substituted, as well as disubstituted aromatic carboxylic acids
provided the desired cross-coupling products in good yields.
The reaction tolerated a variety of electron-withdrawing
substituents such as nitro (3b, 3h, 3l–q, 3r, 3v, 3z), mesyl (3i,
3o, 3t), trifluoromethyl (3c, 3k, 3n), fluoro (3y, 3z), chloro
(3r–u, 3w, 3aa, 3ac, 3ad), bromo (3j, 3u, 3v–x), cyano (3a,
3s, 3y), ester (3d), acetyl (3e) and benzoyl (3 f) groups.
Although the ortho substituents were not necessary in this
protocol, benzoic acids containing ortho substituents gave
higher yields (3l versus 3b, 3h).
Picolinic acid, which can stabilize high-valent silver
species,[18] also participated in the decarboxylative cross-
coupling reaction and gave reasonable yields (3aa–ad;
Scheme 2). Notably, the substituents influenced the efficiency
of the decarboxylative cross-coupling reaction because of the
difference in decarboxylation rates. For example, benzoic
acids bearing an ortho-chloro substituent gave better results
compared with ortho-nitro- and ortho-bromo-substituted
2
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formed a mixture of isomers (4c–e). Unfortunately, arenes
bearing electron-rich substituents did not work well in this
reaction. In the case of pyridines, trifluoroacetic acid (TFA)
was essential for the decarboxylative arylation, presumably
because the introduction of TFA enhances the reactivity of
the pyridines towards capturing nucleophilic aryl radical
intermediates generated from decarboxylation.[14e] Similar to
Minisci reactions with alkyl carboxylic acids, arylation
occurred preferentially at the C2-position of pyridines over
the C3- and C4-positions (4 f–j). For 4-tert-butylpyridine and
4-methylpyridine, only the C2-arylated products were formed
as a result of steric and electronic effects (4k,l). In contrast,
electron-withdrawing groups such as CF3, CN, and an ester
obviously reduced the reaction selectivity (4m–p). Pyridines
containing two electron-withdrawing groups at C3 and C5
were observed to afford higher yields (4q–t). With the
exception of CF3-substutied pyridines, the reaction selectivity
exhibited by all substituted pyridines can be predicted using
guidelines put forward by Baran, Blackmond, et al.[3d]
The previous studies on silver-catalyzed Minisci reac-
tions[14,17] revealed that these reactions involved silver(II)-
promoted oxidative decarboxylation of alkyl carboxylic acids
to generate alkyl radicals. The reactivity and selectivity
observed in the silver/persulfate-catalyzed arylation reaction
of electron-deficient heterocycles with arylboronic acids
points to the possibility that this reaction involves the
generation of an aryl radical.[3a] To gain mechanistic insights,
additional experiments were performed. When the reaction
was conducted in the presence of radical scavengers, such as
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 2,6-di-
tert-butyl-4-methylphenol (BHT), none of decarboxylative
cross-coupling products were observed. Competition experi-
ments between benzene and [D6]benzene showed a negligible
kinetic isotope effect (kH/kD = 1.3), thus suggesting that the
À
rate-limiting step does not involve C H bond cleavage of
benzene.
On the basis of the preliminary results and previous
reports, a possible mechanism is proposed (Scheme 4). At the
beginning of the reaction, the silver(I) salt is oxidized to
a silver(II) species by the persulfate anion. Then, the silver(II)
species oxidizes the aromatic carboxylic acids 1, thus leading
to decarboxylation and aryl radical (A) generation. The aryl
radical subsequently adds to benzene or pyridine to generate
Scheme 3. Scope of (hetero)arenes. Reaction conditions: aromatic
carboxylic acid (0.25 mmol), (hetero)arenes (5.625 mmol), silver(I) salt
(5–25 mol%), K2S2O8, 1208C, 24 h, N2. Yields are those of isolated
products and the relative ratio of regioisomers was calculated using
GC-MS data. [a] 10 mol% Ag2SO4 (0.025 mmol). [b] 5 mol% AgNO3
(0.0125 mmol). [c] 5 mol% AgTFA (0.0125 mmol). [d] 25 mol%
Ag2SO4 (0.0625 mmol). [e] K2S2O8 (0.75 mmol), without TFA. [f] K2S2O8
(1.5 mmol), TFA (5.625 mmol). [g] MeCN (0.5 mL). [h] MeCN
(0.75 mL). [i] MeCN (1 mL). [j] MeCN (1.5 mL).
benzoic acids. The latter produced more of the protodecar-
boxylation side-products.
The decarboxylative arylation reaction was applicable to
benzenes bearing electron-withdrawing groups and pyridines
(Scheme 3), but reaction conditions needed to be adjusted
slightly to obtain satisfying yields and depended on the
(hetero)arenes (see the Supporting Information for details).
Although an excess of (hetero)arene (22.5 equiv) was used,
more than 90% starting materials could be recovered through
column chromatography. 1,4-Dichloro- and 1,4-difluoro-sub-
stituted benzenes offered pure products (4a, 4b) while 1,2-
disubstituted benzenes and monosubstituted benzenes
Scheme 4. The proposed mechanism for silver-catalyzed decarboxyla-
tive arylation.
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Received: August 28, 2014
Revised: November 16, 2014
Published online: && &&, &&&&
À
Keywords: C H activation · heterocycles · oxidation · radicals ·
silver
.
À
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4
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Angewandte
Chemie
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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Angewandte
Communications
Communications
Cross-Coupling
J. Kan, S. Huang, J. Lin, M. Zhang,
W. Su*
&&&& — &&&&
Silver-Catalyzed Arylation of
(Hetero)arenes by Oxidative
Decarboxylation of Aromatic Carboxylic
Acids
Silver hammer: The silver-catalyzed
decarboxylative arylation of electron-defi-
cient (hetero)arenes has been success-
fully developed using aromatic carboxylic
acids as arylating reagents. For most of
aromatic carboxylic acids evaluated,
5 mol% of the silver(I) salt was enough
for the oxidative decarboxylation. An
ortho substituent was not necessary for
this decarboxylative cross-coupling pro-
tocol.
6
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