Palladium (II)-Catalyzed Decarboxylative Cross-Dehydrogenative Coupling: Direct Synthesis of meta-Substituted Biaryls from Aromatic Acids

Article abstract of DOI:10.1002/adsc.201800333

A palladium-catalyzed tandem process of simple aromatic acids has been achieved to afford meta-substituted biaryls in moderate to good yields. The reaction proceeds via carboxyl-directed intermolecular cross-dehydrogenative coupling and subsequent decarboxylation. The new C?C bonds in this transformation are formed in the ortho position of carboxyl and the reaction tolerates electron-rich acids. Both symmetrical and unsymmetrical meta-substituted biaryls can be directly synthesized via this method. (Figure presented.).

Full text of DOI:10.1002/adsc.201800333

Title: Palladium (II)-Catalyzed Decarboxylative Cross-Dehydrogenative
Coupling: Direct Synthesis of meta-Substituted Biaryls from
Aromatic Acids
Authors: Fan Pu, Lin-Yan Zhang, Zhong-Wen Liu, and Xian-Ying Shi
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800333
Link to VoR: http://dx.doi.org/10.1002/adsc.201800333

10.1002/adsc.201800333
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium (II)-Catalyzed Decarboxylative Cross-
Dehydrogenative Coupling: Direct Synthesis of meta-
Substituted Biaryls from Aromatic Acids
Fan Pu,a Lin-Yan Zhang,a Zhong-Wen Liu,a and Xian-Ying Shia,*
a
Key Laboratory for Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid
Chemistry (Ministry of Education), School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an
710062, China
Fax: (86)-29-8153-0726; e-mail: shixy@snnu.edu.cn
Received:((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
triflates[5] or phenylboronic acid[6] have been
Abstract: A palladium-catalyzed tandem process of
demonstrated to afford various biaryls, in which
simple aromatic acids has been achieved to afford meta-
carboxyl served as a versatile arylating reagent via
substituted biaryls in moderate to good yields. The reaction
metal mediated decarboxylation to generate aryl-
proceeds via carboxyl-directed intermolecular cross-
metal intermediates.
A
direct C‒H bond
dehydrogenative coupling and subsequent decarboxylation.
The new C‒C bonds in this transformation are formed in
the ortho position of carboxyl and the reaction tolerates
electron-rich acids. Both symmetrical and unsymmetrical
meta-substituted biaryls can be directly synthesized via this
method.
functionalization combining the decarboxylation
offers an alternative atom economical route
delivering biaryls.[7-9] These decarboxylative cross
couplings do not require any harmful halogenated
benzene as starting materials. However, the arenes
were restricted to a narrow range of substrates, such
as benzene,[7] polyfluoroarenes,[8] or heteroaromatic
compounds.[9]
Keywords: biaryls; cross-dehydrogenative coupling;
decarboxylation; palladium catalysis; aromatic acids
In fact, biaryl compounds can also be formed via
decarboxylative cross-coupling of aromatic acids
(Scheme 1, a). Larrosa and Cai reported the
decarboxylative homocoupling of (hetero)aromatic
carboxylic acids to afford symmetrical biaryls
catalyzed by Pd(TFA)2 or CuI, respectively.[10]
Unsymmetrical biaryls can also be synthesized
through the palladium-catalyzed decarboxylative
heterocoupling of substituted benzoic acids.[11] In
these cases, ortho-substituted electron-deficient
carboxylic acids or heteroaromatic acids are
necessary to facilitate the coupling process, and the
reaction takes place at the original position of the
carboxyl. Thus, ortho-substituted biaryls could be
generated via this decarboxylative coupling.
Employing traceless directing decarboxyltive
coupling, Lorrosa, Su and You reported sequentially
the meta-seletive arylation of aromatic acids with
iodoarenes or heteroarenes as partners to produce
meta-substituted biaryl compounds, which are
difficult to access from inexpensive precursors by
traditional methods.[12] In 2016, Li’s group developed
a regiospecific dimerization of simple aromatic acids
to generate 2,2’-diaryl acids, which proceed through
two rhodium-catalyzed C‒H activation.[13] This result
and our understanding on the decarboxylative
reactions inspire us to conceive that meta-substituted
biaryls can be generated through the cross-
The biaryl motifs are vital intermediates for
synthesizing many fine chemicals applied in various
areas, such as natural products, pharmaceuticals,
pesticides, dyes, and polymer materials.[1] Transition-
metal-catalyzed cross coupling reactions such as
Suzuki, Stille, Kumada-Corriu, Negishi, and Hiyama
reactions, have become the most commonly used
tools for the synthesis of biaryls.[2] However, these
processes generally employ aryl halides as coupling
partners. The requirements for pre-functionalized
substances together with the generation of undesired
byproducts arise as main limitations for these
transformations. Therefore, the development of more
efficient approaches for the synthesis of valuable
biaryls from simple and readily available starting
materials by concise steps continues to be an intense
interest.[1]
In the last decade, the transition-metal-catalyzed
decarboxylative coupling reactions have become an
attractive approach for the regioselective formation of
C–C bonds owing to that carboxylic acids are cheap
synthons and can be easily prepared by a number of
methods.[3] Various synthetic methodologies to
generate biaryls have been developed based on this
strategy. For example, decarboxylative cross-
couplings of carboxylic acids with aryl halides,[4] aryl
1
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10.1002/adsc.201800333
Advanced Synthesis & Catalysis
dehydrogenative coupling of ortho-substituted
aromatic acids and the subsequent decarboxylation
(Scheme 1, b). However, to realize this
entries 18-21).
Subsequently, various conditions concerning the
amount of catalyst, Ag2CO3 and K2HPO4, reaction
time and reaction temperature were examined to
further optimize the formation of this homocoupling
product with DME as a solvent. When 1.25 equiv
Ag2CO3 and 0.25 equiv K2HPO4 were utilized, 69%
yield of 2a could be obtained (Table 1, entry 22).
Questions may be raised from the increased 2a yield
by decreasing the amount of K2HPO4 or increasing
the amount of Ag2CO3 (Table 1, entries 6 and 22). To
confirm this, the experiment employing 1.0 equiv
Ag2CO3 and 0.25 equiv K2HPO4 was carried out
(entry 23), the yield of 2a was almost kept the same
even if the K2HPO4 was decreased from 1.25 equiv to
0.25 equiv (entries 6 and 23). Thus, the enhancement
of the yield lies in the increase of the oxidant.
However, when the amount of Pd(OAc)2 was reduced,
unsatisfactory yield of 2a was observed (see
Supporting Information, Table S1, entries 24-26).
transformation,
the
challenges
including
protodecarboxylation and decarboxylative ipso-
homocoupling of starting aromatic acids must be
surmounted.[14] In this work, we describe an efficient
and regioselective synthesis of symmetrical and
unsymmetrical meta-substituted biaryls from simple
aromatic acids (Scheme 1, b). The reaction proceeds
through an intermolecular cross-dehydrogenative
coupling directed by carboxyl and subsequent
decarboxylation. This method is complementary to
palladium-catalyzed cross-coupling between aryl
halides and organometallic reagents.
Table 1. Selected results for optimizing reaction
conditions.[a]
Scheme 1. Decarboxylative coupling of aromatic acids.
Yield
(%)
9
14
6
Entry
Catalyst
Oxidant
Additive Solvent
To validate our hypothesis, the reaction of 2,4-
dimethylbenzoic acid (1a) was selected as a model
substrate to optimize reaction conditions (Table 1).
Gratifyingly, although the yield was rather low (9%),
the desired homocoupling decarboxylative product
was obtained when 1a was treated with Ag2CO3 in
DME employing Pd(TFA)2 as a catalyst (Table 1,
entry 1). 5-Methylisobenzofuran-1(3H)-one (2a’) was
isolated as the main byproduct in this process (See
Supporting Information, Table S1). Inspired by this
result, different palladium catalysts were evaluated
firstly (Table 1, entries 2-5), and Pd(OAc)2 gave a
slightly higher yield of 2a (Table 1, entry 2).
Considering the beneficial effect of K2HPO4 on the
1
2
3
Pd(TFA)2
Pd(OAc)2
PdCl2(PPh3)2
PdCl2
(C6H5CN)2
Ag2CO3
Ag2CO3
Ag2CO3
-
-
-
DME
DME
DME
4
Ag2CO3
-
DME
9
5
6
7
8
Pd(PPh3)4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Ag2CO3
Ag2CO3
Ag2CO3 Na2HPO4 DME
Ag2CO3
Ag2CO3
-
DME
5
K2HPO4 DME
45
24
16
23
KH2PO4 DME
NaOAc DME
9
10
11
12
13
14
15
16
17
18
19
20
21
Ag2CO3 NaH2PO4 DME trace
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
Na2CO3
K2CO3
K2HPO4
K2HPO4
K2HPO4 DMSO
K2HPO4 dioxane
K2HPO4 MePh
K2HPO4 DME
DME trace
DME
THF
ND
ND
14
5
3
4
carboxyl-directed
ortho-functionalization
and
DMF
subsequent decarboxylation,[12b-c,15] K2HPO4 (1.25
equiv) was introduced to the reaction system
expecting to improve the selectivity of desired
product. To our delight, a moderate yield of 2a was
achieved in the presence of K2HPO4 (Table 1, entry
6). Other alkali salts such as Na2HPO4, KH2PO4,
NaOAc afforded lower yields of the product
compared with K2HPO4 whereas NaH2PO4 or
Na2CO3 only gave trace amounts of 2a, K2CO3 failed
to afford the product (Table 1, entries 7-12). Next,
several commonly used solvents were screened.
Unfortunately, no improvement in yield was observed
and DME appeared to be the best solvent for this
transformation (Table 1, entries 13-17). Among the
oxidants investigated, Ag2CO3 was clearly the best
choice. However, some oxidants such as AgOAc,
Cu(OAc)2, CuO, and (NH4)2S2O8 were found to be
ineffective in this homocoupling reaction (Table 1,
ND
Cu(OAc)2 K2HPO4 DME trace
CuO
(NH4)2S2O8 K2HPO4 DME
Ag2CO3
Ag2CO3
K2HPO4 DME
ND
ND
69
22[b] Pd(OAc)2
23[c]
Pd(OAc)2
Reactions were carried out with 1a (0.2 mmol), catalyst
(10 mol%), Ag2CO3 (1 equiv), additive (1.25 equiv),
solvent (0.6 mL) at 150 °C for 24 h, under Ar in
K2HPO4 DME
K2HPO4 DME
46
[a]
1
pressure tubes. Yields were determined by H NMR
analysis of the crude reaction mixture using 1,3,5-
trimethoxybenzene as internal standard.
[b] With Ag2CO3 (1.25 equiv), K2HPO4 (0.25 equiv).
[c] With Ag2CO3 (1.0 equiv), K2HPO4 (0.25 equiv).
2
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10.1002/adsc.201800333
Advanced Synthesis & Catalysis
This can be explained as that Pd(OAc)2 not only
activates two C‒H bonds but also catalyzes
decarboxylation in this transformation (see
Supporting Information, Table S2).
dimethylbenzoic acid, gave the expected product 2i in
46% yield. However, 2,5- or 3,5- di-substituted
aromatic acids failed to afford the desired coupling
products. The reason might lie in the steric effect,
which hinders the formation of the spiro Pd
intermediate. Ortho mono-substituted acids such as 2-
ethoxybenzoic acid and 2-ethylbenzoic acid delivered
the good yields of homocoupling products (2j, 72%
and 2k, 80%), whereas moderate yield was obtained
employing 2-methoxybenzoic acid (2l, 44%). It's
good to see that 2-methoxyl benzoic acid and 2-
ethoxybenzoic acid were active in this transformation
because 2-methoxyl benzoic acid retards the C‒H
functionalization in the previous carboxyl-directing
reactions.[16] In the case of aromatic acids bearing a
With the establishment of the optimal conditions,
the scope and limitation of this homocoupling
reaction were next explored and the results are
presented in Table 2. As summarized in Table 2, the
substituents’ properties and their position had
significant effect on the reactivity. For instance, the
2,3- or 2,4-di-substituted aromatic acids bearing a
methyl at the ortho position of carboxyl all
underwent this transformation smoothly to afford the
corresponding biaryls in moderate to good yields (2b-
2g, 42%-71%). This reaction was also found to be
tolerant of 1-naphthoic acid (2h, 47%).It is worth to
point out that the halogen at meta or ortho-position of
the methyl were all compatible with the catalytic
conditions and afforded 42-71% yields, which
provides the possibility for further useful
transformations through common cross-coupling
method. 3,4-Disubstituted aromatic acids such as 3,4-
Table 3. Substrate scope for the heterocoupling reaction.[a]
Table 2. Substrate scope for the homocoupling reaction.[a]
[a] Reactions were carried out with acids 1m-1z (0.2 mmol),
acids 1m’-1z’ (0.24 mmol), Pd(OAc)2 (20 mol%),
Ag2CO3 (2.5 equiv), K2HPO4 (0.5 equiv), DME (0.6
mL) at 150 °C for 24 h, under Ar in pressure tubes,
isolated yields.
[a]
Reactions were carried out with acids (0.2 mmol),
Pd(OAc)2 (10 mol%), Ag2CO3 (1.25 equiv), K2HPO4
(0.25 equiv), DME (0.6 mL) at 150 °C for 24 h, under
Ar in pressure tubes, isolated yields.
[b] 0.2 mmol acids (1p’, 1r’, 1u’, 1w’, 1x’) were used.
[b] K2HPO4 (1 equiv), Ca(H2PO4)·H2O (0.5 equiv), 12 h.
[c] 12 h.
[c] 0.3 mmol 1t’ was used, 28 h.
[d]
[d] Ag2CO3 (1.5 equiv), K2HPO4 (1.25 equiv), Cu2O (0.25
equiv), 21 h.
Ag2CO3 (3 equiv), K2HPO4 (2.5 equiv), Cu2O (0.5
equiv), 21 h.
3
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10.1002/adsc.201800333
Advanced Synthesis & Catalysis
methyl at the ortho position of the carboxyl,
substituted isobenzofuran-1(3H)-ones were formed as
the main byproducts. In most cases, unreacted acids
were observed after the reaction (See Supporting
Information, Table S4). In contrast, self-
decarboxylation of aromatic acids was not observed
in this catalytic system.
Information, Table S3).
On the basis of the above experiment results as
well as previous reports about palladium-catalyzed
cross-dehydrogenative coupling and decarboxylation
reactions,[17,18] a plausible mechanism is proposed in
Scheme 3. With the help of base, the formed
carboxylate would undertake
a
regioselective
To our delight, the scope of this transformation
could be further extended to the decarboxylative
heterocoupling reaction between different aromatic
acids delivering unsymmetrical biaryls (Table 3).
However, the undesired byproducts via homo-
coupling or intramolecular cyclization reactions were
formed in this transformation (see Supporting
Information, Table S5). It was found that utilizing
equal or slightly excessive substrates, 4-chloro-2-
methylbenzoic acid, 4-bromo-2-methylbenzoic acid,
4-fluoro-2-methylbenzoic acid and 3-chloro-2-
methylbenzoic acid could be reacted with electron-
rich disubstituted benzoic acids and furnished
moderate yields of heterocoupling products (2m-2w,
45%-66%). Heterocoupling between two electron-
rich benzoic acids were also compatible with this
catalytic system. For instance, 4-methoxy-2-
methylbenzoic acid, 2-ethoxybenzoic acid and 1-
electrophilic C‒H substitution by Pd(II) via the C‒H
activation directed by carboxyl to generate a dual
cyclometallation A. Then, the mono-cyclemetallation
B formes in the presence of proton with a CO2
released. The reductive elimination and protonation
of B deliver the biphenyl-2-carboxylic acid C and
Pd(0). The Pd(0) is oxidized by Ag2CO3 to regenerate
the Pd species. The reaction of biphenyl-2-carboxylic
acid C with Pd(II) affords the intermediate D. Finally,
the carboxylate D undergo another decarboxylation
and protonation to produce the desired product, and
regenerates the active Pd species. In this cylce, the
Pd(OAc)2 serves as two functions, i.e., the activation
of two C‒H bonds and
decarboxylation.
the promotion of
naphthoic
acid
underwent
cross-
dehydrogenative/decarboxylative reaction smoothly
with 2,4-dimethylbenzoic acid and 2-ethylbenzoic
acid, respectively, affording 2x (42%), 2y (61%) and
2z (52%) in moderate yields.
Further experiments were performed to gain
mechanism insight into this coupling system. When
optimizing the reaction conditions, 33% yield of
3,3',5,5'-tetramethylbiphenyl-2-carboxylic acid (2a’’)
was observed in a shorter reaction time of 5 h.
However, the yield of 2a’’ decreased to trace with the
extension of reaction time to 12 h. To test whether
2a’’ was an intermediate to form 2a via
decarboxylation, we prepared 2a’’ and treated it
under different conditions. The results are given in
Table S2 (see Supporting Information). As illustrated
in Scheme 2a, a high yield of the product 2a was
obtained in the presence of Pd(OAc)2 (Scheme 2a),
indicating that 2a’’ is very probably the intermediate
of the catalytic cycle. More importantly, Pd(OAc)2
played a key catalytic role in the decarboxylation.
However, 3,3'-dimethoxybiphenyl-2,2'-dicarboxylic
acid (2m’) failed to generate the decarboxylative
product, although the reaction conditions were
changed in a broad range (Scheme 2b, see Supporting
Scheme 3. A plausible mechanism.
In Summary, a novel synthetic route to meta-
substituted biaryls has been discovered and
developed via the dimerization of simple aromatic
acids catalyzed by palladium catalyst. The reaction
proceeds via intermolecular cross-dehydrogenative
coupling directed by a carboxyl, and subsequent
decarboxylation.
Both
symmetrical
and
unsymmetrical meta-substituted biaryls can be
efficiently synthesized in moderate to good yields.
Unlike previous in-situ decarboxylative coupling
protocols, the new C‒C bonds are formed in the ortho
position of carboxyl in this transformation, and the
reaction tolerates electron-rich acids. Further studies
on the detailed mechanistic investigations are
currently in progress in our laboratory.
Experimental Section
A typical experimental procedure: An oven-dried reaction
vessel was charged with Pd(OAc)2 (4.5 mg, 10 mol%, 0.02
mmol), Ag2CO3 (68.9 mg, 0.25 mmol), aromatic acids (0.2
Scheme 2. Mechanistic investigations.
4
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10.1002/adsc.201800333
Advanced Synthesis & Catalysis
mmol), K2HPO4 (8.7 mg, 0.05 mmol), 1,2-
dimethoxyethane (0.6 mL). The mixture was stirred at
150 °C for 24 h under Ar. When the reaction was complete,
the resulting mixture was cooled to room temperature, and
filtered through a short silica gel pad. Then, the mixture
was concentrated in vacuo to give a residue, which was
purified by preparative thin-layer chromatography (TLC)
on silica gel to afford the corresponding product.
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Acknowledgements
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21776171), the Fundamental Research Funds for the Central
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COMMUNICATION
Palladium(II)-Catalyzed Decarboxylative Cross-
Dehydrogenative Coupling: Direct Synthesis of
meta-Substituted Biaryls from Aromatic Acids
Adv. Synth. Catal.Year, Volume, Page – Page
Fan Pu, Lin-Yan Zhang, Zhong-Wen Liu, and
Xian-Ying Shi*
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