Rhodium(I)-catalyzed regiospecific dimerization of aromatic acids: Two direct C-H bond activations in water

Article abstract of DOI:10.1002/anie.201500220

2,2'-Diaryl acids are key building blocks for some of the most important and high-performance polymers such as polyesters and polyamides (imides), as well as structural motifs of MOFs (metal-organic frameworks) and biological compounds. In this study, a direct, regiospecific and practical dimerization of simple aromatic acids to generate 2,2'-diaryl acids has been discovered, which proceeds through two rhodium-catalyzed C-H activations in water. This reaction can be easily scaled up to gram level by using only 0.4-0.6 mol% of the rhodium catalyst. As a proof-of-concept, the natural product ellagic acid was synthesized in two steps by this method. On the double: An efficient, regiospecific, and general oxidative dimerization of simple aryl acids to generate diaryl acids was developed. The reaction involves two direct aryl C-H activations catalyzed by rhodium, uses water as the solvent, and can be easily scaled up. The natural product ellagic acid was obtained in only two steps by using this method.

Full text of DOI:10.1002/anie.201500220

Angewandte
Chemie
DOI: 10.1002/anie.201500220
Cross-Coupling
Rhodium(I)-Catalyzed Regiospecific Dimerization of Aromatic Acids:
À
Two Direct C H Bond Activations in Water**
Hang Gong, Huiying Zeng, Feng Zhou, and Chao-Jun Li*
Abstract: 2,2’-Diaryl acids are key building blocks for some of
the most important and high-performance polymers such as
polyesters and polyamides (imides), as well as structural motifs
of MOFs (metal–organic frameworks) and biological com-
pounds. In this study, a direct, regiospecific and practical
dimerization of simple aromatic acids to generate 2,2’-diaryl
acids has been discovered, which proceeds through two
À
rhodium-catalyzed C H activations in water. This reaction
can be easily scaled up to gram level by using only 0.4–
0.6 mol% of the rhodium catalyst. As a proof-of-concept, the
natural product ellagic acid was synthesized in two steps by this
method.
2,2’-Diaryl acids are the key building blocks for some of
the most important and high-performance polymers (high
thermal stability, low coefficient of thermal expansion, and
Figure 1. Representative compounds with a 2,2’-diaryl acids frame-
work.
excellent mechanical/electrical properties; Figure 1a).[1]
Moreover, these motifs also play an important role in the
construction of MOFs (Figure 1b)[2] and are widely present in
natural products (Figure 1c),[3] pharmaceutical agents (Fig-
ure 1d),[4] and C2-symmetric ligands (Figure 1e).[5]
Despite their importance, the synthetic methods for
making 2,2’-diaryl acids are unsatisfactory. One of the most
popular methods is the oxidative cleavage of phenanthrene,[6]
phenanthrene-9,10-dione,[7] or phenanthrene-9,10-diol[8]
(Scheme 1a). Various catalysts such as Pd/In, Ru, Os, or N-
heterocyclic carbenes (NHC), along with a variety of oxidants
including H2O2, KO2, NaClO, KMnO4, and air, have been
reported for this method. However, the requirement of a pre-
synthesized biaryl motif limits its practicability and generality.
Another well-known method is the Ullmann-type coupling
reaction (Scheme 1b),[9] however, the required halogen
prefunctionalization of the aromatic acids as well as the
intolerance of free acids under the classical Ullmann reaction
conditions limit its applications to only a few diaryl acids. A
[*] Dr. H. Gong, Dr. H. Zeng, Dr. F. Zhou, Prof. Dr. C.-J. Li
Department of Chemistry and FQRNT Centre for Green Chemistry
and Catalysis, McGill University
801 Sherbrooke St. W., Montreal, Quebec H3A 0B8 (Canada)
E-mail: cj.li@mcgill.ca
Dr. H. Gong
The Key Laboratory of Environmentally Friendly Chemistry and
Application of the Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105 (China)
Scheme 1. Strategies for the construction of diaryl acid motif.
nbd=norbornadiene.
[**] We thank the CSC (China Scholarship Council) and FQRNT for
a postdoctoral fellowship (to H.G.), as well as NSERC, FQRNT, CFI,
third common strategy towards diaryl acids is the diazotiza-
tion/coupling sequence, which requires multistep synthetic
operations to generate the amino aromatic acids, and there
are also the potential hazards of handling diazo compounds
and the Canada Research Chair (to C.-J.L.) for their support of our
research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500220.
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(Scheme 1c).[10] In addition, oxidative coupling of stoichio-
metric amounts of lithium reagents (Scheme 1d)[11] and the
Suzuki cross-coupling reaction[12] have also been used to
synthesize this motif. However, the required stoichiometric
organolithium reagent as well as the necessary pre-synthesis
of the haloaryl acids and carboxyl aryl boronic acids are
shortcomings of these methods. In light of the principles of
green and sustainable chemistry, a highly desirable alternative
for the construction of diaryl acid moieties would involve two
(41%; entry 7). Either decreasing the MnO2 loading or
lowering the reaction temperature resulted in lower yields
(entries 8–10). When the amount of catalyst was further
decreased to 2.5 mol%, a slightly lower yield (73%) was
obtained (entry 11). Finally, control experiments revealed
that both the rhodium catalyst and MnO2 are essential to this
reaction (entries 12 and 13). By using the optimized reaction
conditions [5 mol% [{Rh(nbd)Cl}2], 3 equiv MnO2, 1508C
under air in water for 24 h], the generality of this method was
then explored.
Gratifyingly, benzoic acid with electron-donating substi-
tutents underwent the dehydrogenative coupling reaction in
high yields (Table 2; 2a–d). However, with 3,4,5-trimethoxy-
benzoic acid (1e), the reaction afforded the product in 40%
yield, presumably owing to the steric hindrance of the meta-
methoxy group of 1e. Importantly, this transformation was
successful for gram-scale preparation, giving a comparable
À
C H activations. Despite of the obvious advantages, for
example, shorter reaction steps and reduced waste, multiple
obstacles such as unfavorable thermodynamics, the generally
À
low reactivity of C H bonds, and selectivity issues make this
strategy particularly challenging.[13,14] To the best of our
knowledge, no known general method for dimerization of
simple aryl acids has been reported. As part of our continuous
[15]
À
interest in C H functionalizations
and aqueous reac-
tions,[16] herein we describe an efficient, regiospecific, and
general oxidative dimerization of simple aryl acids to
generate a wide range of diaryl acids for the first time
(Scheme 1e).[17]
Table 2: Scope of homodehydrogenative coupling reactions of aromatic
carboxylic acids[a]
Our investigation commenced with the reaction of o-
anisic acid using [RhH(Ph3P)4] (10 mol%) as the catalyst,
MnO2 as the oxidant, and water as the solvent at 1508C under
an atmosphere of air (Table 1). Gratifyingly, the desired
Table 1: Selected optimization results.[a]
Entry
Cat. (mol%)
Oxidant
T [8C]
Yield [%]
1
2
3
4
5
6
7
[RhH(Ph3P)4] (10)
[Rh2(AcO)4] (5)
[{Cp*RhCl2}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (5)
[{Rh(nbd)Cl}2] (2.5)
–
MnO2
MnO2
MnO2
MnO2
Na2S2O8
KMnO4
AgOAc
MnO2
MnO2
MnO2
MnO2
MnO2
–
150
150
150
150
150
150
150
150
90
120
150
150
150
56
20
60
85
2
7
41
53
50
75
73
0
8[b]
9
10
11
12
13
[{Rh(nbd)Cl}2] (5)
0
[a] Unless otherwise noted, all reactions were conducted on a 0.2 mmol
scale with 3 equiv of oxidant in a sealed tube in 0.5 mL H2O. Yields are
those of isolated products. For other results from the optimization of the
reaction conditions please see the Supporting Information. [b] 1 equiv of
MnO2 was used. Cp*=C5Me5.
homocoupling product 3,3’-dimethyl-2,2’-dibenzoic acid was
formed in 56% yield (entry 1). Encouraged by this result,
various rhodium catalysts were examined (entries 2–4) and it
was found that [{Rh(nbd)Cl}2] (5 mol%) was the most
effective, affording 85% yield of the isolated diaryl acid
(entry 4). Next, various oxidants were evaluated. Only a trace
amount of the product was detected in most cases (entries 5–
7) except for silver acetate, which gave a moderate yield
[a] All reactions were carried out with aromatic acid (0.2 mmol),
[{Rh(nbd)Cl}2] (5 mol%), and MnO2 (3 equiv) in a sealed tube in 0.5 mL
H2O at 1508C for 24 h unless otherwise stated. All yields are those for
isolated products. [b] 1 gram (4.7 mmol) of aromatic acid, 0.6 mol%
catalyst and 5 mL H2O was used, reaction time=72 h; [c] 1 gram
(7.2 mmol) of aromatic acid, 0.4 mol% catalyst and 5 mL H2O was used,
reaction time=72 h.
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yield by using a much lower amount of catalyst (0.6 mol%)
with a longer reaction time (72 h). For substrates bearing
cyclopentyloxyl or tert-butyl groups, the reactions also
proceeded smoothly to furnish the desired products 2 f
(60%) and 2g (40%), respectively. The slightly lower yields
are most likely the result of their poor solubility in water. In
contrast, for substrates bearing electron-withdrawing groups
(1i–l), the reaction afforded the desired product in yields
ranging from 30 to 59% (2i–l); with ortho-fluorobenzoic acid
(1h), the reaction yield increased to 89%. Notably, the
corresponding gram-scale synthesis of 2h was achieved in
80% yield by using 0.4 mol% of the rhodium catalyst.
Interestingly, the substrate with a hemiacetal was also well
tolerated (2l). Under the optimized reaction conditions, b-
naphthoic acid (1m) could be converted into 2m exclusively
at the b-position. Interestingly, consecutive coupling products
were observed in the absence of ortho-substituents (2n–p).
Based on our previous reports on rhodium-catalyzed
unoptimized reaction conditions. This result shows the
potential extension of this method to the synthesis of non-
symmetric diaryl acids as well.
The simplicity and convenience of this novel method to
access diaryl diacids can be readily applied in the synthesis of
natural products. As a proof-of-concept, the antitumor
natural product ellagic acid can be effectively assembled
from 3,4,5-trimethoxybenzoic acid in two steps on a gram
scale (Scheme 4), although concise syntheses of ellagic acid
have been reported previously.[19]
[15b]
À
carboxyl-directed C H bond activation
and other litera-
ture studies,[14] a plausible mechanism for this transformation
is proposed in Scheme 2. We hypothesized that a rhodium-
Scheme 4. Total synthesis of ellagic acid.
In conclusion, an efficient and practical synthetic
approach towards highly important diaryl acids has been
developed for the first time by using a rhodium-catalyzed
oxidative homocoupling of simple aromatic acids in water.
The reaction is environmentally friendly, operationally
simple, not sensitive to air, and compatible with water.
Additionally, this reaction can be easily scaled up to the gram-
scale level with low rhodium catalyst loading (0.4–0.6 mol%).
As an example, this method also provides an avenue for the
rapid assembly of natural products such as ellagic acid, and
unlocks the potential for the preparation of axial chiral
compounds, which are widely used as chiral ligands in
asymmetric catalysis. Further studies on the scope and
mechanism of this method are in progress.
Scheme 2. Proposed mechanism for the rhodium-catalyzed aromatic
dehydrogenative coupling strategy for the construction of the diaryl
acid motif.
Experimental Section
A typical experimental procedure: A solution of an aromatic acid
(0.2 mmol), [{Rh(nbd)Cl}2] (4.6 mg, 0.01 mmol) and activated MnO2
(purchased from Aldrich and used as received, 52.2 mg, 0.6 mmol) in
distilled water (0.5 mL) was stirred in a sealed tube under an
atmosphere of air at 1508C for 24 h. The reaction mixture was then
cooled to room temperature and acidified by dilute HCl to pH < 3,
and then the solvent was evaporated in vacuo. The residue was
purified by preparative thin-layer chromatography (TLC) on silica gel
with diethyl ether containing an appropriate amount of formic acid to
give the pure product.
(III)-initiated dual cyclometallation and a subsequent reduc-
tive elimination would produce the desired coupling product
dicarboxylic acid and a rhodium(I) species, which was
reoxidized to the active rhodium(III) catalyst by the oxidant
MnO2 (Scheme 2).
The cross-dehydrogenative coupling[18] reaction of aro-
matic acids was also examined briefly (Scheme 3), and it gave
a good yield (72%) of the isolated product when using the
Received: January 9, 2015
Published online: && &&, &&&&
À
Keywords: C H activation · cross-coupling · rhodium ·
synthetic methods · water chemistry
.
Scheme 3. Extension to nonsymmetric diaryl diacids.
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Cross-Coupling
H. Gong, H. Zeng, F. Zhou,
C.-J. Li*
&&&& — &&&&
On the double: An efficient, regiospecific,
and general oxidative dimerization of
simple aryl acids to generate diaryl acids
was developed. The reaction involves two
rhodium, uses water as the solvent, and
Rhodium(I)-Catalyzed Regiospecific
Dimerization of Aromatic Acids: Two
can be easily scaled up. The natural
product ellagic acid was obtained in only
two steps by using this method.
À
Direct C H Bond Activations in Water
À
direct aryl C H activations catalyzed by
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