Facile synthesis of 2,2′-dinitrosubstituted biaryls through Cu-catalyzed ligand-free decarboxylative homocoupling of ortho-nitrobenzoic acids

Article abstract of DOI:10.1039/c5ra07771k

A novel waste-free Cu-catalyzed decarboxylative homocoupling of ortho-nitrobenzoic acids has been developed, and diverse substituents on the phenyl core of ortho-nitrobenzoic acid are compatible with the transformation. This method provides a practical alternative to synthesize valuable 2,2′-dinitrosubstituted biaryls from cheap and readily available ortho-nitrobenzoic acids.

Full text of DOI:10.1039/c5ra07771k

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DOI: 10.1039/C5RA07771K
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Facile synthesis of 2,2’-dinitrosubstituted biaryls through Cu-
catalyzed ligand-free decarboxylative homocoupling of ortho-
nitrobenzonic acids
Received 00th January 20xx,
Accepted 00th January 20xx
Zhengjiang Fu,a Zhaojie Li,a Qiheng Xiong,a and Hu Cai*a
DOI: 10.1039/x0xx00000x
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A novel waste-free Cu-catalyzed decarboxylaitve homocoupling of dilemmas have impeded to widely prepare biaryls via the above
ortho-nitrobenzoic acids has been developed, and diverse protocols, and have continued to stimulate the development of
substituents on the phenyl core of the ortho-nitrobenzoic acid are facile methods for the synthesis of biaryls with easily available
compatible with the transformations. This method provides a substrates as well as good regio- and chemoselectivities.
practical alternative to synthesize valuable 2,2’-dinitrosubstituted Pioneered by Myers who reported a Pd-catalyzed decarboxylative
biaryls from cheap and readily available ortho-nitrobenzoic acids.
Heck coupling of arene carboxylic acids with olefins,5 carboxylic
acids and transition metal-catalyzed decarboxylative reactions have
been hot research topics in the field of catalysis and organic
synthesis in the last decade,6-10 because of ready availability and
low cost of carboxylic acids as well as nontoxic carbon dioxide as by-
product in the decarboxylative reactions. Therefore, transition
metal-catalyzed decarboxylative coupling reactions of aryl
carboxylic acids have recently emerged as an attractive and
alternative approach to synthesis of biaryl compounds.11 In this
context, taking advantage of aromatic carboxylic acid as arylating
partner to couple with aryl (pseudo)halides, arylboron reagents and
even unfunctionalized (hetero)arenes to deliver biaryl compounds.
Under transition-metal catalysis, the in situ generated aryl-metal
intermediate from decarboxylation of aryl carboxylic acid can serve
as either nucleophile or electrophile depending on the nature of the
employed transition-metal catalyst, which demonstrates the
potential of aromatic carboxylic acid as versatile coupling partner.
In spite of the advantages of these decarboxylative coupling
methods, and although biaryl compounds can be achieved in above
elegant studies, however, there is still significant room for
improvement for the unwelcome waste and/or reaction selectivity.
Most notably, it’s an ideal and much greener route to synthesize
biaryls from the decarboxylative coupling reaction between aryl
carboxylic acids, since the reaction only takes place at the original
aryl position of the carboxylic acid group with high selectivity, and
only innocuous carbon dioxide is extruded as by-product from the
reaction. In the year of 2011, the group of Larrosa developed the
first Pd/Ag-catalyzed decarboxylative homocoupling of substituted
aromatic and heteroaromatic carboxylic acids to prepare
symmetrical biaryls in good yields.12 Tan, and Deng et al. further
It’s well known that biaryls not only constitute important structural
motifs in many functional materials, natural products and
pharmaceuticals,1 but also are usually employed as useful building
blocks in organic catalysts and ligands in organic synthesis to
construct complex molecules,2 therefore, synthesis of biaryl scaffold
has been
a matter of great interest in organic synthesis
community.3 In this regard, considerable studies has been devoted
into developing quantities of transformations to create various
biaryl compounds, and transition metal-catalyzed coupling
procedures have exhibited as preferential choice to synthesize
biaryls for their straightforward and concise routes. Traditionally,
great progress has been achieved through transition metal-
catalyzed coupling of haloarenes or arylmetallic reagents, including
well-known Ullmann coupling reaction.3a-b Nevertheless, the
drawbacks including the prefunctionalization of arenes and
generation of unwanted byproducts, to some extent, limit the
widespread application of the protocols in organic synthesis and
industrial production. To address these issues, reactions involving
transition metal-promoted oxidative coupling of arene C-H bond
activation have emerged as atom- and step-economic alternatives
to synthesize biaryls. Thus, transition metal-promoted oxidative
coupling of arene C-H bond activation is one of the most widely
accepted green transformations to form biaryls.4 In spite of
effectiveness of direct functionalization of arene C-H bonds, the
challenges associated with these transformations are the difficulty
to control the regio- and chemoselectivity of arene C-H bond
activation processes, as well as tedious procedure to remove or
modify the directing groups in many cases. Consequently, these
reported
a
Pd/Ag-mediated decarboxylative homo- and
heterocoupling of substituted benzoic acids to deliver symmetrical
and unsymmetrical biaryl compounds.13 Remarkably, Su disclosed
Pd/Ag-promoted decarboxylative cross coupling between various
a.College of Chemistry, Nanchang University, Nanchang, Jiangxi, China. E-mail:
caihu@ncu.edu.cn, http://chem.ncu.edu.cn/Teacher_show.asp?id=13.
Electronic Supplementary Information (ESI) available: Copy of all the
compounds 1H, 13C and 19F spectra. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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substituted arene carboxylic acids to synthesize asymmetrical Table 1 Selected results for decarboxylative homocoupling of 4-
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a
biaryls exclusively in synthetically useful yields together with only chloro-2-nitrobenzoic acid 1a under nitrogenDaOtmI: o10s.p10h3e9r/eC5RA07771K
trace amount of homocoupling by-products.14 In view of high
loading and expensiveness of silver salts in their protocols,
Djakovitch developed copper as a replacement for silver salt in the
above mentioned protocol for decarboxylative heterocoupling of
substituted benzoic acids, although narrow in substrate scope and
low in reaction yield.15 Therefore, an efficient method for the
formation of biaryls via decarboxylative coupling reaction between
aryl carboxylic acids in the absence of noble metal catalyst is still
highly desirable. Inspired by these works, aiming at pursuing our
efforts to develop sustainable methods for the synthesis of biaryl
compounds16 and exploit new catalytic methods for
environmentally friendly decarboxylative coupling,17 herein, we
firstly describe the reliable synthesis of valuable 2,2’-
dinitrosubstituted biaryls through decarboxylative homocoupling of
2-nitrobenzonic acids with cheap CuI as the sole catalyst under Pd-
or Ag-free conditions.
We commenced our investigation by taking decarboxylative
homocoupling of 4-chloro-2-nitrobenzoic acid (1a) as the model
reaction for the optimization studies. Table 1 presented some
selected results from these optimization studies, which showed the
effects of the catalyst, reaction temperature and other reaction
parameters on the reaction outcome. Initially, in the presence of 2
equivalents of CuI as a mediator, the decarboxylative homocoupling
of 4-chloro-2-nitrobenzoic acid (1a) was carried out in DMSO at
160 °C under nitrogen atmosphere and furnished desired product
a Conditions: 1a (0.2 mmol), solvent (2 mL), nitrogen atmosphere,
4,4'-dichloro-2,2'-dinitrobiphenyl (2a) in 24% isolated yield (entry 1,
c
140 °C, 20 h. b Average of two runs. Reaction conducted at 160 °C.
Table 1). Remarkably, the influence of reaction temperature
showed that lowering reaction temperature to 140 °C led to a much
d Reaction conducted at 120 °C. e 0.15 equiv of 1,10-phenanthroline
was added into the reaction. 0.15 equiv of 2,2'-bipyridine was
added into the reaction.
f
higher efficiency (82%), however, essential trace of the target
product provided when the reaction was performed at 120 °C
(entries 2 and 3). The result indicated the reaction temperature was
an important factor to influence the overall yield. Then, the effect
of different loading of CuI was elaborately evaluated. It’s found that
the reaction offered mild yield (45%) in the presence of a catalytic
amount of 0.4 equiv of CuI. Pleasingly, the yield increased to 75%
when 4 Å molecular sieves (MS) were introduced into the reaction
system (entries 4 and 5). In this regard, MS presumably functioned
as a water scavenger to avoid protonation of decarboxylative aryl-
copper intermediate.11a-b Under otherwise identical conditions,
decreasing the loading of CuI to 0.3 equivalents gave comparable
yield (72%), nevertheless, further reducing CuI loading led to a
slightly lower yield (61%) (entries 6 and 7). Subsequently, the
influence of counterion of the Cu catalyst on the reaction was
examined, and CuI was proved to be the best choice for this
decarboxylative homocoupling transformation (entries 8-10). A
brief survey of different solvent systems under otherwise equal
conditions announced that the combination of CuI and DMSO was
clearly the optimized selection for this catalytic system, because
either inferior yield (11%) exhibited when the reaction carried out
in DMF or no product was detected completely when employing
NMP as the solvent (entries 11-12). Finally, studies indicated that
the additional nitrogen ligands did not display any beneficial effect
on the reaction (entries 13-14). On the basis of these results, we
decided to perform decarboxylative homocoupling of 4-chloro-2-
nitrobenzoic acid (1a) in the presence of 0.4 equiv of CuI in DMSO
at 140 °C with 4 Å MS as additive under nitrogen atmosphere and
use these conditions as our standard conditions.
We next evaluated the substrate scope of this novel Cu-catalyzed
decarboxylative homocoupling protocol with respect to aromatic
carboxylic acids. As depicted in Table 2, ortho-nitrobenzoic acid 1h
was an effective substrate to afford 53% isolated yield under the
standard conditions, and all of the ortho-nitrobenzoic acids
substrates bearing electron-deficient (1a-g) and -rich group (1i, 1k-l)
directly provided the corresponding symmetrical biaryl compounds
in moderate or satisfactory yields. Generally speaking, this protocol
worked for a variety of ortho-nitrobenzoic acids with electron-
withdrawing (chloro, fluoro, bromo, trifluoromethyl, and sulfonyl)
and electron-donating groups (methoxy and methyl). 4-Chloro-2-
nitrobenzoic acid (1a) and its isomer 5-chloro-2-nitrobenzoic acid
(1b) smoothly formed the hoped-for products (2a and 2b) with the
halogen moiety surviving from this catalytic system. Interestingly,
the halogen moiety could be used for late-stage modification via
the transformation of the C–Hal bond. Unlike its isomer 4-methyl-2-
nitrobenzoic acid 1i, 6-methyl-2-nitrobenzoic acid 1j was an inert
substrate toward this process in the presence of CuI catalyst,
indicative of the sensitivity of this transformation to steric
hindrance. Unfortunately, the attempt to employ a broad range of
other benzoic acids in this transformation failed, regardless of the
presence other groups (chloro, methoxy, fluoro, etc.) or absence of
2 | J. Name., 2012, 00, 1-3
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Science Foundation of Jiangxi Province (20142BAB213001) and
Science Foundation of State Key Laboratory DofOSI:t1r0u.c1t0u3r9a/lCC5hRAe0m7i7s7t1rKy
(20150022) is greatly appreciated.
Table
2 Synthesis of symmetrical biaryls via Cu-sataylzed
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decarboxylative homocoupling of ortho-nitrobenzoic acidsa
Notes and references
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a Conditions: 1 (0.2 mmol), CuI (30 mol %), 4 Å MS, DMSO (2 mL),
nitrogen atmosphere, 140 °C, 20 h.
nitro group at the ortho-position of benzoic acid substrate, this
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for Cu-catalyzed decarboxylaitve homocoupling of ortho-
nitrobenzoic acids under noble metal-free conditions. The
method exhibited good functional tolerance with respect to
both electron-donating and -withdrawing groups and
furnished desired nitro-containing biaryl compounds in
moderate or satisfactory yields with high selectivity. Thus, the
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dinitrosubstituted biaryls. Investigation on Cu-catalyzed
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Financial support from the National Key Basic Research Program of
MOST of China (2012CBA01204), National Natural Science
Foundation of China (NSFC) (21301088 and 21162015), Natural
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