A simple protocol for Cu-catalyzed protodecarboxylation of (hetero)aromatic carboxylic acids

Article abstract of DOI:10.1039/c5nj02792f

A simple and practical protodecarboxylation of o-nitrobenzoic acids as well as heteroaromatic carboxylic acids with various substituents via using CuI/Et₃N has been established. This transformation provides a viable and low-cost approach to generating previously unavailable substituted arenes from readily accessible aryl carboxylic acids as the starting materials.

Full text of DOI:10.1039/c5nj02792f

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DOI: 10.1039/C5NJ02792F
Journal Name
ARTICLE
A simple protocol for Cu-catalyzed protodecarboxylation of
hetero)aromatic carboxylic acids
(
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
a
a
a
a
Zhaojie Li, Zhengjiang Fu,* Haixia Zhang,‡ Jiao Long,‡ Yuanyuan Song, and Hu Cai*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A simple and practical protodecarboxylation of o-nitrobenzoic as
well as heteroaromatic carboxylic acids with various substituents
via using CuI/Et N has been established. This transformation
3
highlights a viable and low-cost approach to generate unavailable
substituted arenes from readily accessible aryl carboxylic acids as
starting materials.
Owing to the attractive features of low cost, innocuity, high
stability and easy availability, carboxylic acid and carboxylate Scheme 1 Protodecarboxylation of aryl aromatic aicds via different
represent valuable starting materials in synthetic organic chemistry methods.
to generate high value products such as medicinally relevant
8
molecules and biofuel compounds via powerful tools of transition nitrobenzoic acids with DMSO to synthesize aryl methyl thioethers,
1
metal-catalyzed decarboxylative reactions. During the past years, and Cu-catalyzed decarboxylative homocoupling of 2-nitrobenzoic
9
Myers, Goossen, Forgione and others have well established the acids to generate biaryls, as well as Cu-mediated decarboxylative
transition metal-catalyzed decarboxylative coupling of arene halogenation and cyanation of various aryl carboxylic acids to form
2
10
carboxylic acids as well as α-oxocarboxylic acids with alkenes, aryl aryl halides and aryl nitriles. In this context, we have also
3
4
5
2h,6
halides, alkynes, arenes, organoboron reagents,
and other observed decarboxylative methylthiolation of o-nitrobenzoic acids8
7
reagents (eq 1). In these reactions, organometallic intermediates with DMSO by using Cu as the sole promoter, albeit in a mild yield.
generated from the decarboxylative transformations served as Aiming at pursuing our efforts to well understand our recent
8-10
either nucleophiles or electrophiles relying on the nature of works
and further exploit new catalytic methods for
employed transition-metal catalysts, which have exhibited environmentally friendly decarboxylative coupling, thus,
carboxylic acids as versatile coupling partners in organic synthesis. protodecarboxylation of arene carboxylic acids has appeared as an
To some degree, transition metal-catalyzed decarboxylative ideal model reaction to achieve more effective catalyst systems and
coupling for C-C and C-X (X = heteroatom) bond formation has realize more broader range of carboxylic acids substrates.
become crucial transformations in various scientific disciplines, the
In addition, protodecarboxylation reactions are also useful
key reasons are free of preparing or employing moisture-sensitive synthetic tools to remove surplus carboxyl moieties used as
organometallic reagents and in situ extrusion of CO as the nontoxic directing groups to direct regioselectivity in the synthesis of
2
waste instead of usually toxic by-products. Very recently, our group
complex compounds. For these reasons, much effort has been
has developed the bimetallic Pd/Cu promoted decarboxylation of o- devoted into the development of effective methods for transition
metal-catalyzed protodecarboxylation reactions (Scheme 1). Some
11
12
seminal studies include the stoichiometric amounts of Hg, Cu,
12c
and Ag reagents mediated protodecarboxylation of carboxylic
acids substrates, however, the above reports generally suffer from
limitations with regard to harsh reaction conditions and
impracticableness. Moreover, Kozlowski has reported Pd-catalyzed
protodecarboxylation of particularly electron-rich bis-ortho-
substituted benzoic acids with ten equivalents of trifluoroacetic acid
a.
College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031 (China).
E-mail: fuzhengjiang@ncu.edu.cn, caihu@ncu.edu.cn
State Key Laboratory of Structural Chemistry, Fuzhou, Fujian 350002 (China).
b.
13
‡
.Authors contributed equally to this work.
as additive. Zhao has developed a Rh-based catalyst system to
Electronic Supplementary Information (ESI) available.See
DOI: 10.1039/x0xx00000x
promote hydrodecarboxylation of ortho-substituted benzoic acids
14
12c
and nitrophenylacetic acids. Basing on previous studies,
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J. Name., 2013, 00, 1-3 | 1
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1
5
16
Goossen and Larrosa have almost simultaneously presented Ag- Table 1. Selected results for protodecarboxylation of 4,5-
View Article Online
DOI: 10.1039/C5NJ02792F
a
catalyzed protodecarboxylation of benzoic acids which were dimethoxy-2-nitrobenzoic acid 1o under nitrogen atmosphere.
compatible with a range of functionalities. Furthermore, Larrosa has
disclosed Ag-catalyzed selective monoprotodecarboxylation of
(
hetero)aromatic dicarboxylic acids via the activating effects of
17
18
ortho substituents or α heteroatoms. Efforts made by Jafarpour
19
and Greaney also have led to the development of Ag-catalyzed
protodecarboxylation of coumarin-3-carboxylic acids and benzoic
acids respectively via different mechanisms. In comparison with Ag
10
1
species, Au element also has (n-1)d ns electron configuration,
generally, the protodecarboxylation reaction should also proceed
smoothly when the metal center is changed to Au compound. Just
as theoretical analysis, Nolan and coworkers have showcased a Au-
catalyzed protodecarboxylation of (hetero)aromatic carboxylic acids
20
involving activated and de-activated benzoic acids. Despite the
progress has made by recent literatures on catalytic
protodecarboxylative reactions, the common challenge for recently
existing methods involved the requirement of noble metal catalysts.
Therefore, there has been still significant room for improvement for
the catalytic protodecarboxylative reaction. Compared to the above
commonly used transition-metals, copper compounds are surely
much more abundant and inexpensive, which are widely used as
a
b
21
Conditions: 1o (0.2 mmol), DMSO (2 mL), N , 140 °C, 20 h.
catalysts in different types of C-C and C-X bond forming reactions.
In this regard, Goossen has established
2
c
d
Average of two runs. Reaction conducted at 120 °C. Reaction
conducted at 100 °C. Reaction conducted at 120 °C for 10 h.
Reaction conducted under air atmosphere. L1 = 4,7-diphenyl-1,10-
phenanthroline, 120 °C.
a
e
f
copper/phenanthroline/quinoline as the catalyst system for an
effective decarboxylation of a wide spectrum of aromatic carboxylic
g
22
acids at the temperature as high as 170 °C. Considering that
neither phenanthroline nor quinoline is a cheap additive for the
catalytic protodecarboxylative reaction, a mild, simple and general
method for the protodecarboxylation of aromatic carboxylic acids is
therefore still highly demanded. Herein, we describe a study on
effective protodecarboxylation of benzoic acids with the
yield (30%) was obtained when the reaction carried out in NMP or
trace amount of desired product was detected when employing
toluene as the solvent (entries 6-7). Subsequently, the influence of
reaction temperature on the catalytic process was also evaluated,
and study illustrated that lowering reaction temperature to 120 °C
provided a comparable efficiency (93%) (entry 8). Unfortunately,
disappointed reaction conversions were furnished in the case of
further lowering temperature to 100 °C or shortening reaction
time to 10 hours at 120 °C or swithching the reaction atmosphere
to air (entries 9-11). Although high yield (93%) was obtained with
CuI/Et N in DMSO at 120 °C (entry 8), nevertheless, applying
previously reported Cu-catalyzed protodecarboxylation reaction
conditions in DMSO or other solvent to this model reaction,
inferior yield or no product was detected (entries 12-15).
Furthermore, control experiments demonstrated that both CuI and
employment of economical CuI/Et N as catalyst system under
3
relatively mild conditions (120 °C).
We
commenced
our
investigation
by
performing
protodecarboxylation of 4,5-dimethoxy-2-nitrobenzoic acid (1o) as
the model reaction for the optimization studies, due in part to
conveniently manipulate and analyze of resultant decarboxylation
product 3,4-dimethoxynitrobenzene (2o) for its solid state
associated with relatively high boiling point under reduced pressure
at ambient temperature. Table 1 presented some selected results
from the optimization studies that showed the effects of the
catalyst, additive and other parameters on the reaction outcome.
At the outset of the study, one equivalent of acetic acid as proton
donors, the protodecarboxylation of 4,5-dimethoxy-2-nitrobenzoic
acid (1o) in the presence of CuI (0.3 equiv) in DMSO at 140 °C for
3
2
2,23
Et N played an essential role in this protocol (entries 16-17). On the
basis of above results, we decided to perform protodecarboxylation
of 4,5-dimethoxy-2-nitrobenzoic acid (1o) in the presence of 0.3
equiv of CuI with 0.5 equiv of Et N in DMSO at 120 °C for 20 hours
3
2
0 hours under nitrogen atmosphere furnished none of desired 3,4-
3
under nitrogen atmosphere (entry 8) and use these data as our
standard conditions.
dimethoxynitrobenzene (2o) product (entry 1, Table 1). To our
delight, replacing acetic acid with equal amounts of triethylamine
led to a drastically increased yield 94% (entry 2). Under otherwise
identical conditions, halving triethylamine loadings to 0.5
equivalents did not affect the efficiency and led to an essentially
quantitative yield (95%) (entry 3). Nevertheless, further decreasing
Subsequently, the optimal conditions of entry 8 in Table 1
inspired us to explore the scope of this protodecarboxylation
reaction with respect to benzoic acids substrates. As outlined in
8-10
Table 2, similarly to our recent observation,
this Cu-catalyzed
protodecarboxylation of arene carboxylic acids was shown to be
effective for a range of ortho-nitrobenzoic acids (1a-1q) as well as
O- and S-containing heteroaromatic carboxylic acids (1s-1v). Ortho-
nitrobenzoic acid 1i was an effective substrate to straightforward
the loadings of CuI or taking Cu O instead of CuI as catalyst resulted
2
in inferior yields (entries 4-5). A brief solvent screening using 0.3
equiv CuI catalyst announced that DMSO was clearly the best
choice for this catalytic vesion reaction, because either very low
2
| J. Name., 2012, 00, 1-3
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a
Table 2. Protodecarboxylation of various aryl carboxylic acids.
and/or regioselectivity of transformation has been well
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2-3
DOI: 10.1039/C5NJ02792F
established, and it seems that the nitro group at the ortho-
position of benzoic acid substrate facilitate the decarboxylative
reaction to stabilize the transition structure of decarboxylation
procedure and enhance the formation rate of aryl-copper
intermediate in this process. Unfortunately, except for ortho-
nitrobenzoic acids (1a-1q) and 3-chloro-2,6-dimethoxy-5-
nitrobenzoic acid 1r, other aryl carboxylic acids illustrated to be
troublesome substrates in this protocol, regardless of the presence
or absence of a substituent group on the position ortho to the
carboxy.
In conclusion, we have developed a simple and efficient protocol
for Cu-catalyzed protodecarboxylation of ortho-nitrobenzoic acids
and heteroaromatic carboxylic acids under relative mild conditions.
The process demonstrated a practical approach to obtain arenes in
a
Reaction conditions: aryl carboxylic acid (0.2 mmol), CuI (0.3 moderate to good yields (30 - 93%) with high selectivity and
equiv), Et N (0.5 equiv), DMSO (2 mL), nitrogen atmosphere, 120 ° exhibited a broad functional-group tolerance with respect to both
3
b
C, 20 h. Reaction conducted at 140 °C.
electron-donating (Me and MeO) and -withdrawing groups (-Cl, F,
CF , SO Me and NO ), even including unprotected functional groups
3
2
2
afford desired nitrobenzene in 63% yield in the presence of 0.3 (-COOH, NH
and CH
CONH). Practical simplicity, the cheapness of
N and relative mild temperature are the
2
3
equiv CuI together with 0.5 equiv Et N, and it’s the divide catalytic amount of CuI/Et
3
3
between the electron-withdrawing groups (chloro, sulfonyl, nitro, advantages of this method. Thus, the procedure provides a viable
carboxyl, trifluoromethyl and fluoro) and electron-donating (methyl, and low-cost alternative to existing methods for the preparation of
methoxy, amino and acetamido) on the aromatic ring of 2- unavailable substituted arenes from easily available aromatic
nitrobenzoic acid substrate. Except excellent yields from the case of carboxylic acids. Ongoing efforts in our laboratory exploit to further
2
-nitrobenzoic acids bearing chloro and sulfonyl groups (1a-c), decrease catalyst loadings and reduce reaction temperature with
ortho-nitrobenzoic acids tolerated other electron-withdrawing high catalytic efficiency.
groups (1d-h) furnished corresponding products in moderate to
good yields. An amount of 0.3 equiv CuI with 0.5 equiv Et N Experimental section
3
furnished acceptable yields with 4-fluoro-2-nitrobenzoic acid (1g) General
procedure
for
Copper-Catalyzed
and its isomer 5-fluoro-2-nitrobenzoic acid (1h) in DMSO at 140 °C. Protodecarboxylation of (Hetero)Aromatic Carboxylic Acids.
Thus, the electronic nature of substituent on the ortho-nitrobenzoic An oven-dried Schlenk tube equipped with a stir bar was
acid had an influence on the transformation. It is noteworthy that charged with aryl carboxylic acid (0.2 mmol) and CuI (11.4 mg,
complete regioselectivity was obtained for aromatic dicarboxylic 0.06 mmol, 0.3 equiv). The tube was fitted with a rubber
acid 1e to afford monoacid 2e in moderate yield (41%) via removing septum, and then it was evacuated and refilled with nitrogen
the carboxyl ortho to nitro substituent and leaving the other three times. Under nitrogen, the solution of Et N in DMSO (2
3
carboxyl intact. Remarkably, ortho-nitrobenzoic acids bearing mL) was added via syringe. The rubber septum was replaced
electron-donating groups (1j-q) swimmingly afforded corresponding with a Teflon screwcap under nitrogen flow, and the Schlenk
products in satisfied to excellent yields except that the substrate of tube was pressurized to 1 atm. With stirring, the reaction
3
-methylbenzoic acid 1j. It is impressive that even unprotected mixtures were heated at 120 °C for the indicated amount of
nitrogen-based electron-donating groups such as free amino and time (unless otherwise specified), and then cooled down to
acetamido (1p and 1q) were found to be compatible with the room temperature. The resultant mixture was filtered through
reaction conditions. Of unexpected, electron-rich bis-ortho- a short plug of silica gel and then concentrated in vacuo. The
substituted 3-chloro-2,6-dimethoxy-5-nitrobenzoic acid 1s exhibited residue was then purified by flash chromatography on silica gel
as
an
effective
substrate
to
give
5-chloro-2,4- to provide the corresponding product.
dimethoxynitrobenzene 2r in an excellent yield as high as 89%. O-
and S-containing heteroaromatic carboxylic acids (1s-v) proved to
be suitable substrates in this protodecarboxylation transformation,
although 1u and 1v provided target products in good and moderate
yields respectively at a slightly elevated temperature. Notably,
ortho-nitrobenzoic acid and ortho-thiophenecarboxylic acid
tolerated chloro group (1a-b, 1t, 1v) formed the hoped-for products
Acknowledgements
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
Science Foundation of Jiangxi Province (20142BAB213001) and
(
2a-b, 2t, 2v) with the chloro moiety surviving from this system,
which could be used for late-stage modification via the cleavage of Science Foundation of State Key Laboratory of Structural Chemistry
the C–Cl bond, providing the great potential for application of this (20150022) is greatly appreciated.
methodology. The use of ortho weakly coordinating substitutent on
the aromatic ring of benzoic acid core to control the reactivity
This journal is © The Royal Society of Chemistry 20xx
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Page 5 of 5
New Journal of Chemistry
View Article Online
DOI: 10.1039/C5NJ02792F
Simple and practical protodecarboxylation of (hetero)aromatic
carboxylic acids with various substituents through using CuI/Et N has
3
been advantageously achieved.