Pd-Catalyzed Decarboxylative Sonogashira Reaction via Decarboxylative Bromination

Article abstract of DOI:10.1021/acs.orglett.8b00772

The decarboxylative alkynylation of (hetero)aryl carboxylic acids with terminal alkynes has been achieved by using a Pd(PPh₃)₄/PCy₃ catalyst. This Pd-catalyzed method exhibits good functional group tolerance for both coupling partners and enables chemical modification of complex molecules. The establishment of this decarboxylative alkynylation reaction is attributed to the discovery of a highly selective decarboxylative bromination of (hetero)aryl carboxylic acids with NBS (N-bromosuccinimide).

Full text of DOI:10.1021/acs.orglett.8b00772

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd-Catalyzed Decarboxylative Sonogashira Reaction via
Decarboxylative Bromination
Quandi Jiang,†,‡ Hongyi Li,†,‡ Xiaofeng Zhang,† Biping Xu,†,‡ and Weiping Su*,†
†State Key Laboratory of Structural Chemistry, Center for Excellence in Molecular Synthesis, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China
‡University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
S
* Supporting Information
ABSTRACT: The decarboxylative alkynylation of (hetero)aryl carboxylic acids with
terminal alkynes has been achieved by using a Pd(PPh3)4/PCy3 catalyst. This Pd-catalyzed
method exhibits good functional group tolerance for both coupling partners and enables
chemical modification of complex molecules. The establishment of this decarboxylative
alkynylation reaction is attributed to the discovery of a highly selective decarboxylative
bromination of (hetero)aryl carboxylic acids with NBS (N-bromosuccinimide).
Successful strategies to achieve the C−H alkynylation
reactions led us to consider whether the decarboxylative
Sonogashira reaction of an aryl carboxylic acid with a terminal
alkyne could be realized via in situ generation of an alkynyl
bromide from a terminal alkyne or conversion of an aryl
carboxylic acid to a more active reaction intermediate. The
implementation of such a cascade reaction for decarboxylative
Sonogashira reaction represents a great challenge due to the
following reasons: (1) compatibility beween the formation
reaction intermediate process and subsequent cross-coupling
process; (2) lack of methods for conversion of relatively inert
benzoic acids to an active intermediate. In this regard, Gooßen
et al. reported pioneering work on the decarbonylative Heck
reaction of aryl esters,18 and very recently, Larrosa and co-
workers established the oxidative cross-coupling reactions of
benzoic acids via decarboxylative iodination of benzoic acids.19
These seminal studies encouraged us to develop a tandem
sequence for a decarboxylative Sonogashira reaction.
Herein, we provide a solution to this long-standing challenge
and present a palladium-catalyzed method for synthesis of
alkynylated (hetero)arenes via a decarboxylative cross-coupling
of (hetero)aryl carboxylic acids with terminal alkynes. Initially,
we chose the reaction of phenylacetylene 1a with 2,4-
dimethoxybenzoic acid 2a as a model system to examine the
possibility of our proposed decarboxylative Sonogashira
reaction (Table 1). After screening palladium precursors,
ligands, solvents, oxidants, and additives, we found that the
use of Pd(PPh3)4 as the precatalyst, tricyclohexylphosphine
(PCy3) as the ligand, Cs2CO3 as the base, NBS (N-
bromosuccinimide) as the oxidant, and DBU (1,8-diazabicyclo-
[5.4.0]-7-undecene) as the additive in DMF was optimal to
generate the desired product. With the optimized conditions,
the desired decarboxylative alkynylation product 3a could be
isolated in 75% yield from the reaction system that was stirred
(Hetero)aryl carboxylic acids are readily available in large
structural diversity from nature and synthetic sources. Efforts to
exploit the applications of (hetero)aryl carboxylic acids in
organic syntheses have led to the discovery of a wealth of
decarboxylative cross-coupling reactions.1 In this context, aryl
carboxylic acids are able to cross-couple with aryl halides,2
organoboron reagents,3 olefins,4 and even aryl carboxylic acids5
and functionalize the C−H bond of (hetero)arenes6 through
metal-promoted decarboxylation to form an aryl-metal
intermediate. (Hetero)aryl carboxylic acids also participate in
various metal-catalyzed carboxyl-directed ortho-C−H function-
alization reactions,7 of which products can be further elaborated
via decarboxylative cross-coupling8 or protodecarboxylation.9
These decarboxylative cross-coupling reactions complement
conventional cross-coupling reactions such as Suzuki−Miyaura
coupling and provide a set of new possibilities for construction
of the targeted compounds, especially in the cases where aryl
halide starting materials are not easy to access. Despite this
substantial progress, the decarboxylative cross-coupling of
(hetero)aryl carboxylic acids with terminal alkynes to generate
alkynylated (hetero)arenes has remained untouched to date.
In fact, alkynylated (hetero)arenes represent a class of
attractive structural motifs due to their prevalence in natural
products, pharmaceuticals, and organic materials10 and their
ability to function as versatile synthetic intermediates in useful
chemical transformations.11 The most popular and reliable
method for the installation of alkyne moieties to aromatic rings
is the Sonogashira reaction.12 Recently, a series of metal-
catalyzed C−H alkynylation reactions have emerged as a viable
alternative to the Sonogashira reaction.13−17 Generally, these
C−H alkynylation reactions use the preactivated alkynylating
reagents, such as alkynyl halides13 and benziodoxolone-based
hypervalent iodine reagents,14 instead of terminal alkynes to
avoid formation of a 1,3-diyne side product, with the exception
of polyfluoroarenes, azoles,15 thiophenes, indoles,16 and arenes
bearing bidentate directing groups.17
Received: March 7, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00772
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Selected Reaction Development
Scheme 1. Substrate Scope of Alkynes in Pd-Catalyzed
a
Decarboxylative Sonogashira Reaction
entry
variations from standard conditions
isolated yield (%)
1
2
3
4
5
none
75
48
0
w/o 20 min stirring at rt
w/o Pd(PPh3)4
w/o NBS
0
w/o DBU
25
for 20 min at room temperature and then heated at 120 °C for
24 h (Table 1, entry 1). Omitting the 20 min of stirring at room
temperature led to a decrease in yield (Table 1, entry 2).
Control experiments revealed that no product was formed in
the absence of Pd(PPh3)4 or NBS, implying the vital roles of
these two reaction components (Table 1, entries 3 and 4).
Removing DBU from the reaction system significantly reduced
the yield to 25% (Table 1, entry 5). Although DBU was present
as a base in the reaction system, Cs2CO3 is essential for the
desired reaction; a 53% yield was obtained in the absence of the
PCy3 ligand, indicating that an additional ligand exerted a
beneficial effect on this reaction. Surprisingly, the choice of
palladium source is important for this reaction. The use of
Pd(OAc)2 and Pd2(dba)3 gave no product. Analogues of NBS,
namely, NIS and NCS, did not work for this reaction.
With this established reaction system in hand, we evaluated
the substrate scope with respect to alkynes (Scheme 1). As
illustrated, the phenylacetylenes contains electron-donating
groups such as methyl, tert-butyl, or methoxy at the para or
meta positions of the aryl rings were efficiently coupled with
the benzoic acid, producing the desired products with good
yields (3b, 3c, 3d, 3k). Electron-rich disubstituted phenyl-
acetylene also furnished high yield product (3h). Notably, the
hydroxymethyl group, which is rarely tolerated by the Pd-
catalyzed oxidative cross-coupling reaction, remained intact in
this decarboxylative alkynylation reaction with a 55% yield
obtained (3j). Phenylacetylenes bearing electron-withdrawing
groups such as chloro, fluoro groups at the para or meta
positions also smoothly underwent reaction to furnish the
corresponding products in good yields (3e, 3g, 3l, 3m). 1-
Ethynyl-4-(trifluoromethyl)benzene, which is a weak nucleo-
phile because of its strong electron-withdrawing trifluoromethyl
group, could be used as an alkynylating reagent, albeit in
slightly diminished yield (3f). The heteroaryl-substituted
alkynes also served as suitable coupling partners to offer high-
to-good yields, as exemplified by 3n, 3o, and 3q.
Further, we expanded the scope of alkynes beyond
(hetero)aryl acetylenes. The silyl-protected (triisopropylsilyl)-
acetylene produced an alkynylated arene in synthetically useful
yield (3r). A broad range of aliphatic alkynes proved to be
efficient alkynylating reagents (3s−3y) and cyclohexenyl-
substituted alkynes also smoothly participated in the reaction
(3z), fully exhibiting the generality of this decarboxylative
alkynylation reaction. It is noteworthy that the decarboxylative
cross-coupling product from methyl hex-5-ynoate (3x), which
is a useful synthetic intermediate,20 could be prepared on gram
scale in 42% yield by virtue of this decarboxylative alkynylation
reaction. To further investigate the flexibility of this protocol in
the synthesis of natural product analogues or the decoration of
a
Reaction conditions: 2a (0.25 mmol), 1 (2.2 equiv), Pd(PPh3)4 (10
mol %), PCy3 (20 mol %), Cs2CO3 (3 equiv), NBS (2.3 equiv), DBU
(2.3 equiv), DMF (1.5 mL). After being stirred for 20 min at room
temperature, the reaction system is heated with stirring at 120 °C for
b
24 h, under a N2 atmosphere. All yields are isolated ones. 5 mmol
scale, yield of isolated product: 0.55 g.
the pharmaceuticals, our protocol was applied to the synthesis
of 3aa that was the analogue of Antrocamphin A, which is a
potentially useful chemical compound for anti-inflammatory
activities.21 As demonstrated in the cases of 3ab and 3ac, our
protocol also enabled decoration of complex pharmaceutical
molecules containing a terminal alkyne moiety, that is,
Levonorgestrel and Mestranol.22
Subsequently, we investigated the substrate scope of this
decarboxylative Sonogashira reaction with regard to the benzoic
acids (Scheme 2). Electron-rich aryl carboxylic acids underwent
decarboxylative cross-coupling with phenylacetylene to give the
corresponding products in good-to-excellent yields (3a, 4b, 4c).
Naphthoic acids also participated in this transformation to
obtain satisfying yields (4d, 4e). Aryl carboxylic acids
containing chloro and fluoro groups worked very well in 73%
yield (4h). Heteroaryl carboxylic acids were also suitable
substrates in this decarboxylative Sonogashira reaction (4f, 4g,
4i−4k). Thiophene containing dicarboxyl groups were
observed to produce a mixture of mono- and dialkynylated
products 4i. Pyrrol-substituted thiophene-2-carboxylic acid,
which was very reluctant to undergo decarboxylation,
participated in the decarboxylative alkynylation to give the
corresponding product in synthetically useful yield (4k).
Currently, 2-arylbenzoic acids are readily accessible through
B
DOI: 10.1021/acs.orglett.8b00772
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Substrate Scope of (Hetero)aryl Carboxylic Acids
in Pd-Catalyzed Decarboxylative Sonogashira Reaction
Scheme 3. Mechanism Study in Pd-Catalyzed
Decarboxylative Sonogashira Reaction
a
aryl bromides with good selectivity starting from abundant aryl
carboxylic acids.
Furthermore, we observed the aryl bromide species in the
reaction mixture when the reaction was carried out for 2 h. The
cross-coupling of aryl bromide with a terminal alkyne could
take place in 77% isolated yield (Scheme 3, eq 3). These
findings suggested that our Pd-catalyzed decarboxylative
Sonogashira reaction likely proceeded via decarboxylative
bromination of aryl carboxylic acid with NBS to generate aryl
bromide and a subsequent Sonogashira reaction of aryl bromide
with terminal alkyne. In the absence of NBS and an alkyne, the
decarboxylative protonation of aryl carboxylic acids did not
occur under otherwise identical conditions, which excluded the
possibility that the decarboxylative alkynylation reaction
proceeded through decarboxylative protonation and the
subsequent bromination of the resultant arene with NBS.
In conclusion, we have developed the first example of the Pd-
catalyzed decarboxylative alkynylation reaction of aryl carbox-
ylic acids with terminal alkynes. This method exhibited a broad
substrate scope with respect to both coupling partners and
enabled the late-stage elaboration of complex pharmaceutical
molecules and the preparative synthesis of useful compounds.
The mechanistic investigations disclosed that this decarbox-
ylative cross-coupling reaction proceeded via decarboxylative
bromination of (hetero)aryl carboxylic acid with NBS and
Sonogashira reaction of the in situ generated aryl bromide. The
decarboxylative bromination discovered by us, in combination
with the cross-coupling of aryl bromides, would allow for the
development of a variety of decarboxylative cross-coupling
reactions since aryl bromides are versatile building blocks in
organic synthesis.
a
Reaction conditions: 2 (0.25 mmol), 1 (2.2 equiv), Pd(PPh3)4(10
mol %), PCy3 (20 mol %), Cs2CO3 (3 equiv), NBS (2.3 equiv), DBU
(2.3 equiv), DMF (1.5 mL). After being stirred for 20 min at room
temperature, reaction system is heated with stirring at 120 °C for 24 h,
under a N2 atmosphere. All yields are isolated ones. 1a (4 equiv),
Cs2CO3 (4 equiv), NBS (4.2 equiv), DBU (3 equiv). 1a (4 equiv),
b
c
NBS (4 equiv), and 4′-methoxy-3-morpholino-[1,1′-biphenyl]-2-
carboxylic acid used as a substrate.
carboxyl-directed C−H arylation.7a,c,e A series of 2-arylbenzoic
acids were synthesized and verified to undergo the decarbox-
ylative alkynylation reaction (4l−4r) to furnish ortho-
alkynylated biphenyls that are useful precursors to organic
materials and bioactive molecules.23 2-Arylbenzoic acid
containing an amino substituent afforded dialkynylated product
4r.
Mechanistic investigations were performed to understand the
decarboxylative alkynylation of (hetero)aryl carboxylic acids
with terminal alkynes. (Bromoethynyl)benezene was used in
place of phenylacetylene to react with 2.4-dimethoxybenzoic
acid under the standard conditions, which did not give any of
the decarboxylative alkynylation product (Scheme 3, eq 1).
Moreover, after removal of NBS or both NBS and DBU from
the reaction system, the reaction of (bromoethynyl)benezene
with 2.4-dimethoxybenzoic acid under otherwise identical
conditions did not give any alkynylation product either.
These observations ruled out the possibility that the
decarboxylative alkynylation reaction occurs via in situ
generation of alkynyl bromide from the reaction of a terminal
alkyne, NBS, and DBU.24 However, we found that the reaction
of 2.4-dimethoxybenzoic acid (0.25 mmol) with 2.3 equiv of
NBS in the presence of Cs2CO3 (3 equiv) and DBU (2.3 equiv)
at 120 °C for 24 h produced 1-bromo-2,4-dimethoxybenzene in
70% GC yield (Scheme 3, eq 2). Notably, the newly found
decarboxylative bromination reaction of aryl carboxylic acids
with NBS has an advantage of high selectivity for mono-
brominated arenes over the previously reported methods,25
exhibiting great potential to simplify the synthesis of valuable
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00772.
Experimental procedures and spectral data for all new
compounds (PDF)
Accession Codes
CCDC 1815934 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
C
DOI: 10.1021/acs.orglett.8b00772
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wpsu@fjirsm.ac.cn.
ORCID
Weiping Su: 0000-0001-5695-6333
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSFC (21431008, 21332001,
u1505242, and 21602221), the CAS/SAFEA International
Partnership Program for Creative Research Teams, the
Strategic Priority Research Program of the Chinese Academy
of Sciences (XDB20000000), and the Key Research Program of
Frontier Sciences, CAS (QYZDJ-SSW-SLH024).
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