A Convenient and Efficient Palladium-Catalyzed Carbonylative Sonogashira Transformation with Formic Acid as the CO Source

Article abstract of DOI:10.1002/ejoc.201700076

A practical, convenient, and efficient palladium-catalyzed carbonylative Sonogashira reaction of aryl iodides was developed. With formic acid as the CO source and dicyclohexylcarbodiimide as the activator, various alkynones were produced in good to excell

Full text of DOI:10.1002/ejoc.201700076

A Journal of
Accepted Article
Title: A Convenient and Efficient Palladium-Catalyzed Carbonylative
Sonogashira Transformation with Formic Acid as the CO Source
Authors: Xiao-Feng Wu, Jin-Bao Peng, Fu-Peng Wu, Chong-Liang li,
and Xinxin qi
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700076
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700076
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10.1002/ejoc.201700076
European Journal of Organic Chemistry
COMMUNICATION
A Convenient and Efficient Palladium-Catalyzed Carbonylative
Sonogashira Transformation with Formic Acid as the CO Source
Jin-Bao Peng,*a Fu-Peng Wu,a Chong-Liang Li,a Xinxin Qia and Xiao-Feng Wu*a,b
Abstract: A practical, convenient and efficient palladium-catalyzed
carbonylative Sonogashira reaction of aryl iodides has been
developed. With formic acid as the CO source and using DCC as the
activator, various alkynones were produced in good to excellent
yields.
still highly desired. We recently reported the first example on
carbonylative Sonogashira coupling with formic acid as the CO
source.9h With acetic anhydride as the activator, good yields of
the desired alkynones can be obtained. However, the pre-
preparation of acetic formic anhydride and large excess of base
to neutralize the in situ formed acetic acid is required.
Dicyclohexylcarbodiimide (DCC) has been known as a good
dehydration reagent in organic synthesis. We assume that
employing DCC as the activator of formic acid, CO can be in-situ
generated and DCC will be hydrolyzed to its urea. Due to the
poor solubility of urea, it will precipitate out and would not
interrupt the following transformation and therefore provide a
clean environment for the carbonylative Sonogashira reaction.
With this idea in mind, we present here our newly
developed palladium-catalyzed carbonylative Sonogashira
method with formic acid as the CO source and
dicyclohexylcarbodiimide (DCC) as the activator. The reaction
was performed in a one-step one-pot manner; good to excellent
yields of the desired alkynones can be prepared under mild
conditions with good functional group tolerance.
Carbonyl groups are important structural units exist in a large
number of organic compounds such as aldehydes, ketones,
carboxylic acids and their derivatives, and usually serve as
valuable building blocks in the synthesis of natural products,
pharmaceuticals, agrochemicals and materials. Among all these
carbonyl compounds, alkynones have attracted great attention
owing to their multi-functional property and numerous
applications in synthetic chemistry.1 Among them, alkynones
represent a powerful platform for the synthesis of numerous
heterocyclic compounds,2 such as pyrroles, pyrazoles,
pyrimidines, and quinolones. Traditionally, alkynones were
prepared via the cross-coupling reaction of acid chlorides with
terminal alkynes3 (or alkynyl metal4). However, despite good
reactivity, acid chlorides are not stable and usually need a
preparation process which limited the scope of the functional
groups tolerance. An alternative approach for the synthesis of
alkynones is the transition-metal catalyzed carbonylative
Sonogashira coupling reaction,5 which employs readily
accessible aryl halides (or pseudohalides) and terminal alkynes
as the starting materials. Since Kobayashi and Tanaka
published the first carbonylative Sonogashira reaction in 1981,5a
a great number of methods have been developed on this subject.
However, large percentage of those procedures employs carbon
monoxide as the CO source. Although CO gas holds many
advantages, its high toxicity and odourless characters hindered
the extended applications of those procedures in synthetic
chemistry.
Table 1 Optimization of the reaction conditionsa
Temp.
(oC)
30
30
30
30
60
80
100
30
30
30
30
30
30
30
30
30
Yieldb
(%)
79c
98(97d)
38e
75f
74
23
8
0
52
5
3
59
42
80
84
23
13
Entry
Ligand
Base
solvent
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
PPh3
PPh3
PPh3
PPh3
PPh3
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
DBU
DIPEA
K2CO3
Et3N
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
PPh3
PPh3
Under these backgrounds, several CO surrogates have
been developed and applied in carbonylation chemistry during
the past decades including metal carbonyls,6 formamides,7
aldehyde,8 formic acid,9 and oxalyl chloride.10 However, some
drawbacks are still existing, such as stoichiometric metal
residuals or acids by-products, special reactors, harsh reaction
conditions and so on. The development of general and
convenient gaseous CO-free carbonylation methods under mild
condition without the influence of accompanied by-product are
DPPE
DPPF
Xantphos
BINAP
PPh3
PPh3
PPh3
PPh3
PPh3
DMF
CH3CN
dioxane
Toluene
Toluene
Toluene
Toluene
PPh3
PPh3
PPh3
30
30
30
0
93g
[a]
[b]
Dr. J.-B. Peng, F.-P. Wu, C.-L. Li, Dr. X. Qi, Prof. Dr. X.-F. Wu
Department of Chemistry, Zhejiang Sci-Tech University, Xiasha
Campus, Hangzhou 310018, People’s Republic of China
E-mail: pengjb_05@zstu.edu.cn; xiao-feng.wu@catalysis.de
Prof. Dr. X.-F. Wu, Leibniz-Institut für Katalyse e.V. an der
Universität Rostock, Albert-Einstein-Straße 29a, 18059 Rostock,
Germany, E-mail: xiao-feng.wu@catalysis.de
a
Reaction conditions: Iodobenzene (1.0 mmol), phenyl acetylene (2.0
mmol), Pd(OAc)2 (3 mol%), ligand (6 mol %), base (2.0 mmol), HCOOH (2.0
mmol), DCC (2.0 mmol), solvent (4 mL), 30 oC, 24 h. GC yield, with
b
c
dodecane as the internal standard. HCOOH (1.5 mmol), DCC (1.5 mmol).
d Isolated yield. e HCOOH (2.0 mmol), DCC (3.0 mmol). f Et3N (1.0 mmol). g
DIC (2.0 mmol) instead of DCC.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201700076
European Journal of Organic Chemistry
COMMUNICATION
Initially, iodobenzene 1a and phenyl acetylene 2a were
selected as model substrates to this idea. To our delight, using
1.5 equivalent of HCOOH and 1.5 equivalent of DCC as the CO
source, the desired product 1,3-diphenylprop-2-yn-1-one 3a was
obtained in 79% yield in the presence of 3 mol% Pd(OAc)2, 6
were well tolerated and delivered the corresponding alkynones
in good to excellent yields (3b-3h). Generally, electron-donating
substituents (3b-3e) gave slightly higher yields than electron-
withdrawing substituents (3f-3h). Meanwhile, the position of
substituents on the iodobenzene influenced the yields
significantly due to the steric effect (3i-3k). Para-
o
mol% PPh3 and Et3N in toluene at 30 C (Table 1, entry 1). The
yield can be further improved to 98% (Table 1, entry 2) when 2
equivalent of CO (2 eq. HCOOH and 2 eq. DCC) was used. The
ratio of formic acid and DCC turned out to be crucial for this
reaction, excessive DCC decreased the yield dramatically to
38% (Table 1, entry 3). Reducing the amount of base gave a
lower yield of 75% (Table 1, entry 4). It should be noted that the
reaction temperature also played an important role, when the
reaction was performed at higher temperature, the yields of
alkynone decreased significantly (Table 1, entries 5-7). This
might be resulted from the decomposition of DCC at high
temperature. Screening of other phosphine ligands showed that
bidentate phosphine ligands are not suitable for this reaction and
only a few or trace amount of product was obtained (Table 1,
entries 8-11). Further investigations revealed that toluene and
Et3N are the optimal solvent and base (Table 1, entries 12-18).
Additionally, the use of DIC (diisopropylcarbodiimide) instead of
DCC as the activator also works well, and the desired product
can be obtained in 93% yield (Table 1, entry 19).
chloroiodobenzene gave
a
higher yield than meta-
chloroiodobenzene, while ortho-chloroiodobenzene needs to be
performed at a higher temperature to give the desired product in
moderate yield. Furthermore, 2-naphthyl and heteroaryl iodides
were also amenable to the carbonylative coupling reaction, and
the corresponding products were conveniently generated in
good yields (3l and 3m).
a
Scheme
2
Substrate scope of the carbonylative Sonogashira reaction
:
a
variation of alkynes. Reaction conditions: iodobenzene (1.0 mmol), alkynes
(2.0 mmol), Pd(OAc)2 (3 mol%), PPh3 (6 mol %), Et3N (2.0 mmol), HCOOH (2.0
mmol), DCC (2.0 mmol), toluene (4 mL), 30 oC, 24 h, isolated yields. The
b
reaction was performed at 80 oC.
Then we turned attention to test the generality of the
alkyne coupling partner with various substituted alkynes. As
demonstrated in Scheme 2, alkyl- and alkoxy-substituted phenyl
acetylenes were successfully applied to the carbonylative
coupling reaction and gave the corresponding products in good
to excellent yields (3n-3r). Interestingly, compared to ortho- and
para-substituents, meta-substituted aromatic alkyne gave even
higher yields (3n and 3p vs 3o). This might be resulted from the
combined effect of steric and conjugate effect. Besides, when
halogenated substrates were subjected to this transformation,
the coupling reaction proceeded smoothly and delivered the
corresponding alkynones. The most electronegative fluoro
containing substrate worked better than that of chloro- and
bromo-substituents (3s vs 3t and 3u). This can be explained by
the stronger electron-withdrawing effect which is benefit for the
insertion of alkyne to the acyl palladium intermediate. In addition,
3-thienyl (3v) and alkenyl alkynes (3w) underwent this reaction
Scheme 1 Substrate scope of the carbonylative Sonogashira reactiona: variation
of aryl iodides. a Reaction conditions: aryl iodides (1.0 mmol), phenyl acetylene
(2.0 mmol), Pd(OAc)2 (3 mol%), PPh3 (6 mol %), Et3N (2.0 mmol), HCOOH (2.0
b
mmol), DCC (2.0 mmol), toluene (4 mL), 30 oC, 24 h, isolated yields. The
reaction was performed at 60 oC.
With the optimized conditions in hand (Table 1 entry 2), we
began to investigate the substrates scope of the reaction with
various aryl iodides and terminal alkynes. First, as summarized
in Scheme 1, a series of aryl iodides was successfully applied to
this reaction. Both electron-donating groups and electron-
withdrawing groups at the para position of the iodobenzenes
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10.1002/ejoc.201700076
European Journal of Organic Chemistry
COMMUNICATION
at a higher temperature and afforded the desired products in
49% and 85% yields respectively. Moreover, two aliphatic
alkynes were successively transformed to their corresponding
alkynones in good yields (3x and 3y).
A plausible mechanism was proposed in Scheme 3.
Initially, the oxidative addition of the Pd0 to the aryl iodide 1 give
an aryl-palladium complex 4. Then coordination and insertion of
4 with carbon monoxide, which was generated in-situ from
formic acid, forms an acyl-palladium intermediate 5. The acyl-
palladium 5 is then attacked by the terminal alkyne under the
assistance of base to generate an alkynyl-palladium complex 6.
Finally, reductive elimination produces the final product 3 and
meanwhile regenerates Pd0 for the next catalyst cycle.
Keywords: Palladium Catalyst • Carbonylation • Alkynone •
Formic acid • CO Surrogate
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derivatives were produced in good to excellent yields with good
functional group tolerance. No pre-preparation steps and large
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Experimental Section
Pd(OAc)2 (3 mol %), PPh3 (6 mol %) were transferred into an oven-dried
tube which was filled with nitrogen. Toluene (4.0 mL), formic acid (2.0
mmol), alkyne (2.0 mmol) and aryl iodide (1.0 mmol) were added to the
reaction tube. After DCC (2.0 mmol) and Et3N (2.0 mmol) were added,
the tube was sealed and the mixture was stirred at 30 °C for 24 h. After
the reaction was completed, the reaction mixture was filtered and
concentrated under vacuum. The crude product was purified by column
chromatography (EtOAc/hexane = 5/95) on silica gel to afford the
alkynone product.
Acknowledgements
The authors thank the financial supports from NSFC (21472174,
21602201, 21602204) and Zhejiang Natural Science Fund for
Distinguished Young Scholars (LR16B020002). X. -F Wu
appreciates the general support from Professor Matthias Beller
in LIKAT.
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COMMUNICATION
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10.1002/ejoc.201700076
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
Jin-Bao Peng,* Fu-Peng Wu, Chong-
Liang Li, Xinxin Qi and Xiao-Feng Wu*
Page No. – Page No.
A Convenient and Efficient Palladium-
Catalyzed Carbonylative Sonogashira
Transformation with Formic Acid as
the CO Source
See you, CO!
A convenient and efficient gaseous CO-free carbonylative
Sonogashira reaction has been developed. Employing formic acid as the CO
source and DCC as the activator, a series of synthetically useful alkynones were
produced in good to excellent yields. Tedious pre-preparation steps and the usage
of large excess of reagents can be successfully avoided here.
This article is protected by copyright. All rights reserved.