Silver-mediated oxidative C-H/C-H functionalization: A strategy to construct polysubstituted furans

Article abstract of DOI:10.1021/ja301153k

A novel silver-mediated highly selective oxidative C-H/C-H functionalization of 1,3-dicarbonyl compounds with terminal alkynes for the creation of polysubstituted furans and pyrroles in one step has been demonstrated. Promoted by the crucial silver specie

Full text of DOI:10.1021/ja301153k

Communication
pubs.acs.org/JACS
Silver-Mediated Oxidative C−H/C−H Functionalization: A Strategy
To Construct Polysubstituted Furans
Chuan He,† Sheng Guo,† Jie Ke,† Jing Hao,† Huan Xu,† Hongyi Chen,† and Aiwen Lei†,‡,*
†College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, P. R. China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, 730000, P. R. China
S
* Supporting Information
Scheme 1. Silver-Mediated Oxidative C−H/C−H
Functionalization Route to Polysubstituted Furans
ABSTRACT: A novel silver-mediated highly selective
oxidative C−H/C−H functionalization of 1,3-dicarbonyl
compounds with terminal alkynes for the creation of
polysubstituted furans and pyrroles in one step has been
demonstrated. Promoted by the crucial silver species, perfect
selectivity and good to excellent yields could be achieved.
This protocol represents an extremely simple and atom-
economic way to construct polysubstituted furans and
pyrroles from basic starting materials under mild conditions.
reaction. Undoubtedly, the best choice in constructing a Csp−
3
3
Csp bond is the direct Csp−H and Csp −H functionalization by
using carbonyl compounds and terminal alkynes. Our initial
efforts focused on the reaction of phenylacetylene 1a and ethyl
acetoacetate 2a. Using copper salts as the promoter, most of the
reactions afforded the homocoupling 1,4-diphenylbuta-1,3-
diyne as the major product. To our delight, simply changing
Cu salts to Ag salts in the reaction, a new product with the
furan structure was obtained in perfect selectivity and moderate
yield (Scheme 1 and Table 1). It is reasonable that, in the
presence of silver species, the cycloisomerization process could
be facile.8 This unanticipated and interesting one-step trans-
formation to the polysubstituted furans encouraged us to
further examine the feasibility to the efficient furan synthesis.
The combination of Ag2CO3 and KOAc in DMF at 80 °C was
found to be the best reaction conditions for this Ag-mediated oxida-
tive C−H/C−H functionalization (Table 1, entries 1 and 16−17).
Ag2O could also provide the furan product, but other Ag(I)
salts such as AgOAc and AgI were totally ineffective (Table 1,
entries 2−4). The polar solvent such as DMSO also gave the
desired products but in low yield, while no reactions occurred in
THF and toluene (Table 1, entries 5−7). Cs2CO3 and K2CO3
turned out to be effective while reducing the yields obviously
(Table 1, entries 8 and 9). Meanwhile, diluting the reaction could
dramatically improve the yields due to the better stirring of the
reaction mixture (Table 1, entry 10 vs 1, and entries 15−17).
Notably, the employment of a 1:1 ratio between 1a and 2a could
also afford the product with perfect selectivity in moderate yield
(Table 1, entry 11). Employing just 2- or 3-fold 2a could enhance
the conversion of 1a satisfactorily (Table 1, entries 10 and 12).
A lesser amount of Ag2CO3 would reduce the reaction yield (Table 1,
entry 13), but 3.0 equiv of Ag2CO3 did not improve the yield
compared with 2.0 equiv of Ag2CO3 (Table 1, entries 12 and 14).
he direct utilization of two different hydrocarbons as
reagents for coupling reactions to form C−C bond (C−
T
H/C−H functionalization) is of great significance and a challenge,
and application of these strategies to construct polysubstituted
furans is especially attractive. The possibility of direct C−H func-
tionalization, especially the oxidative coupling between two C−H
bonds, provides a highly attractive strategy for an ideal chemical
synthesis.1 Although some progress has been made in this
emerging field,1,2 there still remains a great challenge in achieving
a highly efficient and selective cross-coupling utilizing two different
hydrocarbons as the reagents. A typical example is related to the
oxidative coupling of an acetylene C−H bond and other types of
C−H bonds. This is due to the facile homocoupling of terminal
alkynes in most cases under the oxidative conditions (Glaser−Hay
coupling).3 Recently, some insights for the alkynylation of aryl,
alkyl C−H and N−H, P−H bonds have been reported.1f,4 One
often employs a large excess of reagents (vs terminal alkynes) or
conducts the reactions by using a syringe pump to partially address
the challenge. Improving the synthetical efficiency is still imperative.
Polysubstituted furans and pyrroles not only represent an
important class of five-membered heterocycles prevalent in natural
products, pharmaceuticals, and agrochemicals (Scheme 2) but also
represent useful intermediates in organic synthesis.5 In spite of the
myriad methods available for constructing the furan and pyrrole
scaffold,5c−e,6 the direct, region-defined synthesis of polysubsti-
tuted furans and pyrroles from basic chemical materials has always
drawn chemists’ attention. Herein, we would like to communicate
our results regarding a silver-mediated highly selective oxidative
C−H/C−H functionalization reaction between 1,3-dicarbonyl
compounds and terminal alkynes, which illustrated an efficient
example of achieving polysubstituted furans and pyrroles in one
step under mild conditions (Scheme 1).
Due to the interest in α-functionalization of carbonyl
Received: February 4, 2012
Published: March 19, 2012
compounds,7 recently we focused on the direct alkynylation
© 2012 American Chemical Society
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Communication
Table 1. Conditions Optimization for Silver-Mediated
Oxidative C−H/C−H Functionalization of 1a and 2a
Table 2. Substrates Scope for the Silver-Mediated Oxidative
a
C−H/C−H Functionalization of 1a and 1, 3-Dicarbonyl
a
Compounds 2
V
yield
a
entry
[Ag]
base
solvent
1a:2a (mL)
(%)
b
1
Ag2CO3
Ag2O
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
Cs2CO3
K2CO3
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
DMF
DMF
DMF
DMF
DMSO
THF
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
1:3
1:3
1:3
1:3
1:3
1:3
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
6
8
70
2
3
4
5
6
7
8
9
48
AgOAc
AgI
0
0
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
48
0
toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
0
40
24
c
10
87
11
12
50
94
d
13
69
e
14
93
15
16
17
94
96 (89)
100 (92)
a
Yield determined by GC analysis with biphenyl as the internal standard.
b
Isolated yields for entries 16 and 17 in the parentheses. Reactions were
carried out on the scale of 0.5 mmol of 1a in the presence of 2.0 equiv
of [Ag] and 2.0 equiv of base in 4 mL of solvent in Schlenk tubes
c
at 80 °C for 12 h (entries 1−9). Reactions were carried out on the
scale of 0.25 mmol of 1a in the presence of 2.0 equiv of Ag2CO3
and 2.0 equiv of KOAc in DMF in Schlenk tubes at 80 °C for 12 h
d
e
(entries 10−17). 1.5 equiv of Ag2CO3. 3.0 equiv of Ag2CO3.
The reaction was highly selective. For the oxidative reactions
involving a terminal alkyne, one often employed an excess of
the other substrate to suppress the homocoupling of the
terminal alkyne and achieve high selectivity, or even conducted
the reactions by using a syringe pump.4e,9 In this oxidative
C−H/C−H functionalization, no alkyne homocoupling prod-
uct was observed (even the ratio of the two substrates was 1:1
and added in one pot).
Varied 1,3-dicarbonyl compounds were suitable reaction
partners with phenylacetylene 1a to form the polysubstituted
furans (Table 2). Promoted by Ag2CO3, almost all of the β-keto
esters are effective under the standard conditions. As shown in
Table 2, methyl, tert-butyl, n-otcyl, benzyl, and cyclohexyl
acetoacetates all reacted smoothly affording the desired products
in good to excellent yields (Table 2, 3ab−3af). Ethyl
isobutyrylacetate was also suitable for this reaction (Table 2,
3ag). In addition, β-diketones could be readily introduced in the
reaction, providing the corresponding furans 3ah, 3ai, and 3aj, but
the yields were lower. A 1,3-dicarbonyl compound bearing an
amide group was also allowed to react with phenylacetylene 1a
and afford the furan product 3ak in 65% yield. Interestingly, the
reaction of 4-methyl-3-oxopentanenitrile with 1a resulted in a
mixture of the two regioisomeric products 3am in 46% yield (ratio =
5:1). Unfortunately, diethyl malonate and 2-nitro-1-phenyl-
ethanone were not suitable for the reaction under the current
conditions (Table 2, 3al and 3an).
a
Reactions conditions: 0.25 mmol of 1a and 0.75 mmol of 2 in the
presence of 2.0 equiv of Ag2CO3 and 2.0 equiv of KOAc in 6 mL of
DMF in Schlenk tubes at 80 °C for 12 h. Isolated yields. Reaction was
carried out on the scale of 0.5 mmol of 1a; isolated combined yield;
b
ratio was determined by GC-MS.
variety of aryl terminal alkynes in high yields. Both electron-
withdrawing and -donating substituted groups were well tolerated
under the conditions, such as Me, Et, t-Bu, OMe, COOEt, CN,
and Ac groups (Table 3, 3ba−3da, 3ha−3ka). Aryl alkynes
bearing halo substituted groups such as fluoro, chloro, and bromo
could also be introduced to give the desired corresponding furans
(Table 3, 3ea−3ga). Moreover, alkynes containing thiophene and
naphthalene moieties could also be employed to the furan scaffold
without any difficulties (Table 3, 3la and 3ma).
Although good to excellent yields were achieved by the
reaction of aryl terminal alkynes and β-keto esters, unfortu-
nately, most of the alkyl terminal alkynes did not perform well
under the current conditions. A selected example was shown in
eq 1 in which prop-2-ynyl pivalate 1n could afford the target
furan 3na albeit in a lower yield.
Varied terminal alkynes were also suitable partners with 1,3-
dicarbonyl compound 2a to access the polysubstituted furans
(Table 3 and eq 1). The reaction was readily extended to a
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Journal of the American Chemical Society
Communication
Table 3. Substrates Scope for the Silver-Mediated Oxidative
C−H/C−H Functionalization of Aryl Acetylenes 1 and Ethyl
a
Acetoacetate 2a
cheapest silver sources, we sought to reduce the cost and
waste. Actually, excess Ag2CO3 and all silver species after the
reaction could be recycled conveniently by filtration and
treating with nitric acid and Na2CO3. The regenerated Ag2CO3
could still promote this oxidative C−H/C−H functionalization
in comparable yield without loss of activity (Scheme 3).
Scheme 3. Recovery Experiment of Ag Salts
To gain some preliminary understanding of the mechanism,
we synthesized the silver acetylide11 and investigated its
reaction with 2a under the standard conditions. The results
were listed in Scheme 4. In the presence of additional Ag2CO3,
a
Scheme 4. Reaction of Silver Phenylacetylide and 2a
Reactions conditions: 0.25 mmol of 1 and 0.75 mmol of 2a in the
presence of 2.0 equiv of Ag2CO3 and 2.0 equiv of KOAc in 6 mL of
DMF in Schlenk tubes at 80 °C for 12 h. Isolated yields.
Notably, polysubstituted pyrroles could also be achieved via
this oxidative C−H/C−H functionalization (eq 2). The
multisubstituted pyrrole is a basic scaffold of numerous
biologically active alkaloids and pharmaceutical drugs, such as
the hugely successful and valuable molecule Lipitor (Scheme 2).
silver phenylacetylide could react with 2a and afford the furan
product in moderate yield. Without the additional silver
oxidant, the reaction yield was lower. Meanwhile, some internal
alkynes have also been tested in the reaction instead of terminal
alkynes. However, only a trace of corresponding products could
be observed (see more details in the Supporting Information).
These results indicated that silver acetylide might be the
intermediate in the reaction, and the additional silver oxidant
must be needed to achieve this oxidative coupling. Notably,
high-valent transition metal induced oxidative radical cycliza-
tions between 1,3-dicarbonyl compounds and alkenes are
already known in literature, and the mechanism was regarded as
a radical process.12 By employing styrene as the substrate
instead of 1a to react with 2a under the standard conditions, no
corresponding dihydrofuran product was produced. These
results showed that the mechanism of this silver-mediated
oxidative protocol might not proceed via the previous oxidative
radical cyclization.
Scheme 2. Examples of Significant Furan- and Pyrrole-
Containing Pharmaceuticals
In summary, we have developed a novel silver-mediated
highly selective oxidative C−H/C−H functionalization of 1,3-
dicarbonyl compounds with terminal alkynes, which provided a
direct and efficient entry to the polysubstituted furans and
pyrroles in one step. Promoted by the crucial silver species,
perfect selectivity and good to excellent yields could be
achieved in this reaction. From the synthetic point of view, this
protocol represents an extremely simple and atom-economic
way to construct polysubstituted furans and pyrroles from basic
To our delight, simply changing β-keto esters to enamines
could readily provide the desired pyrrole products under similar
conditions. A selected example (eq 2) shows that a moderate
yield of 3ao could be obtained by the reaction of (Z)-ethyl 3-
(phenylamino)but-2-enoate 2o and 1a in DMSO.
The silver salts could be recycled.10 One inevitable problem
in this reaction was related to the employment of
stoichiometric Ag2CO3. Although Ag2CO3 is one of the
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experiment details and spectral data for all compounds are
provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
aiwenlei@whu.edu.cn
■
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21025206, 20832003, and 20972118) and
the “973” Project from the MOST of China (2011CB808600).
The authors are also thankful for the support from “the Funda-
mental Research Funds for the Central Universities”, Program
for New Century Excellent Talents in University (NCET) and
Program for Changjiang Scholars and Innovative Research Team
in University (IRT1030).
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