Silver-Catalyzed Coupling of Two Csp3-H Groups and One-Pot Synthesis of Tetrasubstituted Furans, Thiophenes, and Pyrroles

Article abstract of DOI:10.1002/chem.201501410

Silver-catalyzed coupling of two Csp3-H groups to form 1,4-diketones have been developed for the first time. The resultant ketones then undergo cyclization to synthesize tetrasubstituted furans, thiophenes, and pyrroles from benzyl ketone derivatives in a one-pot reaction process. This highly-efficient synthetic method, which utilizes air as the terminal oxidant and readily accessible starting materials, displays a wide substrate scope and broad functional-group tolerance.

Full text of DOI:10.1002/chem.201501410

DOI: 10.1002/chem.201501410
Communication
&
Heterocycle Synthesis |Hot Paper|
Silver-Catalyzed Coupling of Two Csp3ÀH Groups and One-Pot
Synthesis of Tetrasubstituted Furans, Thiophenes, and Pyrroles
Shuai Mao, Xue-Qing Zhu, Ya-Ru Gao, Dong-Dong Guo, and Yong-Qiang Wang*[a]
and modern methods developed for the construction of these
Abstract: Silver-catalyzed coupling of two Csp3ÀH groups
to form 1,4-diketones have been developed for the first
time. The resultant ketones then undergo cyclization to
synthesize tetrasubstituted furans, thiophenes, and pyr-
roles from benzyl ketone derivatives in a one-pot reaction
process. This highly-efficient synthetic method, which uti-
lizes air as the terminal oxidant and readily accessible
starting materials, displays a wide substrate scope and
broad functional-group tolerance.
five-membered heterocycles and their derivatives,[6–8] there is
no general approach to access all three compounds. Therefore,
a general and flexible approach to synthesize the three poly-
substituted five-membered heterocycles from easily accessible
starting materials with a simple operation is of great signifi-
cance. Herein, we report the first Ag-catalyzed coupling reac-
tions of two Csp3ÀH groups to form 1,4-diketones with air as
the terminal oxidant.[9] The resultant diketones then undergo
cyclization to synthesize tetrasubstituted furans, thiophenes,
and pyrroles from benzyl ketone derivatives in a one-pot pro-
cess.
Transition metal-catalyzed reactions have become versatile
tools in organic synthesis.[1] In comparison with other transition
metals, silver(I) species have long been believed to have low
catalytic efficiency and have often been used as cocatalysts or
Lewis acids.[2] Nevertheless, there has recently been great
growth in Ag-catalyzed reactions,[3] and silver catalysis has
become an important field in organic synthesis. Generally, the
Ag-catalyzed reactions reported have been initiated by coordi-
nation of silver(I) at multiple bonds, such as alkynes, alkenes,
and allenes, followed by nucleophilic attack and subsequent
protonolysis to afford the products (Scheme 1).[4] Despite these
At the outset of the study, deoxybenzoin 1a was treated
with silver carbonate (10 mol%) and TFA (trifluoroacetic acid,
1 equiv) in xylene at 1408C under an O2 (1 atm) atmosphere
for 10 h. To our delight, not only did the desired coupling of
Csp3ÀH groups occur, but the resulting coupling product was
also subsequently cyclized to afford the tetrasubstituted furan
compound 2a, albeit only in 15% yield (Table 1, entry 1). This
result encouraged us to further examine the feasibility of this
method for tetrasubstituted furan synthesis. The synthesis of
tetrasubstituted furans is known to be a challenge due to
steric hindrance from substituents. Interestingly, when 1a was
treated with silver carbonate (10 mol%) in xylene at 1408C
under an O2 (1 atm) atmosphere for 7 h, before TFA (1 equiv)
was added to the reaction mixture and the reaction continued
for 3 h, the yield was significantly improved to 45% (Table 1,
entry 2). Among other AgI catalysts tested, AgF was found to
be most effective, providing 2a in 60% yield (Table 1, entries 3
and 4). To our surprise, using air to take the place of oxygen as
the oxidant led to an appreciable improvement in the yield,
probably because air is a milder oxidant (Table 1 entry 5). Next,
a series of acids and the amount of acids were investigated
(Table 1, entries 6–8). TsOH (1.5 equiv) proved to be the best
choice, affording the desired product in 89% yield (Table 1
entry 8). Changing solvent and adding base or other oxidants
could not further improve the reactivity (Table 1, entry 9). Re-
ducing the catalyst loading and reaction temperature led to
significantly decreased yields (Table 1, entries 10 and 11).
Under argon atmosphere, 2a was obtained only in 32% yield
along with 65% of unreacted starting material recovered
(Table 1, entry 12). Accordingly, the optimized conditions were
as follows: AgF (10 mol%), TsOH (1.5 equiv), xylene as the sol-
vent at 1408C under air atmosphere.
Scheme 1. Previously reported on silver(I)-catalyzed reactions.
significant progresses relevant to sp2 and sp carbon centers, to
our knowledge, no efficient Ag-catalyzed coupling reaction of
two Csp3ÀH groups has been reported to date. Therefore, the
realization of such a transformation will constitute an impor-
tant advance among Ag-catalyzed reactions.
Polysubstituted furans, thiophenes, and pyrroles are valuable
five-membered heterocycles, since they are not only prevalent
in natural products, pharmaceuticals, agrochemicals, and mate-
rials, but also useful and versatile building blocks in organic
synthesis.[5] Despite a variety of well-documented traditional
[a] S. Mao, X.-Q. Zhu, Y.-R. Gao, D.-D. Guo, Prof. Dr. Y.-Q. Wang
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry
of Ministry of Education, Department of Chemistry & Materials Science
Northwest University, Xi’an 710069 (PR. China)
With the optimized reaction conditions in hand, the scope
of the reaction was investigated (Scheme 2). Pleasingly, deoxy-
benzoins with either electron-withdrawing or electron-donat-
E-mail: wangyq@nwu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501410.
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Therefore, we investigated the Ag-catalyzed cross-coupling
reaction. To our delight, the Ag-catalyzed system was indeed
able to effectively accomplish cross-coupling reactions when
the substrates with electron-withdrawing groups (1d, 1m, 1l,
and 1t) were added to their corresponding reaction systems at
the appropriate rate, providing the desired cross-coupling
products in good yields (Scheme 3). Remarkably, furans with
four different aromatic rings, such as 2u, could be effectively
accessed, which provides enormous scope to synthesize func-
tionalized furan cores, suggesting great potential for applica-
tion.[10]
Table 1. Optimization of conditions for the synthesis of tetrasubstituted
furan[a,b]
Entry
Catalyst
Ag2CO3
Ag2CO3
other Ag catalysts
Acid (equiv)
Solvent
xylene
Yield [%]
1[c]
2[c]
3[d]
4[d]
5
6[d]
7
TFA (1)
TFA (1)
TFA (1)
TFA (1)
TFA (1)
15
45
xylene
xylene
xylene
ꢀ40
AgF
AgF
AgF
AgF
AgF
AgF
AgF
AgF
AgF
60
xylene
65
Acids (1)
TsOH (1)
TsOH (1.5)
TsOH (1.5)
TsOH (1.5)
TsOH (1.5)
TsOH (1.5)
xylene
xylene
xylene
ꢀ60
74
8
89
9[d]
10[e]
11[f]
12[g]
other solvents
xylene
ꢀ65
67
xylene
xylene
40
32
[a] Unless otherwise noted, the reaction was carried out with 1a
(1 mmol), [Ag] (10 mol%) in solvent (3 mL) under air atmosphere at
1408C, 10–28 h. [b] Yields of isolated product. [c] Under O2 (1 atm) atmos-
phere. [d] See the Supporting Information. [e] 5 mol% of AgF was em-
ployed. [f] T=1208C. [g] Under argon (1 atm) atmosphere.
Scheme 3. Synthesis of tetrasubstituted furans by cross-coupling reactions.
Reaction conditions: 1 A (0.5 mmol), 1B (0.5 mmol), AgF (10 mol%), air,
TsOH (1.5 equiv) in xylene (3 mL) at 1408C. Yields of isolated product are
given.
Regarding the mechanism of the one-pot synthesis of tetra-
substituted furans, we proposed that 1,4-dicarbonyl com-
pounds were first generated by a Ag-catalyzed oxidative radi-
cal coupling reaction, which was demonstrated by the 1,4-di-
carbonyl compound (3a)[11] isolated before the addition of
TsOH into the reaction system (Scheme 4).
Scheme 2. Substrate scope for the synthesis of tetrasubstituted furans. Reac-
tion conditions: 1 (1 mmol), AgF (10 mol%), air, TsOH (1.5 equiv) in xylene
(3 mL) at 1408C. Yields of isolated product are given.
ing groups on either aromatic ring substituent were found to
react smoothly and provided tetrasubstituted furans in good
to excellent yields (71–91%, Scheme 2, 2b–p). The position of
the substituent on the aromatic rings had almost no effect on
the reactivity (Scheme 2, 2g–i). Halide substituents on the aro-
matic rings, including F, Cl, Br, and I, were well-tolerated in the
reaction system, providing the possibility for further functional-
ization of the polysubstituted furans (Scheme 2, 2l–n, 2p). A
deoxybenzoin incorporating a biphenyl substituent was also
suitable for the reaction and provided the desired product in
91% yield (Scheme 2, 2o). Dialkyl ketones, such as benzyl
butyl ketone, and 2-bromophenylacetone were also suitable
substrates and afforded the corresponding products in good
yields (Scheme 2, 2q and 2r), whereas 3-pentanone was inert
under these conditions. It is worth noting that deoxybenzoins
bearing electron-withdrawing substituents on either aromatic
ring reacted faster than those with electron-donating groups,
which suggests the possibility of cross-coupling reactions.
Scheme 4. Synthesis of 1,4-dicarbonyl compound 3a.
In addition, a radical-trapping experiment was carried out.
The radical inhibitor TEMPO (2,2,6,6-tetramethylpiperidine-1-
oxyl) was introduced into the standard reaction system of 1a
(Scheme 5). The oxidative homo-coupling reaction of 1a did
not occur. Instead, the coupling reaction of 1a with TEMPO
happened, producing 4a in 65% yield.
These results inspired us to study Ag-catalyzed coupling of
two Csp3ÀH groups as a means to synthesize 1,4-diketones,
Scheme 5. Radical trapping experiment.
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beacuse1,4-diketones are ubiquitous substructures of natural
products and pharmaceuticals, as well as highly useful synthet-
ic building blocks for various valuable compounds.[11] Pleasing-
ly, under the optimized conditions without TsOH, all of the re-
actions afforded the 2,3-disubstituted-1,4-dicarbonyl com-
pounds in excellent yields (Scheme 6). Functional groups in-
applications in the areas of biology, material science, and
chemistry, efficient and viable routes to synthesize polysubsti-
tuted thiophenes, especially tetrasubstituted thiophenes,
remain very limited.[14]
After careful investigations, an efficient one-pot synthesis of
tetrasubstituted thiophenes was achieved (Scheme 8). Deoxy-
Scheme 8. One-pot synthesis of tetrasubstituted thiophenes. Reaction condi-
tions: i) 1 (1 mmol), AgF (10 mol%), air in xylene (3 mL) at 1408C; ii) addition
of Lawesson’s reagent (1.5 mmol), toluene (5 mL), reflux, 3 h. Yields of isolat-
ed product are given.
benzoin compounds 1 were treated with AgF (10 mol%) in
xylene under air atmosphere at 1408C for 4–10 h. The solvent
was then removed by evaporation under reduced pressure;
Lawesson’s reagent with toluene was added to the reaction
mixtures, which were then heated at 1208C for 3 h. Pleasingly,
all of the one-pot reactions proceeded smoothly and afforded
tetrasubstituted thiophenes 5 in good yield. The reaction also
showed great functional-group tolerance (e.g. Me, OMe, F, Cl,
Br) and was not affected by steric hindrance of the substituted
group. The structure of 5g was confirmed by single-crystal X-
ray diffraction[18] (see the Supporting Information).
Scheme 6. Substrate scope for the synthesis of 2,3-disubstituted-1,4-dike-
tones. Reaction conditions: 1 (1 mmol), AgF (10 mol%), air in xylene (3 mL)
at 1408C. Yields of isolated product are given. Diastereomeric ratios (d.r.)
1
were determined by H NMR spectroscopy.
cluding methyl, methoxy, fluoro, chloro, bromo, and nitro
groups were tolerated under the reaction conditions. Similarly,
Dialkyl ketones also provided the desired products in good
yields (Scheme 6, 3q, 3r), whereas 3-pentanone was once
again unreactive. Pleasingly, by using similar conditions to
those discussed above, cross-coupling reactions also proceed-
ed well to afford the products in good yields (Scheme 7). The
successful syntheses of 1,4-diketones prompted us to synthe-
size other polysubstituted heterocycles, thiophenes and pyr-
roles, in a one-pot reaction from ketone compounds.
We then turned our focus towards polysubstituted pyrroles,
another important compounds with widespread applications
in medicinal chemistry, supramolecular chemistry, and material
science.[15,16] By using a similar procedure to the above synthe-
sis of tetrasubstituted thiophenes, ketone compounds 1 under-
went reaction for 4–10 h; after solvent removal, ammonium
acetate was introduced neat into the reaction mixtures, which
were then heated at 1208C for 8 h. As expected, all reactions
provided the desired products 6 in excellent yields (Scheme 9).
Using primary amines in place of ammonium acetate did not
yield fully substituted pyrroles under the current reaction con-
ditions. The structure of 6e was confirmed by single-crystal X-
ray diffraction[18] (see the Supporting Information).
Polysubstituted thiophene structures have shown broad and
excellent biological properties and are found in some top-sell-
ing marketed drugs such as Plavix and Spiriva.[12] Due to their
structural rigidity and specific electronic properties, polysubsti-
tuted thiophenes have also found wide applications in organic
solar cells and organic semiconductors.[13] Despite these wide
On the basis of the above experimental results together
with the related reports,[19] a mechanism for the Ag-catalyzed
one-pot synthesis of tetrasubstituted furans, thiophenes, and
pyrroles is proposed in Scheme 10. Substrate 1 undergoes iso-
merization, followed by an oxidative single electron transfer
(SET) by AgI and deprotonation to generate the a-carbonyl
radical B. One possible subsequent pathway is homo-coupling
of radical B to give 1,4-dicarbonyl product 2 (path a). Another
is reaction of radical B with enolate A to afford intermediate C,
followed by a second SET to yield 2 (path b).[17] Under acid
conditions, 2 cyclizes to produce tetrasubstituted furan 3,
whereas in the presence of Lawesson’s reagent or ammonium
Scheme 7. Synthesis of 2,3-disubstituted-1,4-diketones by cross-coupling re-
actions. Reaction conditions: 1 A (0.5 mmol), 1B (0.5 mmol), AgF (10 mol%),
air in xylene (3 mL) at 1408C. Yields of isolated product are given. Diastereo-
1
meric ratios (d.r.) were determined by H NMR spectroscopy.
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Scheme 10. Proposed mechanism.
acetate, cyclization yields tetrasubstituted thiophenes or tetra-
substituted pyrroles, respectively.
In summary, we have developed the first Ag-catalyzed cou-
pling reactions of two Csp3ÀH groups to construct 1,4-diketones
with air as the terminal oxidant. The couplings were followed
by cyclization reactions to efficiently synthesize tetrasubstitut-
ed furans, thiophenes, and pyrroles, depending on the reaction
conditions, from benzyl ketone derivatives in a one-pot pro-
cess. This highly-efficient synthesis exhibited a wide substrate
scope and high functional-group tolerance. The method
broadens the chemistry of silver catalysis and should have
many applications in organic chemistry. Due to the products
possessing multiply arylated p-electron systems, which are
privileged structures in material science,[10] the approach could
be easily used in the development of new functional materials.
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This research was supported by National Natural Science Foun-
dation of China (NSFC-20972126, 21272185), the Program for
New Century Excellent Talents in University of the Ministry of
Education China (NCET-10-0937).
Keywords: CÀH activation · domino reactions · heterocycles ·
homogeneous catalysis · silver
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Received: April 10, 2015
Published online on June 10, 2015
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