Modular synthesis of all-substituted furans through oxidative carbonylation of cyclopropenes with tandem metal relay catalysis

Article abstract of DOI:10.1039/c3ob41155a

A modular synthesis of tetrasubstituted furan carboxylates using a tandem cycloisomerization/oxidative carbonylation sequence of cyclopropenes has been successfully developed. The Tandem Metal Relay Catalysis (TMRC) pathway accounted for the high efficiency of this transformation.

Full text of DOI:10.1039/c3ob41155a

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Modular synthesis of all-substituted furans through
oxidative carbonylation of cyclopropenes with tandem
metal relay catalysis†
Cite this: Org. Biomol. Chem., 2013, 11,
6258
Received 5th June 2013,
Accepted 30th July 2013
Chuanling Song, Shuli Dong, Lei Feng, Xianglong Peng, Mingchao Wang,
Jianwu Wang and Zhenghu Xu*
DOI: 10.1039/c3ob41155a
www.rsc.org/obc
A modular synthesis of tetrasubstituted furan carboxylates using on account of its high atom and step economy. Thus a series
a tandem cycloisomerization/oxidative carbonylation sequence of of directed or non-directed C–H carbonylation reactions
cyclopropenes has been successfully developed. The Tandem have been developed.7,8 The general mechanism of all
Metal Relay Catalysis (TMRC) pathway accounted for the high these reactions is initiated by the metal insertion into the C–
efficiency of this transformation.
H bond generating carbon–metal species and then they
undergo the normal carbonylation process with CO and finally
regenerate the metal catalyst by the oxidant. In these reactions,
the C–H metallation is usually the rate determining step
because of the high activation energy of the inert C–H bond
and thus very harsh conditions such as a high temperature
and an acid additive are required. In consequence the develop-
ment of the C–H carbonylation reaction under very mild con-
ditions which can be easily used in complex molecular
synthesis is highly desirable. An alternative approach is to
avoid the direct C–H metallation step, but form the key
carbon–metal species indirectly; the reaction efficiency could
be greatly improved in this way.
Recently, we developed an efficient copper–palladium relay
catalysis for the synthesis of tetrasubstituted alkene functiona-
lized furans from cyclopropenes under very mild reaction
conditions (Scheme 1).9 Cyclopropene10 underwent a copper-
catalyzed ring-opening reaction11 and subsequent transmetal-
lation to palladium generating the key furan palladium inter-
mediate M, which underwent oxidative Heck reaction with
Furans represent an important class of five-membered hetero-
cycles which are prevalent in a number of biologically active
natural products as well as pharmaceuticals.1 Therefore signifi-
cant efforts have been made to synthesize this heterocyclic
ring.2,3 All-substituted furans which possess four substituents
are even more difficult to access synthetically.4 Recently
elegant transition-metal catalyzed cyclization/cycloaddition
reactions of yne-enones to construct polycyclic furans and
other heterocycles were developed by the Larock group and the
Zhang group.4 Thus modular synthesis of tetrasubstituted
furans from simple and readily available starting materials
and the possibility to install the four substituents very freely
are still a great challenge. Herein, we report the modular syn-
thesis of tetrasubstituted furan carboxylates via the oxidative
carbonylation reaction of cyclopropenes.
Since carbon monoxide is one of the most important
readily available C1 feedstocks, transition-metal-catalyzed
carbonylation with CO became a very important method
to introduce carbonyl groups into the parent molecules.
Palladium-catalyzed carbonylation of organic halides in the
presence of various nucleophiles have undergone rapid devel-
opment and are widely used in both the laboratory and indus-
try.5 In recent years, much progress has been made in the C–H
bond activation and functionalization.6 In this context, the
direct C–H carbonylation reaction became a highly desirable
and much “greener” approach to construct the carbonyl group
Key Lab for Colloid and Interface Chemistry of Education Ministry, School of
Chemistry and Chemical Engineering, Shandong University, Jinan 250100,
People’s Republic of China. E-mail: xuzh@sdu.edu.cn
†Electronic supplementary information (ESI) available: Detailed experimental
procedures and analytical data. CCDC 930436 for compound 3f. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ob41155a
Scheme 1 Copper–palladium relay strategy to construct tetrasubstituted
furans.
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investigation of different copper salts demonstrated that
Cu(OH)2, Cu(OTf)2, and Cu(acac)2 were almost ineffective and
Cu(OPiv)2 was not as good as Cu(OAc)2 for this transformation
(Table 1, entries 1–5). DMSO might act as a ligand to prevent
the formation of palladium black. The reaction was less
efficient when conducted in the absence of DMSO (entry 6).
Attempts to replace DMSO by other sulfoxides resulted in no
coupled product (entries 7–9). To our delight, when cyclo-
propene 1a was slowly injected into the reaction system in 1 h
using a syringe pump and then reacted another 3 h, the
desired product was isolated in 73% yield (Table 1, entry 10).
Under these conditions, other Pd species including Pd(PPh3)4,
Pd(TFA)2, PdCl2, and others were screened and Pd(OAc)2 was
still the most effective (entries 11–13). Increasing the amount
of DMSO to 7 equivalents led to the highest 84% yield (entry
14). Control experiments confirmed that both copper and
palladium catalyst were necessary in this process. Only the
isomerized product 4a was formed in the presence of copper
without palladium catalyst (entry 15). No product was formed
with 5 mol% palladium catalyst alone (entry 16).
Scheme 2 Initial formation of the tetrasubstituted furan carboxylates.
alkenes generating the tetrasubstituted furans. In this Tandem
Metal Relay Catalysis (TMRC) pathway,12 the key carbon–palla-
dium bond was formed through the transmetallation strategy
under very mild conditions. Herein, we wish to extend this
methodology to the C–H carbonylation reaction. Since cyclo-
propene can be easily prepared from different terminal
alkynes and different diazo compounds, a modular synthesis
of furan carboxylates can be developed from alkyne, diazo
compound, CO and alcohol, and this methodology makes it
possible to freely install the four substituents onto the furan
ring (Scheme 1).
With the optimized carbonylation conditions established,
the scope of the reaction was investigated (Table 2). The new
reaction displayed high functional group tolerance and proved
We began our study with the reaction of cyclopropene 1a with
EtOH in the presence of balloon pressure CO under the standard
conditions for oxidative Heck reaction (Scheme 2). To our
delight, the expected tetra-substituted furan carboxylate 3a was
isolated in 35% yield. The structure was unambiguously charac-
terized by NMR, mass analysis and single X-ray crystallography.
Inspired by the observed results, we next set out to optimize
reaction conditions to develop an efficient oxidative carbonyla-
tion reaction of cyclopropenes toward the tetra-substituted
furan carboxylates (Table 1, for more details, see ESI†). An
Table 2 Substrate scope of carbonylation reactionsa
Table 1 Optimization of reaction conditionsa
Entry
Catalyst
Oxidant
Additive
Yieldb
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PPh3)4
Pd(TFA)2
PdCl2
Cu(OAc)2
Cu(OPiv)2
Cu(OH)2
Cu(OTf)2
Cu(acac)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
/
DMSO (3 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
/
PhSOCH3 (3 eq.)
PhSOPh (3 eq.)
BnSOBn (3 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
DMSO (7 eq.)
DMSO (3 eq.)
DMSO (3 eq.)
35%
23%
7%
0%
3%
30%
Trace
Trace
Trace
73
65
31
9
10c
11c
12c
13c
14c
15d
16e
10
84
0 (42)
0
Pd(OAc)2
/
Pd(OAc)2
a Reaction conditions: cyclopropene 1a (0.2 mmol), palladium catalyst
(5 mol%), oxidant (0.4 mmol), EtOH (0.2 mL), CH3CN (1.5 mL), 60 °C.
b Isolated yield. c 1a was slowly injected in 1 h. d Cu(OAc)2 only, the a Reaction conditions: cyclopropene 1 (0.2 mmol), Pd(OAc)2 (5 mol%),
number in parentheses represents the isolated yield of byproduct 4a. Cu(OAc)2 (0.4 mmol), EtOH (0.2 mL), CH3CN (1.5 mL), DMSO
e 5 mol% Pd(OAc)2 only.
(1.4 mmol), 60 °C, 1 was slowly injected in 1 h.
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Table 3 Substrate scope of carbonylation reactionsa
Scheme 3 Proposed reaction mechanism.
a Reaction conditions: cyclopropene 1 (0.2 mmol), Pd(OAc)2 (5 mol%),
Cu(OAc)2 (0.4 mmol), EtOH (0.2 mL), CH3CN (1.5 mL), DMSO
(1.4 mmol), 60 °C, 1 was slowly injected in 1 h.
In summary, a very general modular synthesis of tetrasub-
stituted furan carboxylates has been successfully developed.
This methodology makes it possible to introduce four substitu-
ents very freely under very mild conditions. Tandem Metal
Relay Catalysis (TMRC) accounted for the high efficiency of
this transformation, which is not only complementary to the
current transition-metal catalyzed C–H activation, but also a
novel fundamentally different strategy. Further work aiming to
gain a detailed mechanistic understanding of this process and
the other synthetic applications of this strategy is currently
underway in our laboratory.
We are grateful for financial support from the Natural
Science Foundation of China (grant no. 21102085), the
Science Foundation of the Ministry of Education of China
(no. 20110131120049), the Natural Science Foundation of
Shandong Province (BS2012YY006), and Shandong University.
to be quite a general methodology for the synthesis of all-sub-
stituted furan carboxylates. Both aliphatic cyclopropenes and
aromatic cyclopropenes could generate the corresponding pro-
ducts in good to excellent yields (3a–3m). Different substitu-
ents on the phenyl ring such as Br, F and methyl were all
tolerated (3g–3i). The substrate bearing the phenol group was
also suitable for this carbonylation reaction, giving the desired
furan ester 3j in 71% yield. Other electron-withdrawing groups
such as CN could also be installed onto the furan ring in good
yield if started with cyano group substituted cyclopropene (3k,
3l). Cyclopropene derived from ethyl acetoacetate was also sub-
jected to this reaction and afforded 2-alkyl-tetrasubstituted
furan (3m) in relatively lower 35% yield under the standard
conditions. Non-substituted cyclopropene dicarboxylate sub-
strates could also provide the 2,3,5-trisubstituted furan 3n in
84% yield.
Notes and references
Furthermore, we investigated the scope of ROH (Table 3).
Aliphatic alcohols, such as methanol, n-butanol, benzyl
alcohol, and isopropanol, were all suitable nucleophiles under
the optimized conditions and produced the corresponding
products in good yields (3o–3r). More importantly, the big ter-
tiary butanol, which is normally unreactive in other carbonyla-
tion systems,8b could also act as a nucleophile and gave the
product 3s in 41% yield.
A tentative mechanism is proposed in Scheme 3. Cyclopro-
pene reacted with Cu(OAc)2 and then underwent transmetalla-
tion to Pd(OAc)2, generating the key furan palladium M. This
intermediate M underwent CO insertion, ligand exchange with
alcohol, and reductive elimination to afford the carbonylation
product 3 and the Pd(0), which is oxidized to Pd(II) by Cu-
(OAc)2. Since we did see the isomerized furan 4a during our
reactions, we carried out the carbonylation reaction with 4a
under the standard reaction conditions; the target product 3a
was formed, but the reaction was very slow. Thus Tandem
Metal Relay Catalysis (TMRC) is the major pathway, and the
isomerization/palladium catalyzed C–H carbonylation pathway
cannot be excluded.
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