An efficient nanoparticle-supported and magnetically recoverable copper(I) catalyst for synthesis of furans from ene-yne-ketone

Article abstract of DOI:10.1002/cjoc.201400730

A magnetic nano-supported Cu(I) catalyst was prepared and showed high activity for cyclization of ene-yneketone to synthesize furans. The catalyst was easily recovered from the reaction by using external magnets and reused 8 times without significant loss

Full text of DOI:10.1002/cjoc.201400730

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DOI: 10.1002/cjoc.201400730
An Efficient Nanoparticle-Supported and Magnetically Recov-
erable Copper(I) Catalyst for Synthesis of Furans from
Ene-yne-ketone
Yi Liu,a Zhan Liu,*,b and Yingde Cui*,a,c
a School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi 710072,
China
b Departmenl of Chemistry and Chemical Engineering, Huaihua University, Huaihua, Hunan 418000, China
c Guangzhou Vocational College of Science and Technology, Guangzhou, Guangdong 510550, China
A magnetic nano-supported Cu(I) catalyst was prepared and showed high activity for cyclization of ene-yne-
ketone to synthesize furans. The catalyst was easily recovered from the reaction by using external magnets and re-
used 8 times without significant loss of its catalytic activity.
Keywords furans, ene-yne-ketone, magnetic nano-supported, copper(I) catalyst, cyclization
Introduction
from the reaction medium by external permanent mag-
nets for the preparation of furans[5] via domino reaction.
Domino reactions[6] have been developed to enable
the rapid construction of complex and diverse furan
molecules starting from simple substrates. What’s more,
they have increased synthetic efficiency by obviating the
isolation and purification of intermediates, decreasing
the number of laboratory operations required and the
quantities of chemicals and solvents used. Recently,
transition-metals are widely used for the formation of
furan derivatives, such as Pd,[7] Cu,[8] Au,[9] and Ag.[10]
Although a range of domino reactions have been devel-
oped to prepare the furans, the development of magnetic
nanoparticle is still highly favorable for the construction
of furans.
Transition-metal-catalyzed organic transformations
are powerful tools for the formation of new carbon-
heteroatom and carbon-carbon bonds to construct or-
ganic functional molecules from readily available start-
ing materials under mild conditions.[1] Many successful
strategies have been developed with a variety of homo-
geneous metal catalysts.[2] However, these reactions, in
general, suffered from the deficits in recovery of homo-
geneous catalysts, the pollution of environment and the
separation of products. The development of heteroge-
neous catalytic systems[3] that can be efficiently reused
is the most efficient routes to avoid and solve these
problems. Nanoparticles catalysts have been extensively
studied and shown good catalytic activities due to their
high surface areas during the last decades.[4] There has
been increasing emphasis on the use and development of
environmentally friendly solid catalyst systems to
maximize resource utilization and decrease waste, espe-
cially for environmental health and safety.
Although many heterogeneous catalytic transforma-
tions for preparation of functionalized molecules have
been developed, the catalyst that can be efficiently re-
used while keeping the inherent activity of the catalytic
center is still far from being solved. In addition, separa-
tion and recovery of the catalyst constitutes to be im-
portant scaffolds and blocks for synthetic chemistry,
which is generally performed by filtration with reduced
efficiency. Herein, we hope to develop magnetic
nanoparticle-supported catalyst that can be separated
Experimental
General
Transmission electromicroscopy (TEM) was per-
formed at the research resources center of South China
Normal University, with a JEM-2100HR microscope
operating at an accelerating voltage of 100－200 kV.
The samples were dispersed in high-purity ethanol un-
der sonication, after which a carbon-coated 400 mesh
copper grid was immersed in the suspension and then
air-dried. A slight aggregation of the nanoparticles was
observed due to magnetic interactions with the electron
beam. Powder X-ray diffraction (XRD) measurements
were performed at room temperature over the range
*
E-mail: andygle2013@gmail.com, zhanliu2008@126.com; Tel.: 0086-0760-88207939, 0086-0745-2851014; Fax: 0086-0760-882007899, 0086-
0745-2851014
Received October 26, 2015; accepted January 4, 2015; published online XXXX, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201400730 or from the author.
Chin. J. Chem. 2015, XX, 1—6
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Liu, Liu & Cui
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product was separated by column chromatography
[eluted with V(petroleum ether)∶V(ethyl acetate)＝6∶
1] to give a pure sample of 2a.
2θ＝15°－80° with a BRUKER D8 ADVANCE dif-
fractometer. Fourier transform IR (FTIR) spectra were
collected with a BRUKER FT/IR-460 Plus spectropho-
tometer in the 450－4000 cm−1 range. The spectra of the
samples were obtained in KBr pellets (Merck, spectro-
scopic grade) containing 1 wt% MNPs. 1H NMR spectra
and 13C NMR spectra were recorded using a Bruker
Avance 400 MHz NMR spectrometer (100 MHz for
carbon) and respectively referenced to δ 7.26 and 77.0
for chloroform-d solvent with TMS as internal standard.
Mass spectra were recorded on a Shimadzu GCMS-
QP5050A at an ionization voltage of 70 eV equipped
with a DB-WAX capillary column (internal diameter＝
0.25 mm, length＝30 m). TLC was performed using
commercially prepared 100－400 mesh silica gel plates
(GF254), and visualization was effected at 254 nm.
Synthesis of 3a according to the following proce-
dure
3-(Oct-2-ynylidene)pentane-2,4-dione (0.5
mmol), 1 mol% L-proline, 5 mol% Cu2O@Fe3O4 and
THF (3 mL) were stirred for 12 h in a closed system
under protection of purified argon at r.t. After comple-
tion of the reaction (monitored by TLC), the catalyst
could be easily separated by external permanent mag-
nets. And then the solvent was removed and the crude
product was separated by column chromatography
(eluted with V(petroleum ether)∶V(ethyl acetate)＝
12∶1) to give a pure sample of 3a.
Results and Discussion
At the outset of our studies, 3-(oct-2-ynylidene)-
pentane-2,4-dione 1a was chosen as model substrates to
optimize the suitable reaction conditions and the results
are summarized in Table 1. A variety of nano-copper(I)
catalyst, solvents, additives, oxidant, temperatures and
time were screened. Various nanoparticle-supported
catalysts, such as Cu2O@Fe3O4, CuBr@Fe3O4, CuCl@
Fe3O4, CuO@Fe3O4 and nano-Cu2O, were firstly tested
by using air as oxidant in THF at room temperature for
12 h (Table 1, Entries 1－5). The results showed that the
product 2a was formed in 45% yield in the presence of
Cu2O@Fe3O4. Other catalysts also generated 2a in 10%
－42% yields. Notably, the side product 3a was also
detected. We next hope to optimize the suitable condi-
tions for the formation of 2a and 3a respectively in good
yields. Among solvents surveyed, the moderate yield of
2a was formed in CH3CN, CH2Cl2, CH3OH, DMSO and
DMF (Table 1 Entries 6－10). And it was indicated that
THF was the most effective media for this domino reac-
tion. Subsequently, additives were examined (Table 1
Entries 11－13) and the results indicated that higher
yield of 2a was obtained by using L-proline as additive
(Table 1 Entry 13). We next tried to improve the yields
of 2a by using oxygen with increasing oxygen pressure.
The results indicated that the products were formed in
82%, 73% yields by using 1 atm and 2 atm of O2 re-
spectively (Table 1, Entries 14, 15). To our delight, the
products 2a and 3a were formed in 10% and 79% yields
respectively in the protection of N2 (Table 1, Entry 16).
The good yield was obtained in the protection of Ar
(Table 1, Entry 17). The formation of Cu2O@Fe3O4 was
confirmed by TEM (Figure 1a) and XRD (Figure 2).
Having in hand a highly efficient catalytic system,
we then investigated the scope of the reaction for cycli-
zation of ene-yne-ketone to synthesize 2-carbonyl furans
(Table 2). First, alkyl-substituted alkyne (R＝CH3-
(CH2)4) was examined for this domino process. We
were pleased to find that this cyclization/oxidation se-
quence tolerated a wide range of group on the carbonyl
carbon (R1, R2). Different substituted substrates, such as
R1＝CH3, ClCH2; R2＝CH3, CH3O, (CH3)3CO, EtO,
Synthesis of catalysts
Synthesis of Fe3O4 MNPs In the case of ammonia
water or diethylamine syntheses, 20 mmol of
FeCl3•6H2O and 10 mmol of FeCl2•4H2O were dis-
solved in 25 mL of deoxygenated 0.5 mol/L HCl solu-
tion. This solution was quickly added to 250 mL of a
deoxygenated 3.0 mol/L solution of ammonia water or
diethylamine at r.t. with vigorous mechanical stirring
under an argon atmosphere. A black precipitate formed
immediately, and stirring was continued for 2 h at that
temperature under an argon atmosphere. After that time,
the black precipitate was magnetically separated,
washed with deoxygenated water several times, and
stored in 0.1 mol/L TMAOH for future use. A similar
procedure was followed for the synthesis of Fe3O4 with
the inorganic base NaOH (pH 14).
Synthesis of Cu2O@Fe3O4 MNPs In the case of
the syntheses with ammonia water or diethylamine, 5
mmol of Cu2Cl2•6H2O was dissolved in a solution of 1
mL of 37% HCl in 4 mL of water, and 20 mmol of
FeCl3•6H2O and 10 mmol of FeCl2•4H2O were dis-
solved in 40 mL of water. The two solutions were mixed
at r.t., and then quickly added to 200 mL of 3.0 mol/L
ammonia water or diethylamine at 85 ℃ with vigorous
mechanical stirring under an argon atmosphere. A
brownish black precipitate formed immediately, and
stirring was continued for 2 h at 85 ℃. After that time,
the reaction mixture was cooled to room temperature,
and the precipitate (Cu2O@Fe3O4 MNPs) was magneti-
cally separated, washed with water several times, and
dispersed in 0.1 mol/L TMAOH. For comparison, the
nanomaterials were also prepared using aqueous NaOH
solution (pH 14) under the same experimental condi-
tions.
Synthesis of 2a according to the following proce-
dure
3-(Oct-2-ynylidene)pentane-2,4-dione (0.5
mmol), 5 mmol% L-proline and 5 mol% Cu2O@Fe3O4
were stirred for 12 h in THF (3 mL) at r.t. After comple-
tion of the reaction (monitored by TLC), the catalyst
could be easily separated by an external permanent mag-
nets. And then the solvent was removed and the crude
2
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Magnetically Recoverable Copper(I) Catalyst for Synthesis of Furans from Ene-yne-ketone
Table 1 Optimization of reaction conditionsa
O
O
O
cat.
(CH2)4CH3
+
O
H3C(H2C)4
O
1a
2a
O
Figure 1 TEM images of Cu2O@Fe3O4 before reaction (a) and
after reuse eight times (b).
(CH2)3CH3
O
3a
Yieldb/%
2a 3a
45 trace
42 ＜5
23
Entry Catalyst
Solvent Additive Oxidant
1
Cu2O@Fe3O4 THF
CuBr@Fe3O4 THF
CuCl@Fe3O4 THF
CuO@Fe3O4 THF
－
－
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
2
3
4
－
－
10 ＜5
30 ＜5
26 ＜5
24 ＜5
5
nano-Cu2O
THF
6
Cu2O@Fe3O4 CH3CN －
Cu2O@Fe3O4 CH2Cl2
Cu2O@Fe3O4 CH3OH －
7
－
8
31
－
9
Cu2O@Fe3O4 DMF
Cu2O@Fe3O4 DMSO
Cu2O@Fe3O4 THF
Cu2O@Fe3O4 THF
Cu2O@Fe3O4 THF
－
35 trace
38 trace
26 ＜5
22 ＜5
88 ＜5
Figure 2 XRD pattern of Cu2O@Fe3O4.
10
11
12
13
CH3CO2H Air
PhCO2H Air
L-proline Air
p-ClPhNH, o-CH3OPhNH led to a beneficial effect on
the reaction outcome, and in most cases the correspond-
ing products (2a－2k) were obtained in good yields.
Commercially available cyclic 1,3-dicarbonyl com-
pounds were next tested to explore the scope of this
transformation. To our delight, the results indicated that
the reaction would be successfully extended to cyclic
1,3-dicarbonyl compounds under the optimum condi-
tions and afforded the corresponding products in good
yields.
For further investigation, phenyl-substituted alkyne
(R＝Ph) was also employed for this Cu2O@Fe3O4 cata-
lyzed domino reaction (Table 2). It was pleased to find
that this cyclization/oxidation sequence also tolerated a
wide range of group on the carbonyl carbon (R1, R2).
These results indicated that this transformation was ap-
plicable to both open and cyclic 1,3-dicarbonyl com-
pounds and the desired products were formed in good
yields. Interestingly, the group (R2 ＝ p-ClPhNH,
c-CH3OPhNH) led to a beneficial effect on the reaction
outcome, and the corresponding products were obtained
in good yields. All these results demonstrated that this
domino reaction could occur with functional groups at
different positions of the alkyne terminus and carbonyl
carbon. It was worthy to note that all of the desired
products were formed with high regioselectivity.
O2 (1
14
Cu2O@Fe3O4 THF
L-proline
L-proline
82 trace
73 trace
atm)
O2 (2
atm)
15
16
Cu2O@Fe3O4 THF
Cu2O@Fe3O4 THF
L-proline N2
10 79
17c Cu2O@Fe3O4 THF
L-proline Ar
＜5 89
a Reaction conditions: 1a (0.5 mmol), catalyst (5 mol%), additive
(1 mol%), r.t., solvent (2.0 mL), 12 h. b GC Yields. c The reaction
was performed in a closed system under protection of purified
argon.
cycloalkyl and carboxamide groups remained unaffected
under these reaction conditions. The method indeed was
nice, especially as vinylfurans were not easy to obtain.
Finally, we turned our attention to reusability of
catalyst. It was found that catalyst could be recovered by
using external magnets, with high efficiency and up to
99% recovery of catalyst. After completion of the reac-
tion, the mixture turned clear and catalyst was deposited
on the magnetic bar within 30 s, and then re-used in the
next reaction (Figure 3). Noteworthily, 72% yield was
maintained even when the catalyst was reused for 8
times.
We next explored the scope of this remarkable
transformation for synthesis of 2-vinyl furans (Table 3).
This reaction was compatible with a variety of func-
tional groups (R1, R2), including methyl, ester, cycloal-
kyl, and carboxamide and gave the corresponding prod-
ucts 3a－3k in 41%－87% yields. Notably, the ester,
The most plausible Cu(I)-catalyzed reaction mecha-
nism for cyclization/oxidation ofene-yne-ketone has
been depicted in Scheme 1. The substrates 1a coordi-
nated with Cu(I) species to form the metal complexes A.
Subsequently, an intramolecular 5-exo-dig cyclization
by nucleophilic attack of the oxygen of carbonyl onto
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Liu, Liu & Cui
COMMUNICATION
Table 2 Cu2O@Fe3O4-catalyzed synthesis of 2-carbonyl furans
O
R2
R1
O
R1
Cu2O@Fe3O4
O
O
O
R2
R
L-proline, air, THF, r.t.
R
O
O
O
O
O
O
O
EtO
(CH2)4CH3
2b 86%
O
O
O
(CH2)4CH3
(CH2)4CH3
2a 82%
2c 81%
O
Cl
O
O
O
O
(CH2)4CH3
EtO
Cl
O
O
N
H
O
(CH2)4CH3
O
O
(CH2)4CH3
2d 87%
2f 85%
2e 81%
O
O
O
O
O
O
O
(CH2)4CH3
N
H
O
(CH2)4CH3
O
(CH2)4CH3
O
2i 79%
2h 80%
2g 84%
O
O
O
O
O
O
O
(CH2)4CH3
O
(CH2)4CH3
O
Ph
Ph
2j 81%
2l 83%
2k 80%
O
O
O
O
O
O
EtO
O
O
Ph
Ph
O
O
O
Ph
2o 86%
2m 84%
2n 85%
Cl
O
O
O
O
O
O
O
N
H
N
H
Ph
O
O
Ph
Ph
O
2r 78%
2p 80%
2q 79%
O
O
O
O
O
O
O
Ph
O
Ph
O
Ph
Ph
2s 82%
2t 81%
2u 82%
Isolated yields.
Scheme 1 Proposal mechanism
O
O
O
O
Cu2O@Fe3O4, air
L-proline, THF, r.t.
(CH2)4CH3
Cu2O@Fe3O4, Ar
O
L-proline, THF, r.t.
(CH2)3CH3
O
O
(CH2)4CH3
2a
3a
CuL
O
air
1,2-H shift
O
O
O
-
Ar
CuL
(CH2)3CH3
CuL
CuL
O
+
O
(CH2)3CH3
B
(CH2)4CH3
H
H
A
C
4
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Magnetically Recoverable Copper(I) Catalyst for Synthesis of Furans from Ene-yne-ketone
Table 3 Cu2O@Fe3O4-catalyzed synthesis of 2-vinyl furans
O
O
R2
R1
O
R2
Cu2O@Fe3O4, Ar
(CH2)3CH3
L-proline, THF, r.t.
R1
R
O
1
3
O
O
O
O
EtO
(CH2)3CH3
3a 82%
(CH2)3CH3
(CH2)3CH3
(CH2)3CH3
(CH2)3CH3
O
O
O
3b 86%
3c 81%
O
O
O
O
(CH2)3CH3
(CH2)3CH3
O
O
O
3d 87%
3e 81%
3f 85%
O
O
O
EtO
Ph
(CH2)3CH3
(CH2)3CH3
O
O
O
Ph
3i 85%
3k 85%
3g 87%
3h 81%
O
O
H
H
N
N
(CH2)3CH3
(CH2)3CH3
O
Cl
O
O
3j 87%
Isolated yields.
Further studies will focus on cyclization of ene-yne
compounds and synthetic applications of this reaction
are being carried out in our laboratory.
Acknowledgement
The work was financially supported by the Natural
Science Foundation of Guangdong Province (No.
10151022501000033).
Figure 3 Photographs showing the separation of catalyst.
References
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sponding product 2a and release the copper catalyst.
While the desired product 3a was formed via 1,2-H mi-
grations of intermediate C and copper catalyst was re-
leased.
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Chin. J. Chem. 2015, XX, 1—6
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Liu, Liu & Cui
COMMUNICATION
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