A heterogeneous gold(I)-catalyzed ring expansion of unactivated alkynylcyclopropanes with sulfonamides leading to (E)-2-alkylidenecyclobutanamines

Article abstract of DOI:10.1016/j.tetlet.2016.08.062

A heterogeneous gold(I)-catalyzed ring expansion of unactivated alkynylcyclopropanes was achieved in 1,2-dichloroethane at 100?°C in the presence of sulfonamides by using a magnetic nanoparticle-supported phosphine gold(I) complex [Fe₃O₄@SiO₂-P-AuOTf] as catalyst, yielding a variety of (E)-2-alkylidenecyclobutanamines in moderate to high yields. The new heterogeneous gold catalyst can easily be separated from the reaction mixture by applying an external magnet and recycled at least 10 times without a significant loss of activity.

Full text of DOI:10.1016/j.tetlet.2016.08.062

Tetrahedron Letters 57 (2016) 4405–4410
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Tetrahedron Letters
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A heterogeneous gold(I)-catalyzed ring expansion of unactivated
alkynylcyclopropanes with sulfonamides leading
to (E)-2-alkylidenecyclobutanamines
⇑
Feiyan Yi, Bin Huang, Quan Nie, Mingzhong Cai
Key Laboratory of Functional Small Organic Molecule, Ministry of Education and College of Chemistry & Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A heterogeneous gold(I)-catalyzed ring expansion of unactivated alkynylcyclopropanes was achieved in
1,2-dichloroethane at 100 °C in the presence of sulfonamides by using a magnetic nanoparticle-sup-
ported phosphine gold(I) complex [Fe3O4@SiO2-P-AuOTf] as catalyst, yielding a variety of (E)-2-alkyli-
denecyclobutanamines in moderate to high yields. The new heterogeneous gold catalyst can easily be
separated from the reaction mixture by applying an external magnet and recycled at least 10 times
without a significant loss of activity.
Received 22 June 2016
Revised 16 August 2016
Accepted 20 August 2016
Available online 22 August 2016
Keywords:
Gold
Ó 2016 Elsevier Ltd. All rights reserved.
Ring expansion
Four-membered carbocycle
Magnetic nanoparticle
Heterogeneous catalysis
Introduction
variety of cyclobutanes, a wide range of transition metal catalysts,
such as Co,8 Ru,9 Rh,10 Fe,11 Pd,12 and Pt13 complexes have been
Four-membered carbocycles are important structural units
frequently present in many natural products of significant biolog-
ical activities.1 In addition, substituted cyclobutanes constitute
useful building blocks for organic synthesis because of their rich
chemistry, they can undergo facile and unique ring-opening and
ring-expansion reactions driven by the relief of ring strain.2
Despite the high preparative utility of four-membered carbocycles,
there are only a limited number of practical and efficient methods
for their preparation, which is mainly due to the following reasons.
utilized successfully in numerous ring expansion reactions.
Homogeneous gold catalysis has attracted much attention and
cationic gold(I) complexes have evolved as mild Lewis acid cata-
lysts for organic transformations requiring the activation of
p
bonds in the past decade.14 Since Toste and co-workers reported
the first gold(I)-catalyzed ring expansion of alkynylcyclopropanols
to alkylidenecyclobutanones,15 gold(I)-catalyzed ring expansion
reactions of cyclopropane derivatives have received great attention
in recent years,16 providing a novel and powerful method to con-
struct synthetically useful four-membered carbocycles. However,
industrial applications of these homogeneous gold catalysts
remain a challenge because they are quite expensive, cannot be
recycled, and difficult to separate from the product mixture, which
is a particularly significant drawback for their application in the
pharmaceutical industry. In contrast, heterogeneous catalysts have
received more and more attention because of the advantages of
high catalytic efficiency and easy recycling, which are important
for precious metal catalysts and flow chemistry processes.17 To
overcome these limitations, magnetic nanoparticles-supported
catalysts are a better choice because their magnetic separation is
an alternative to filtration or centrifugation as it prevents loss of
catalyst and increases the reusability.18 In continuing of our efforts
to develop economical and eco-friendly synthetic pathways for
organic transformations,19 in this letter, we report the synthesis
of a magnetic nanoparticle-supported phosphine gold(I) complex
Thermal or catalyzed (2 + 2) cycloaddition reactions of two
p com-
ponents are limited to some special substrates.3 Photochemical
(2 + 2) cycloaddition reaction is another route to substituted
cyclobutanes, however, only the intramolecular cycloaddition
reaction is efficient and the intermolecular reaction has not been
well developed.4 Among the various approaches to four-membered
carbocycles, ring expansion of cyclopropanes is considered one of
the most powerful and versatile methods because the starting
materials can be easily prepared by many known methods, such
as Simmons–Smith,5 Kulinkovich,6 de Meijere-Kulinkovich,7 and
others. Transition metal-promoted ring expansion reactions of
cyclopropanes provide a powerful method for construction of a
⇑
Corresponding author.
E-mail address: caimzhong@163.com (M. Cai).
http://dx.doi.org/10.1016/j.tetlet.2016.08.062
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

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F. Yi et al. / Tetrahedron Letters 57 (2016) 4405–4410
[Fe3O4@SiO2-P-AuOTf] and its successful application to ring expan-
sion of unactivated alkynylcyclopropanes. To the best of our
knowledge, this is the first example of using a heterogeneous gold
catalyst for the ring expansion reaction of cyclopropane derivatives
with high efficiency. The catalyst could be easily separated from
the reaction mixture with the simple application of an external
magnet, and its catalytic efficiency remains unaltered even after
recycling ten times.
and 64.6° should be attributed to (111), (200) and (220)
reflections of the Au species, respectively. EDS analysis of the fresh
Fe3O4@SiO2-P-AuOTf complex shows the presence of Si, O, C, P, S, F,
Fe, and Au elements (Fig. 3).
The magnetically separable nanocomposite Fe3O4@SiO2-P-
AuOTf was then used as catalyst for the ring expansion reaction
Results and discussion
The magnetic nanoparticle-supported phosphine gold(I) com-
plex was synthesized according to the procedure summarized in
Scheme 1. Firstly, Fe3O4 nanoparticles were coated with a thin
layer of silica through a sol–gel process using Si(OEt)4 as the silica
source and aqueous NH3 as the hydrolyzing agent to give silica-
coated Fe3O4 (Fe3O4@SiO2). The latter was treated with commer-
cially available 2-(diphenylphosphino)ethyltriethoxysilane in
toluene under reflux for 24 h to afford the phosphino-functional-
ized magnetic nanoparticles (Fe3O4@SiO2-P), the phosphine con-
tent was determined to be 0.44 mmol gÀ1 by elemental analysis.
The supported gold(I) complex (Fe3O4@SiO2-P-AuOTf) was
obtained by the reaction of Fe3O4@SiO2-P with AuCl in methanol
at 65 °C, followed by the treatment with AgOTf in dichloromethane
(DCM), the gold content was determined to be 0.38 mmol gÀ1 by
ICP-AES.
Figure 2. XRD patterns of Fe3O4@SiO2 (a), Fe3O4@SiO2-P-AuOTf (b), and recycled
The TEM image revealed that the diameter of the Fe3O4@SiO2-P-
AuOTf complex was approximately 30 nm (Fig. 1a). Figure 2 shows
the XRD spectra of Fe3O4@SiO2, Fe3O4@SiO2-P-AuOTf, and the recy-
cled Fe3O4@SiO2-P-AuOTf. XRD patterns of both Fe3O4@SiO2-P-
AuOTf and the recycled Fe3O4@SiO2-P-AuOTf obviously show two
sets of diffraction peaks, compared with the XRD pattern of
Fe3O4@SiO2, the three peaks that appeared at 2h = 38.0°, 44.3°,
Fe3O4@SiO2-P-AuOTf (c).
O
O
(EtO)3Si(CH2)2PPh2
Toluene, reflux, 24 h
Si(OEt)4
NH4OH
PPh2
Fe O
Si
OEt
Fe O
Fe O
4 4
Fe O
3
3
3
4 4
3
Fe3O4
SiO2
SiO2
Fe3O4@SiO2
Fe3O4@SiO2-P
PPh2AuOTf
O
O
AuCl
AgOTf
Fe O
Fe33O4 4
Si
OEt
CH3OH, reflux
12 h
DCM, r.t., 2 h
SiO2
Fe3O4@SiO2-P-AuOTf
Scheme 1. Preparation of Fe3O4@SiO2-P-AuOTf complex.
Figure 3. Energy dispersive spectra (EDS) of Fe3O4@SiO2-P-AuOTf.
Figure 1. TEM images of the fresh Fe3O4@SiO2-P-AuOTf (a) and recycled Fe3O4@SiO2-P-AuOTf after 10th run (b).

F. Yi et al. / Tetrahedron Letters 57 (2016) 4405–4410
4407
Table 1
Optimization of the reaction conditions for the ring expansiona
NHTs
Catalyst
+
TsNH2
Solvent, Temp.
1a
2a
3a
Entry
Catalyst (mol %)
Solvent
Temp. (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuCl (5)
Ph3P-AuCl (5)/AgOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (5)
Fe3O4@SiO2-P-AuOTf (10)
Fe3O4@SiO2-P-AuOTf (2)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Dioxane
THF
40
60
80
36
36
36
24
24
24
24
24
24
24
24
16
48
0
Trace
45
66
62
Trace
67
0
100
110
100
100
100
100
100
100
100
100
9
0
10
11
12
13
Toluene
TCE
DCE
23
58
65
49
DCE
a
All reactions were performed using phenylethynylcyclopropane (0.5 mmol), TsNH2 (0.6 mmol) in solvent (5 mL) under Ar.
Isolated yield based on phenylethynylcyclopropane.
b
of alkynylcyclopropanes with sulfonamides. Initial experiments
with phenylethynylcyclopropane (1a) and 4-methylbenzenesul-
fonamide (2a) were performed to optimize the reaction conditions,
and the results are summarized in Table 1. At first, the temperature
effect was examined by using 5 mol % of Fe3O4@SiO2-P-AuOTf as
catalyst in 1,2-dichloroethane (DCE) (entries 1–5). It is evident that
the reaction did not occur at 40 °C and the reaction proceeded very
slowly even at 60 °C. When the temperature was raised to 80 °C,
the desired product 3a was obtained in 45% yield (entry 3). Further
raising the temperature to 100 °C gave the highest efficiency with a
66% yield (entry 4). When Fe3O4@SiO2-P-AuCl was employed as the
catalyst, only a trace amount of 3a was detected, while a homoge-
neous Ph3P–AuOTf complex, generated in situ from Ph3P–AuCl and
AgOTf, also afforded good yield, which indicated that the effective
component should be the phosphine–AuOTf complex (entries 6
and 7). Our next studies focused on the effect of solvent on the
model reaction and a significant solvent effect was observed.
Among the solvents examined, dioxane and THF were inefficient
and toluene gave a low yield (entries 8–10). When 1,1,2,2-tetra-
chloroethane (TCE) was used as solvent, the desired product 3a
was obtained in only 58% yield (entry 11), so DCE was the best
choice. Increasing the amount of the catalyst could shorten the
reaction time, but did not improve the yield (entry 12). Reducing
the amount of the catalyst to 2 mol % resulted in a lower yield
and a long reaction time was required (entry 13). Thus, the
optimized reaction conditions for this transformation are the
Fe3O4@SiO2-P-AuOTf (5 mol %) in DCE at 100 °C under Ar for 24 h
(entry 4).
With the optimized conditions in hand, we then studied the
scope and limitation of this heterogeneous gold-catalyzed ring
expansion reaction using a range of alkynylcyclopropanes and var-
ious sulfonamides as the substrates20 and the results are listed in
Table 2. As shown in Table 2, the reactions of alkynylcyclopropanes
1b–1f containing electron-deficient aryl groups such as 4-chloro or
3-chlorophenyl, 4-bromophenyl, 4-(trifluoromethyl)phenyl and 4-
(methoxycarbonyl)phenyl with TsNH2 (2a) proceeded smoothly
under the optimized conditions to afford the desired products
3b–3f in good yields. We speculated that electron-rich aryl groups
in alkynylcyclopropanes would increase the nucleophilicity of the
carbon–carbon triple bond toward cationic gold(I) complex due
to conjugation with it, making these substrates more reactive.
However, it was found that, the more electron-donating the aryl
group in the substrate was, the worse the result was. For instance,
p-tolylethynylcyclopropane 1g afforded a 45% yield and the reac-
tion of (p-methoxyphenyl)ethynylcyclopropane with 2a did not
occur at all. Interestingly, the o-chlorophenyl group was also
demonstrated as a suitable substituent in the substrate and the
desired product 3h was obtained in 77% yield. The bulky and elec-
tron-rich 1-naphthyl group turned out to be less suitable, and the
reaction gave 3i in only 21% yield. In addition, 3,4-disubstituted
phenylethynylcyclopropane 1j could be well transformed into the
expected product 3j in 86% yield. Alkyl-substituted alkynylcyclo-
propanes were also suitable for the ring expansion in addition to
aryl-substituted substrates. For example, 1-cyclopropyl-1-octyne
1k underwent the ring expansion reaction with 2a smoothly to
furnish the desired product 3k in 54% yield. Furthermore,
other sulfonamides such as 4-bromobenzenesulfonamide (2b),
2-nitrobenzenesulfonamide (2c), and methanesulfonamide (2d)
also underwent the ring expansion reactions with various
alkynylcyclopropanes effectively to afford the corresponding
products 3l–3u in good yields. It is noteworthy that, N-methyl-p-
tolylsulfonamide (TsNHMe) (2e) was proved to be highly effective
trapping reagent and underwent the ring expansion reactions with
different alkynylcyclopropanes smoothly to give the desired
products 3v–3y in high yields.
To verify whether the observed catalysis was due to the hetero-
geneous catalyst Fe3O4@SiO2-P-AuOTf or to a leached gold species
in solution, the reaction of phenylethynylcyclopropane (1a) with
4-methylbenzenesulfonamide (2a) was carried out until an
approximately 50% conversion of 1a. Then the Fe3O4@SiO2-P-
AuOTf catalyst was separated magnetically from the solution and
the solution was transferred to another sealed tube and stirred
again at 100 °C for 12 h. In this case, no significant increase in con-
version was observed, indicating that leached gold species from
the catalyst (if any) are not responsible for the observed activity.
It was confirmed by ICP-AES analysis that no gold species could
be detected in the solution. These results rule out any contribution
to the observed catalysis from a homogeneous gold species,
demonstrating that the observed catalysis was intrinsically
heterogeneous.
A possible mechanism for this heterogeneous gold(I)-catalyzed
ring expansion of unactivated alkynylcyclopropanes is outlined in
Scheme 2. Firstly, coordination of the magnetic nanoparticles-
immobilized cationic gold(I) catalyst to alkyne moiety in alkynyl-
cyclopropane (1) and subsequent reaction generates a magnetic
nanoparticle-immobilized vinylgold cation intermediate A. Then,

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F. Yi et al. / Tetrahedron Letters 57 (2016) 4405–4410
Table 2
Ring expansions of alkynylcyclopropanes with sulfonamides catalyzed by Fe3O4@SiO2-P-AuOTfa,b
R2
N
O
S
Fe3O4@SiO2-P-AuOTf
(5 mol%)
O
O
O
S
R
+
R1
R
R2HN
DCE, 100 oC, 24 h
R1
1
2
3
O
S
O
S
O
S
O
H
O
H
N
O
H
N
N
Cl
Br
3a
3c
66%
3b 73%
68%
Me
O
Me
Me
O
S
O
S
O
O
H
O
H
N
H
N
N
S
O
O
Cl
F3C
MeO2C
3e 83%
3d 79%
3f
69%
Me
Me
Me
O
S
O
S
Cl
O
H
N
O
S
O
H
N
H
N
Me
3g
45%
3h
3i
77%
21%
Me
Me
O
Me
O
S
O
S
O
S
H
N
O
O
H
H
N
N
Cl
Cl
Me
3j 86%
3l 61%
3k 54%
Br
NO2
Me
Me
NO2
O
S
O
S
O
S
H
N
O
H
N
O
H
N
Cl
Me
Cl
Cl
3m 71%
3n 72%
3o 69%
Br
NO2
O
S
O
S
O
S
O
H
N
O
O
H
H
N
NO2
N
Cl
Me
Cl
Cl
Me
3p
79%
3r
67%
3q
64%
O
S
O
S
O
S
O
H
O
H
N
O
H
N
N
Cl
Me
Me
Me
Me
3s
82%
Me
3t 69%
3u 89%
O
S
O
S
O
S
Me
N
Me
N
Cl
O
O
O
N
Cl
3v 90%
3w 92%
3x 89%
Me
O
S
Me
Me
Me
N
O
Cl
Me
3y 94%
Me
a
All reactions were performed using alkynylcyclopropane
1
(0.5 mmol), sulfonamide 2 (0.6 mmol), and
Fe3O4@SiO2-P-AuOTf (0.025 mmol) in DCE (5 mL) in a sealed tube at 100 °C under Ar for 24 h.
b
Isolated yield based on alkynylcyclopropane 1 used.
intermediate A undergoes cation-triggered ring expansion by a
1,2-alkyl shift to give a magnetic nanoparticle-immobilized vinyl-
gold cation intermediate B. Finally, the cation intermediate B is
trapped by TsNH2 (2) to afford the desired product (3) and regen-
erate the gold(I) catalyst.
For a heterogeneous precious metal catalyst, it is important to
examine its ease of separation, good recoverability, and reusability.
We next investigated the recyclability of the supported phosphine
gold(I) catalyst. More than 99% of the catalyst could simply be
recovered by fixing a magnet near to the reaction tube (Fig. 4).
After the completion of the reaction, the reaction mixture was
cooled to room temperature and the catalyst was easily separated
from the product by exposure to an external magnet and decanta-
tion of the reaction solution. The remaining magnetic nanoparti-
cles were further washed with DCE to remove residual product,
air-dried, and subjected to the next run. As shown in Figure 5,
the supported gold catalyst could be reused ten times without a
significant loss of activity for the ring expansion reaction of
(2-chlorophenyl)ethynylcyclopropane (1h) with TsNHMe (2e). In
addition, from the TEM image of the recovered catalyst, much

F. Yi et al. / Tetrahedron Letters 57 (2016) 4405–4410
4409
Conclusions
R
[LAu]+
OTf
In summary, we have developed a novel heterogeneous Au(I)-
catalyzed ring expansion reaction of unactivated alkynylcyclo-
propanes, which can be easily prepared from commercially readily
available starting materials. Sulfonamides can efficiently trap the
in situ generated cation from the ring expansion. The reaction toler-
ates a range of aryl and alkyl substituents with moderate to high
yields, providing an efficient and practical method to construct syn-
thetically useful (E)-2-alkylidenecyclobutanamines. More impor-
tantly, this new heterogeneous gold(I) catalyst can simply be
recovered by applying an external magnet and recycled at least
10 times without a significant loss of catalytic activity, thus making
this procedure economically and environmentally more acceptable.
R
1
+[LAu]
+
TfO
[LAu]+
OTf
L = Fe3O4@SiO2-P
R
H
A
NHTs
R
[AuL]+
OTf
R
+
3
2
TsNH2
B
Scheme 2. A possible mechanism for this Au-catalyzed expansion reaction.
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21462021) and Key Laboratory of Functional Small Organic
Molecule, Ministry of Education (No. KLFS-KF-201409) for financial
support.
Supplementary data
Supplementary data (experimental procedures for the prepara-
tion of Fe3O4@ SiO2-P-AuOTf catalyst and 1H, 13C NMR spectra of
the products are described) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.
2016.08.062.
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Figure 5. Recycle of the Fe3O4@SiO2-P-AuOTf catalyst.
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change in the morphology and dispersion of nanoparticles was not
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20. General procedure for the heterogeneous gold(I)-catalyzed ring expansion
reaction of alkynylcyclopropanes with sulfonamides: To a resealable Schlenk
tube was added Fe3O4@SiO2-P-AuOTf (66 mg, 0.025 mmol). The reaction tube
was evacuated and back-filled with argon and this evacuation/back-fill
procedure was repeated one additional time. Alkynylcyclopropane
(0.5 mmol), sulfonamide (0.6 mmol) and DCE (5 mL) were then added
under
a stream of argon. The reaction tube was quickly sealed and the
contents were stirred while heating in an oil bath at 100 °C for 24 h. After the
reaction mixture was cooled to room temperature, the supported gold
catalyst was magnetically separated and the reaction mixture was
concentrated under vacuum and then purified by flash chromatography
(petroleum ether/ethyl acetate = 7:1) to afford the corresponding product.
The recovered gold catalyst was washed with DCE (2 Â 2 mL), air-dried and
used directly for the next run.
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