1D Fe3O4&at;CuSiO3 composites catalyzed decarboxylative A3-coupling for propargylamine synthesis

Article abstract of DOI:10.1016/j.cclet.2019.11.004

Highly active and stable magnetic copper catalysts were successfully achieved by magnetic induced St?ber method and subsequent hydrothermal reaction with copper ions in alkaline condition. The high content of Cu²⁺ as well as the unique structures of hierarchical copper silicate in the as-prepared catalysts endowed their outstanding catalytic performance. Efficient decarboxylative A³-coupling of α-keto acid, amine and alkyne was realized with the low Fe₃O₄&at;CuSiO₃ loading. A range of propargylamines were produced in good to excellent yields under solvent-free condition. Moreover, the catalyst can be easily separated from the final organic product with an external magnet. Also, this kind of catalyst could be recycled up to six times while maintaining its activity.

Full text of DOI:10.1016/j.cclet.2019.11.004

Journal Pre-proof
1D Fe3O4@CuSiO3 composites catalyzed decarboxylative A3-coupling
for propargylamine synthesis
Fang Wang, Huangdi Feng, Huiqiong Li, Teng Miao, Tiantian Cao,
Min Zhang
PII:
S1001-8417(19)30651-5
https://doi.org/10.1016/j.cclet.2019.11.004
CCLET 5311
DOI:
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Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
17 September 2019
3 November 2019
5 November 2019
Please cite this article as: Wang F, Feng H, Li H, Miao T, Cao T, Zhang M, 1D
Fe3O4@CuSiO3 composites catalyzed decarboxylative A3-coupling for propargylamine
synthesis, Chinese Chemical Letters (2019), doi: https://doi.org/10.1016/j.cclet.2019.11.004
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Communication
1D Fe3O4@CuSiO3 composites catalyzed decarboxylative A3-coupling for
propargylamine synthesis
Fang Wang, Huangdi Feng * hdfeng@sues.edu.cn , Huiqiong Li, Teng Miao, Tiantian Cao, Min Zhang*
zhangmin@sues.edu.cn
College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
 Corresponding authors.(H. Feng); (M. Zhang)
Graphical abstract
1D Fe3O4@CuSiO3 composite has been designed, synthesized and successfully used in the decarboxylative
A3 reactions for the generation of propargylamines without a solvent. Results also showed the magnetic
catalyst could be readily separated and reused.
ABSTRACT
Highly active and stable magnetic copper catalysts were successfully achieved by magnetic induced Stöber method and
subsequent hydrothermal reaction with copper ions in alkaline condition. The high content of Cu2+ as well as the unique
structures of hierarchical copper silicate in the as-prepared catalysts endowed their outstanding catalytic performance.
Efficient decarboxylative A3-coupling of α-keto acid, amine and alkyne was realized with the low Fe3O4@CuSiO3 loading.
A range of propargylamines were produced in good to excellent yields under solvent-free condition. Moreover, the
catalyst can be easily separated from the final organic product with an external magnet. Also, this kind of catalyst could be
recycled up to six times while maintaining its activity.
Keywords:
Magnetic copper catalyst
Heterogeneous catalysis
One-dimensional

Decarboxylative coupling
Propargylamines
Propargylamines, a versatile class of frameworks, are widely used in medical chemistry and synthetic chemistry [1]. As such,
different strategies have been developed so far to construct these compounds. In view of the synthetic efficiency and procedural
simplicity of multicomponent reactions [2], transition metal-catalyzed A3-coupling of aldehydes, amines and alkynes is now widely
employed [3]. Similarly, transition metal-catalyzed decarboxylative reactions, avoiding the use of organometallic reagents, have
attracted significant attention [4]. As an alternative to classical A3 coupling, decarboxylative A3-type reaction, usually catalyzed by a
transition metal such as copper, ruthenium or iridium is then performed to obtain the corresponding propargylamines [5]. However,
these procedures involving the homogeneous copper salt as a catalyst generally suffered from reusability of catalyst [6]. Recently,
heterogeneous catalysis for organic transformations has been of great interest. Solid-supported copper catalysts and copper
nanoparticles currently being applied in the field of the A3 reaction possessed several advantages such as easy separation of reactants
and reusability of catalyst than homogeneous catalysts [7]. As an alternative, the strategy by coupling copper catalyst with magnetic
support has attracted great attention and has been strengthened in recent years. For instances, Aliaga et al. impregnated copper on
magnetite as a catalyst for the multicomponent acetylene-Mannich reaction between terminal alkynes, secondary amines, and
aldehydes [8a]. Song also utilized CuFe2O4 nanoparticles for catalyzing multicomponent synthesis of propargylamines [8b]. However,
the specific surface area and active sites of copper-based magnetic catalysts is low, greatly decreasing the catalysis performance.
Additionally, a series of copper-based nanocomposites such as Cu/HM, Chit@CuI, Cu@SiO2-NS and RGO@CuO have been
developed for the application of decarboxylative A3 reactions (Scheme 1a) [9]. In these cases, the copper catalyst can be separated from
the reaction system with centrifugation or filtration process, but the catalyst losing and time-energy consumption are inevitable.
Nevertheless, the substrate scope of the process is quite limited thus far. Only propiolic acid and its derivatives, and amino acids were
found to be the precursors for the synthesis of propargylamines. Therefore, introduction of a new carboxylic acid substrate to the
decarboxylative A3-coupling, involving a new kind of magnetic copper catalyst with superior catalytic activity, cost economy as well
as high stability loading for producing titled compounds, is still in high demand.
Scheme 1. Heterogeneous copper-catalyzed decarboxylative A3-coupling.
The well-designed copper silicates have recently aroused considerable attention due to their excellent physical and chemical
properties, highly abundant of copper ions, and low cost [10]. Srinivas and coworkers used two-dimensional mesoporous copper
silicate catalyst with ordered two-dimensional pore structure for the efficient synthesis of propargylamines and their derivatives by
A3-coupling [11]. Our group has recently synthesized a series of magnetic copper silicate with high specific surface area, showing
exceptional performance as protein adsorbents [12]. Inspired by the above works, a creative idea is proposed to prepare a novel
magnetic copper silicate composite which is used to catalyze the decarboxylative A3-coupling. Herein, we present a mild route to
construct one dimensional (1D) magnetic copper silicate catalyst using a magnetic field-induced Stöber method followed by in-situ
hydrothermal reaction with copper ions in ammonia buffer solution. This not only provides high content of copper ions of the copper
silicates nanotubes, but also facilitating the recycle of catalyst with an external magnet. Catalytic performance tests demonstrate that
the as-prepared Cu-based catalysts exhibit excellent activity and stability for introduction of a new carboxylic acid substrate to the
decarboxylative A3-coupling reaction. The catalytic activity of Fe3O4@copper silicate is higher than that of Fe3O4@SiO2, Fe3O4,
respectively. Moreover, the Fe3O4@CuSiO3 catalysts reported in this work hold great advantages over conventional Cu-based catalysts
such as easy separation, higher activity etc. To the best of our knowledge, renewable magnetic copper catalysts have not been
documented for decarboxylative A3-coupling of α-keto acids (Scheme 1b).
In order to achieve a heterogeneous copper-catalyzed decarboxylative A3 reaction of α-keto acids, amines and alkynes, we started
our studies with the preparation of a highly active, selective and recoverable Fe3O4-supported Cu catalyst. Firstly, the 1D Fe3O4@SiO2
nanochains were synthesized by a magnetic-induced modified Stöber method under moderate magnetic field. Once a magnetic field
was applied in the process of silica coating, the magnetic NPs were brought together to form 1D Fe3O4@SiO2 nanochains by means of
magnetic dipole-dipole interaction. SEM (Fig. 1A) and TEM (Fig. 1B) images confirmed that the magnetically assembled Fe3O4@SiO2
nanochains exhibited linear alignments with a smooth SiO2 shell about 30 nm in thickness. This indicates that the SiO2 layer plays vital

role in the synthesis of 1D Fe3O4@SiO2 nanochains. Then the 1D Fe3O4@CuSiO3 nanochains were prepared by hydrothermal reaction
between the 1D Fe3O4@SiO2 nanochains and mixture of aqueous CuCl2 in NH3-NH4Cl buffer system at 140 °C for 12 h. The surface
and morphologies of 1D Fe3O4@SiO2 nanochains and 1D Fe3O4@CuSiO3 nanochains were characterized from SEM and TEM
techniques respectively, as shown in Figs. 1D and E. The as-obtained 1D hierarchical Fe3O4@CuSiO3 nanochains exhibit the hairy
structures. Thanks for its one-dimensional hierarchical structures, the specific surface area of 1D hierarchical Fe3O4@CuSiO3
nanochains was as high as 179.33 m2/g, which was tested by the N2 adsorption-desorption isotherm (Fig. 1C). And pore size
distribution was 5.46 nm, greatly facilitating the catalysis performance of organic molecules. X-ray photoelectron spectroscopy was
also utilized to analyze the surface electronic states and the chemical elemental composition of the as-prepared composites. As shown
in Fig. 1F, the detected signal peaks of Cu, Si, C, O elements were clearly observed in the survey XPS spectrum of 1D hierarchical
Fe3O4@CuSiO3 hybrid. Owing to a loose shell thickness of copper silicate layer above the 10 nm (the limit detection of XPS) coating
on Fe3O4 cores, so the signal of Fe element was almost disappeared. Notably, the atom ratio of copper ions is as high as 18.56%. In the
Cu 2p core level spectrum, Cu 2p normally spin-orbit coupled into two peaks as well as several shake-up satellite peaks at 941.8 eV,
944 eV and 963eV. As depicted in Fig. 1D, the binding energy of Cu 2p3/2 and Cu 2p1/2 at 935 eV and 955 eV could be addressed to
copper ions, which confirmed the existence of CuSiO3 [13].
Fig. 1. SEM (A, D) and TEM (B, E) images of 1D Fe3O4@SiO2 (A, B), 1D Fe3O4@CuSiO3 (D, E). (C) The N2 adsorption-desorption
isotherms and pore size distribution curves (inset) of 1D Fe3O4@CuSiO3. (F) XPS of 1D Fe3O4@CuSiO3 and Cu 2p (inset).
Subsequently, the catalytic activities were evaluated in a model reaction of phenylglyoxylic acid 1a, N-methylbenzylamine 2a and
phenylacetylene 3a in the presence of Fe3O4@CuSiO3 (Table 1). To our delight, the desired product 4a was obtained in a yield of 34%
using toluene as a solvent at 100 °C for 16 h (entry 1). Other solvents such as DCE, THF, MeCN, dioxane and EtOH were screened,
and EtOH provided the highest yield (entries 2-6). Further optimization showed that the experimental process of solvent-free
significantly increased the yield to 74% (entry 7). The reaction afforded 83% yield under an Ar protecting atmosphere (entry 8). And
then we controlled the amount of catalyst employed (entries 9-12). Indeed, the reaction proceeded smoothly in the presence of 3 mg
(<3 mol%) Fe3O4@CuSiO3 to produce 4a in excellent yield (entry 11), whereas previously reported gave almost the similar yield
through the use of 20mol% CuBr [5c]. These results indicated that heterogeneous copper catalyst played a significant role in enhancing
the reaction. The importance of the catalyst Fe3O4@CuSiO3 was again confirmed when the comparative experiments were carried out
using Fe3O4@SiO2 (3 mg), or Fe3O4 (3 mg), or without catalyst (entries 13-15). Switching the reaction temperature to 85 oC or 115 oC
resulted in a slightly lower yield (entries 16 and 17). Interesting, a best yield of the target product (94%) was achieved when the
amount of 3a was increased to 0.8 mmol (entries 18 and 19). With conditions for the catalytic process have been identified, a
gram-scale reaction was done to address the practical utility of this class of catalysts, giving a 93% yield of corresponding product 4a
with the slight lower Fe3O4@CuSiO3 loading (entry 20).
Table 1
Optimization of the conditions.a
Yield
Entry
Catalyst (mg)
Solvent
Temp. (oC)
(%)b
34
1
2
3
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (20)
Toluene
DCE
THF
100
100
100
32
36

4
5
6
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (20)
Fe3O4@CuSiO3 (10)
Fe3O4@CuSiO3 (5)
Fe3O4@CuSiO3 (3)
Fe3O4@CuSiO3 (1)
Fe3O4@SiO2 (3)
MeCN
Dioxane
Ethanol
100
100
100
100
100
100
100
100
100
100
100
100
85
43
42
65
74
83
85
87
86
77
10
8
n.d.
82
80
73
94
92
7
--
--
--
--
--
--
--
--
--
--
--
--
--
--
8c
9c
10c
11c
12c
13c
14c
15c
16c
17c
18c,d
19c,e
20c,f
Fe3O4 (3)
no catalyst
Fe3O4@CuSiO3 (3)
Fe3O4@CuSiO3 (3)
Fe3O4@CuSiO3 (3)
Fe3O4@CuSiO3 (3)
Fe3O4@CuSiO3 (20)
115
100
100
100
a Reactions were performed using phenylglyoxylic acid 1a (0.5 mmol), N-methylbenzylamine 2a (0.55 mmol), phenylacetylene 3a (0.65
mmol), solvent (1 mL), 16 h.
b Isolated yields.
c Under Ar atmosphere.
d 2a (0.45 mmol) was used.
e 3a (0.8 mmol) was used.
f 1a (5 mmol), 2 (5.5 mmol) and alkynes 3 (6.5 mmol).
The substrate scope of decarboxylative A3-coupling, employing 1a with various amines 2 and alkynes 3, was surveyed (Scheme 2).
Firstly, we started our investigations with the reaction of 15 different amine coupling partners including acyclic and cyclic aliphatic
amines. It was found that in all cases the corresponding propargylamines 4a–4o were isolated in good to excellent yields. Additionally,
functionalized substrates with CN or Br group were used to expand the application of this method, providing the corresponding
products 4p and 4q. Then, a wide range of alkynes were investigated. Electron-rich as well as electron-poor groups were tolerated on
the phenyl ring, delivering the desired compounds 5a–5k in 77%–98% yield. Notably, the reaction also worked well with aliphatic
alkynes, giving the corresponding products 5l–5n in excellent yield.

Scheme 2. Scopes of amines and alkynes. Reaction conditions: 1a (0.5 mmol), 2 (0.55 mmol), alkynes 3 (0.65 mmol), Fe3O4@CuSiO3
catalyst (3 mg) were stirred at 100 °C for 16 h under Ar atmosphere; isolated yield based on 1a.
After these two initial evaluations, several α-keto acids were employed for the formation of the titled compounds (Scheme 3). In the
case of the aromatic α-keto acids with different substituents, good to excellent yields of the corresponding products 6a–6h were
observed. To our delight, 2-oxobutanoic acid also underwent this transformation to generate 55% yield of the desired propargylamine
6i.
Scheme 3. Scopes of α-keto acids. Reaction conditions: α-keto acids
1 (0.5 mmol), 2a (0.55 mmol), 3a (0.65 mmol), Fe3O4@CuSiO3
catalyst (3 mg) were stirred at 100 °C for 16 h under Ar atmosphere; isolated yield based on 1.

Furthermore, the recyclability of Fe3O4@CuSiO3 catalyst was evaluated for the generation of product 4a under the standard
conditions. After completion of reaction, the catalyst was separated by employing an external magnet, washed several times with ethyl
acetate, and reused in a model reaction to provide 4a in 95% yield (Figs. 2A-C). This procedure was repeated for 5 more times as
shown in Fig. 2D. The result indicated that its catalytic activity slightly lost, the isolated yield of the product was slowly decreased
from 95% to 84%, owing to a little change of the morphologies of catalyst with the physical loss of catalyst amount after recycling and
washing (see supporting information: SEM image of the reused Fe3O4@CuSiO3).
D 100%
95%
93%
93%
92%
86%
84%
80%
60%
40%
20%
0%
1
2
3
4
5
6
No. of cycles
Fig. 2. Photo of (A): Fe3O4@CuSiO3 dispersion in ethyl acetate; (B) and (C): a magnet attracted the magnetic stirring bar and 1D
Fe3O4@CuSiO3; (D) Recyclability of 1D Fe3O4@CuSiO3 catalyst for the synthesis of 4a.
Based on experimental observation and previous reports [5c, 14], a plausible mechanism for this 1D Fe3O4@CuSiO3 catalyzed
decarboxylative A3-coupling is depicted in Fig. 3. Initially, the alkyne 3 is activated by a Fe3O4@CuSiO3 catalysis system, providing
the corresponding the copper acetylide B through the intermediate A. Then, this species B reacts with α-keto acids 1 and amines 2 to
produce the intermediate copper-complex C by a decarboxylative process, followed by the formation of the final products 4-6 along
with the regeneration of the Fe3O4@CuSiO3-catalyst.
Fig. 3. Proposed mechanism.
In summary, taking advantage of the magnetic induced Stöber method as well as the copper ion assisted hydrothermal process, one
dimensional hierarchical Fe3O4@CuSiO3 was successfully fabricated. Owing to the large surface area, hierarchical copper silicate
structure, and magnetic support, the as-prepared Fe3O4@CuSiO3 exhibits excellent performance for the decarboxylative A3-coupling.
Moreover, compared with the other copper-based catalysts, the decarboxylative A3-coupling in this work is carried out under milder
reaction conditions, and the resultant organic product can be easily separated with Fe3O4@CuSiO3 catalysts using an external magnetic
field, which greatly facilitate the process of purification and the recycle of magnetic catalyst. We believe that this kind of magnetic
catalyst could be extended to other organic reaction systems which are catalyzed by transition metal ions.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have
appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be considered as
potential competing interests:
Acknowledgments
This research was supported by the National Natural Science Foundation of China (Nos. 21601121, 21305086), the Natural Science
Foundation of Shanghai (No. 18ZR1416400). We also acknowledge the support of the Shanghai University of Engineering Science
(No. 201810856017, A1-0601-19-01017), and the Opening Project of Shanghai Key Laboratory of Chemical Biology for financial
support.
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