Magnetic nanoparticle-supported phosphine gold(i) complex: A highly efficient and recyclable catalyst for the direct reductive amination of aldehydes and ketones

Article abstract of DOI:10.1039/c5cy02159f

The direct reductive amination of aldehydes and ketones has been achieved in dichloromethane at room temperature by using a magnetic nanoparticle-supported phosphine gold(i) complex [Fe₃O₄@SiO₂-P-AuCl] as the catalyst and ethyl Hantzsch ester as the hydrogen source, yielding a variety of secondary amines in excellent yields under neutral conditions. The new heterogeneous gold catalyst can be prepared by a simple procedure from commercially readily available reagents and can easily be separated from the reaction mixture by applying an external magnet and recycled at least 10 times without any loss of activity.

Full text of DOI:10.1039/c5cy02159f

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Magnetic nanoparticle-supported phosphine
goldIJI) complex: a highly efficient and recyclable
catalyst for the direct reductive amination of
aldehydes and ketones†
Cite this: DOI: 10.1039/c5cy02159f
a
a
a
Weisen Yang,ab Li Wei, Feiyan Yi and Mingzhong Cai
*
The direct reductive amination of aldehydes and ketones has been achieved in dichloromethane at room
temperature by using a magnetic nanoparticle-supported phosphine goldIJI) complex [Fe3O4@SiO2-P–AuCl]
as the catalyst and ethyl Hantzsch ester as the hydrogen source, yielding a variety of secondary amines in
excellent yields under neutral conditions. The new heterogeneous gold catalyst can be prepared by a sim-
ple procedure from commercially readily available reagents and can easily be separated from the reaction
mixture by applying an external magnet and recycled at least 10 times without any loss of activity.
Received 13th December 2015,
Accepted 6th February 2016
DOI: 10.1039/c5cy02159f
www.rsc.org/catalysis
mild as well as environmentally benign conditions is highly
desirable.
Introduction
Amine functionality plays a vital role in organic chemistry
due to its prominence in natural products, pharmaceuticals
and agrochemicals.1 Direct reductive amination of carbonyl
compounds, mostly aldehydes or ketones, with primary or
secondary amines is one of the most attractive methods for
the synthesis of substituted amines.2 This is particularly ad-
vantageous because the carbonyl compound and the amine
with a suitable chemoselective reducing agent are treated in a
one-pot fashion such that the isolation of the intermediary
imine or iminium is avoided. So far, there have been many re-
agents developed to effect the direct reductive amination of
carbonyl compounds, including active borane,3 tin,4 indium5
and iridium6 compounds. Recently, direct one-pot reductive
amination of aldehydes with anilines or nitroarenes promoted
by heterogeneous palladium catalysts has also been reported.7
Although these methods are encouraging, most of them have
some drawbacks in one or another respect, such as acidic
conditions, limited substrate scope, excess amount of re-
agents, poor stability and toxic by-products. Therefore, the de-
velopment of an alternative method that employs simple and
The past decade has witnessed a dramatic growth in re-
ports on organic transformations catalyzed by gold complexes
owing to their unique reactivity as soft and carbophilic Lewis
acids.8 Reactions catalyzed by gold generally proceed under
facile conditions with high yields and chemoselectivity with-
out the exclusion of water and oxygen. To date, gold-catalyzed
carbon–carbon9 and carbon–heteroatom10 bond formation re-
actions and oxidation11 and reduction12 reactions have suc-
cessfully been applied in organic synthesis. However, the in-
dustrial applications of these homogeneous gold catalysts
remain a challenge because they are quite expensive, cannot
be recycled, and are 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 atten-
tion because of the advantages of high catalytic efficiency and
easy recycling, which are important for precious metal catalyst
and flow chemistry processes.13 Although gold nanoparticles
supported on different matrices have been widely used as
heterogeneous catalysts in recent years,14 little progress has
been made on the supported gold complex-catalyzed reactions
so far.15 Recently, magnetic nanoparticles have emerged as
smart and promising supports with great industrial potential
for immobilization because the magnetic nanoparticle-
supported catalysts can easily be separated from the reaction
mixture by using an external magnet, which prevents loss of
catalyst and improves the reusability.16 During recent years,
magnetic nanoparticle-supported palladium,17 ruthenium,18
rhodium,19 and copper20 complexes have been prepared and
successfully used in some organic reactions. However, to the
a Key Laboratory of Functional Small Organic Molecule, Ministry of Education and
College of Chemistry & Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, PR China. E-mail: mzcai@jxnu.edu.cn;
Fax: +86 791 8812 0388
b College of Ecology and Resources Engineering, Wuyi University, Wuyishan City
354300, PR China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy02159f
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best of our knowledge, no example of
a magnetic
nanoparticle-supported gold complex has been reported until
now.
Cao et al. have reported that supported gold nanoclusters
are efficient heterogeneous catalysts for transfer hydrogena-
tion.21 Bao and co-workers have described a chemoselective
reduction of imines in the coexistence of aldehydes using the
nanoporous gold skeleton (AuNPore) catalyst.12a In addition,
Hantzsch ester 1,4-dihydropyridine (HEH), which is a safe,
easy-to-handle and commercially available reagent, has been
widely used as a reducing agent in organic synthesis.22 In
continuing our efforts to develop economical and eco-
friendly synthetic pathways for organic transformations,23 we
here report the first synthesis of magnetic nanoparticle-
supported phosphine goldIJI) complex and its successful appli-
cation to the direct reductive amination of aldehydes or ke-
tones with amines using inexpensive ethyl Hantzsch ester as
the hydrogen source. The new heterogeneous gold catalyst ex-
hibits excellent catalytic activity in the reaction under mild
reaction conditions and can easily be separated from the re-
action mixture with the simple application of an external
magnetic field, and its catalytic efficiency remains unaltered
even after recycling ten times.
Fig. 1 TEM image of Fe3O4@SiO2-P–AuCl catalyst.
30 nm (Fig. 1). X-ray diffraction (XRD) and energy dispersive
spectroscopy (EDS) were also used to characterize the
supported phosphine goldIJI) complex. Fig. 2 shows the XRD
spectra of Fe3O4@SiO2 and Fe3O4@SiO2-P–AuCl complex. The
XRD pattern of Fe3O4@SiO2 exhibits diffraction peaks corre-
sponding to a typical spinel maghemite structure, whereas
the diffraction peak of the layered amorphous silica was not
obvious. The XRD pattern of Fe3O4@SiO2-P–AuCl complex ob-
viously shows two sets of diffraction peaks; the new peaks
that appeared at 2θ = 38.3°, 44.4° and 64.5° are attributed to
the (111), (200) and (220) reflections of the Au species, re-
spectively. The results from XRD imply that the Au complex
has been successfully immobilized on the surface of MNPs.
Energy dispersive X-ray spectroscopy (EDS) shows the ele-
ments present in the material. EDS analysis of fresh
Fe3O4@SiO2-P–AuCl complex shows the presence of Si, O, C,
P, Cl, Fe, and Au elements (Fig. 3).
The magnetically separable nanocomposite Fe3O4@SiO2-
P–AuCl was then used as the catalyst for the direct reductive
amination of aldehydes and ketones with amines using ethyl
Hantzsch ester as the hydrogen source. Initial experiments
with benzaldehyde (1a) and benzenamine (2a) were
performed to optimize the reaction conditions, and the re-
sults are summarized in Table 1. At first, the solvent effect
was examined in the presence of 3 mol% Fe3O4@SiO2-P–AuCl
at room temperature and a significant solvent effect was
Results and discussion
The magnetic nanoparticle-supported phosphine goldIJI) 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 SiIJOEt)4 as
the silica source and aqueous NH3 as the hydrolyzing agent
to give silica-coated Fe3O4 (Fe3O4@SiO2). The latter was
treated with commercially available 2-(diphenylphosphino)-
ethyltriethoxysilane in toluene under reflux for 24 h to afford
the
phosphino-functionalized
magnetic
nanoparticles
(Fe3O4@SiO2-P); the phosphine content was determined to be
0.44 mmol g−1 by elemental analysis. The supported goldIJI)
complex (Fe3O4@SiO2-P–AuCl) was obtained by the reaction
of AuCl with Fe3O4@SiO2-P in methanol at 65 °C with a load-
ing of 0.39 mmol of gold per gram as determined by ICP-AES.
The transmission electron microscopy (TEM) image
showed that the diameter of the complex was approximately
Scheme 1 Synthesis of magnetic nanoparticle-supported phosphine goldIJI) complex.
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observed. DCM gave better conversions than other solvents,
such as toluene, THF, dioxane, MeCN and MeOH; the desired
product 3a was isolated in 67% yield (Table 1, entries 1–6).
To our delight, the reaction was promoted effectively by the
addition of AgOTf and the yield of 3a was increased to 95%
(entry 7), whilst the use of AgOTf alone as catalyst resulted in
a low yield (entry 8). Thus, Fe3O4@SiO2-P–AuOTf, generated
in situ from Fe3O4@SiO2-P–AuCl and AgOTf, was found to be
the most efficient for this reaction. The role of AgOTf is to
convert Fe3O4@SiO2-P–AuCl into Fe3O4@SiO2-P–AuOTf. We
next screened the amount of the supported gold catalyst and
were pleased to find that there was no change in yield when
the amount of catalyst was decreased to 1 mol% (entry 9).
The reaction still proceeded smoothly even with only 0.5
mol% catalyst (entry 10). No reaction was observed in the ab-
sence of any catalyst (entry 11). In order to characterize the
Fe3O4@SiO2-P–AuOTf, we prepared the true catalyst by reac-
tion of Fe3O4@SiO2-P–AuCl with AgOTf in DCM. The TEM im-
age revealed that the diameter of the Fe3O4@SiO2-P–AuOTf
complex was approximately 30 nm (Fig. 4a). Fig. 5 shows the
XRD spectra of Fe3O4@SiO2, Fe3O4@SiO2-P–AuOTf and the
recycled 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 2θ = 38.0°, 44.3° and 64.6° should be attributed
to the (111), (200) and (220) reflections of the Au species, re-
spectively. EDS analysis of the fresh Fe3O4@SiO2-P–AuOTf
complex shows the presence of Si, O, C, P, S, F, Fe, and Au el-
ements (Fig. 6). The structure of the supported gold catalyst
was also verified by X-ray photoelectron spectroscopy (XPS).
The XPS spectra of the fresh Fe3O4@SiO2-P–AuOTf (Fig. 7a)
displayed a spin–orbit pair at 85.0 and 88.7 eV that can
Fig. 2 XRD patterns of Fe3O4@SiO2 (a) and Fe3O4@SiO2-P–AuCl (b).
Fig. 3 Energy dispersive spectra (EDS) of Fe3O4@SiO2-P–AuCl.
Table 1 Optimization of the reaction conditions of the direct reductive amination reaction catalyzed by Fe3O4@SiO2-P–AuCla
Entry
Catalyst (mol%)
Solvent
t (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Fe3O4@SiO2-P–AuCl (3)
Fe3O4@SiO2-P–AuCl (3)
Fe3O4@SiO2-P–AuCl (3)
Fe3O4@SiO2-P–AuCl (3)
Fe3O4@SiO2-P–AuCl (3)
Fe3O4@SiO2-P–AuCl (3)
Fe3O4@SiO2-P–AuCl (3) + AgOTf (3)
AgOTf (3)
Fe3O4@SiO2-P–AuCl (1) + AgOTf (1)
Fe3O4@SiO2-P–AuCl (0.5) + AgOTf (0.5)
None
Fe3O4@SiO2-P–AuOTf (1)
AuCl (1) + AgOTf (1)
Toluene
THF
DCM
MeCN
Dioxane
MeOH
DCM
DCM
DCM
DCM
DCM
2
2
2
2
2
2
1
2
1
45
41
67
31
39
36
95
32
95
91
0
9
10
11
12
13
2
24
1
DCM
DCM
94
41
2
a
b
Reaction conditions: 1a (1 mmol), 2a (1 mmol), ethyl Hantzsch ester (1.3 mmol) in solvent (3 mL), rt. Isolated yields.
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Fig. 4 TEM images of the fresh Fe3O4@SiO2-P–AuOTf (a) and recycled Fe3O4@ SiO2-P-AuOTf after the 10th run (b).
be assigned to AuIJI).24 The binding energies of Au 4f7/2 and
Au 4f5/2 of the recycled Fe3O4@SiO2-P–AuOTf (after the 10th
run) are 84.9 and 88.5 eV, respectively (Fig. 7b), indicating
that the oxidation state of gold in the used catalyst is still
AuIJI). In order to further confirm that the supported AuOTf is
the real catalyst, freshly prepared Fe3O4@SiO2-P–AuOTf (1
mol%) was used as the catalyst; the desired product 3a was
isolated in 94% yield after 1 h (entry 12). However, when
AuCl/AgOTf (1 mol%) was used as the catalyst, only 41% yield
of 3a was obtained (entry 13). These results indicated that
the Fe3O4@SiO2-P–AuOTf complex is the real catalyst.
With the optimized conditions established (1 mol%
Fe3O4@SiO2-P–AuCl/AgOTf, DCM, rt), we tried to investigate
the scope of this heterogeneous gold-catalyzed direct reduc-
tive amination reaction. Firstly, reactions of benzaldehyde
with various amines were examined and the results are sum-
marized in Table 2. The reactions of a variety of substituted
anilines bearing both electron-donating and electron-
withdrawing groups with benzaldehyde (1a) proceeded
smoothly, leading to the desired products 3b–3o in high
yields (89–95%) within 1–3 h. The results indicated that the
electronic effect of substituents on the benzene ring has
limited influence on this heterogeneous gold-catalyzed direct
reductive amination reaction. It is noteworthy that, for
4-nitrobenzenamine with strong electron-withdrawing
a
group, the reaction required a longer time (3 h) to obtain a
high yield of 3h due to lower reactivity. The o-substituted ani-
lines such as 2-chlorobenzenamine, 2-hydroxybenzenamine,
2,4-dichlorobenzenamine, and 2,4-difluorobenzenamine also
displayed lower reactivity and the reactions afforded the de-
sired products 3l–3o in 89–93% yields after 3 h. In addition
to substituted anilines, bulky naphthalidine and a heterocy-
clic amine were also applicable under the optimized reaction
conditions to afford the corresponding products 3p and 3q in
87% and 93% yields, respectively.
Encouraged by the above results, reactions of different al-
dehydes or ketones with benzenamine were then examined
and the results are listed in Table 3. Substituted benzalde-
hydes with various substituents, regardless of their electronic
properties and substitution positions, all afforded the expected
products 3r–3y in good to excellent yields (88–96%). Compared
to the p-substituted benzaldehydes, the o-substituted ben-
zaldehydes such as 2-bromobenzaldehyde also gave the de-
sired product 3x in 93% yield within 1 h, but electron-
deficient 2-nitrobenzaldehyde was an exception. The phenom-
enon observed here was quite different from that of the
o-substituted anilines in Table 2. Heteroaromatic aldehydes
Fig. 5 XRD patterns of Fe3O4@SiO2 (a), Fe3O4@SiO2-P–AuOTf (b) and
recycled Fe3O4@SiO2-P–AuOTf (c).
Fig. 6 Energy dispersive spectra (EDS) of Fe3O4@SiO2-P–AuOTf.
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smoothly at higher reaction temperature (50 °C), affording
the desired products 3b′–3d′ in high yields in longer times.
To verify whether the observed catalysis was due to the
heterogeneous catalyst Fe3O4@SiO2-P–AuOTf or to a leached
gold species in solution, the reaction of benzaldehyde (1a)
with benzenamine (2a) was carried out until an approxi-
mately 50% conversion of 1a was reached. Then the
Fe3O4@SiO2-P–AuOTf catalyst was separated magnetically
from the solution and the solution was transferred to another
reaction tube and stirred again at room temperature for 2 h.
In this case, no significant increase in conversion was ob-
served, 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 (below the detection limit). These re-
sults rule out any contribution to the observed catalysis from
a homogeneous gold species, demonstrating that the ob-
served catalysis was intrinsically heterogeneous.
Fig. 7 XPS of the supported Au catalysts.
such as 4-pyridinealdehyde and 2-furaldehyde could also react
with benzenamine effectively to give the corresponding prod-
ucts 3z and 3a′ in 92% and 94% yields, respectively. Aliphatic
ketones such as butanone, cyclopentanone and cyclohexa-
none are also good substrates under the optimized condi-
tions and the reactions with benzenamine proceeded
It is well documented that some homogeneous gold-
catalyzed reactions might in fact be proton catalyzed.25 The
mechanism for this heterogeneous gold-catalyzed reductive
amination reaction may be through a proton-promoted hydride
transfer process (Scheme 2). The catalyst Fe3O4@SiO2-P–AuOTf
Table 2 One-pot reductive amination of benzaldehyde with various aromatic amines catalyzed by Fe3O4@SiO2-P–AuCl/AgOTfa b
,
3d, 92% yield, 1h
3b, 92% yield, 1 h
3g, 95% yield, 1 h
3c, 91% yield, 1 h
3e, 91% yield, 1.5h
3a, 95% yield, 1 h
3j, 92% yield, 1 h
3o, 93% yield, 3 h
3h, 90% yield, 3 h
3m, 89% yield, 3 h
3i, 91% yield, 1 h
3n, 92% yield, 3 h
3f, 92% yield, 1.5 h
3k, 94% yield, 1 h
3l, 91% yield, 3 h
3q, 93% yield, 2 h
3p, 87% yield, 3 h
a
Reaction conditions: 1a (1 mmol), 2 (1 mmol), ethyl Hantzsch ester (1.3 mmol), Fe3O4@SiO2-P–AuCl (1 mol%), AgOTf (1 mol%) in DCM
(3 mL), rt. b Isolated yields.
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Table 3 One-pot reductive amination of aromatic aldehydes and ketones with benzenamine catalyzed by Fe3O4@SiO2-P–AuCl/AgOTfa b
,
3r, 94% yield, 1 h
3w, 90% yield, 2 h
3s, 96% yield, 1 h
3u, 92% yield, 1 h
3z, 92% yield, 3 h
3v, 92% yield, 1 h
3a′, 94% yield, 1 h
3t, 94% yield, 2 h
3x, 93% yield, 1 h
3c′,c 93% yield, 3 h
3y, 88% yield, 3 h
3d′,c 91% yield, 3 h
3b′,c 88% yield, 6 h
a
Reaction conditions: 1 (1 mmol), 2a (1 mmol), ethyl Hantzsch ester (1.3 mmol), Fe3O4@SiO2-P–AuCl (1 mol%), AgOTf (1 mol%) in DCM
(3 mL), rt. b Isolated yields. c 50 °C.
Scheme 2 A possible mechanism for this Au-catalyzed reductive amination reaction.
is partially hydrolyzed by traces of water, generating the proton,
which can protonate the imine.26 The resulting iminium can
then be reduced by the ethyl Hantzsch ester as the hydride
source.27 For a supported precious metal catalyst, it is important
to examine its ease of separation, good recoverability and reus-
ability. The recyclability of the supported phosphine goldIJI)
catalyst was also investigated. More than 99% of the catalyst
could simply be recovered by fixing a magnet near the reaction
vessel (Fig. 8b and c). The recovered catalyst was washed with
CH2Cl2, air-dried and used directly in the next run without fur-
ther purification. As shown in Fig. 8a, the supported gold cata-
lyst could be reused ten times without any loss of activity for
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Fig. 8 Recycle of the Fe3O4@SiO2-P–AuCl catalyst.
the direct reductive amination reaction of benzaldehyde (1a)
with benzenamine (2a). In addition, from the TEM image of the
recovered catalyst, no obvious change in the morphology and
dispersion of particles was observed after ten reaction cycles
(Fig. 4b). The observed excellent catalytic activity and reusability
of the Fe3O4@SiO2-P–AuCl should result from the properties of
the catalyst, such as the nano size (about 30 nm), good disper-
sion and stability of particles. It was noteworthy that the reac-
tion catalyzed by the recovered catalyst did not need the addi-
tion of AgOTf because the Fe3O4@SiO2-P–AuCl had been
changed to the Fe3O4@SiO2-P–AuOTf after the first cycle.
Chemical shifts are reported in δ (ppm) relative to TMS.
HRMS spectra were recorded on a Q-Tof spectrometer with
micromass MS software using electrospray ionization (ESI).
Gold content was determined with inductively coupled
plasma atom emission spectrometry (ICP-AES). TEM images
were recorded on a transmission electron microscope oper-
ated at an accelerated voltage of 200 kV. X-ray diffraction
(XRD) measurements were carried out at room temperature
using an X-ray powder diffractometer. X-ray energy dispersive
spectroscopy (EDS) was performed using a microscope. X-ray
photoelectron spectra (XPS) were recorded on an XSAM 800
spectrometer (Kratos).
Conclusions
Preparation of Fe3O4@SiO2-P17f
In conclusion, we have developed a highly efficient and easily
recoverable magnetic nanoparticle-supported phosphine
goldIJI) catalyst for the direct reductive amination reaction of
aldehydes or ketones with amines using commercially avail-
able, stable, and inexpensive ethyl Hantzsch ester as the hy-
drogen source. The reaction proceeds with high efficiency at
room temperature under neutral conditions, generating a va-
riety of secondary amines in excellent yields, which are ex-
tremely important synthetic intermediates in the construc-
tion of biologically active compounds. More importantly, this
heterogeneous gold catalyst can simply be recovered by apply-
ing an external magnet and recycled at least ten times with-
out any loss of activity. The Fe3O4@SiO2-P–AuCl catalyst not
only solves the basic problems of catalyst separation and re-
covery but also avoids the use of AgOTf in the recycling pro-
cess. This makes our protocol facile, economical, and envi-
ronmentally benign.
Fe3O4 nanoparticles (0.50 g, with a diameter of approximately
20 nm, purchased from Aldrich) were diluted with 5.0 mL of
deionized water and 50 mL of 2-propanol, and the mixture
was sonicated for 40 min. To this well-dispersed nanoparticle
solution, 2.0 mL of NH3·H2O followed by 1.65 g of SiIJOEt)4
were slowly added and stirred for 5 h at room temperature.
The resulting product was washed repeatedly with water until
the solution was neutral, then washed with ethanol and
diethyl ether, respectively, and dried under vacuum to give
1.35 g of the silica-coated magnetic nanoparticles (Fe3O4@SiO2).
To a suspension of 0.75 g of Fe3O4@SiO2 in 30 mL of dry
toluene was added a solution of 2-(diphenylphosphino)-
ethyltriethoxysilane (0.57 g) in 10 mL of dry toluene. The
mixture was stirred at 110 °C under argon atmosphere for
24 h. Then the resulting material was magnetically separated,
washed repeatedly with toluene and CH2Cl2 to remove
any unanchored species and dried at 100 °C under vacuum
for 5 h to afford 0.85 g of the phosphino-functionalized
Fe3O4@SiO2 (Fe3O4@SiO2-P) as brown nanoparticles. The
phosphine content was found to be 0.44 mmol g−1 by ele-
mental analysis.
Experimental
All reagents were obtained from commercial sources without
further purification, and commercially available solvents were
purified before use. The reaction was carried out at room
temperature without the exclusion of air or moisture from
the reaction mixture. All compounds were fully characterized.
1H and 13C NMR spectra were recorded at 400 or 100 MHz
with CDCl3 as the solvent and TMS as an internal standard.
Synthesis of Fe3O4@SiO2-P–AuCl
To a three-necked round bottom flask, AuCl (80 mg, 0.34 mmol)
and MeOH (50 mL) were added. The solution was stirred at
room temperature under argon atmosphere for 10 min, and
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then 0.82 g of the phosphino-functionalized magnetic nano-
particles (Fe3O4@SiO2-P) was added. The mixture was stirred
at 65 °C for 12 h, and then the solid catalyst was magnetically
separated, washed thoroughly with methanol, and dried un-
der vacuum at 100 °C for 3 h to afford 0.88 g of Fe3O4@SiO2-
P–AuCl as brown nanoparticles. The gold content was deter-
mined to be 0.39 mmol g−1 via inductively coupled plasma
atomic emission spectrometry.
NMR (100 MHz, CDCl3): δ 149.2, 138.8, 135.1, 130.2, 128.8,
127.5, 127.4, 117.5, 112.6, 111.2, 48.2.
N-Benzyl-4-methoxybenzenamine (3g)28. White solid. Mp
47–49 °C. 1H NMR (400 MHz, CDCl3): δ 7.39–7.23 (m, 5H),
6.77 (d, J = 8.8 Hz, 2H), 6.59 (d, J = 8.8 Hz, 2H), 4.27 (s, 2H),
4.10 (br, 1H), 3.72 (s, 3H). 13C NMR (100 MHz, CDCl3): δ
152.4, 142.4, 139.7, 128.6, 127.6, 127.2, 115.0, 114.3, 55.9,
49.4.
N-Benzyl-4-nitrobenzenamine (3h)28. Yellow solid. Mp
1
145–146 °C. H NMR (400 MHz, CDCl3): δ 8.06 (d, J = 9.2 Hz,
General procedure for the heterogeneous gold-catalyzed direct
reductive amination of aldehydes or ketones with amines
2H), 7.40–7.30 (m, 5H), 6.56 (d, J = 9.2 Hz, 2H), 4.95 (br, 1H),
4.43 (s, 2H). 13C NMR (100 MHz, CDCl3): δ 153.1, 138.5,
137.5, 128.9, 127.8, 127.3, 126.3, 111.4, 47.7.
A mixture of Fe3O4@SiO2-P–AuCl (26 mg, 0.01 mmol) and
AgOTf (2.6 mg, 0.01 mmol) in dichloromethane (3 mL) was
stirred at room temperature. After 10 minutes, amine (1.0
mmol), aldehyde or ketone (1.0 mmol) and ethyl Hantzsch es-
ter (1.3 mmol) were subsequently added into the stirred solu-
tion. The reaction was monitored by TLC on silica-gel plates
(GF 254). After the reaction was completed, the supported
catalyst was magnetically separated and the reaction mixture
was concentrated under vacuum and then purified by flash
chromatography (petroleum ether: ethyl acetate = 20 : 1) to af-
ford the corresponding product. The recovered catalyst was
washed with dichloromethane (2 × 2 mL), air-dried and used
directly for the next run.
N-Benzyl-4-hydroxybenzenamine (3i)7b. White solid. Mp
85–86 °C. 1H NMR (400 MHz, CDCl3): δ 7.37–7.22 (m, 5H),
6.67 (d, J = 8.8 Hz, 2H), 6.54 (d, J = 8.8 Hz, 2H), 4.25 (s, 2H),
4.14 (br, 2H). 13C NMR (100 MHz, CDCl3): δ 148.2, 139.6,
128.6, 127.6, 127.2, 123.4, 116.3, 114.6, 49.5.
N-Benzyl-3-hydroxybenzenamine (3j)28. Colorless oil. 1H
NMR (400 MHz, CDCl3): δ 7.33–7.22 (m, 5H), 6.98 (t, J = 8.0
Hz, 1H), 6.20 (d, J = 8.0 Hz, 1H), 6.16 (d, J = 8.0 Hz, 1H), 6.08
(s, 1H), 4.38 (br, 2H), 4.24 (s, 2H). 13C NMR (100 MHz,
CDCl3): δ 156.8, 149.7, 139.3, 130.3, 128.7, 127.6, 127.3, 106.1,
105.0, 100.2, 48.4.
1
N-Benzyl-3,4-dichlorobenzenamine (3k)28. Colorless oil. H
N-Benzylbenzenamine (3a)28. Colorless oil. 1H NMR (400
MHz, CDCl3): δ 7.38–7.26 (m, 4H), 7.26 (t, J = 6.8 Hz, 1H),
7.16 (t, J = 8.0 Hz, 2H), 6.71 (t, J = 7.2 Hz, 1H), 6.63 (d, J = 7.6
Hz, 2H), 4.31 (s, 2H), 4.06 (br, 1H). 13C NMR (100 MHz,
CDCl3): δ 148.1, 139.4, 129.3, 128.7, 127.6, 127.3, 117.7, 113.0,
48.4.
NMR (400 MHz, CDCl3) δ 7.37–7.25 (m, 5H), 7.15 (d, J = 8.8
Hz, 1H), 6.68 (d, J = 2.4 Hz, 1H), 6.43 (dd, J = 8.8, 2.8 Hz,
1H), 4.27 (s, 2H), 4.13 (br, 1H). 13C NMR (100 MHz, CDCl3) δ
147.5, 138.3, 132.8, 130.6, 128.8, 127.6, 127.5, 120.1, 114.0,
112.6, 48.2.
N-Benzyl-2-chlorobenzenamine (3l)29. Colorless oil. 1H
NMR (400 MHz, CDCl3): δ 7.37–7.25 (m, 6H), 7.10–7.05 (m,
1H), 6.65–6.60 (m, 2H), 4.77 (br, 1H), 4.39 (s, 2H). 13C NMR
(100 MHz, CDCl3): δ 143.9, 138.8, 129.1, 128.7, 127.8, 127.4,
127.3, 119.2, 117.5, 111.6, 47.9.
N-Benzyl-4-methylbenzenamine (3b)28. Colorless oil. 1H
NMR (400 MHz, CDCl3): δ 7.36–7.23 (m, 5H), 6.97 (d, J = 8.0
Hz, 2H), 6.54 (d, J = 8.4 Hz, 2H), 4.28 (s, 2H), 3.89 (br, 1H),
2.22 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 146.0, 139.7,
129.8, 128.7, 127.6, 127.2, 126.9, 113.2, 48.8, 20.5.
N-Benzyl-2-hydroxybenzenamine (3m)30. White solid. Mp
81–83 °C. 1H NMR (400 MHz, CDCl3): δ 7.39–7.23 (m, 5H),
6.83–6.75 (m, 1H), 6.73–6.58 (m, 3H), 4.50 (br, 2H), 4.32 (s,
2H). 13C NMR (100 MHz, CDCl3): δ 143.7, 141.0, 137.2, 128.9
128.6, 127.5, 127.1, 117.8, 114.4, 112.5, 48.6.
N-Benzyl-3-methylbenzenamine (3c)28. Colorless oil. 1H
NMR (400 MHz, CDCl3): δ 7.37–7.24 (m, 5H), 7.05 (t, J = 7.6
Hz, 1H), 6.54 (d, J = 7.6 Hz, 1H), 6.47–6.41 (m, 2H), 4.29 (s,
2H), 3.98 (br, 1H), 2.26 (s, 3H). 13C NMR (100 MHz, CDCl3): δ
148.1, 139.5, 139.1, 129.2, 128.7, 127.6, 127.3, 118.7, 113.8,
110.1, 48.5, 21.7.
N-Benzyl-2,4-dichlorobenzenamine (3n). Colorless oil. 1H
NMR (400 MHz, CDCl3) δ 7.31–7.25 (m, 6H), 7.03 (d, J = 8.8
Hz, 1H), 6.51 (d, J = 8.8 Hz, 1H), 4.70 (br, 1H), 4.36 (s, 2H).
13C NMR (100 MHz, CDCl3) δ 142.6, 138.3, 128.8, 128.7,
127.8, 127.5, 127.2, 112.1, 48.0. HRMS calcd for C13H12Cl2N+
[M+ + H]: 252.0347, found 252.0344.
N-Benzyl-4-bromobenzenamine (3d)28. White solid. Mp
49–50 °C. 1H NMR (400 MHz, CDCl3): δ 7.35–7.31 (m, 4H),
7.30–7.26 (m, 1H), 7.22 (d, J = 8.8 Hz, 2H), 6.49 (d, J = 8.8 Hz,
2H), 4.28 (s, 2H), 4.13 (br, 1H). 13C NMR (100 MHz, CDCl3): δ
147.0, 138.9, 132.0, 128.7, 127.5, 127.4, 114.5, 109.2, 48.3.
N-Benzyl-4-chlorobenzenamine (3e)28. White solid. Mp 42–
N-Benzyl-2,4-difluorobenzenamine (3o). Colorless oil. 1H
NMR (400 MHz, CDCl3) δ 7.36–7.24 (m, 5H), 6.81–6.77 (m,
1H), 6.76–6.66 (m, 1H), 6.60–6.55 (m, 1H), 4.32 (s, 2H), 4.15
(br, 1H). 13C NMR (100 MHz, CDCl3) δ 154.4 (dd, J = 247.2,
11.1 Hz), 150.9 (d, J = 228.8 Hz), 138.8, 133.1 (d, J = 11.5 Hz),
128.8, 127.5, 127.4, 112.3 (dd, J = 8.7, 4.6 Hz), 110.6 (dd, J =
21.4, 3.7 Hz), 103.4 (dd, J = 26.5, 22.6 Hz), 48.3. HRMS calcd
for C13H12F2N+ [M+ + H]: 220.0938, found 220.0932.
1
43 °C. H NMR (400 MHz, CDCl3): δ 7.35–7.26 (m, 5H), 7.09
(d, J = 8.8 Hz, 2H), 6.53 (d, J = 8.4 Hz, 2H), 4.28 (s, 2H), 4.11
(br, 1H). 13C NMR (100 MHz, CDCl3): δ 146.6, 138.9, 129.1,
128.7, 127.5, 127.4, 122.3, 114.1, 48.5.
N-Benzyl-3-chlorobenzenamine (3f)28. Colorless oil. 1H
NMR (400 MHz, CDCl3): δ 7.35–7.26 (m, 5H), 7.05 (t, J = 8.0
Hz, 1H), 6.66 (dd, J = 8.0, 1.2 Hz, 1H), 6.60 (t, J = 2.0 Hz, 1H),
6.47 (dd, J = 8.4, 1.6 Hz, 1H), 4.28 (s, 2H), 4.13 (br, 1H). 13C
N-Benzylnaphthalidine (3p)31. White solid. Mp 68–69 °C.
1H NMR (400 MHz, CDCl3): δ 7.78 (d, J = 8.0 Hz, 2H), 7.51–
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7.23 (m, 9H), 6.61 (d, J = 7.6 Hz, 1H), 4.70 (br, 1H), 4.46 (s,
2H). 13C NMR (100 MHz, CDCl3): δ 143.2, 139.1, 134.4, 129.9,
128.8, 127.8, 127.4, 126.6, 125.8, 124.8, 123.5, 119.9, 117.8,
105.0, 48.7.
(t, J = 7.4 Hz, 2H), 6.72 (t, J = 7.4 Hz, 1H), 6.57 (d, J = 7.6 Hz,
2H), 4.72 (s, 2H), 4.30 (s, 1H). 13C NMR (101 MHz, CDCl3): δ
147.4, 135.6, 133.5, 129.9, 129.3, 128.8, 128.0, 125.1, 118.1,
113.0, 45.8.
N-(2-Pyridyl)benzylamine (3q)29. White solid. Mp 92–93 °C.
1H NMR (400 MHz, CDCl3): δ 8.08 (d, J = 3.6 Hz, 1H), 7.39–
7.24 (m, 6H), 6.57 (t, J = 5.8 Hz, 1H), 6.36 (d, J = 8.0 Hz, 1H),
5.02 (br, 1H), 4.49 (d, J = 5.4 Hz, 2H). 13C NMR (100 MHz,
CDCl3): δ 158.7, 148.1, 139.3, 137.5, 128.6, 127.4, 127.2, 113.1,
106.8, 46.3.
N-((Pyridin-4-yl)methyl)benzenamine (3z)3c. White solid.
1
Mp 100–102 °C. H NMR (400 MHz, CDCl3): δ 8.53 (d, J = 4.4
Hz, 2H), 7.29–7.24 (m, 2H), 7.16 (t, J = 7.8 Hz, 2H), 6.72 (t, J =
7.2 Hz, 1H), 6.57 (d, J = 7.6 Hz, 2H), 4.36 (s, 2H), 4.28 (br,
1H). 13C NMR (100 MHz, CDCl3): δ 149.6, 147.4, 129.4, 129.3,
122.2, 118.1, 112.9, 47.1.
N-(4-Bromobenzyl)benzenamine (3r)28. White solid. Mp
50–51 °C. 1H NMR (400 MHz, CDCl3): δ 7.44 (d, J = 8.4 Hz,
2H), 7.24–7.20 (m, 2H), 7.16 (t, J = 8.0 Hz, 2H), 6.72 (t, J = 7.4
Hz, 1H), 6.59 (d, J = 8.0 Hz, 2H), 4.27 (s, 2H), 4.10 (br, 1H).
13C NMR (100 MHz, CDCl3): δ 147.8, 138.5, 131.7, 129.3,
129.1, 121.0, 118.0, 113.1, 47.8.
N-((Furan-2-yl)methyl)benzenamine (3a′)4c. Colorless oil.
1H NMR (400 MHz, CDCl3): δ 7.35 (s, 1H), 7.18 (t, J = 7.8 Hz,
2H), 6.74 (t, J = 7.4 Hz, 1H), 6.67 (d, J = 7.6 Hz, 2H), 6.31 (s,
1H), 6.23 (s, 1H), 4.31 (s, 2H), 3.94 (br, 1H). 13C NMR (100
MHz, CDCl3): δ 152.8, 147.6, 141.9, 129.2, 118.1, 113.3, 110.3,
107.0, 41.6.
N-(4-Methylbenzyl)benzenamine (3s)28. Colorless oil. 1H
NMR (400 MHz, CDCl3): δ 7.21 (d, J = 8.0 Hz, 2H), 7.16–7.08
(m, 4H), 6.68 (t, J = 7.4 Hz, 1H), 6.58 (d, J = 8.0 Hz, 2H), 4.21
(s, 2H), 3.85 (br, 1H), 2.31 (s, 3H). 13C NMR (100 MHz,
CDCl3): δ 148.4, 136.9, 136.6, 129.4, 129.3, 127.6, 117.6, 113.1,
48.2, 21.2.
N-sec-Butylbenzenamine (3b′)32. Colorless oil. 1H NMR
(400 MHz, CDCl3): δ 7.08 (t, J = 8.0 Hz, 2H), 6.59 (t, J = 7.4
Hz, 1H), 6.51 (d, J = 8.4 Hz, 2H), 3.59 (br, 1H), 3.36–3.27 (m,
1H), 1.56–1.47 (m, 1H), 1.43–1.34 (m, 1H), 1.09 (d, J = 6.4 Hz,
3H), 0.88 (t, J = 7.4 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ
146.1, 128.3, 116.3, 112.6, 49.2, 28.5, 19.0, 9.3.
N-(4-Chlorobenzyl)benzenamine (3t)3c. White solid. Mp
N-Cyclopentylbenzenamine (3c′)32. Colorless oil. 1H NMR
(400 MHz, CDCl3): δ 7.15 (dd, J = 8.4, 7.2 Hz, 2H), 6.66 (t, J =
7.2 Hz, 1H), 6.59 (d, J = 7.6 Hz, 2H), 3.81–3.74 (m, 1H), 3.59
(br, 1H), 2.05–1.95 (m, 2H), 1.76–1.66 (m, 2H), 1.64–1.57 (m,
2H), 1.49–1.41 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 148.1,
129.2, 117.0, 113.3, 54.7, 33.6, 24.1.
1
48–49 °C. H NMR (400 MHz, CDCl3): δ 7.28 (s, 4H), 7.15 (t, J
= 7.6 Hz, 2H), 6.71 (t, J = 7.4 Hz, 1H), 6.59 (d, J = 7.6 Hz, 2H),
4.28 (s, 2H), 3.98 (br, 1H). 13C NMR (100 MHz, CDCl3): δ
147.9, 138.1, 132.9, 129.3, 128.8, 128.7, 117.9, 113.0, 47.7.
N-(4-Dimethylaminobenzyl)benzenamine (3u)12a
.
White
1
solid. Mp 58–60 °C. H NMR (400 MHz, CDCl3): δ 7.23 (d, J =
8.4 Hz, 2H), 7.16 (t, J = 7.8 Hz, 2H), 6.74–6.68 (m, 3H), 6.65–
6.61 (m, 2H), 4.19 (s, 2H), 4.01 (br, 1H), 2.93 (s, 6H). 13C
NMR (100 MHz, CDCl3): δ 150.0, 148.4, 131.0, 129.3, 128.8,
117.4, 112.9, 112.8, 48.0, 40.8.
N-Cyclohexylbenzenamine (3d′)33. Colorless oil. 1H NMR
(400 MHz, CDCl3): δ 7.17–7.12 (m, 2H), 6.65 (t, J = 7.4 Hz,
1H), 6.58 (d, J = 7.6 Hz, 2H), 3.44 (br, 1H), 3.28–3.20 (m, 1H),
2.06–2.01 (m, 2H), 1.77–1.71 (m, 2H), 1.38–1.26 (m, 2H),
1.25–1.14 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 147.4,
129.3, 116.9, 113.3, 51.8, 33.5, 26.0, 25.1.
N-(3-Hydroxybenzyl)benzenamine (3v)29. White solid. Mp
1
101–102 °C. H NMR (400 MHz, CDCl3): δ 7.19–7.12 (m, 3H),
6.89 (d, J = 7.6 Hz, 1H), 6.77 (s, 1H), 6.74–6.66 (m, 2H), 6.61
(d, J = 8.0 Hz, 2H), 4.29 (br, 1H), 4.23 (s, 2H), 4.06 (br, 1H).
13C NMR (100 MHz, CDCl3): δ 156.0, 148.0, 141.3, 129.9,
129.4, 119.7, 117.9, 114.4, 114.3, 113.2, 48.2.
Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21462021) and Key Laboratory of Functional Small Or-
ganic Molecule, Ministry of Education (No. KLFS-KF-201409)
for financial support.
N-(3-Nitrobenzyl)benzenamine (3w)32. Yellow solid. Mp
1
82–83 °C. H NMR (400 MHz, CDCl3): δ 8.22 (s, 1H), 8.10 (d, J
= 8.0 Hz, 1H), 7.70 (d, J = 7.6 Hz, 1H), 7.49 (t, J = 7.6 Hz, 1H),
7.16 (t, J = 7.8 Hz, 2H), 6.74 (t, J = 7.2 Hz, 1H), 6.60 (d, J = 8.0
Hz, 2H), 4.45 (s, 2H), 4.22 (br, 1H). 13C NMR (100 MHz,
CDCl3): δ 148.6, 147.4, 142.1, 133.3, 129.6, 129.4, 122.3, 122.1,
118.2, 113.0, 47.6.
Notes and references
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