Reusable Nickel Nanoparticles-Catalyzed Reductive Amination for Selective Synthesis of Primary Amines

Article abstract of DOI:10.1002/anie.201812100

The preparation of nickel nanoparticles as efficient reductive amination catalysts by pyrolysis of in situ generated Ni-tartaric acid complex on silica is presented. The resulting stable and reusable Ni-nanocatalyst enables the synthesis of functionalized and structurally diverse primary benzylic, heterocyclic and aliphatic amines starting from inexpensive and readily available carbonyl compounds and ammonia in presence of molecular hydrogen. Applying this Ni-based amination protocol, -NH₂ moiety can be introduced in structurally complex compounds, for example, steroid derivatives and pharmaceuticals.

Full text of DOI:10.1002/anie.201812100

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Accepted Article
Title: Reusable Nickel Nanoparticles-catalyzed Reductive Amination for
Selective Synthesis of Primary Amines
Authors: Rajenahally Jagadeesh, ꢀKathiravan Murugesan, and
Matthias Beller
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812100
Angew. Chem. 10.1002/ange.201812100
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10.1002/anie.201812100
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Reusable Nickel Nanoparticles-catalyzed Reductive Amination for
Selective Synthesis of Primary Amines
Kathiravan Murugesan, Matthias Beller*, and Rajenahally V. Jagadeesh*
Abstract: The preparation of nickel nanoparticles as efficient
reductive amination catalysts by pyrolysis of in situ generated Ni-
tartaric acid complex on silica is presented. The resulting stable and
reusable Ni-nanocatalyst enables the synthesis of functionalized and
structurally diverse primary benzylic, heterocyclic and aliphatic
amines starting from inexpensive and readily available carbonyl
compounds and ammonia in presence of molecular hydrogen.
Applying this Ni-based amination protocol, -NH2 moiety can be
introduced in structurally complex compounds, e.g. steroid
derivatives and pharmaceuticals.
precursors and optimal methods play an important role.
[5a, 9]
Compared to simple metal salts, metal complexes
as well
[1j,10]
as metal organic frameworks
were found to be more
effective precursors to create active nanocatalysts. In this
respect, the use of simple and cost-effective ligands or metal
complexes is desirable. As an example, tartaric acid-based
metal complexes are more expedient because this di-acid is
inexpensive, abundant and occurs naturally from fruit biomass.
Herein, we demonstrate the immobilization of in situ generated
Ni-tartaric acid complexes on silica and subsequent pyrolysis
under inert atmosphere to produce Ni-nanoparticles. These
supported nanoparticles create stable and reusable catalysts for
the selective synthesis of functionalized and structurally diverse
linear and branched primary benzylic, heterocyclic and aliphatic
amines from aldehydes and ketones and ammonia in the
presence of molecular hydrogen. The resulting primary amines
constitute valuable fine and bulk chemicals extensively used as
central starting materials and key intermediates for advanced
chemicals, pharmaceuticals, agrochemicals and materials.[11]
To prepare supported Ni-nanoparticles, first we immobilized
Ni-tartaric acid complex on silica by solvothermal synthesis and
then applied pyrolytic processes at different temperatures (See
section S2 in supporting information). In particular, a mixture of
Catalytic reductive aminations using molecular hydrogen
represent an essential class of reactions widely applied for the
synthesis of different kinds of amines.[1-5] In general, especially
the synthesis of primary amines from carbonyl compounds and
5a]
ammonia is of central importance.[2-4,
However, these
reactions often are non-selective and suffer from over-alkylation
and reduction of carbonyl compounds to the corresponding
alcohols. In order to avoid these problems, the development of
suitable catalysts is crucial and continues to be of major
challenge. Regarding potential catalysts for reductive aminations,
[2-3]
precious metal-based (Ru, Ir, Pt, Rh)
materials
heterogeneous
[2]
[3]
and homogeneous metal complexes
are mainly
.
o
Ni(NO3)2 H2O and DL-tartaric acid in DMF was stirred at 150 C
along with aerosil silica. After slow evaporation of the solvent
and drying, the corresponding Ni-tartaric acid complex was
immobilized on silica and forms a stable template (Fig. 1).
Subsequently, this templated solid compound was pyrolyzed at
400-1000 oC under argon atmosphere. Hereafter, these Ni-
materials are labelled as Ni-TA@SiO2-x, where TA and x defines
tartaric acid and pyrolysis temperature respectively.
used to access primary amines. Apart from these, Raney
[5a]
nickel,[4] as well as Ni-
and Co- [1j] based catalysts were also
applied. Despite these achievements, still the development of
selective base-metal catalysts is important and continues to
attract scientific interest. In this respect, here we report the
development of supported nickel nanoparticles as earth
abundant reusable catalysts for reductive amination to
synthesize primary amines.
In the past Raney nickel was considered to be the state-of-
the-art catalyst for various industrially relevant hydrogenation
[6]
processes
as well as for reductive aminations of non-
functionalized molecules.[4] Despite the valuable applications
and low-cost of Raney Ni, it is often non-selective and sensitive
that leads to difficulties in handling. In general, Raney-type
catalysts cannot be applied for the refinement of complex
molecules. These drawbacks inspired chemists to look for
alternative Ni-based catalysts, which should be easy to handle
as well as more selective towards functional molecules.
Consequently, in recent years, significant efforts have been
made on the development of alternative Ni-based hydrogenation
catalysts.[7] Interestingly, very recently Ni-based nanocatalysts
were also reported by Kempe group for reductive aminations to
prepare primary amines applying aqueous ammonia.[5a]
Fig. 1 Preparation of Ni-nanoparticles supported on silica by the pyrolysis of
nickel-tartaric acid complex.
The prepared new materials were tested for the reductive
amination of 2-bromobenzaldehyde (1) with ammonia in
presence of molecular hydrogen as bench mark reaction (Table
1). Neither the homogeneous Ni-tartaric acid complex nor the
supported one on silica under non-pyrolyzed condition was
active (Table 1; entries 1-3). Similarly, pyrolysis of simple nickel
nitrate on silica also gave no active catalyst (Table 1, entry 4).
However, materials obtained by pyrolysis (400-1000 oC) of nickel
nitrate-tartaric acid on silica exhibited significant activities (Table
1, entries 5-8). Among these, the one pyrolyzed at 800°C (Ni-
TA@SiO2-800) showed maximum activity and produce 2-
bromobenzylamine in 96% yield (Table 1, entry 7). We also
prepared other Ni-based materials using different di-carboxylic
acid ligands (eg. malic acid, malonic acid and succinic acid) and
were tested for their activity in the bench mark reaction.
Interestingly, all of these materials displayed similar activity and
selectivity (95-96% yield; Table S2). Next, different mole ratio of
nickel nitrate to tartaric acid were tested and found that 1:3 mole
Among the different materials, especially supported
nanoparticles-based heterogeneous catalysts are of interest due
to their easy separation, potential activity and selectivity.[8-10] For
the preparation of active nanoparticles-based catalysts, suitable
K. Murugesan, Prof. Dr. M. Beller, and Dr. R. V. Jagadeesh
Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert
Einstein-Str. 29a, 18059 Rostock, Germany
*E-mail: matthias.beller@catalysis.de; jagadeesh.rajenahally@catalysis.de
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201812100
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COMMUNICATION
ratio is required to produce the optimal catalyst (Table S3). In
addition to SiO2, the influence of other oxide supports such as
Al2O3, ZrO2, TiO2 and ZSM-5 have also been tested (Table S4).
Among these, Ni-TA@Al2O3-800 and Ni-TA@ ZrO2-800 exhibit
poor activity (<20% yield), while Ni-TA@TiO2-800 and Ni-
TA@ZSM5-800 were found to be active by yielding 75% and
88% of 2-bromobenzylamine, respectively.
peak at 852.6 eV is clearly indicates the predominant presence
of metallic Ni. Although small amount of Ni2+ is evident from
XRD, but due to the interference of Ni(II) 2p3/2 peak with the
satellite peak of Ni(0) 2p3/2, complete deconvolution of the XPS
spectrum is not possible. Ni2p1/2 main and satellite peaks were
also visible above the background at 870.8 eV and 876.8 eV but
their intensity was very low and precise peak fitting is not
possible.[12] BET surface area of Ni-TA@SiO2-800 and
Ni(NO3)2@SiO2-800 are found to 58.98 m2/g and 38.79 m2/g
respectively (S3.4; Figs. S9-S10). All of these characterization
data revealed that the use of tartaric acid has a significant effect
on the nature, size and shape of Ni-nanoparticles.
Table 1. Reductive amination of 2-bromobenzaldehyde: Activity of different Ni-
catalysts.
Entry
Catalyst
Conv
(%)
-
Yield of
2 (%)
-
Yield of
3 (%)
-
Yield of
4 (%)
-
1 [a]
2 [a]
3 [b]
4 [b]
5 [b]
6 [b]
7 [b]
8 [b]
Ni(NO3)2.H2O
Ni(NO3)2.6H2O + TA
Ni-TA@SiO2
-
-
-
-
-
-
-
-
Ni(NO3)2@SiO2-800
Ni-TA@SiO2-400
Ni-TA@SiO2 -600
Ni-TA@SiO2 -800
Ni-TA@SiO2 -1000
<5
>99
>99
>99
>99
-
<2
39
30
2
<2
4
5
-
52
60
96
90
4
5
Reaction conditions: [a] 0.5 mmol substrate, 6 mol% Ni(NO3)2.H2O, 12 mol %
DL-tartaric acid, 120°C, 5-7 bar NH3, 20 bar H2, 15 h, 3 ml t-BuOH, GC-
yields). [b] Same as [a] with 25-30 mg heterogeneous catalyst (6 mol% Ni). A
preliminary kinetic investigation is outlined in Section S7 of the Supporting
Information.
Fig. 2. TEM images of Ni-TA@SiO2-800: (a & b) = distribution of Ni-
nanoparticles, (c) = HRTEM, (d) = selected area electron diffraction (SAED) of
Ni-nanoparticles.
In order to know the structural features and the role of
tartaric acid in the creation of active catalyst, we conducted
characterization using TEM (Transmission Electron Microscopy),
EDX (Energy Dispersive X-Ray Spectroscopy), XRD (X-Ray
Diffraction) spectral analysis), XPS (X-ray Photoelectron
Spectroscopy) and BET (Brunauer–Emmett–Teller). TEM
analysis of the most active catalyst (Ni-TA@SiO2-800) showed
the formation of mainly metallic Ni-nanoparticles with sizes of 5-
15 nm (Fig. 2). In addition, the presence of a small quantity of
NiO-particles was also observed. The HR-TEM image of Ni-
particles, which are in spherical shape, is shown in Fig. 2c.
Lattice spacing of 0.24 nm and 0.21 nm can be indexed to (111)
and (200) crystal planes of the cubic Ni-phase respectively,
which further confirms the formation of a crystalline structure.
The corresponding SAED pattern (Selected Area Electron
Diffraction) is shown in Fig. 2d, which specifies the
polycrystalline nature of Ni-nanoparticles. The diffraction rings
are allocated to (111), (200) and (220) planes and in good
agreement with XRD results (Fig S3). In contrast, the inactive
material prepared without tartaric acid (Ni(NO3)2@SiO2-800)
contains mainly NiO-particles with nanorod-like shapes (Fig. S1).
In this sample, metallic Ni-particles with spherical shape were
rarely observed. Enlarged TEM image (Fig. S1b) showed the
morphology of nanorods with 170-190 length and 15-20 nm
thickness. Different phases of Ni-particles were analyzed by
XRD (Fig. S4) and this data are in agreement with TEM analysis.
XPS analysis of Ni-TA@SiO2-800 showed the presence of
Ni, Si, C, O and N elements. The deconvolution of Ni displayed
Ni2p3/2 and Ni2p1/2 peaks (Fig. S4). The Ni2p spectrum is quite
complex and intensity is low. However, Ni2p3/2 spectrum with
After having an active catalyst (Ni-TA@SiO2-800), we
studied its general applicability for the synthesis of primary
amines. As shown in Schemes 1-3, all kinds of aldehydes and
ketones successfully underwent reductive aminations to produce
the corresponding industrially important as well as
pharmaceutically and biological relevant linear and branched
primary amines. Halogenated benzaldehydes were selectively
aminated to primary linear benzylic amines in high yields (>92%),
without any significant dehalogenation (Scheme 1; 11-14).
Achieving a high degree of chemoselectivity is an important
aspect for a given catalyst to be implemented in organic
synthesis and drug discovery. To establish this aspect, 9
aldehydes containing functional groups such as ester, C-C
double and triple bonds were tested. In all these cases, primary
amines were selectively obtained without affecting other
functional groups (18-26). In addition, heterocyclic and aliphatic
aldehydes also smoothly reacted and yielded the corresponding
primary amines in up to 92% (Scheme 1; 27-34).
Gratifyingly, Ni-TA@SiO2-800 exhibited excellent activity
and selectivity towards reductive amination of ketones, too
(Schemes 2-3). As a result, branched benzylic and heterocyclic
primary amines were obtained in up to 94% yields (35-43). In
addition, Ni-TA@SiO2-800 was also active for the reductive
amination of aliphatic ketones to obtain various branched
araliphatic and aliphatic amines in 88-94% yields (44-53). Next,
we investigated the introduction of –NH2 moiety into structurally
complex molecules (Scheme 3). Amination of important drugs
such as Nabumetone, Pentoxifylline, Azaperone, and
Haloperidol proceeded smoothly and the corresponding
aminated products (55-58) were obtained in up to 93% yield.
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Scheme 3. Application of Ni-nanoparticles for the introduction of –NH2 moiety
in pharmaceuticals and steroid derivatives. [a] Reaction conditions: 0.5 mmol
ketone, 30 mg catalyst (6 mol%), 5-7 bar NH3, 20 bar H2, 3 mL t-butanol,
120°C, 24 h, isolated yields as free amines and converted to hydrochloride
salts. NMR spectra were recorded for corresponding hydrochloride salts of
amines. [b] Diastereomeric ratio.
In addition, steroid derivatives, which are vital biologically active
compounds, also reacted with ammonia and produced the
corresponding –NH2 substituted products (59-61).
To showcase the generality and selectivity of Ni-TA@SiO2-
800, its activity was compared with Raney Ni. As shown in Table
S5, under our standard reaction conditions Raney Ni worked
well to obtain 90% of 2-bromobenzylamine with a small amount
of debromination (5%). However, for the remaining 4 substrates
such as 4-stilbenecarboxaldehyde, Pentoxifylline, dihydro-beta-
ionone and 6-methyl-5-hepten-2-one, it was non-selective and
produced only 14-25% of the desired primary amines. In all
these substrates, the corresponding alcohols were mainly
formed in 73-85% yields. Advantageously, Ni-TA@SiO2-800
catalyst successfully worked for all of these carbonyl compounds
to produce the corresponding primary amines in 84-96% yields.
Notably, our Ni-TA@SiO2-800 catalyst exhibits generality and
selectivity with broad substrate scope similar to Ni/Al2O3, which
Scheme 1. Ni-TA@SiO2-800 catalyzed synthesis of linear primary amines
form aldehydes[a]. [a] Reaction conditions: 0.5 mmol aldehyde, 30 mg catalyst
(6 mol%), 5-7 bar NH3, 20 bar H2, 3 mL t-butanol, 120°C, 24 h, isolated yields
as free amines and converted to hydrochloride salts. NMR spectra were
recorded for corresponding hydrochloride salts of amines. [b] Yields were
determined by GC using 100 µLn-hexadecane as standard.
was reported by Kempe and co-workers.[5a]
In addition,
Ni-TA@SiO2-800 showed high selectivity for the preparation of
silylated ethynyl benzylamine (product 26) without being the
reduction of C-C triple bond as well as to obtain other
structurally diverse amines such as products 23, 39, 41, 44, 45,
50, 56. However, Ni/Al2O3 works under mild reaction conditions
(10 bar H2, 80 oC) and more active for aliphatic aldehydes. [5a]
The utility of this reductive amination has been demonstrated
by upscaling the reactions to gram scale and by catalyst
recycling. Interestingly, 4 substrates in 2-25g scale were
successfully aminated to their corresponding primary amines in
similar yields to that of 50-100 mg scale reactions (Scheme S1).
Furthermore, Ni-TA@SiO2-800 was highly stable and
successfully recycled and reused up to 10 times without any
significant loss of both activity and selectivity (Fig. S14).
Scheme 2. Reductive amination of ketones for the synthesis of branched
primary amines using Ni-TA@SiO2-800 [a]. [a] Reaction conditions: 0.5 mmol
ketone, 30 mg catalyst (6 mol%), 5-7 bar NH3, 20 bar H2, 3 mL t-butanol,
120°C, 24 h, isolated yields as free amines and converted to hydrochloride
salts. NMR spectra were recorded for corresponding hydrochloride salts of
amines. [b] Yields were determined by GC using 100 µL n-hexadecane as
standard.
Finally, we proposed plausible mechanism for this Ni-based
reductive amination protocol (See, Section S8 and Scheme S5).
Initially, carbonyl compound condenses with NH3 and forms the
corresponding unstable primary imine A (I). Next, adsorption of
imine A and hydrogen to catalyst takes place that leads in their
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tartaric acid complex on silica produced highly active and
selective catalysts for reductive amination to access various
primary amines. This Ni-based amination process enables the
preparation of functionalized and structurally diverse linear and
branched benzylic, heterocyclic and aliphatic primary amines
starting from carbonyl compounds, ammonia using molecular
hydrogen. We have also shown the possibility of upscaling this
amination protocol up to 25 g as well as catalyst recycling.
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Acknowledgements
We gratefully acknowledge the European Research Council (EU
project 670986-NoNaCat), and the State of Mecklenburg-
Vorpommern for financial and general support. We thank
analytical staff of Leibniz Institute for Catalysis for their excellent
service.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: Ni-nanoparticles • reductive amination• primary
amines • carbonyl compounds • ammonia
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10.1002/anie.201812100
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Kathiravan Murugesan, Matthias Beller*,
and Rajenahally V. Jagadeesh*
No. – Page No.
Reusable
Nickel
Nanoparticles-
catalyzed Reductive Amination for
Selective Synthesis of Primary
Amines
Tartaric acid derived Ni-nanoparticles: Pyrolysis of Ni-tartaric acid on silica produces stable and reusable Ni-nanoparticles, which
empowers the synthesis of linear and branched benzylic, heterocyclic and aliphatic amines form easily accessible and inexpensive
carbonyl compounds and ammonia using molecular hydrogen.
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