Iron-catalyzed synthesis of secondary amines: On the way to green reductive aminations

Article abstract of DOI:10.1002/cssc.201402413

Amines represent important intermediates in chemical and biological processes. Herein, we describe the use of a nanostructured iron-based catalyst for the tandem reductive amination between nitroarenes and aldehydes using hydrogen as reductant. The nanostructured iron-catalyst is prepared by immobilization of an iron-phenanthroline complex onto a commercially available carbon support. In the reaction sequence a primary amine is formed in situ from the corresponding nitro compound. Reversible condensation with aldehydes forms the respective imines, which are finally reduced to the desired secondary amine. This synthesis of secondary amines is atom-economical and environmentally attractive using cheap and readily available organic compounds as starting materials.

Full text of DOI:10.1002/cssc.201402413

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DOI: 10.1002/cssc.201402413
Iron-Catalyzed Synthesis of Secondary Amines: On the
Way to Green Reductive Aminations
Tobias Stemmler, Annette-Enrika Surkus, Marga-Martina Pohl, Kathrin Junge, and
[a]
Matthias Beller*
Amines represent important intermediates in chemical and
biological processes. Herein, we describe the use of a nano-
structured iron-based catalyst for the tandem reductive amina-
tion between nitroarenes and aldehydes using hydrogen as re-
ductant. The nanostructured iron-catalyst is prepared by im-
mobilization of an iron–phenanthroline complex onto a com-
mercially available carbon support. In the reaction sequence
a primary amine is formed in situ from the corresponding nitro
compound. Reversible condensation with aldehydes forms the
respective imines, which are finally reduced to the desired sec-
ondary amine. This synthesis of secondary amines is atom-eco-
nomical and environmentally attractive using cheap and readi-
ly available organic compounds as starting materials.
Scheme 1. Envisioned tandem reductive amination.
This synthesis of secondary amines is atom-economical and
environmentally attractive using cheap and readily available or-
ganic compounds as starting materials. Nitroarenes are the
[12]
classic and most economical feedstock to provide aromatic
[13]
primary amines by hydrogenation. Such catalytic reductions
proceeds via several intermediates as exemplarily described by
[14–16]
Haber.
In general, the selective hydrogenation of nitroar-
enes to anilines is achieved in the presence of modified metal
[
17]
catalysts such as Au/TiO ,
palladium or platinum on
2
[18–20]
IV
[21]
[22]
[23]
carbon, Pt oxide, as well as Raney-nickel or Ni/TiO2.
Also the use of rhodium or ruthenium sulfides have been de-
Amines represent important intermediates in chemical and
[
1,2]
[24–26]
biological processes.
In industry, they are applied as ubiqui-
scribed.
However, these materials are costly and they have
tous building blocks of functional materials, agrochemicals, or
practical drawbacks regarding preparation, reusability, and the
requirement of modifying reagents. In the past, stoichiometric
reducing agents such as iron in acidic media or zinc in ammo-
nium formate prevailed for nitro reduction, but lost their rele-
[
3]
fine chemicals. Hence, there is a continuing interest in sus-
tainable, cost-effective, and selective synthesis of functional-
[
4]
ized amines. For the synthesis of secondary amines several
[27]
methodologies, such as N-alkylation of primary amines with al-
vance.
[
5]
[6]
[7]
cohols or alkyl halides, direct hydrogenation of nitriles, ad-
For our investigations, the nanostructured iron-based cata-
lyst was prepared from Fe(OAc)2 and 1,10-phenanthroline,
which were coated and subsequently pyrolyzed onto Vulcan
XC72R as carbon support under an argon atmosphere (metal/
[
8]
[9]
dition reactions to imines, reduction of amides as well as
[
10]
solid phase reactions, have been reported. In general, these
reactions are realized either in homogeneous phase or in the
presence of supported catalysts. Notably, as catalysts mainly
noble metal-based complexes or materials are used. However
in recent years, an important and actual goal in catalysis re-
search is the replacement of such noble metals with more
earth-abundant and less toxic elements. In this respect, iron is
an ideal candidate to replace precious metals, if comparable
activities and selectivities can be achieved.
[28,29]
ligand (M/L) ratio 1:3).
Notably, the most active hydrogena-
tion catalyst was prepared by pyrolysis at 8008C for 2 h. Simul-
taneous thermal analyses showed that the nitrogen and
oxygen ligands are mainly decomposed inbetween 200 and
4008C. From 4008C on an endothermal transformation is ob-
served (Supporting Information, Figure S1). As a result of this
heat treatment, we obtained widely distributed fractions of
iron-based nanoparticles and agglomerates with varying sizes
from 5 to 200 nm. Bright field (BF) and high-angle annular dark
field (HAADF) examinations using spherical aberration (Cs)-cor-
rected transmission electron microscopy showed the complete
size spectra of iron-oxide nanocomposites (Figure 2A and B).
Interestingly, the metal-based particles are surrounded by indi-
vidually nitrogen-enriched graphene-type layers (NGr), which
are formed through the carbonization of the nitrogen ligand
Herein, we describe the use of a nanostructured iron-based
catalyst for the tandem reductive amination between nitroar-
[
11]
enes and aldehydes using hydrogen as reductant. In general,
in this reaction sequence a primary amine is formed in situ
from the corresponding nitro compound. Reversible condensa-
tion with aldehydes forms the respective imines, which are fi-
nally reduced to the desired secondary amine (Scheme 1).
[29–31]
(
Figure 2C).
An illustration of the obtained core–shell-
[
a] T. Stemmler, Dr. A.-E. Surkus, Dr. M.-M. Pohl, Dr. K. Junge, Prof. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein Strasse 29a, Rostock 18059 (Germany)
E-mail: matthias.beller@catalysis.de
structured nanoiron-based catalyst is shown in Figure 1.
The presence of such a composite is confirmed by the X-ray
diffraction (XRD) powder pattern (Supporting Information, Fig-
ure S7) as well as energy dispersive X-ray (EDX) spectroscopy
with the observed elements (Figure 3). X-ray photoelectron
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402413.
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2 3
Figure 1. Illustration of core–shell-structured Fe O /NGr@C nanoparticles.
spectroscopy (XPS) revealed two distinct nitrogen species (pyri-
dinic, pyrrolic) incorporated in the graphene lattice as well as
nitrogen atoms bound to the iron (Supporting Information,
Figure S2). So far, the electronic influence of the doped gra-
phene layers on the particle is still not well understood.
Recently, we used such iron-based materials for a general
and highly chemoselective hydrogenation of nitroarenes to
[
29]
functionalized anilines.
Based on this work, the reductive
amination of nitrobenzene with benzaldehyde using different
ratios (1:1, 1:1.5, 1:2, and 1:2.5) was studied. An initial evalua-
tion of the catalyst performance was carried out using 5 mol%
catalyst loading in a THF/H O (1:1)-mixture at 1208C and
2
50 bar hydrogen pressure for 24 h (Scheme 2).
Scheme 2. Tandem reductive amintion of benzaldehyde with nitrobenzene.
As expected under these conditions, the formation of aniline
was quantitative and also the subsequent condensation reac-
tion was completed with 2.0 equivalents of benzaldehyde.
However, the reduction of the formed imine intermediate pro-
ceeded slowly and the desired product N-benzylaniline was de-
tected only in traces by GC measurements (Table 1, entry 1).
Gratifyingly, increasing the temperature and hydrogen pressure
led to 85% yield of the desired N-benzylaniline after 30 h (en-
tries 2–5). Notably, the catalytic hydrogenation of the Schiff
base represents the rate-limiting step in the overall reaction
process. As minor side products, tertiary amines were ob-
served, which result from dialkylation of the product. It is
known, that the formation of such side-products occurs via
Figure 2. Selected a) BF- and b) HAADF-TEM images of the carbon support-
ed iron-based catalyst. c) Visualization of graphene-layers of Fe
BF-TEM.
2 3
O /NGr@C by
nations are in general favored under anhydrous conditions due
to the elimination of a water molecule, here water has a posi-
[
32,33]
hemiaminal or gem-diamines.
Notably, when running the
[34,35]
catalytic experiment with an excess of benzaldehyde, also
small amounts of benzyl alcohol (<40%) have been obtained
in addition to the remained benzaldehyde.
tive influence.
Apparently, it promotes association of the
substrates at the hydrophobic catalyst surface and might also
[5a,36,37]
suppress catalyst poisoning.
Next, the influence of the solvent was investigated. To our
surprise, the most suitable solvent for this reaction is a mixture
of tetrahydrofuran and water (Table 1, entry 5). The individual
solvents as well as mixtures of dioxane or toluene and water
did not show similar effects (entry 5). Although reductive ami-
Moreover, we performed the benchmark reaction in the
presence of catalytic amounts of acids (e.g. p-TsOH) to pro-
[15,38]
mote the imine formation/conversion (Table 1, entry 6).
However, no significant effect on the yield of the desired prod-
uct was detected.
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catalyst was reused up to five times without significant loss of
activity (reaction conditions: 2.5 mmol nitrobenzene, 5.0 mmol
benzaldehyde, 1708C, 50 bar H , 30 h). Selectivity and yield of
2
N-benzylaniline remained constant through all cycles
(
Figure 4). The leaching of the active metal from the supported
material was not observed by ICP analysis. Also, a hot filtration
test showed no activity of the filtered reaction solution.
Figure 4. Recycling experiments of the benchmark reaction in the presence
of the nanoiron-based catalyst.
To demonstrate the general applicability of the catalytic
system, the N-alkylation of various aromatic aldehydes with ni-
troarenes was investigated (Tables 2 and 3). As a general trend,
the reaction is favored using electron-rich aromatic aldehydes
or nitroarenes. Common alkyl- and alkoxy-substituted nitroar-
enes or aromatic aldehydes are reduced to give the desired
amine in good yields (Table 2, entry 6 and 7; Table 3, entries 1,
2 3
Figure 3. Selected EDX spectrum of Fe O /NGr@C catalyst.
Table 1. Optimization of the benchmark reaction.
2
and 8). Nitroarenes with electron-withdrawing substituents
were less reactive and the subsequent hydrogenation of the
imine intermediate to the secondary amine is interfered
(Table 2, entries 8–10). Even after longer reaction times up to
40 h, the imine conversion was not significantly increased.
Furthermore, the electronic and steric influences of substitu-
ents on the aromatic ring were confirmed by reactions of o-,
m-, and p-nitrochlorobenzene. As expected the reaction with
the ortho-isomer is significantly slower than in case of the
para- or meta-isomer due to the increased steric effect
Entry
T
P
H2
Solvent
Yield
Yield
[
d]
[d]
[
8C] [bar]
imine 1 [%] amine 2 [%]
[
[
[
[
a]
b]
b]
b]
1
2
3
4
120 50
140 50
150 50
160 70
THF/H
THF/H
THF/H
THF/H
THF/H
THF/H
THF/H
2
2
2
2
2
2
2
O (1:1)
O (1:1)
O (1:1)
O (1:1)
O (1:1)
O (1:2)
O (2:1)
94
42
26
12
–
–
52
–
69
73
22
7
–
–
<3
22
46
68
85
33
16
76
27
16
6
dioxane/H
dioxane/H
dioxane/H
toluene/H
THF
2
2
2
O (1:2)
O (1:1)
O (2:1)
(
Table 2, entries 3–5). Thus, the desired secondary amine was
[
b]
5
170 70
formed in lower yield (entry 5). Di- and trihalogenated nitroar-
enes exhibited similar reactivity to that of ortho-halogenated
derivatives and ended up with a mixture of the imine and the
wanted product. Analogues results were obtained with the dif-
ferent substituted aromatic aldehydes (Table 3, entries 4–6).
Among the various halogenated aldehydes p-fluorobenzalde-
hyd showed the best result (entry 3). In case of bromo-substi-
tuted substrates, also the dehalogenated product was ob-
served in traces (Table 2, entry 1).
2
O (1:1)
73
55
20
80
dioxane
[
e]
H
2
O
[c]
6
170 70
THF/H
2
O (1:1)
<5
[a] Standard conditions: 0.5 mmol of nitrobenzene, 1.0 mmol of benzalde-
hyde, 5 mol% Fe /NGr@C, 24 h, 3 mL solvent (1:1). [b] Standard condi-
2
O
3
tions, 30 h. [c] Addition of 10 mol% TsOH. [d] Determined by GC using n-
hexadecane as an internal standard. [e] Non-converted nitroarene re-
mained.
Interestingly, even more demanding aliphatic aldehydes can
be used for the alkylation of nitroarenes in the presence of the
iron-based catalyst. Similarly, aliphatic branched or cyclic nitro-
compounds reacted well (Scheme 3). Finally, nitro-substituted
N-heterocycles such as quinoline, isoquinoline, acridine, or car-
An important aspect for the use of heterogeneous materials
is the convenient recycling of the active catalyst. In fact, our
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Table 2. N-alkylation of benzaldehyde with nitroarenes.
Table 3. N-alkylation of aromatic aldehydes with nitrobenzene.
[
a]
[b]
[a]
[b]
Entry
Substrate
Mole ratio
imine/2
Yield 2
[%]
Entry
1
Substrate
Mole ratio
imine/2
Yield 2
[%]
[
c]
1
2
3
4
5
6
7
8
1:3
1:3
1:3
1:3
3:1
1:7
1:3
1:1
58
64
65
61
25
89
69
45
1:4
72
2
3
4
1:5
1:7
1:5
71
84
[
[
c]
d]
79 (67)
[
[
d]
d]
5
1:5
75 (62)
[
c]
6
7
1:4
1:2
64 (51)
47
8
9
1:40
1:3
94
65
[c]
9
1:3
1:1
51
42
[
a] Reaction conditions: 0.5 mmol of nitrobenzene, 1.0 mmol of aromatic
aldehyde, 5 mol% Fe /NGr@C, 3 mL solvent. [b] Determined by GC
using n-hexadecane as an internal standard. [c] Isolated yield by flash
2
O
3
[c]
1
0
chromatography. [d] Up scaling by factor 4.
[a] Reaction conditions: 0.5 mmol of nitrobenzene, 1.0 mmol of benzalde-
hyde, 5 mol% Fe /NGr@C, 3 mL solvent. [b] Determined by GC using n-
2
O
3
hexadecane as an internal standard. [c] Isolated yield by flash chromatog-
raphy.
ated by the iron-based catalyst to give the corresponding
amines selectively (Table 2, entries 8–10; Scheme 3). However,
reacting ethyl-4-nitrocinnamate, 1-vinyl-4-nitrobenzene, or 1-
ethynyl-4-nitrobenzene with benzaldehyde led to isomeric mix-
tures of the expected imines and secondary amines.
In conclusion, a nanostructured iron-catalyst is prepared by
immobilization of the corresponding iron–phenanthroline com-
plex onto a commercially available carbon support. This cata-
lyst allows for the selective hydrogenation of nitroarenes as
well as imines with molecular hydrogen to give diverse secon-
dary amines. Our protocol for the heterogeneous catalysis
does not require any additives and can be proposed as an en-
vironmentally friendly method for the selective synthesis of
secondary amines.
Experimental Section
General procedure for the one-pot reductive amination: In a re-
action vial (8 mL), nitroarene (0.5 mmol), and aromatic aldehyde
(
1.0 mmol) were dissolved in 3 mL THF/H O (1:1) solvent mixture.
2
Scheme 3. Selection of diverse N-alkylated amines.
The carbon-supported iron catalyst (5 mol% Fe O /NGr@C) was
2
3
added. The closed reaction vials (up to 6) were placed via a pre-
pared metal plate into a 300 mL autoclave. The autoclave was
flushed with hydrogen twice (ca. 40 bar) and pressurized to 70 bar
hydrogen. Then, it was placed into an aluminium block, heated to
bazole were investigated, also. The best result was obtained
for benzothiazole with moderate yields of the desired product
(Scheme 3). In all cases, mixtures of the imines and the corre-
1
708C, and stirred for the indicated time of 30 h. After the reaction
sponding secondary amines were observed. From a synthetic
point of view, it is important to note that reducible functionali-
ties such as ketones, esters, amides, and nitriles are well toler-
was complete, the autoclave was cooled to room temperature and
the hydrogen was released. To the crude reaction mixture was
added n-hexadecane (52 mL) as an internal standard. Subsequently,
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Published online on && &&, 0000
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
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5
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These are not the final page numbers!

COMMUNICATIONS
T. Stemmler, A.-E. Surkus, M.-M. Pohl,
K. Junge, M. Beller*
&& – &&
Iron-Catalyzed Synthesis of Secondary
Amines: On the Way to Green
Reductive Aminations
A well-defined carbon-supported iron-
based catalyst is investigated for a one-
pot multistep synthesis of secondary
amines. The reductive amination of al-
dehydes includes the industrially rele-
vant hydrogenation of nitroarenes to
functionalized anilines as well as the
conclusive reduction of the formed
Schiff base. The catalytic activity of this
cheap, promising catalyst is caused by
a core–shell-structured iron oxide nano-
composite, which is encapsulated by in-
dividually nitrogen-enriched graphene-
type layers.
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!