Multisite solid (NHC)NN-Ru-catalysts for cascade reactions: Synthesis of secondary amines from nitro compounds

Article abstract of DOI:10.1016/j.jcat.2012.04.015

Supported ruthenium complexes based on electron-rich (NHC)NN-pincer-type ligands selectively catalyze the formation of substituted amines from nitroaromatics and carbonyl compounds through a series of consecutive steps which involves the reduction of nitro group to amine, the formation of an imine or iminium ion intermediate, followed by in situ reduction to an alkylated amine of higher order in a single operation. Solid complexes result active and recyclable catalysts and no deactivation was observed after repeated recycling.

Full text of DOI:10.1016/j.jcat.2012.04.015

Journal of Catalysis 291 (2012) 110–116
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Multisite solid (NHC)NN-Ru-catalysts for cascade reactions: Synthesis
of secondary amines from nitro compounds
c,
Carolina del Pozo a,c, Avelino Corma b, Marta Iglesias a, , Félix Sánchez
⇑
⇑
a Instituto de Ciencia de Materiales de Madrid, CSIC, C/Sor Juana Inés de la Cruz 3, Cantoblanco, 28049 Madrid, Spain
b Instituto de Tecnología Química, CSIC-UPV, Avenida de los Naranjos s/n, 46022 Valencia, Spain
c Instituto de Química Orgánica General, CSIC, C/Juan de la Cierva 3, 28006 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Supported ruthenium complexes based on electron-rich (NHC)NN-pincer-type ligands selectively cata-
lyze the formation of substituted amines from nitroaromatics and carbonyl compounds through a series
of consecutive steps which involves the reduction of nitro group to amine, the formation of an imine or
iminium ion intermediate, followed by in situ reduction to an alkylated amine of higher order in a single
operation. Solid complexes result active and recyclable catalysts and no deactivation was observed after
repeated recycling.
Received 29 November 2011
Revised 22 March 2012
Accepted 18 April 2012
Available online 21 May 2012
Keywords:
MCM-41
Ó 2012 Elsevier Inc. All rights reserved.
One-pot
Domino
Ruthenium
NHC pincer ligand
Supported catalysts
Catalyst recycling
1. Introduction
the reductive alkylation of amines with aldehydes is a common pro-
cess, only few papers describe the reaction under hydrogenation
One-pot reactions, where several steps are performed in the
same reaction vessel, are of much interest since they are more sus-
tainable process [1] by avoiding intermediate separation, purifica-
tions steps, and minimizing energy.
conditions, for example with Pd/C [8,9], but the reported substrate
scope is limited to aliphatic aldehydes. Very recently, imine forma-
tion from aldehydes and nitroarenes has been reported with Au/
TiO2 catalysts [10] or Au/Fe2O3 [11].
Compounds containing a nitro group are valuable substrates in
organic synthesis. In particular, nitroarenes are one of the most
readily available starting materials in organic synthesis since they
can be produced from a range of aromatic starting materials [2].
One of the major pathways for the transformation of nitroarenes
is their hydrogenation into anilines, and further transformation
by alkylation, reductive alkylation, hydroamination, etc. to obtain
their derivatives including heterocyclic compounds [3].
Amines and their derivates are a highly versatile building blocks
for various substrates and they also are essential precursors to a
variety of biologically active compounds such as pharmaceuticals
[4] and agrochemicals [5,6]. Therefore, the reductive amination of
carbonyl compounds has found much attention as a direct-
atom-economical route to get amines and the search for new
catalysts remains an important area of research. Several reagents
for reductive amination have recently been reported [7]. Although
A pincer organometallic is generally described as a tridentate
ligand bound to a metal center via one sigma and two dative bonds
[12]. Pincer metal complexes have been applied in a wide range of
catalytic reactions: aldol condensation [13], transfer hydrogena-
tion [14], etc. Immobilization has been achieved through the
4-position of the aromatic central ring [15]. Here, we report mono
and bifunctional catalysts based of ruthenium complexes with
electron-rich (NHC)NN-pincer-type ligands with an N-heterocyclic
carbene substituent, and the immobilization on MCM-41 [16],
MCM-41/Al, or MCM-41/Sn for application in organic synthesis.
More specifically, we have investigated the (NHC)NNRu-catalyzed
chemoselective hydrogenation of nitro groups and have applied
this reaction to one-pot synthesis of aniline derivatives by succes-
sive reductive amination. The recycling of the catalytic materials
has been studied and shows that the highly stable organometallic
catalysts are stable to multiple recycles [17]. It is demonstrated
that catalyst immobilization can be a simple, high-yielding pro-
cess, without complex degradation, and that catalysts immobilized
can be recycled.
⇑
Corresponding authors.
E-mail address: marta.iglesias@icmm.csic.es (M. Iglesias).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.04.015

C. del Pozo et al. / Journal of Catalysis 291 (2012) 110–116
111
2. Experimental
2.1. Materials
(NHC)NNRuHClCO complexes were obtained as we have re-
ported previously. All the preparations and manipulations were
performed using standard Schlenk techniques under an atmo-
sphere of dinitrogen [16a,b]. The Supplemental Material includes
details for the synthesis and characterization procedures. The inor-
ganic supports for anchoring were purely siliceous MCM-41, Al-
containing MCM-41 (Si/Al ꢀ 15), and Sn-containing MCM-41 (Si/
Sn ꢀ 20 Sn) with a pore diameter of 3.5 nm were prepared at Insti-
tuto de Tecnología Química (Valencia Spain) in accordance with lit-
erature [18]. Nitro and carbonyl compounds were obtained from
Aldrich.
2.2. Heterogenized complexes
General method: A solution of trialkoxysilyl-ruthenium com-
plexes (0.022 mmol in 5 mL ethanol) was added to a suspension
of the mesoporous solid MCM-41 (150 mg) in wet toluene (25
mL) (details for the synthesis and characterization data for were
presented in Supplementary Information). The slurry was heated
at 100 °C for 3 h. The mixture was cooled, and the solid was filtered
off and washed thoroughly with ethanol and diethyl ether. Details
Fig. 1. (NHC)NNRu-catalysts. Grafting of the alcoxysilane moieties is represented as
a bipodal attachment to the surface, for simplification. Both mono- and tripodal
anchoring modes are also likely to occur.
added directly to the dichloromethane solution of corresponding
silver complex. Precipitation of silver chloride was observed imme-
diately, but the mixtures were allowed to stir at room temperature
for 2–24 h before workup. Ru-complexes were isolated upon pre-
cipitation with diethyl ether as brown powders in >90% yield.
The IR spectra show two absorptions due to the hydride V(RuAH)
of characterization data for Si(OR3)
Ru-complexes and its het-
erogenized complexes were included in Supplementary Material.
2.3. Cascade reaction procedure
Catalytic reactions were performed in a reactor (Autoclave Engi-
neers, 100 mL capacity, 1500 RPM). Conversions and selectivities
were measured by NMR and gas chromatographic techniques. All
hydrogenated products were initially identified by using authentic
commercial samples of the expected products. A mixture of the
appropriate nitroaromatic (1 mmol) and aldehyde (1 mmol) was
introduced into the reactor together with a catalytic amount of
Ru-catalyst (0.02 mmol, 2 mol%) and solvent (ethanol or isopropa-
nol, 40 ml). Afterward, the reactor was sealed and air was purged
by flushing three times with 1.5 bar of hydrogen. Then, the reac-
tion mixture was stirred, heated at 40 °C, and the reactor was pres-
surized with H2 at the required pressure. Ones the nitro compound
is hydrogenated to amine, the dihydrogen pressure increased until
5 bar and the temperature at 80 °C. The progress of the reaction
was monitored by GC. When the hydrogenation reaction was fin-
ished, the reactor was depressurized. Finally, the catalysts were fil-
tered and the organic solution was concentrated under vacuum
and analyzed by GC–MS.
at 2043–2048 cmꢁ1 and the V(CO) appears at 1932–1938 cmꢁ1
.
Grafting experiments (Scheme 1) were carried out using a mod-
ular system that combines functionalized ligands with MCM-41, in
order to have systematic access to a variety of immobilized chiral
catalysts. In a typical process, attachment of (NHC)NNRu-com-
plexes containing pendant alkoxy silane groups [16a,b] to MCM-
41 or MCM-41/Al or MCM41/Sn was achieved by heating overnight
150 mg silica with ꢂ16 mg (0.022 mmol) of [RuꢂSi(OR)3] com-
plexes in toluene at 100 °C. The original colored solution became
colorless and the silica acquired a yellow color. Continuous extrac-
tion (Soxhlet) of the resulting material with CH2Cl2 or ethanol was
performed for 24 h in order to remove any non-covalently attached
organic material. After drying in vacuo, the functionalized silicas
Ru were obtained. The mesoporous structure of the supports re-
tains long-range order as indicated by PXRD. The catalyst prepared
on this way presented metal loading of 0.06–0.08 mmol-metal/g
support as it was determined by atomic absorption analysis. Heter-
ogenization process yielded materials with the expected ratios of
metal to nitrogen loadings. The impact of the immobilization of
Ru-complex on the structure of complexes was studied by FTIR
spectroscopy, electronic spectra CP/MAS NMR spectroscopy
2.4. Recycling experiments
At the end of the process, the reaction mixture was centrifuged,
and the solid residue washed to completely remove any remaining
products and/or reactants. The solid was used again without
changes in the catalytic activity.
(
13C, 1H), and elemental analysis. Peaks due to the support domi-
nate the spectra (around 1240, 1060, and 806 cmꢁ1). Some of the
bands characteristic of the complexes could be, however, distin-
guished. A weak band at 1942 cmꢁ1 corresponds to V(CO) stretch-
ing. The 1640–1600 cmꢁ1 frequencies may be assigned to C@N,
C@C. A strong V(OAH) stretch appeared in heterogenized com-
pounds from 3200 to 3700 cmꢁ1 confirming the presence of sur-
face silanol groups and the inability to react all silanols with
bulky Ru-complex. The complexes immobilized on supports have
also been characterized by diffuse reflectance UV spectroscopy.
The diffuse reflectance spectrum of Ru-complex is almost identical
before and after heterogenization process, indicating that the com-
plexes maintain their geometry and their electronic surrounding
even after heterogenization without significant distortion. Grafting
3. Results and discussion
Intermolecular reductive amination of aldehydes with nitroa-
renes was examined with soluble and heterogenized Ru-catalysts
shown in Fig. 1.
Ruthenium complexes were synthesized by transmetalation
from intermediate silver(I) precursor complexes [19] obtained by
metalation of the respective imidazolium salts using Ag2O. The sil-
ver complexes were not isolated. Instead, starting Ru-complex was

112
C. del Pozo et al. / Journal of Catalysis 291 (2012) 110–116
Scheme 1. Immobilization of trialkoxysilyl–ruthenium complexes on MCM-41, MCM-41/Al, or MCM-41/Sn.
of Ru species on the silica surface was supported by cross-polariza-
tion magic-angle spinning (CP MAS) 13C and 29Si NMR spectra (see
Supporting Information). 13C NMR spectrum is dominated by the
resonances of the aliphatic and aromatic groups. Carbon reso-
nances appeared in the aliphatic region d = 8–60 ppm (SiACH2,
CMe3, CH2, NCH2, PyACH2), aromatic region d = 120–140 ppm,
imine region d = 160 ppm (C@N), d = 170 (C@O), and d = 175 ppm
(RuAC). The majority peaks corresponding to the 13C NMR spec-
trum of homogeneous complex were present in the 13C spectrum
of their heterogenized counterpart. The introduction of the organo-
metallic moieties did not induce drastic changes. In fact, the metal
content is small and therefore since the spectra are dominated by
the aromatic signals it will mask any resonances from other species
in lower concentration. In addition to framework, silicon
Q2 [(HO)2Si(OSi)2], Q3 [(HO)Si(OSi)3], and Q4 [Si(OSi)4] resonances
(d = ꢁ92.1, ꢁ101.0, ꢁ111.1 ppm), weak T (T2: R(HO)Si(OSi)2, T3:
RSi(OSi)3) signals corresponding to condensation of two to three
ethoxy groups of Ru-compound (d ꢂ ꢁ52, ꢁ67 ppm) appeared in
the 29Si NMR spectrum.
The final objective of the work was to find a multisite selective
catalyst that allows catalyzing the reductive amination of carbonyl
compounds through a series of consecutive reactions in a cascade
mode with high selectivity and using very mild conditions. The
process involves, firstly, reduction of the correspondent nitro com-
pound to amine. After that, the carbonyl compound condenses
with the amine to give an imine, which is hydrogenated to a final
amine (Scheme 2). The result for the one-pot synthesis of substi-
tuted anilines from nitroarenes catalyzed by (NHC)NNRu-catalysts
under hydrogenation conditions is shown in Table 1. In the first
step, the nitroaromatics would be chemoselectively hydrogenated
with hydrogen in the presence of the carbonyl substrate on the
hydrogenation sites, while in a consecutive step the condensation
between the aromatic amine and the carbonyl group would occur
on the acid sites of the catalyst (support) (Scheme 2). Finally, the
hydrogenation of the resulting imine with the Ru-complexes gives
the secondary amine.
aldehydes and nitroarenes, most of them giving high yield of the
desired product.
3.1. Solvent effects
We observed that N-ethylanilines were formed in a reasonable
yield (22–49%) together with the desired substituted anilines (15–
42%) (Table 1, entries 1, 3, and 5; Fig. 2) when ethanol was used as
solvent. To elude this problem, we made the reaction with isopro-
panol as solvent; in this case resulted in clean conversion of the
nitrobenzene to aniline (Table 1, entries 2, 4, and 6). In all experi-
ments conducted in isopropanol, the reactions proceed a little bit
slower compared with the same reaction when performed in eth-
anol. More specifically, the initial reduction of the nitro groups
was complete in all cases, but the concomitant imine formation
proceeded more slowly in isopropanol. No trace of the correspond-
ing isopropylaniline could be detected (TLC and 1H NMR analyses)
even after prolonged reaction times (5 days). It is also worth noting
that no trans-esterification was observed in these experiments
[20].
An evaluation of the proposed reductive amination reaction was
performed with benzaldehyde and nitrobenzene, under standard
conditions (isopropanol, 1.5–5 bar PH2, and temperature: 40–
80 °C) using different ruthenium catalysts, homogeneous, or
bifunctional organic–inorganic catalysts obtained by heterogeniza-
tion of Ru-complexes on different structured mesoporous silicates
(MCM-41, Si/Al = 1, MCM-41/Al, Si/Al = 15 and MCM-41/Sn, Si/
Sn = 20). Table 2 shows that all ruthenium catalysts are active for
the initial reduction of nitro group; thus, we find that at 1.5 bar
of H2 and 40 °C, the corresponding imine can be obtained with
93% selectivity at 94% conversion (4 h), while the carbonyl group
of aldehyde remains unaffected but the reaction did not signifi-
cantly continue (<3%) toward the secondary amine. However,
when temperature and hydrogen pressure were increased to
80 °C and 5 bar further hydrogenation of the intermediate imine
took place and gave N-benzylaniline through a cascade reaction.
The roles of the support material and linkers have been investi-
gated, and it was found that the support/tether can have a signifi-
cant effect on the catalytic reactions. To evaluate the influence the
An initial evaluation of the reductive amination was performed
with benzaldehyde and nitrobenzene using 2RuMCM-41 as cata-
lyst (Table 1). After that, the reaction will be extended to different
Scheme 2. One-pot synthesis of substituted aromatic amines.

C. del Pozo et al. / Journal of Catalysis 291 (2012) 110–116
113
Table 1
Solvent effect over 2RuMCM-41-catalyzed reaction.a
Entry
1
Aldehyde
Solvent
EtOH
Conv. (%)b
70
Yield (%)
Imine, A
33
Amine,c
15 (57)
B
N-ethylaniline, C
22
2
i-PrOH
98
50
48 (100)
–
3
4
EtOH
91
Traces
40
42 (46)
49
–
i-PrOH
100
60 (100)
5
6
EtOH
79
79
11
7
41 (60)
27
–
i-PrOH
72 (100)
a
b
c
Reaction conditions: nitrobenzene (1.0 mmol), aldehyde (1.5 mmol), [Ru] (2 mol%) in ethanol or isopropanol (40 mL), 1st: 1.5 bar, 40 °C, (4 h); 2nd: 5 bar, 80 °C, (12 h).
Referred to nitrobenzene.
In parenthesis, selectivity to amine B in %.
controlled by modifying the framework and molecular sieve struc-
ture (the condensation between the aromatic amine and the car-
100
90
80
70
60
50
40
30
20
10
0
bonyl group would occur on the acid sites of the support), while
the pore diameter and topology of the catalyst could be adapted
to the size of reactants and products. In this case, the relative large
size of the product led us to select three silicate molecular sieves as
acid catalysts that are structured mesoporous silicates: MCM-41
(Si/Al = 1, 2RuMCM-41), MCM-41 containing Brønsted acid sites
(Si/Al = 15, 2RuMCM-41/Al), and MCM-41 containing Sn-Lewis
acid sites (Si/Sn = 20, 2RuMCM-41/Sn) as potential candidates to
catalyze the condensation reaction. MCM-41 is a short-range
amorphous material that contains a large number of silanol groups
available for grafting; however, it presents a long-range ordering of
hexagonal symmetry with regular, monodirectional channels of
3.5 nm diameter (BET Surface Area (m2 gꢁ1) = 1030, Micropore Sur-
face (t-plot m2 gꢁ1) = 0, External (or mesoporous) Surface Area
(m2 gꢁ1) = 1030).
Yield
Imine, A
amine, B
N-ethyl aniline,C
0
100
200
300
400
500
600
From the catalytic point of view, we have a bifunctional catalyst
that combines the activity of the ruthenium metal complex, with
the promoting effect of the Brønsted or Lewis acid sites of the sup-
port. The results (Table 2, entry 6) clearly show that 2RuMCM-41/
Al do not catalyze the reduction of imine under our reaction
conditions but it is really good bifunctional catalyst for the forma-
tion of imine. We expected that a Lewis acid would improve both
time (min.)
Fig. 2. Reaction between nitrobenzene (1.0 mmol), benzaldehyde (1.5 mmol), [Ru]
(2% mol) in ethanol (40 mL): 1st: 1.5 atm, 40 °C, (4 h); 2nd: 5 bar, 80 °C, (12 h).
support, silicate molecular sieves were selected since with these
materials the number and strength of the acid sites can be

114
C. del Pozo et al. / Journal of Catalysis 291 (2012) 110–116
Table 2
[Ru]-catalyzed one-pot reductive amination of benzaldehyde with nitrobenzene.a
Entry
Cat.
T (h)
Conv. (%)b
Yield (%)
N-benzylideneaniline, A
N-benzylaniline,c
B
1
2
3
4
5
6
7
8
9
1Ru
1RuMCM-41
2Ru
12
12
12
12
24
12
24
12
24
85
93
86
98
99
65
75
97
98
Traces
10
61
50
10
60
65
33
5
Traces
–
25 (31)
48 (49)
89 (90)
– (0)
10
64 (66)
93 (95)
2RuMCM-41
2RuMCM-41/Al
2RuMCM-41/Sn
a
b
c
Conditions: nitrobenzene (1.0 mmol), aldehyde (1.5 mmol), [Ru] (2 mol%), isopropanol (40 mL), 1st: 1.5 bar, 40 °C (3–4 h); 2nd: 5 bar, 80 °C (12 h).
Referred to nitrobenzene.
In parenthesis, selectivity to desired amine in % at 24 h.
processes [21] formation and reduction of resulting imine. To
check this hypothesis, the Sn-silicate support, prepared by direct
synthesis, with Lewis acidity rather than Brønsted acidity was
tested. The results (Table 2, entry 8) show that 2RuMCM-41/Sn
gives better yield (64%) and selectivity (66%) than 2RuMCM-41
(50%). No product was formed when the reaction was run in ab-
sence of the catalyst. Soluble 2Ru-catalyst gives lower yield (entry
3). 1Ru-compounds failed to form any perceptible amount of the
condensation product, N-benzylideneaniline (entries 1 and 2).
The structure as well as the acidity of supports provides a favor-
able environment for the direct transformation of imine to amine
[21]. To explain this behavior, one has to take into account the
adsorption–desorption properties of the material. Indeed, if we
consider that the reactant (aniline), the intermediate (imine), and
the product (amine) have basic character, the presence of stronger
acid sites on the crystalline molecular sieve [22] will produce a
stronger adsorption of the intermediate and final product on
MCM-41/Al, which will strongly compete with the adsorption of
the double bonds on the active sites. That stronger adsorption of
reactant and product will be responsible for a lower reaction rate
and faster catalyst deactivation with MCM-41/Al than with pure
silica and mild acidity MCM-41. These results suggest that the
condensation reaction does not require strong acid sites and even
silanol groups may carry out the reaction, though at a slower rate
than the bridging hydroxyl groups of the alumino-silicate or Lewis
sites of tin-silicate [21b].
Under the optimized reaction conditions, the scope of the reac-
tion was explored with structurally and electronically diverse alde-
hydes with different nitroarenes. As revealed in Table 3, various
aldehydes were reductively aminated with nitrobenzene. The alde-
hydes used for this study included aromatic and aliphatic exam-
ples. Irrespective of the electronic nature of the substituent,
aromatic aldehydes reacted smoothly to give the corresponding
products in good yields (Table 3, entries 3–6). On the other hand,
aliphatic aldehydes underwent the reductive amination rapidly
and gave the products in excellent yield without any byproducts
(Table 3, entries 1 and 2). The benefits of this catalytic process
are more evident when other easily reducible groups are present
Table 3
Reductive amination catalyzed by 2Ru and 2RuMCM-41 in isopropanol.a
Entry
R
R0
Conv.b (%)
2RuMCM-41
Imine A
Conv.b (%)
2Ru
Aminec
B
Imine A
Aminec
B
1
2
3
4
5
6
C6H13
C6H13
OMePh
OMePh
Ph
I
H
I
H
I
H
100
79
71
100
100
98
–
7
5
40
50
50
100 (100)
72 (91)
66 (93)
60 (60)
50 (50)
48 (49)
20
30
64
85
82
73
12
27
14
7
52
25
8 (40)
3 (10)
50 (78)
78 (92)
30 (37)
48 (66)
Ph
a
b
c
Conditions: nitrobenzene (1.0 mmol), aldehyde (1.5 mmol), [Ru] (2 mol%), isopropanol (40 mL), 1st: 1.5 bar, 40 °C (4 h); 2nd: 5 bar, 80 °C (12 h).
Referred to nitrobenzene.
In parenthesis, selectivity to desired amine in %.

C. del Pozo et al. / Journal of Catalysis 291 (2012) 110–116
115
Table 4
a
100
90
80
70
60
50
40
30
20
10
0
Recycling experiments in the reductive amination of aldehydes with nitrobenzene
catalyzed by 2RuMCM-41 over fourth cycles.
N-benzylideneaniline
N-benzylaniline
Entry
Aldehyde
Cycle
(yield %)
Average yield (%)
First
Second
Third
Fourth
1
2
3
C6H13CHO
C6H5CHO
CH3OC6H4CHO
75
62
48
71
57
49
72
65
48
72
63
48
72
60
48
2bRuMCM-41 (fresh)
2bRuMCM-41 (1 cycle)
2bRuMCM-41 (5 cycle)
0
100
200
300
400 950 955 960 965 970 975
Time (min)
100
90
80
70
60
50
40
30
20
10
0
b
N-heptylideneaniline
N-heptylaniline
2
3
4
5
6
7
8
9
10
11
2θ (º)
Fig. 4. PXRD spectra of fresh 2bRuMCM-41 and this material recovered after fifth
runs.
0
100
200
300
700
800
900
1000
The results displayed in Fig. 3 show that the kinetic behavior of
ruthenium during this process allows the selective production of
either the substituted imine by stopping the hydrogenation just
before complete exhaustion of the nitroaromatic compound or,
on the contrary, the substituted secondary amine if the reduction
reaction is allowed to proceed further. Thus, in Fig. 3a we see that
the N-benzylideneaniline is obtained selectively in 3 h at 40 °C and
1.5 bar H2 and when the temperature is increased to 80 °C and
dihydrogen pressure to 5 bar, the imine is reduced selectively to
give N-benzylaniline. Fig. 3b shows the kinetic profile of the reac-
tion between nitrobenzene and heptaldehyde; in this case, we ob-
serve that the rate of reduction of N-heptylideneaniline is higher
than with N-benzylideneaniline and we get better yields.
time (min)
Fig. 3. (a) Conditions: nitrobenzene (1.0 mmol), benzaldehyde (1.5 mmol), [Ru]
(2 mol%), isopropanol (40 mL), 1st: 1.5 bar, 40 °C (3 h); 2nd: 5 bar, 80 °C (12 h). (b)
Reaction between nitrobenzene (1.0 mmol), heptaldehyde (1.5 mmol), [Ru]
(2 mol%), isopropanol (40 mL), 1.5 bar, 40 °C (3 h) to 5 atm, 80 °C (12 h).
in the nitroaromatic compound or in the aldehyde. For instance,
high chemoselectivity is observed when halide functional groups
are included within the reactants (Table 3, entry 1), when an iodide
group is bonded to the aromatic ring of nitrobenzene (entries 1, 3,
and 5) or to the aromatic ring of benzaldehyde (methoxybenzalde-
hyde, entry 3). Amines are obtained with high selectivity and at
conversion levels above 90% in some cases, whereas the hydroge-
nation of the carbonyl groups is avoided.
3.2. Recycling of heterogenized catalysts
It should be considered that the selectivity of formation of the
imine may change depending on the electron-withdrawing or elec-
tron-donating character of the groups attached to the aromatic
rings. Indeed, an increase in the rate of hydrolysis of the resultant
imine can be expected when electron-withdrawing groups are
present in the nitroaromatic compound used as the initial reactant.
In order to quantify this potential drawback, we treated 4-iodoni-
trobenzene with benzaldehyde. However, 100% selectivity at 99%
conversion of the intermediate aniline (total disappearance of the
nitroaromatic starting material) was still achieved in this case,
which indicates that the corresponding imine is not hydrolyzed
by water and no dehalogenation of the iodide group is observed.
It has been found that, if the reaction is continued after complete
exhaustion of the nitroaromatic compound, the imine group can
also be chemoselectively hydrogenated by the Ru-catalyst; this
leads to the preferential formation of N-(R-benzyl)aniline with
90% selectivity.
The heterogeneity of the catalyst is also evaluated to study
whether the reaction using solid RuMCM-41 catalysts occurred
on the solid surface or was catalyzed by Ru species in the liquid
phase. To address this issue, we conducted two separate experi-
ments with benzaldehyde and nitrobenzene. In the first experi-
ment, the reaction was terminated after 1 h, and the conversion
of nitrobenzene was found to be 30%. At this juncture, the catalyst
was separated from the reaction mixture and the reaction was con-
tinued with the filtrate for an additional 5 h. In the second exper-
iment, the reaction was terminated after 4 h at 55% conversion,
and the catalyst was removed. The reaction was continued with
the filtrate for an additional 5 h. In both the cases, the conversion
remained almost unchanged. Ru was not detected in the filtrate
in either experiment by ICP-AES. These studies demonstrate that
only the Ru bound to support during the reaction is active and
the reaction proceeds on the heterogeneous surface.

116
C. del Pozo et al. / Journal of Catalysis 291 (2012) 110–116
(2007) 1226;
As can be seen from Table 4, for both aliphatic and aromatic
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6988;
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1781;
aldehydes, 2RuMCM-41 is recyclable and can be recovered by sim-
ple filtration in air and reused without significant loss of catalytic
activity (Table 4).
Soluble complexes could be used only once, because they dete-
riorate completely by the end of the first catalytic run, supported-
complexes could be recovered for recycling, and reused retaining
most of their catalytic activity. The activity of the recovered cata-
lyst do not decrease after 4 cycles at least. Analysis of recycled cat-
alyst from the reaction showed that the overall structure was
largely maintained (Table 4, Fig. 4).
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4. Conclusions
In summary, we have synthesized and immobilized on MCM-41
Ru(II) hydride complexes based on pincer-type pyridine-function-
alized N-heterocyclic carbene ligands and we have developed a
simple and efficient method for the synthesis of N-alkyl amines
via reductive amination using a new readily recyclable hybrid cat-
alyst that combines the catalytic properties of transition-metal
complexes with the architecture of mesoporous solids. The reac-
tion is carried out in the presence of hydrogen by means of a
bifunctional catalyst that chemoselectively performs the hydroge-
nation of the nitro group to the amino group, which subsequently
condensates with the aldehyde on the acid sites of the composite
catalyst and finally the reduction of imine leads to the correspond-
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Sajiki, T. Ikawa, K. Hirota, Org. Lett. 6 (2004) 4977;
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source, see: R. Nacario, S. Kotakonda, D.M.D. Fouchard, L.M.V. Tillekeratne, R.A.
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Acknowledgment
(j) for
a related methodology where nitro aryls were converted to the
corresponding carbamates (Boc and CO2Et) using Sn/NH4Cl and Boc2O or
ClCO2Et, see: S. Chandrasekhar, Ch. Narsihmulu, V. Jagadeshwar, Synlett (2002)
771.
We thank the Dirección General de Investigación Científica y
Técnica of Spain (Projects MAT2011-29020-C02-02, Consolider-
Ingenio 2010-(CSD-0050-MULTICAT) for financial support.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.04.015.
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