Chemoselective hydrogenation of nitriles to secondary or tertiary amines catalyzed by aqueous-phase catalysts supported on hexagonal mesoporous silica

Article abstract of DOI:10.1002/aoc.4547

The first supported aqueous-phase catalyst for the hydrogenation of nitriles is revealed. The catalyst prepared from Pd(PhCN)₂Cl₂, water-soluble ligand 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt and mesoporous silica is a highly efficient catalyst for the selective formation of secondary or tertiary amines from aromatic or aliphatic nitriles. The catalytic system is stable and can be recycled and reused three times without loss of activity and selectivity. This environmentally friendly process is, in addition, an attractive alternative to many homogeneous and heterogeneous catalysts because of its easy preparation and the moderate operational conditions under which it is highly active.

Full text of DOI:10.1002/aoc.4547

Received: 28 April 2018
DOI: 10.1002/aoc.4547
Revised: 4 July 2018
Accepted: 13 July 2018
C O M M U N I C A T I O N
Chemoselective hydrogenation of nitriles to secondary or
tertiary amines catalyzed by aqueous‐phase catalysts
supported on hexagonal mesoporous silica
1
2
Abdelaziz Nait Ajjou
| André Robichaud
1
Department of Chemistry and
The first supported aqueous‐phase catalyst for the hydrogenation of nitriles is
revealed. The catalyst prepared from Pd(PhCN) Cl , water‐soluble ligand
Biochemistry, Université de Moncton,
Moncton, New‐Brunswick, Canada, E1A
2
2
3E9
2,2′‐biquinoline‐4,4′‐dicarboxylic acid dipotassium salt and mesoporous silica
2
Health Canada, 1001 Saint‐Laurent
is a highly efficient catalyst for the selective formation of secondary or tertiary
amines from aromatic or aliphatic nitriles. The catalytic system is stable and
can be recycled and reused three times without loss of activity and selectivity.
This environmentally friendly process is, in addition, an attractive alternative
to many homogeneous and heterogeneous catalysts because of its easy prepara-
tion and the moderate operational conditions under which it is highly active.
Ouest Boulevard, Longueuil, Québec,
Canada, J4K 1C7
Correspondence
Abdelaziz Nait Ajjou, Department of
Chemistry and Biochemistry, Université
de Moncton, Moncton, New‐Brunswick,
Canada E1A 3E9.
Email: naitaja@umoncton.ca
KEYWORDS
hydrogenation, mesoporous silica, nitriles, secondary and tertiary amines, supported aqueous‐phase
catalysts
1
| INTRODUCTION
avenues for the production of primary amines with very
[
2,11,12]
good yields.
However, in most cases, the conditions
Despite the recent progress in the synthesis of amines
from nitriles, the selective synthesis of primary, second-
ary or tertiary amines under simple and benign condi-
(high temperature and high hydrogen pressure) and the
solvents used (costly and highly toxic organic solvents
including carcinogenic and mutagenic solvents such as
benzene, tetrahydrofuran and 1,4‐dioxane) are harmful
to the environment and health. In addition, dry condi-
tions are often necessary, and the separation of the cata-
lysts from the reaction products is very difficult and
their quantitative recovery in active form has not been
performed. Most recently, nitriles containing aromatic
and heterocyclic rings were efficiently converted to the
corresponding secondary amines, in good yields, using
Ru(II) catalyst bearing pyridyl‐functionalized and
[1,2]
tions remains challenging.
While stoichiometric
amounts of reducing agents allow the formation of
[3–6]
amines,
their toxicity and the generation of wastes
that are toxic as well limit their application on a large
scale. Catalytic hydrogenation of nitriles catalyzed by
transition metals is an important and preferred alterna-
tive for the formation of amines with more respect for
[1,2]
the environment.
In general, reduction of nitriles
leads to a mixture of primary, secondary and tertiary
amines, with the presence of imines as intermedi-
[13]
alkenyl‐tethered N‐heterocyclic carbene ligand.
Het-
[
1,2,7]
ates.
As regards catalysis, the vast majority of reports
erogeneous catalysis has been extensively used for the
hydrogenation of nitriles to amines. Although some pro-
cesses may be limited by the difficult fabrication of the
catalysts, the literature abounds with excellent heteroge-
neous catalysts for the selective hydrogenation of nitriles
to primary, secondary or tertiary amines, and many
[1,2]
are dedicated to the formation of primary amines,
different trapping agents such as acids,
butyl dicarbonate
and
CO , di‐tert‐
and acetic anhydride have been
[1,8]
[9]
2
[9,10]
[9]
used to improve their yields. Recently, the use of pincer
complexes in homogeneous catalysis has opened new
Appl Organometal Chem. 2018;e4547.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
1 of 8
https://doi.org/10.1002/aoc.4547

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NAIT AJJOU AND ROBICHAUD
[
1,2]
examples have been reported in the latest reviews.
and is well aligned with the concept of green chemis-
[
28–30]
These achievements were reached by the judicious con-
trol of various parameters, depending on the structure
of the nitriles, including the nature of the catalyst and
its support, and the reaction conditions employed
such as reaction temperature, solvent used and hydrogen
pressure. Thus, primary amines have been selectively
prepared in good to excellent yields, under mild condi-
tions, with various supported catalysts, namely Co/
try.
The use of water as solvent is important for
economic, safety and environmental reasons. A water‐sol-
uble catalyst, which operates and resides in water, is
easily separated from reaction products by simple decan-
tation. In addition, the products are generally not
contaminated with traces of metal catalyst, and the use
of toxic organic solvents such as benzene is avoided.
Aqueous‐phase catalysis is limited in some cases by low
reaction rate caused by the insolubility of organic com-
pounds in water. Another effective environmentally
friendly approach to minimize the disadvantages of
homogeneous catalysis is to immobilize water‐soluble
[1]
[
14]
[15]
SiO2,
Co(OAc) /1,10‐phenanthroline/α‐Al O ,
Ru
vitamin
and Pd/MCM‐
Heterogeneous processes concerning highly selec-
2
2 3
[17]
[16]
nanoparticles/K‐doped Al O ,
Cu/MgO,
Pd/Al O3
2
2
3
[18]
[19,20]
B ‐derived Co/CeO ,
1
2
2
[
21]
4
1.
[
31,32]
tive nitrile hydrogenation to secondary or tertiary amines
have been also abundantly investigated. For instance,
Greenfield reported the hydrogenation of butyronitrile
in water or methanol using various supported catalysts,
with or without alkaline additives. While nickel and
cobalt catalyzed selectively the formation of butylamine,
rhodium and platinum were respectively the best
catalysts for the formation of secondary and tertiary
catalysts on hydrophilic supports.
Supported aque-
ous‐phase catalysts (SAPCs), initially developed by Davis
[
31]
and co‐workers, have been used successfully in various
catalytic reactions, particularly hydroformylation of
[
31–33]
alkenes.
In contrast to aqueous‐phase catalysis, the
concept of SAPC reaction allows an increase in the reac-
tion rates because of the wide interface between the
organic phase containing the reactants and the thin
aqueous phase layer enclosing the catalyst. Despite the
ecological and economic advantages of aqueous‐phase
catalysis, reports concerning homogeneous hydrogena-
[22]
amines.
Barnett reported the hydrogenation of
aliphatic nitriles, including butyronitrile, catalyzed by
supported transition metal borides. In this case also,
nickel boride and cobalt boride were the best catalysts
for the formation of primary amines, and rhodium boride
and platinum boride for the synthesis of secondary and
[
34,35]
tion of nitriles in water are limited.
The water‐solu-
ble RuCl /P(m‐C H SO Na) complex in organic/aqueous
3
6
4
3
3
biphasic system converts nitriles selectively to the corre-
sponding alcohols with good yields and no amines were
[23]
tertiary amines, respectively.
NaY zeolite‐supported
[
34]
transition metal catalysts have been also studied for the
hydrogenation of aliphatic nitriles in both liquid and
gas phases. At low temperature, selective formation of
amines is controlled by the nature of metal on NaY: Ru
for primary amines, Pd for secondary amines and Pt for
formed.
Another report shows that the hydrogenation
of benzonitrile catalyzed by the water‐soluble (Sulphos)
Ru(NCMe) (OSO CF ) in aqueous/n‐octane biphasic sys-
3
2
3
tem leads to amine derivatives with very low yields (0.5%
for dibenzylamine and 8% for benzylidenebenzylamine),
and to benzaldehyde (11.6%) and benzyl alcohol (5.3%)
[24]
tertiary amines.
Wang and co‐workers reported that
[
35]
palladium supported on polymeric mesoporous graphitic
carbon nitride is an outstanding catalyst for the selective
hydrogenation of nitriles, under ambient hydrogen pres-
as side products.
Thus the authors stated that the
aqueous–biphasic method is inappropriate for the hydro-
genation of nitriles. In the past few years, we have
[25]
[36]
sure.
Aromatic nitriles and cyclohexanecarbonitrile
developed various catalytic transformations in water,
were converted to secondary amines, while aliphatic
nitriles led to tertiary amines. Other impressive results
were, most recently, obtained in cyclohexane as solvent
including hydration of nitriles to the corresponding
amides, transfer hydrogenation of aldehydes and ketones
with isopropanol, Oppenauer‐type oxidation of alcohols,
oxidation of secondary alcohols, alkynes and alkylarenes
with t‐butyl hydroperoxide and reductive amination of
aldehydes. Most recently, we reported new, simple and
convenient procedures for homogeneous hydrogenation
of nitriles to the corresponding primary amines in aque-
[
26]
and at 25–60°C under ambient hydrogen pressure.
Pd/C catalyzed the selective hydrogenation of aliphatic
nitriles to the corresponding tertiary amines, whereas
Rh/C led to the selective formation of secondary amines.
Sterically hindered cyclohexanecarbonitrile gave selec-
tively secondary amines in the presence of either Pd/C
[
37]
ous ammonia using water‐soluble catalysts.
To the
[26]
or Rh/C.
Finally Pt nanowires have been used as a
best of our knowledge, there are no reports concerning
hydrogenation of nitriles based on SAPCs, and in this
paper we report an unprecedented chemoselective hydro-
genation of nitriles to the corresponding secondary or
tertiary amines catalyzed by water‐soluble transition
metal complexes based on TPPTS (P(m‐C H SO Na) ) or
catalyst for the selective hydrogenation of nitriles to
symmetric secondary amines, under a hydrogen pressure
[27]
of 1–4 bar.
Aqueous‐phase catalysis is an elegant alternative to
the classical homogeneous and heterogeneous catalysis,
6
4
3
3

NAIT AJJOU AND ROBICHAUD
3 of 8
BQC (2,2′‐biquinoline‐4,4′‐dicarboxylic acid dipotassium
salt) supported on hexagonal mesoporous silica (HMS).
cyclohexanemethylamine 6, were detected. Despite low
selectivity for either primary, secondary amine or second-
ary imine, this promising result is still important as it
shows that Pd(PhCN) Cl /BQC, under our conditions, is
2
2
[
34]
2
| RESULTS AND DISCUSSION
much better than RuCl
3
/TPPTS
and (Sulphos)
[
35]
Ru(NCMe) (OSO CF ) for the formation of the desired
3
2
3
In our preliminary experiments, we investigated the
hydrogenation of benzonitrile chosen as a model reaction
that may lead to at least six different products (Scheme 1).
amines. It is well known that for the hydrogenation of
nitriles, chemoselectivity depends on the nature of the
[
1,2]
catalyst and its support.
Also, reaction rates depend
In pure water the hydrogenation (200 psi of H ) cata-
strongly on the contact between catalyst and reactants.
Thus, SAPC based on commercial silica, with a surface
2
lyzed by the water‐soluble system Pd(PhCN) Cl /BQC
2
2
2
−1
−1
offered benzylamine 1 (25%), dibenzylamine 2 (22%),
N‐benzylidenebenzylamine 3 (25%) and a small amount
of benzyl alcohol 4 (3%) (Table 1, entry 1), and the pres-
ence of benzamide 5 in water was not detected. Also no
traces of aromatic ring reduction products, mainly
area of 500 m g , a pore volume of 0.75 ml g and a
pore diameter of 60 Å, was prepared and tested as a cata-
lyst under the same reaction conditions (Table 1, entry 2).
The catalyst was prepared by the incipient wetness tech-
[
33]
nique proposed by Hanson and co‐workers
using a
SCHEME 1 Potential products from the hydrogenation of benzonitrile in the presence of water.
a
TABLE 1 Hydrogenation of benzonitrile catalyzed by ML
n
/water‐soluble ligand/HMS systems
b
Run
Catalyst precursor
Ligand
Conversion (%)
1 (%)
2 (%)
3 (%)
4 (%)
c
1
Pd(PhCN)
Pd(PhCN)
Pd(PhCN)
Pd(PhCN)
Pd(PhCN)
Pd(PhCN)
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
BQC
75
25
22
45
97
97
94
25
56
68
95
87
94
34
25
0
3
0
0
0
0
0
13
2
5
3
1
1
d
2
BQC
78
33
3
BQC
97
0
0
e
4
BQC
97
Trace
0
f
5
BQC
98
4
0
0
0
0
0
0
3
0
6
7
8
9
TPPTS
BQC
25
0
[Rh(COD)Cl]
2
2
100
100
100
90
31
30
0
[Rh(COD)Cl]
TPPTS
BQC
[Ir(COD)Cl]
2
2
1
1
1
0
1
2
[Ir(COD)Cl]
TPPTS
BQC
0
g
Pt(COD)Cl
2
2
100
43
0
h
Pt(COD)Cl
TPPTS
3
a
Reaction conditions: transition metal (0.0125 mmol), ligand (0.15 mmol), H
2
2
O (500 mg), HMS (1 g), hexane (10 ml), benzonitrile (2.5 mmol), H (200 psi),
100°C, 24 h.
b
1
Identifications and quantifications (conversions and yields) were determined by H NMR and GC‐MS analyses.
c
Reaction performed in water without the presence of any support.
d
e
2
−1
−1
Commercial silica, with a surface area of 500 m
g , a pore volume of 0.75 ml g and a pore diameter of 60 Å, was used as the support.
Second cycle of entry 3.
Third cycle of entry 3.
f
g
Aromatic ring hydrogenation products were formed in 5% yield.
h
Aromatic ring hydrogenation products were formed in 2% yield.

4
of 8
NAIT AJJOU AND ROBICHAUD
mixture of water and hexane, and no vacuum or drying
was applied. Benzylamine (33%) and dibenzylamine
demonstrated also that the pore diameter has almost no
influence on SAPC activity. For SAPCs, the amount of
water used to prepare the catalysts is very important for
(45%) were obtained as the only products to the detriment
[
44–46]
of the intermediate N‐benzylidenebenzylamine. In het-
erogeneous catalysis, the support has strong influence
on the catalyst activity and selectivity. Highly ordered
mesoporous materials such as MCM‐41, MCM‐48, MSU,
the catalytic activity.
Thus we investigated the influ-
ence of the HMS‐to‐water ratio on the hydrogenation of
benzonitrile. The study showed that the best results were
obtained with 0.75 g of water per gram of HSM. However,
for a reaction time longer than 10 h the results for 0.5 to
0.75 g of water per gram of HMS are similar. Under the
same hydrogenation conditions, various mesoporous
SAPCs were prepared from HMS, and Pd(PhCN) Cl ,
[38–41]
FSM‐16, SBA‐15 and HMS
have been previously
used as supports and showed excellent perfor-
[42,43]
mances.
HMS is particularly interesting because of
its easy preparation. Consequently, we fabricated HMS
2
2
by the process developed by Pinnavaia and co‐
[Rh(COD)Cl] , [Ir(COD)Cl] or Pt(COD)Cl , and water‐
2
2
2
[39,40]
workers,
using dodecylamine as a neutral template.
soluble ligands TPPTS or BQC. Dibenzylamine was selec-
tively obtained as the major product in all cases with no
formation of benzylamine, except with Pt(COD)Cl2/
TPPTS at very low yield (3%) (Table 1, entry 12). The
bidentate nitrogen ligand BQC generally gave rise to
more active catalytic systems than the phosphorus‐based
TPPTS, and in terms of performance Pd(PhCN) Cl was
The support thus prepared had a surface area of
2
−1
−1
1
150 m g , a pore volume of 1.4 ml g and a pore
[39,40]
diameter of 24 Å.
Subsequently, a new SAPC from
Pd(PhCN) Cl /BQC and HMS was synthesized, then
2
2
tested for the hydrogenation of benzonitrile (Table 1,
entry 3). The hydrogenation proceeded smoothly, with
very high conversion, affording dibenzylamine in excel-
lent yield (97%). This excellent result shows the superior-
ity of HMS over the commercial silica used. The large
surface area of HMS allows the aqueous solution contain-
ing the catalyst to be completely spread out through the
support and to generate a thin aqueous film with high
surface area. In this thin film the catalyst precursor
would have enough space to produce the active catalytic
species with sufficient mobility and contact with the
nitrile. The positive influence of the surface area on the
performance of the hydroformylation reactions catalyzed
2
2
better than [Ir(COD)Cl] and Pt(COD)Cl that led also
2
2
to the formation of other minor products (Table 1, entries
3, 9 and 11). Also, it is important to state that benzamide
was not detected in all cases despite the presence of a
small amount of water.
To gain insights into the reaction pathway, hydroge-
nation of benzonitrile was monitored under milder condi-
tions in order to study the effect of reaction time,
temperature and hydrogen pressure. As evident from
Table 2, benzylamine was formed in very small amounts
for reaction time between 2 and 10 h, with the maximum
reached at around 4 h (Table 2, entry 4). While
[44,45]
by SAPC has already been described.
The studies
a
TABLE 2 Hydrogenation of benzonitrile catalyzed by (PhCN)
2
PdCl
2
/BQC/HMS system
b
Run
Reaction time (h)
Conversion (%)
1 (%)
2 (%)
3 (%)
4 (%)
1
2
3
4
5
6
1
12
23
27
40
45
95
92
83
97
0
12
22
26
38
44
94
80
75
97
0
0
0
0
0
0
0
0
0
0
2
1
0
3
1
0
4
2
0
5
1
0
10
10
10
24
1
0
c
7
12
8
Trace
d
8
0
0
9
0
a
Reaction conditions: (PhCN)
2
PdCl
2
(0.0125 mmol), BQC (0.15 mmol), H
2
O (500 mg), HMS (1 g), hexane (10 ml), benzonitrile (2.5 mmol), H
2
(200 psi), 100°C.
b
c
1
Identifications and quantifications (conversions and yields) were determined by H NMR and GC‐MS analyses.
Reaction performed at 55°C.
d
2
Reaction performed under 100 psi of H .

NAIT AJJOU AND ROBICHAUD
5 of 8
benzylamine is obtained in a yield of only 1% at 100°C
after 10 h, its yield increases to 12% at 55°C (Table 2,
entries 6 and 7). This result is in agreement with reports
indicating that a rise in temperature favors the formation
10 h. At the end of the reaction, the analysis showed only
the presence of benzonitrile, and no traces of amines
were detected.
To evaluate the synthetic utility of the
Pd(PhCN) Cl /BQC/HMS system under the optimized
[1]
of secondary amines. A decrease in hydrogen pressure
allows also an increase in benzylamine yield (Table 2,
entry 8). This behavior has been already reported for
the hydrogenation of valeronitrile catalyzed by Raney
2
2
conditions, various nitriles were subjected to hydrogena-
tion with 200 to 400 psi of H , and the best results are
2
summarized in Table 3. The catalyst gave excellent yields
and selectivities towards symmetric secondary amines
for benzonitrile derivatives bearing electron‐withdrawing
or electron‐donating groups. The catalyst is sensitive to
steric hindrance of the methyl group at the ortho posi-
tion in the case of 2‐methylbenzonitrile since the conver-
sion is only 86% and the yield of secondary amine is 78%
(Table 3, entry 3). The reaction was extended to
heteroaromatic nitriles which gave also secondary
amines as the major products. While an impressive yield
(>99%) was reached for 3‐cyanopyridine, a yield of only
62% was obtained in the case of 2‐cyanopyridine after
48 h (Table 3, entries 5 and 6). This latter result is still
interesting since 2‐cyanopyridine is a difficult substrate,
ignored in the vast majority of nitrile hydrogenation
reports, and most recently, it has been shown that the
hydrogenation of 2‐cyanopyridine catalyzed by
[Ru (CO) ] derivatives led to the formation of 2,2′,2″‐
[1]
nickel. It is important to mention that the reaction
performed at temperatures lower than 55°C and hydro-
gen pressure lower than 100 psi gave low conversion of
benzonitrile with the formation of dibenzylamine as
the sole product (benzylamine is probably formed in
undetectable traces). Also, no other minor product,
such as benzaldimine or N‐benzylidenebenzylamine,
was detected.
An important aspect of SAPCs is the viability of the
catalysts. Interestingly the recycling of the catalyst is
highly effective since almost the same performance was
obtained for three cycles, and no leaching of palladium
from the support to the amines was detected (Table 1,
entries 3–5). The leaching of the catalyst from the support
was measured in three ways. First, the analysis of the
organic phases using atomic absorption indicated no
traces of palladium. Second, the hydrogenation of
benzonitrile was stopped after 3 h; then the SAPC was
removed by filtration. The organic phase thus obtained
did not show any further activity while hydrogenated
for 10 h, since the conversion of benzonitrile was not
improved. Finally, we also performed the reaction with-
out benzonitrile, under the same conditions. After 24 h,
the SAPC was removed, benzonitrile was added to the
organic phase and the reaction was conducted another
3
12
(1H‐imidazole‐2,4,5‐triyl)tripyridine, and no amine or
[
47]
imine was formed.
The synthetic efficiency of the
Pd(PhCN) Cl /BQC/HMS system was also demonstrated
2
2
in the case of aliphatic nitriles (Table 3, entries 6–8).
Despite the sluggishness of the reaction in the case of
phenylacetonitrile, the expected tertiary amine was
obtained with a conversion of 63% and 100% selectivity.
Impressively, hexanenitrile and butyronitrile were
a
2 2
TABLE 3 Hydrogenation of various nitriles catalyzed by Pd(PhCN) Cl /BQC/HMS system
b
Run
Nitrile
H
2
pressure (psi)
Time (h)
Conversion (%)
Amine yield (%): 1°/2°/3°
1
2
Benzonitrile
200
200
400
400
200
200
400
200
200
24
24
24
24
24
48
24
24
5
100
100
86
0/97/0
4‐Methoxybenzonitrile
2‐Methylbenzonitrile
2‐Chlorobenzonitrile
3‐Cyanopyridine
2‐Cyanopyridine
Phenylacetontrile
Hexanenitrile
0/92/0
c
3
8/78/0
c
4
100
100
100
63
5/95/0
5
0/>99/0
Trace/62/0
0/0/63
d
6
e
6
7
8
100
100
0/Trace/>99
0/0/100
Butyronitrile
a
Reaction conditions: Pd(PhCN)
2
Cl
2
(0.0125 mmol), BQC (0.15 mmol), H
2 2
O (750 mg), HMS (1 g), hexane (10 ml), nitrile (2.5 mmol), H (200 to 400 psi), 100°C.
b
1
Identifications and quantifications (conversions and yields) were determined by H NMR and GC‐MS analyses.
c
0
.02 mmol of Pd(PhCN)
2
Cl
Other products were formed including the corresponding alcohol in 8% yield.
00 mg of water was used instead of 750 mg.
2
was used.
d
e
5

6
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NAIT AJJOU AND ROBICHAUD
SCHEME 2 Plausible mechanism for
the formation of secondary amines.
converted solely to the corresponding tertiary amines
with a yield of >99%.
observe the formation of any Schiff base with the sup-
ported aqueous‐phase (BQC)PdCl /HMS system, the
2
The hydrogenation mechanism for the formation of
secondary and tertiary amines has been widely studied
formation of the intermediate 1‐amino‐dialkylamine C
is possible as assumed in the case of the hydrogenation
[1,7,22,24,48–50]
by several authors.
This mechanism
of benzonitrile catalyzed by palladium supported on
[7]
[49]
originally proposed by von Braun et al. assumes the
formation of aldimines, primary amines, alkylidene‐
alkylamines (Schiff bases), enamines, 1‐amino‐
dialkylamines and 1‐amino‐trialkylamines as reaction
intermediates through a set of reactions composed of
hydrogenation, hydrogenolysis and condensation. While
there is more evidence (direct and indirect) for the forma-
tion of primary amines and unsaturated intermediates,
the formation of 1‐amino‐dialkylamines and 1‐amino‐
trialkylamines has not been confirmed. In our case, the
primary amines were observed in the case of benzonitrile
γ‐alumina.
The formation of Schiff base E cannot be
completely excluded if its reaction with hydrogen is faster
than its formation. (BQC)PdCl has been proven to form
2
N‐benzylidenebenzylamine in water (Table 1, entry 1),
and was revealed as an excellent catalyst for the direct
reductive amination of aldehydes with primary and
secondary amines that may procced through Schiff bases
[
51]
as intermediates.
Finally, a similar mechanism is pos-
sible for the formation of tertiary amines by the addition
of secondary amines to the aldimines, followed by
hydrogenolysis of the resulting 1‐amino‐trialkylamines.
The reaction may also proceed through other intermedi-
ates such as enamines.
(
(
Table 2), 2‐methylbenzonitrile and 2‐chlorobenzonitrile
Table 3, entries 3 and 4). The other intermediates have
not been detected. Based on these findings, a plausible
mechanism is suggested in our case (Scheme 2). Initially
the organometallic precursor (PhCN) PdCl reacts with
3 | CONCLUSIONS
2
2
the hydrophilic ligand BQC to form the water‐soluble
compound (BQC)PdCl . This complex has already been
Pd(PhCN) Cl /BQC/HMS represents the first SAPC for
2
2
2
exploited by our research group for the reductive
the hydrogenation of nitriles. The catalyst is very efficient
for the hydrogenation of aromatic and heteroaromatic
nitriles to secondary amines, while it converts selectively
aliphatic nitriles to tertiary amines. The catalyst can be
recycled without loss of its catalytic activity and selectiv-
ity. The results obtained by our method, under moderate
conditions, are very interesting in comparison with those
obtained with different heterogeneous or homogeneous
processes, in terms of yields and chemoselectivity towards
secondary or tertiary amines. Finally, in addition to the
environmental and safety benefits of our catalytic system,
the ease of its preparation makes it an attractive alterna-
tive to many homogeneous and heterogeneous catalysts
whish are difficult to prepare and which operate under
harsh and environmentally unfriendly conditions.
[51]
amination of aldehydes with amines,
and similar
water‐soluble N,N′‐bidentate palladium(II) complexes
[
52]
have been prepared and used for hydrogenation
and
[53]
oxidation reactions. (BQC)PdCl reacts with molecular
hydrogen to form catalytically active palladium hydride
2
[54,55]
complex (BQC)PdHCl.
This later initiates the cata-
lytic cycle for the hydrogenation of a nitrile compound
to primary amine B, based on four consecutive reactions,
with the formation of aldimine A as the organic interme-
diate (equations (3a) to (4b)). Condensation of aldimine
and primary amine may lead to 1‐amino‐dialkylamine C
that gives secondary amine D after hydrogenolysis (equa-
tions (5) and (6)), or to Schiff base that may be hydroge-
nated to D (equations (7) and (8)). Since we did not

NAIT AJJOU AND ROBICHAUD
7 of 8
4
| EXPERIMENTAL
100°C for 24 h. The autoclave was cooled to room temper-
ature, the remaining hydrogen was vented, the mixture
was filtered and the SAPC was washed with ethyl acetate.
The organic filtrate was concentrated, and then the
All substrates, BQC and transition metal precursors were
purchased from Sigma‐Aldrich and used without further
purification. High‐purity grade silica with particle size
of 63–200 μm was also acquired from the same source.
It was characterized by a surface area of 500 m g , a
pore volume of 0.75 ml g and a pore diameter of
1
13
resulting products were analyzed using H NMR,
C
NMR and GC‐MS.
2
−1
For the recycling, 1 g of the collected samples of SAPC
obtained after filtrations was re‐used with a fresh charge
of a solution of benzonitrile as substrate (2.5 mmol) in
degassed hexane (10 ml), and no additional BQC or water
was added.
−
1
6
0 Å. TPPTS was prepared according to a literature
[56]
method.
Routine NMR measurements were performed
with a Bruker AC‐200 spectrometer at 200 and 50 MHz,
respectively, for H and C, using tetramethylsilane as
1
13
internal standard and CDCl in most cases as solvent.
Complementary measurements were also performed
3
ACKNOWLEDGMENTS
using an Agilent GC‐MS chromatograph.
We are grateful to NSERC of Canada and to FESR of
Université de Moncton for financial support of this
research. We are grateful to Professor Ralf Brüning
(Mount Allison University, New‐Brunswick) for X‐ray
diffraction analysis of HMS.
4.1 | Fabrication of HMS
HMS was prepared according to the process developed by
[39,40]
Pinnavaia and co‐workers,
using dodecylamine as a
neutral template.
ORCID
Solid n‐dodecylamine (2.5 g, 13.5 mmol) was added to
a mixture of deionized water (27 ml) and ethanol (27 ml).
Under vigorous stirring, the mixture was gently warmed
for a few minutes to melt and dissolve n‐dodecylamine
and to form a homogeneous solution. After cooling to
room temperature, tetraethyl orthosilicate (Si(OEt)4;
Abdelaziz Nait Ajjou
324
http://orcid.org/0000-0003-0244-
6
REFERENCES
[1] C. De Bellefon, P. Fouilloux, Catal. Rev. Sci. Eng. 1994, 36, 459.
[2] D. B. Bagal, B. M. Bhanage, Adv. Synth. Catal. 2015, 357, 883.
[3] P. L. Beaulieu, P. W. Schiller, Tetrahedron Lett. 1988, 29, 2019.
1
0.4 g, 50 mmol) was added, and after a few minutes,
the stirring was stopped and the mixture was allowed to
age for 18 h at ambient temperature. The white solid thus
obtained was filtered, washed with water and methanol,
and then calcined in air at 630°C for 4 h to remove the
structurally incorporated template affording ca 3 g of sil-
ica (HMS). X‐ray diffraction measurements showed a
characteristic diffraction peak at 2θ = 2.4°.
[4] D. Haddenham, L. Pasumansky, J. DeSoto, S. Eagon, B.
Singaram, J. Org. Chem. 2009, 74, 1964.
[5] J. Z. Saavedra, A. Resendez, A. Rovira, S. Eagon, D.
Haddenham, B. Singaram, J. Org. Chem. 2012, 77, 221.
[6] B. Klenke, I. H. Gilbert, J. Org. Chem. 2001, 66, 2480.
[7] J. von Braun, G. Blessing, F. Zobel, Ber. Dtsch. Chem. Ges. 1923,
5
6, 1988.
4
.2 | Typical Procedure for Hydrogenation
[
8] E. Miller, J. M. Sprague, L. W. Kissinger, L. F. McBurney,
J. Am. Chem. Soc. 1940, 62, 2099.
of Nitriles
In a glass liner of a 45 ml autoclave and under an atmo-
sphere of nitrogen were introduced hexane (10 ml),
distilled water (0.75 g) and BQC (0.15 mmol). After disso-
lution of the ligand, Pd(PhCN) Cl (0.0125 mmol) was
added at room temperature, and the mixture is stirred
until the catalyst was completely dissolved. HMS (1 g)
was then added, the mixture stirred and the suspended
silica became yellow indicating the incorporation of the
catalyst. The nitrile (2.5 mmol) was introduced and the
glass liner was placed in the autoclave. This latter was
flushed three times with 40 psi of nitrogen then purged
with hydrogen. After the autoclave was pressurized to
[
9] X. Xie, C. L. Liotta, C. A. Eckert, Ind. Eng. Chem. Res. 2004, 43,
7907.
[
[
[
10] S. Caddick, A. K. D. K. Haynes, D. B. Judd, M. R. V. Williams,
2
2
Tetrahedron Lett. 2000, 41, 3513.
11] S. Werkmeister, K. Junge, M. Beller, Org. Process Res. Dev. 2014,
1
8, 289.
12] H. A. Younus, W. Su, N. Ahmad, S. Chen, Adv. Synth. Catal.
015, 357, 283.
[13] S. Saha, M. Kaur, K. Singh, J. K. Bera, J. Organometal. Chem.
016, 812, 87.
2
2
[14] D. J. Segobia, A. F. Trasarti, C. R. Apesteguía, Catal. Commun.
2015, 62, 62.
2
00 psi of hydrogen, it was placed in an oil bath at

8
of 8
NAIT AJJOU AND ROBICHAUD
[
15] F. Chen, C. Topf, J. Radnik, C. Kreyenschulte, H. Lund, M.
Schneider, A.‐E. Surkus, L. He, K. Junge, M. Beller, J. Am.
Chem. Soc. 2016, 138, 8781.
[37] A. Nait Ajjou, A. Robichaud, Appl. Organometal. Chem. 2018,
32, e4481.
[
[
[
[
38] P. T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 1994, 368, 321.
39] P. T. Tanev, T. J. Pinnavaia, Science 1995, 267, 865.
40] P. T. Tanev, T. J. Pinnavaia, Chem. Mater. 1996, 8, 2068.
[
16] S. Muratsugu, S. Kityakarn, F. Wang, N. Ishiguro, T. Kamachi,
K. Yoshizawa, O. Sekizawa, T. Uruga, M. Tada, Phys. Chem.
Chem. Phys. 2015, 17, 24791.
41] T. A. Zepeda, J. L. G. Fierro, B. Pawelec, R. Nava, T. Klimova,
[
[
[
17] R. K. Marella, K. S. Koppadi, Y. Jyothi, K. S. R. Rao, D. R.
G. A. Fuentes, T. Halachev, Chem. Mater. 2005, 17, 4062.
Burri, New J. Chem. 2013, 37, 3229.
[42] D. E. De Vos, M. Dams, B. F. Sels, P. A. Jacobs, Chem. Rev.
18] R. Ferraccioli, D. Borovika, A.‐E. Surkus, C. Kreyenschulte, C.
Topf, M. Beller, Catal. Sci. Technol. 2018, 8, 499.
2002, 102, 3615.
[
[
43] A. P. Wight, M. E. Davis, Chem. Rev. 2002, 102, 3589.
19] H. Yoshida, Y. Wang, S. Narisawa, S.‐I. Fujita, R. Liu, Appl.
Catal. A 2013, 456, 215.
44] G. Frémy, E. Monflier, J.‐F. Carpentier, Y. Castanet, A.
Mortreux, J. Catal. 1996, 162, 339.
[
20] L. Hegedűs, T. Máthé, Appl. Catal. A 2005, 296, 209.
[45] Z. Li, Q. Peng, Y. Yuan, Appl. Catal. A 2003, 239, 79.
[
21] M. Chatterjee, H. Kawanami, M. Sato, T. Ishizaka, T.
[
46] J. P. Arhancet, M. E. Davis, J. S. Merola, B. E. Hanson, J. Catal.
Yokoyama, T. Suzuki, Green Chem. 2010, 12, 87.
1990, 121, 327.
[
[
[
[
22] H. Greenfield, Ind. Eng. Chem. Prod. Res. Dev. 1967, 6, 142.
23] C. Barnett, Ind. Eng. Chem. Prod. Res. Dev. 1969, 8, 145.
24] Y. Huang, W. M. H. Sachtler, Appl. Catal. A 1999, 182, 365.
[
47] J. J. García, N. Pérez‐Lezama, A. Arévola, Inorg. Chim. Acta
2017, 464, 55.
[48] P. Schärringer, T. E. Müller, J. A. Lercher, J. Catal. 2008, 253,
67.
1
25] Y. Li, Y. Gong, X. Xu, P. Zhang, H. Li, Y. Wang, Catal.
Commun. 2012, 28, 9.
[49] J. J. W. Bakker, A. G. van der Neut, M. T. Kreutzer, J. A.
Moulijn, F. Kapteijn, J. Catal. 2010, 274, 176.
[
26] Y. Monguchi, M. Mizuno, T. Ichikawa, Y. Fujita, E. Murakami,
[
50] S. Gomez, J. A. Peters, T. Maschmeyer, Adv. Synth. Catal. 2002,
44, 1037.
51] A. Robichaud, A. Nait Ajjou, Tetrahedron Lett. 2006, 47, 3633.
T. Hattori, T. Maegawa, Y. Sawama, H. Sajiki, J. Org. Chem.
3
2
017, 82, 10939.
27] S. Lu, J. Wang, X. Cao, X. Li, H. Gu, Chem. Commun. 2014, 50,
512.
[
[
[
3
[52] A. Bouriazos, S. Sotiriou, P. Stathis, G. Papadogianakis, Appl
Catal B 2014, 150‐151, 345.
28] B. Cornils, W. A. Herrmann, Applied Homogeneous Catalysis
with Organometallic Compounds, Vol. 2, VCH, Weinheim,
Germany 1996.
[53] B. P. Buffin, N. L. Belitz, S. L. Verbeke, J. Mol. Catal. A 2008,
284, 149.
[
29] W. A. Herrmann, C. W. Kohlpaintner, Angew. Chem. Int. Ed.
Engl. 1993, 32, 1524.
[54] V. B. Valodkar, G. L. Tembe, M. Ravindranathan, R. N. Ram,
H. S. Rama, J. Mol. Catal. A 2003, 202, 47.
[
[
30] F. Ungváry, Coord. Chem. Rev. 1997, 160, 123.
[55] Y.‐D. Jo, K.‐S. Park, J.‐H. Ahn, S.‐K. Ihm, React. Funct. Polym.
1996, 29, 91.
31] J. P. Arhancet, M. E. Davis, J. S. Merola, B. E. Hanson, Nature
[56] W. A. Herrmann, G. P. Albanese, R. B. Manetsberger, P. Lappe,
1
989, 339, 454.
H. Bahrmann, Angew. Chem. Int. Ed. 1995, 34, 811.
[
[
32] M. E. Davis, Chemtech 1992, 498.
33] J. P. Arhancet, M. E. Davis, B. E. Hanson, J. Catal. 1991, 129,
9
4.
How to cite this article: Nait Ajjou A,
Robichaud A. Chemoselective hydrogenation of
nitriles to secondary or tertiary amines catalyzed by
aqueous‐phase catalysts supported on hexagonal
mesoporous silica. Appl Organometal Chem. 2018;
e4547. https://doi.org/10.1002/aoc.4547
[
[
[
34] Y. P. Xie, J. Men, Y. Z. Li, H. Chen, P. M. Cheng, X. J. Li, Catal.
Commun. 2004, 5, 237.
35] C. Bianchini, V. D. Santos, A. Meli, W. Oberhauser, R. Psaro, F.
Vizza, Organometallics 2000, 19, 2433.
36] A. Nait Ajjou, Catal. Today 2015, 247, 177.