Ruthenium N-heterocyclic carbene catalysts for selective reduction of nitriles to primary amines

Article abstract of DOI:10.1016/j.tetlet.2009.03.108

Easily accessible in situ catalysts composed of [Ru(cod)(2-methylallyl)₂] and N-heterocyclic carbene ligands have been developed for the environmentally benign hydrogenation of various nitriles to give primary amines. Applying optimized conditions in the presence of SIMesBF₄ as ligand high catalyst activity of up to 392 h^-1 is achieved in the hydrogenation of benzonitrile. The general applicability and functional group tolerance of this novel catalyst system are shown in the reduction of ten different nitriles.

Full text of DOI:10.1016/j.tetlet.2009.03.108

Tetrahedron Letters 50 (2009) 3654–3656
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Tetrahedron Letters
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Ruthenium N-heterocyclic carbene catalysts for selective reduction of nitriles
to primary amines
*
Daniele Addis, Stephan Enthaler, Kathrin Junge, Bianca Wendt, Matthias Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße 29a, D-18059 Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Easily accessible in situ catalysts composed of [Ru(cod)(2-methylallyl)2] and N-heterocyclic carbene
ligands have been developed for the environmentally benign hydrogenation of various nitriles to give pri-
mary amines. Applying optimized conditions in the presence of SIMesBF4 as ligand high catalyst activity
of up to 392 hÀ1 is achieved in the hydrogenation of benzonitrile. The general applicability and functional
group tolerance of this novel catalyst system are shown in the reduction of ten different nitriles.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 2 February 2009
Revised 10 March 2009
Accepted 16 March 2009
Available online 20 March 2009
Keywords:
Homogeneous catalysis
Hydrogenation
Nitriles
Primary amines
Ruthenium
Amines constitute important bulk and fine chemicals as well as
pharmaceuticals. Moreover, nitrogen-based compounds represent
precursors for alkaloids, amino acids, and nucleotides, which play
a key role in manifold natural processes.1 During the last decades
efficient catalytic C–N bond forming methods were developed,
for example, palladium-catalyzed amination of aryl halides,2
hydroamination,3 and hydroaminomethylation of olefins or al-
kynes.4 In addition, the reduction of carbonyl groups,5 and amides
or nitriles offers an attractive access to amines.
sis.16,17 Compared to tertiary phosphines NHC ligands are charac-
terized by a stronger metal-interaction and a minimized ligand
dissociation.
Herein, we describe for the first time catalytic hydrogenations
of nitriles to primary amines in the presence of carbene ligands.
Our initial studies were carried out with benzonitrile as standard
substrate using 0.5 mol% of [Ru(cod)(2-methylallyl)2] and 0.5–
1.0 mol% of 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazoli-
um tetrafluoroborate (SIMesBF4) as in situ catalyst in the presence
of 10 mol% of base. All exploratory hydrogenation reactions were
performed in an eightfold parallel reactor array with a reactor vol-
ume of 3.0 mL.18 The first set of reactions was run in toluene with
various hydrogen pressures at 80 °C at a Ru:SIMesBF4 ratio of 1:1
and 1:2, respectively. To our delight full conversion and excellent
chemoselectivity (>99%) are obtained with this novel carbene-
based catalyst. As depicted in Figure 1, the optimal hydrogen pres-
sure depends on the ratio of Ru:SIMesBF4. Notably, best results are
obtained at 30–35 bar of hydrogen, which is significantly lower
compared to similar hydrogenations in the presence of Ru/phos-
phine catalysts.13,14
In recent years we have developed novel and improved cata-
lysts for the synthesis of benzonitriles.6,7 It is well known that
these nitriles can be reduced with (over)stoichiometric amounts
of metal hydrides, for example, LiAlH4 or in the presence of
heterogeneous catalysts based on Pd, Ni, Co, etc. However, the cat-
alytic hydrogenation of nitriles in the presence of homogeneous
metal complexes has been scarcely investigated compared to
C@C, C@O, and C@N bond reductions.8 Nevertheless, few examples
of nitrile reductions are known with ruthenium,9 rhodium,10 and
iridium11 complexes. In this context, last year we have reported
13
ruthenium phosphine catalysts based on dppf12 or PPh3 for the
hydrogenation of aromatic nitriles to give primary amines with
high chemoselectivity (>99%). More recently, we also became
interested in the potential applicability of carbene ligands in ruthe-
nium-catalyzed hydrogenations.14 Since Arduengo et al.15 intro-
duced the first stable N-heterocyclic carbenes in the early 1990s
this class of ligands has become increasingly popular in cataly-
Next, we tested various imidazolium salts shown in Scheme 1
as carbene precursors to investigate the influence of the ligand
structure on the outcome of the reaction. The best catalyst perfor-
mance is observed in the presence of the mesitylene-based imi-
dazolium salts 1 and 2.
Increasing the bulkiness in 2,6-position of the aryl substituent
by iso-propyl groups gave decreased conversion (Scheme 1, 5). A
further depletion of the yield of benzyl amine is observed by
changing the N-substituent from aryl to alkyl (Scheme 1, 3, 4, 8).
* Corresponding author. Tel.: +49 381 1281 5000; fax: +49 381 1281 113.
E-mail address: matthias.beller@catalysis.de (M. Beller).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.108

D. Addis et al. / Tetrahedron Letters 50 (2009) 3654–3656
3655
0.5 mol% [Ru(cod)methylallyl2]
0.5-1.0 mol% SIMesBF4 (1)
10 mol% KOtBu
Table 1
-
BF4
Screening of reaction temperature and reaction timea
N
N
CN
CH2NH2
0.5 mol% [Ru(cod)methylallyl2]
0.5-1.0 mol% SIMesBF4 (1)
10 mol% KOtBu
-
H2 (15-50 bar)
BF4
N
N
1
( )
SIMesBF4
CN
CH2NH2
H2 (35 bar)
100
90
1
( )
SIMesBF4
80
Entry
Cat. (mol %)
T (°C)
T (h)
Yield (%)
Chemoselectivity (%)
70
1
2
3
4
5
6
7
8
9
10
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
0.125
0.06
20
40
40
40
60
80
80
80
80
80
2
2
6
16
2
0.5
1
1
1
1
5
28
98
98
97
99
>99
95
37
12
99
98
99
99
99
99
60
[%] 50
40
Conversion (ratio 1:1)
Conversion (ratio 1:2)
30
>99
20
99
99
99
10
0
10
15
20
25
30
35
40
45
50
a
Reaction conditions: 20–80 °C, 0.5–16 h, 0.038 mmol [Ru(cod)(2-methylallyl)2],
0.038 mmol SIMesBF4, 10 mol % KOtBu, 7.6 mmol benzonitrile in 20.0 mL toluene
under 35 bar hydrogen atmosphere in a 50 mL Paar autoklave.
Pressure [bar]
Figure 1. Influence of pressure at different ligand:metal ratios (reaction conditions:
80 °C, 1 h, 0.0038 mmol [Ru(cod)(2-methylallyl)2], 0.0038 or 0.0076 mmol
SIMesBF4, 10 mol % KOtBu, 0.76 mmol benzonitrile in 2.0 mL toluene).
Table 2
Scope and limitation of catalysta
BF4
Cl
Cl
0.5 mol% [Ru(cod)methylallyl2]
-
BF4
0.5 mol% SIMesBF4
10 mol% KOtBu
N
N
N
N
N
N
R
N
R
CH2NH2
N
N
H
2 (35 bar)
1
yield: 98% (
)
3
yield: 2% ( )
2
yield: 98% (
)
1
( )
SIMesBF4
Cl
N
Entry
Substrate
CN
T (°C)
T (h)
Yield (%)
Sel. (%)
>99
Cl
Cl
N
N
N
N
N
1
2
80
1
>99
CN
4
)
yield: 1% (
40
6
1
41
36
>99
74c
5
yield: 80% (
)
6
)
yield: 7% (
Br
140
CN
PF6
Br
N
3
4
80
1
78
98
N
N
N
Ph
CN
40
40
6
29
99
>99
>99
8
yield: 1% (
)
MeO
yield: 50% (7)
16
Scheme 1. Different ligands tested in the ruthenium-catalyzed reduction of
benzonitrile (reaction conditions: 80 °C, 1 h 0.0038 mmol [Ru(cod)(2-methylal-
lyl)2], 0.0038 mmol carbene, 10 mol % KOtBu, 0.76 mmol benzonitrile in 2.0 mL
toluene at 35 bar hydrogen atmosphere).
OMe
CN
CN
5
40
80
16
1
92
87
>99
>99
CN
However, in the presence of the simple precursor 7, the desired
product is still obtained in 50% yield. In order to find optimal reac-
tion conditions, the influence of temperature and catalyst concen-
tration on the selectivity and yield was also investigated (Table 1).
Applying the catalyst composed of [Ru(cod)(2-methylallyl)2] and
imidazolium salt 1 excellent yield (98%) and chemoselectivity
(>99%) are observed even at 40 °C (Table 1, entries 3 and 4). On
the other hand at 80 °C a TOF of 392 hÀ1 (Table 1, entry 6) is ob-
served, which reflects an increase in catalytic efficiency by the fac-
tor ten compared to the Ru/PPh3 system.13
Finally, we performed the reduction of different nitriles under
optimized conditions to demonstrate the scope and limitations of
the Ru/carbene catalyst (Table 2).19 It is important to note that in
all cases excellent chemoselectivity with more than 99% of the cor-
responding primary amine is obtained, except for p-bromobenzo-
nitrile (Table 2, entry 2), where a small amount of the diamine is
detected. In general, the hydrogenation of substituted benzonitr-
iles proceeded in good to excellent yields under mild conditions
6
7
40
6
>99
>99
MeO
40
60
16
6
15
31
>99
>99
NC
8
9
40
80
6
1
—
—
N
OMe
CN
10
>99
80
80
1
1
28
12
>99
>99
CN
O
10b
a
Reaction conditions: 40–140 °C, 1–16 h, 0.038 mmol [Ru(cod)(2-methylallyl)2],
0.038 mmol SIMesBF4, 10 mol % KOtBu, 7.6 mmol nitrile in 20.0 mL toluene under
35 bar hydrogen atmosphere.
b
10 mol % NH4Cl.
26% of the corresponding diamine.
c

3656
D. Addis et al. / Tetrahedron Letters 50 (2009) 3654–3656
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In summary, we demonstrated for the first time the possibility
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ious nitriles. While the substrate scope of these catalysts should be
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Acknowledgments
The authors thank Dr. C. Fischer, S. Buchholz, and Dr. D. Micha-
lik (all at the Leibniz-Institut für Katalyse e.V. an der Universität
Rostock) for analytical and technical support. We acknowledge
funding of the BMBF and the State Mecklenburg-Vormpommern
as well as the Deutsche Forschungsgemeinschaft (GRK 1211 and
Leibniz price).
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19. General reaction procedure: A solution of benzonitrile (0.38 mmol) in toluene
(1.0 mL) was transferred by syringe into an autoclave that contained KOtBu
(0.038 mmol) and argon atmosphere. The catalyst was generated in situ by
stirring [Ru(cod)(2-methylallyl)2] (0.0019 mmol) and the carbene ligand
(0.0019 mmol) in toluene (1.0 mL) for 10 min and afterwards transferring the
solution by syringe into the autoclave. Then hydrogen (35 bar) was added to
the autoclave and the mixture was stirred for the respective time at 40–80 °C.
After the predetermined time, the hydrogen was released and nitrobenzene or
diglyme was added as internal standards. After stirring for 10 min, the reaction
mixture was filtered through a short plug of silica gel. The yield was measured
by GC (30 m HP Agilent Technologies column, 50–300 °C, benzonitrile:
7.08 min, benzylamine: 7.60 min).
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