Ruthenium/imidazolylphosphine catalysis: Hydrogenation of aliphatic and aromatic nitriles to form amines

Article abstract of DOI:10.1002/chem.201303989

A convenient and efficient catalyst system for the hydrogenation of aliphatic nitriles towards the corresponding primary amines in high to excellent yields is presented. In addition, aromatic nitriles are reduced smoothly, too. The use of low catalyst loadings and molecular hydrogen make this protocol an attractive methodology. It's not complicated: A general and easy homogeneous catalyst system based on [Ru(cod)(methylallyl)₂] and a cyclohexyl-substituted imidazolylphosphine ligand for selective hydrogenation of aliphatic nitriles is presented. In addition, by using an isopropyl-substituted imidazolylphosphine ligand, selected aromatic nitriles were reduced with excellent yields towards the primary amine. Furthermore, two new crystal structures give an insight of possible pre-catalysts.

Full text of DOI:10.1002/chem.201303989

DOI: 10.1002/chem.201303989
Communication
&
Nitrile Hydrogenation
Ruthenium/Imidazolylphosphine Catalysis: Hydrogenation of
Aliphatic and Aromatic Nitriles to Form Amines
Svenja Werkmeister, Kathrin Junge, Bianca Wendt, Anke Spannenberg, Haijun Jiao,
[
a]
Christoph Bornschein, and Matthias Beller*
The selection of the catalyst and reaction conditions is cru-
cial for obtaining high selectivity to the desired primary
amines. Basically, all known homogeneous catalyst systems for
the hydrogenation of nitriles were developed for the reduction
Abstract: A convenient and efficient catalyst system for
the hydrogenation of aliphatic nitriles towards the corre-
sponding primary amines in high to excellent yields is pre-
sented. In addition, aromatic nitriles are reduced smoothly,
too. The use of low catalyst loadings and molecular hydro-
gen make this protocol an attractive methodology.
[4]
of aromatic nitriles. After the original work by Pez and
[4a,b]
Grey
in the 1980s, further progress applying ruthenium
[4i]
hydride complexes was achieved by Beatty and Paciello,
[4j]
[4k]
[4l]
Frediani et al.,
Morris et al.,
in the last decades. Moreover, various systems
that make use of non-hydride complexes were developed by
Sabo-Etienne et al.
and
[4m]
Leitner et al.
Aliphatic amines are of importance as natural and synthetic
chemicals in industry and everyday life, especially as pharma-
[4c]
[4d]
[4e]
Dewhirst,
Bianchini et al.,
Hidai et al.
and by our
[
1]
[4f–h,n,o]
ceuticals, agrochemicals, and polymers. On a small scale, the
synthesis of amines is often still realized using stoichiometric
amounts of metal hydrides such as LiAlH or NaBH . From eco-
group.
Here, we present the first general protocol for the catalytic
hydrogenation of various aliphatic nitriles by applying a defined
ruthenium/imidazolylphosphine-based catalyst system (ligands
1–7 are depicted in Figure1).
4
4
nomic and ecologic points of view, catalytic methods offer
more effective and versatile approaches to amines. Here, CÀN
bond-forming reactions, such as hydroaminations of olefins
[
1c,2]
[3]
and alkynes,
hydroaminomethylations of olefins, or reduc-
tive aminations of carbonyl compounds have been developed.
In addition, catalytic reductions of nitriles with molecular hy-
drogen allow for a clean and atom economic access to amines.
As shown in Scheme 1, initially the nitrile forms the corre-
Figure 1. Structure of different imidazolylphosphine ligands tested for the
hydrogenation of hexanenitrile.
Recently, we reported the synthesis and successful applica-
tion of imidazolylphosphine ligands for the Ru-catalyzed hy-
[5]
Scheme 1. Catalytic hydrogenation of nitriles and possible side reaction.
drogenation of carboxylic esters.
The combination of
[
{Ru(benzene)Cl } ] with ligands 3 or 6 gave good to excellent
2
2
sponding primary imine, which is subsequently hydrogenated
to the desired primary amine (path A). However, the product
can react further on with the imine to give the secondary
imine by releasing ammonia (path B). Final hydrogenation
leads to the secondary amine as a side-product.
results for the hydrogenation of aromatic, aliphatic and cyclic
lactones and carboxylic esters. Although esters were selectively
reduced to alcohols in the presence of many functional
groups, in case of a nitrile moiety the formation of the corre-
sponding amine was observed. Based on our interest in nitrile
[4f–h,n,o]
reduction
and to the importance of the resulting amines,
[
a] S. Werkmeister, Dr. K. Junge, B. Wendt, Dr. A. Spannenberg, Dr. H. Jiao,
C. Bornschein, Prof. Dr. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
we started to investigate the potential of the imidazolylphos-
phine ligands in the Ru-catalyzed hydrogenation of aliphatic
[4m]
nitriles, which represent more challenging substrates.
At first, different P,N-ligands 1–7 (Figure 1) were tested in
the Ru-catalyzed reduction of hexanenitrile 8b. Preliminary ex-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303989.
periments showed that [Ru(cod)(methylallyl) ] is a suitable
2
Chem. Eur. J. 2014, 20, 4227 – 4231
4227
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
catalyst precursor in this reaction. For convenience, the
active catalyst is generated in situ from 0.025 mmol
As described earlier for the hydrogenation of aromatic ni-
[4f,g]
triles,
using other bases or even in the absence of base the
[
6]
[
Ru(cod)(methylallyl) ] and 0.05 mmol ligand 1–7. At 808C
selectivity to the corresponding primary amine decreases.
To get more insight towards the mechanism we tried to
crystallize Ru/imidazolylphosphine complexes. To our delight,
for Ru/4 (compound 10) and Ru/3 (compound 11), crystals
suitable for X-ray crystal structure analysis were obtained by
overlaying a solution of the complex in dichloromethane with
n-heptane for several weeks (see the Supporting Informa-
2
most of these imidazolylphosphine ligands are able to trans-
form the model nitrile to the desired amine with yields up to
79% (Table 1). In addition to the different substituents on the
Table 1. Ru-catalyzed hydrogenation of hexanenitrile in the presence of
[
a]
[8]
different phosphine/amine-imidazolyl ligands.
tion).
As presented in Figure 2, the structures of these two com-
plexes show very different coordination spheres. Complex 10
has one Ru atom in the octahedral coordination sphere with
two bidentate imidazolylphosphine ligands 4 and two bromine
atoms (Figure 2; top). Considering one bromine atom in the
axial position and another one in the equatorial position, the
Ligand
Ratio
T
Yield
[%]
[
b]
[
8C]
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
3
3
3
3
3
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
1:4
1:2
1:2
1:2
80
80
80
80
80
80
80
80
80
35
79
77
17
–
27
12
21
88
93
–
1
100
40
25
1
1
12
–
[
[
8
a] Reaction conditions: 5 mmol 8b, 0.05 mmol 1-7, 0.025 mmol
Ru(cod)(methylallyl) ], HBraq (48% wt, 0.125 mmol), 0.5 mmol KOtBu,
mL toluene, 50 bar H , T, 5.5 h. [b] Yield determined by GC methods
using hexadecane as an internal standard.
2
2
phosphorus atom in ligands 1–6, a different chain length be-
tween the imidazolyl and the phosphorus moiety was investi-
gated. Comparing the results for the Ru-catalyzed hydrogena-
tion of hexanenitrile 8b with 4 (17%) and 7 (12%), no clear
trend is observed. On the other hand, the substituent on the
phosphorus has a strong influence on the reaction outcome.
In particular, ligands 2 and 3, bearing alkyl groups on the
phosphorus part, gave hexylamine 9b in high yields (Table 1,
entries 2 and 3). Owing to the fact that ligands 2 and 3 led to
similar yields, we decided to use 3 for further studies due to
the easier preparation.
As shown in Table 1 (entries 3 and 8), by changing the cata-
lyst/ligand ratio from 1:2 to 1:1 the yield dropped down to
2
1
2
1%, whilst a high yield of 88% was observed with a ratio of
:4 (Table 1, entry 9). Lowering the temperature to 40 and
58C (Table 1, entries 11 and 12, respectively) did not lead to
Figure 2. ORTEP representation of 10 (top) and 11 (bottom). Thermal ellip-
soids are set at 30% probability. Hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢁ] and angles [8] for 10: N1ÀRu1 2.132(5), N3ÀRu1
any conversion. Best results (93% yield) were obtained when
the hydrogenation reaction was performed at 1008C (Table 1,
entry 10). Notably, under our conditions we did not observe
the formation of the common side-products (secondary imines
and amines). To prove that our reaction proceeds by means of
pathway A, we hydrogenated benzophenone imine successful-
ly under optimized conditions to get selectively the primary
2
.080(5), P1ÀRu1 2.2500(15), P2ÀRu1 2.2731(15), Br1ÀRu1 2.5395(9), Br2ÀRu1
2.5852(9), N3-Ru1-Br1 175.92(14), P1-Ru1-Br2 167.77(5), N1-Ru1-Br1 87.99(13),
P2-Ru1-Br2 88.20(4), Br1-Ru1-Br2 90.96(3); selected bond lengths [ꢁ] and
angles [8] for 11: Br1ÀRu1 2.5322(4), Br2ÀRu1A 2.6141(4), Br2ÀRu1 2.6141(4),
Br3ÀRu1 2.4954(4), Br1AÀRu1 2.5090(4), N1ÀRu1 2.056(2), P1ÀRu1 2.2840(7),
Ru1-Br1-Ru1A 79.176(12), Ru1-Br2-Ru1A 75.818(15), N1-Ru1-P1 82.44(7), N1-
Ru1-Br1A 175.77(7), P1-Ru1-Br1A 100.68(2), P1-Ru1-Br1 99.99(2), Br3-Ru1-Br1
171.779(13).
[
7]
amine.
Chem. Eur. J. 2014, 20, 4227 – 4231
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
two phosphorus atoms are in the equatorial positions, and one
nitrogen atom is in the axial position, while another nitrogen
atom is in the equatorial position. Counting the number of the
For the expected monoruthenium complex with two biden-
tate imidazolylphosphine ligands 3 (R=cyclohexyl) and two
bromine atoms which mimics complex 10/4, we also calculat-
ed the cis-10/3 and trans-10/3 isomers. However, the trans-
10/3 isomer is computed to be more stable than the cis-10/3
II
valence electrons of complex 10 reveals a Ru center with 18-
electrons in a closed-shell electronic configuration.
À1
In contrast, complex 11 has two Ru centers, which are
bridged by three bromine atoms. Each Ru atom bears one ad-
ditional terminal bromine atom and one bidentate imidazolyl-
phosphine ligand 3 (Figure 2, bottom). Both Ru centers have
an octahedral coordination sphere and the two octahedrons
are joined on faces of the bridging bromine atoms. Counting
the number of the valence electrons of compound 11 reveals
one Ru center with 18-electrons and one Ru center with 17-
by 6.27 kcalmol in Gibbs free energy, revealing that only the
trans-10/3 should be possible. This is also in line with the ob-
31
served and estimated P NMR spectrum.
Since the cis-10/4 has been isolated as thermodynamically
stable compound, we are interested in the stability of the ex-
pected and more stable trans-10/3. Indeed, the computed ex-
À1
ergonic free energy (À1.48 kcalmol ) of the ligand exchange
reaction (cis-10/4+3=trans-10/3+4) shows the thermody-
namic possibility and stability of the expected trans-10/3.
Considering the possible cis and trans isomers of the com-
plexes with two imidazolylphosphine ligands and two bromine
atoms, we are confident that cis- and trans-isomers of the com-
plexes should represent pre-catalysts that form in situ with
molecular hydrogen the mono- or dihydrido active species for
hydrogenation of nitriles.
II
III+
electrons. Therefore, a mixed valence complex of Ru /Ru is
[
9]
assumed. Indeed, Cotton et al. reported very similar struc-
tures that bear three bridging chlorine atoms instead of bro-
mine atoms. In addition, the computed energy minimum struc-
ture of complex 11 has a C symmetry, indicating that the two
2
Ru centers are equivalent. Thus, complex 11 mimics the
[
10]
5+
Creutz–Taube complex ion,
[C H N ] [{Ru(NH ) } ], which
4 4 2 3 5 2
also has two equivalent Ru centers.
According to the scope of the catalytic hydrogenation, the
influence of the alkyl chain length of nitriles was examined
As we applied a metal/ligand ratio of 1:2 in the catalytic re-
action, complex 10 has two coordinated imidazolylphosphine
ligands, while only one imidazolylphosphine ligand is found in
complex 11 at each Ru center. This shows that complex 10 is
formed as expected, while the formation of complex 11 is
rather surprising. One Ru atom in complex 11 is oxidized from
using 0.5 mol% [Ru(cod)(methylallyl) ]/1 mol% 3 (Table 2, en-
2
tries 1–5). At 808C, an excellent yield of propyl amine 9a (99%,
Table 2, entry 1) was achieved whilst the yields of hexyl-,
heptyl- and dodecyl amine were between 66 and 77%
(Table 2, entries 2–4). Increasing the temperature to 1008C led
to higher yields up to 93% (Table 2, entries 2, 3, and 5). Addi-
tionally, branched as well as cyclic nitriles can be successfully
reduced at 80 or 1008C in high yields up to 92% (Table 2, en-
tries 7–12).
II
III
Ru to Ru , but we started with [Ru(cod)(methylallyl) ] as an
2
II
Ru precursor without the addition of oxidation reagents in
our reactions. Therefore, one might consider that such oxida-
tion took place accidentally during the very long crystallization
process by air oxygen.
Finally, a small selection of aromatic nitriles was also hydro-
genated in the presence of the related catalytic system
In our NMR measurements with the in situ formed com-
31
plexes we found two main P signals (d=45.5, 64.6 ppm) for
the complexes with imidazolylphosphine ligand 4 (R=phenyl)
and one main signal (d=37.6 ppm) for those with imidazolyl-
phosphine ligand 3 (R=cyclohexyl). To understand these re-
sults, we carried out BP86 density functional theory computa-
tions for the energetics of both types of complexes. Apart
from complex 10 with two bromine atoms in cis-position (cis-
[Ru(cod)(methylallyl) ]/1 (Scheme 3). To our delight, the corre-
2
sponding primary amines are formed in almost quantitative
yields already at room temperature using a low catalyst load-
ing (0.5 mol%). Electron-donating as well as electron-withdraw-
ing groups were accepted and gave the completely reduced
products in excellent selectivity.
10/4), we also found one trans-isomer (trans-10/4) (Scheme 2).
Scheme 2. The cis- and trans-isomers of 10/4 and 10/3.
The energy minimum structure of cis-10/4 has C symmetry,
1
while that of trans-10/4 is C2 symmetrical. The computed
Gibbs free energy (DG) shows that the cis-10/4 is more stable
À1
than trans-10/4 by 0.53 kcalmol , which reveals an equilibri-
um ratio of 70:30. This ratio agrees roughly with the estimated
31
P NMR intensities.
Scheme 3. Hydrogenation of aromatic nitriles with [Ru(cod)(methylallyl)
2
]/1.
Chem. Eur. J. 2014, 20, 4227 – 4231
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4229 ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
ical shifts (d) are reported in ppm and coupling constants (J) in Hz.
[
a]
Table 2. Catalytic hydrogenation of various aliphatic nitriles—scope.
1
All chemical shifts are related to solvent peaks {[D ]DMSO: 2.5 ( H),
6
13
13
1
3
9.52 ppm ( C) or [D ]MeOH: 3.3 ( H), 49.0 ppm ( C)}. All measure-
4
ments were carried out at room temperature unless otherwise
stated. Mass spectra were in general recorded on a Finnigan MAT
9
5-XP (Thermo Electron) or on a 6210 time-of-flight LC/MS (Agi-
lent). Gas chromatography was performed on a HP 6890 with
a HP5 column (Agilent). Unless otherwise stated, commercial re-
agents were used without purification. All catalytic hydrogenation
experiments using molecular hydrogen were carried out in a Parr
Instruments autoclave (25 mL).
Nitrile
Amine
T
Yield
[%]
[
b]
[
8C]
1
2
8a
8b
9a
9b
80
99
8
0
77
93
66
67
1
00
Hydrogenation of nitriles
8
0
3
8c
9c
1
00
Under an argon atmosphere, a Schlenk tube was charged with
[Ru(cod)(methylallyl)2] (0.025 mmol) and the respective ligand
(0.05 mmol). After a stirring under vacuum for a short time, ace-
tone (1 mL) and HBraq (48 wt%, 0.125 mmol) were added and the
mixture was stirred again for 15 min after which a yellow-orange
precipitate was formed. Afterwards, acetone was removed under
vacuum and KOtBu (0.5 mmol) was added. The solid was dissolved
in THF (7 mL) and the liquid nitrile (5 mmol) (solid nitrile: the sub-
strate was directly filled in the autoclave) as well as hexadecane (as
standard) was added. The mixture was transferred into the auto-
clave and the apparatus was flushed three times with hydrogen,
4
5
8d
8e
9d
9e
80
77
[
c]
100
100
86
81
6
7
8
8 f
9 f
8
0
17
85
8g
8h
9g
9h
1
00
8
0
63
83
1
00
8
0
63
89
9
0
8i
9i
100
filled with 50 bar H , and stirred for 5.5 h at 80 or 1008C. After the
2
reaction is finished, the autoclave is cooled down to room temper-
ature, hydrogen is released and the reaction mixture is analyzed by
GC or isolated as HCl salt.
8
0
52
86
1
8j
8k
8l
9j
9k
9l
100
[
c]
11
100
84
92
Computational methods
12
80
[11]
Structure optimizations were carried out at the BP86 density
functional level of theory with the SVP basis set for non-metal ele-
[12]
[13]
[
[
8
a] Reaction conditions: 5 mmol 8, 0.05 mmol 3, 0.025 mmol
Ru(cod)(methylallyl) ], HBraq (48 wt%, 0.125 mmol), 0.5 mmol KOtBu,
mL toluene, 50 bar H , 80 or 1008C, 5.5 h. [b] Yield determined by GC
ments (C, H, P, N, Br) and the LANL2DZ basis set for Ru with
[14]
2
Gaussian 03 program package.
The optimized geometries are
2
characterized as energy minimums at the potential energy surface
from frequency calculations at the same level of theory (BP86/
SVP); that is, energy minimum structure has only real frequencies.
For the discussion and comparison of the relative stabilities single-
point energies were computed at the BP86 level with the TZVP
methods using hexadecane as an internal standard. [c] Yield determined
via isolation as HCl salt.
In summary, we have developed an efficient and effective
protocol for the reduction of aliphatic and aromatic nitriles to
give the corresponding primary amines in high yields. The
[15]
basis set for the non-metal elements. The Gibbs free energies
are scaled with the thermal correction to Gibbs free energies at
298 K. The computed energetic data and Cartesian coordinates are
listed in Supporting Information.
combination of [Ru(cod)(methylallyl) ] and imidazolylphosphine
2
ligands 3 or 1 generate active homogeneous catalyst systems,
which allow hydrogenation of linear, branched, and cyclic ali-
phatic nitriles as well as aromatic nitriles. Furthermore, we ob-
tained two new crystal structures of ruthenium complexes 10
and 11, which represent possible pre-catalysts for active hydro-
genation catalysts.
Acknowledgements
This research has been funded by the State of Mecklenburg-
Western Pomerania, the BMBF, and the DFG (Leibniz Prize). We
thank Dr. W. Baumann, Dr. C. Fischer, S. Buchholz, S. Schareina
and A. Koch (all at the LIKAT) for their excellent technical and
analytical support. We are also thankful to Johannes Thomas
Experimental Section
(
LIKAT) for fruitful scientific discussions.
General information
Keywords: amine · hydrogenation · nitrile · P,N ligands ·
Unless otherwise stated, all reactions were run under an argon at-
mosphere with exclusion of moisture from reagents and glassware
using standard techniques for manipulating air-sensitive com-
ruthenium
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3
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www.chemeurj.org
4230
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
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Received: October 11, 2013
Published online on March 11, 2014
Chem. Eur. J. 2014, 20, 4227 – 4231
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