Tuneable hydrogenation of nitriles into imines or amines with a ruthenium pincer complex under mild conditions

Article abstract of DOI:10.1002/cctc.201403047

The selective hydrogenation of aromatic and aliphatic nitriles into amines and imines is described. Using a ruthenium pincer complex, the selectivity towards amines or imines can be controlled by simple parameter changes. The reactions are conducted under very mild conditions between 50-100°C at 0.4 MPa H₂ pressure without any additives at low catalytic loadings of 0.5-1 mol %, which results in quantitative conversions and high selectivity.

Full text of DOI:10.1002/cctc.201403047

DOI: 10.1002/cctc.201403047
Full Papers
Tuneable Hydrogenation of Nitriles into Imines or Amines
with a Ruthenium Pincer Complex under Mild Conditions
Jong-Hoo Choi and Martin H. G. Prechtl*[a]
Dedicated to Prof. Armando Pombeiro on the occasion of his 65th birthday.
The selective hydrogenation of aromatic and aliphatic nitriles
into amines and imines is described. Using a ruthenium pincer
complex, the selectivity towards amines or imines can be con-
trolled by simple parameter changes. The reactions are con-
ducted under very mild conditions between 50–1008C at
0.4 MPa H2 pressure without any additives at low catalytic
loadings of 0.5–1 mol%, which results in quantitative conver-
sions and high selectivity.
Introduction
Nitrile groups exist in several natural and synthetic organic
compounds, which include pharmaceuticals, and can be pro-
cessed by hydrolysis or hydrogenation reactions.[1–4] Methods
for the catalytic hydrogenation of nitriles are interesting be-
cause of their straightforward approach to obtain primary
amines that are industrially and biologically essential.[5–7] The
general challenge in the hydrogenation of nitriles is to control
the selectivity of the reaction.[8–11] The catalytic hydrogenation
of nitriles is accompanied by many equilibrium reactions, in
particular the equilibrium of the primary imine formed and the
reaction with the primary amine to its corresponding secon-
dary imine and amine (Scheme 1).[12,13]
Under similar conditions, Sabo-Etienne and co-workers applied
a bis(dihydrogen) ruthenium complex for the selective hydro-
genation of benzonitrile to benzyl amine at room temperature
and 0.3 MPa H2.[18] The use of some other Ru complexes was
successful for the selective formation of amines from nitriles
but mostly harsh conditions (e.g., 3.0–7.5 MPa H2) and often
additives such as base were required.[9,13,14,21] For non-noble
metals, Beller et al. reported the first Fe-based catalyst for the
hydrogenation of various nitriles to amines, notable also for
the hydrogenation of adiponitrile into 1,6-hexamethylenedia-
mine.[19] For the formation of secondary imines as major prod-
ucts, only a few non-noble catalysts based on Ni, W and Mo
have been reported. These non-noble metal catalysts are good
alternatives to precious metals, even so relatively high pres-
sures (W, Mo) or high temperatures (Ni) are required.[20,22] Fur-
Scheme 1. A) Hydrogenation of nitriles to primary amines and B) the subse-
quent side-reaction to imines and secondary amines.
Typically, precious-metal hydrides such as Ru or Rh are ap-
plied for the hydrogenation of nitriles, and non-noble metals
such as Ni, W, Mo and Fe have been tested.[9,13–20] The first Rh-
based hydrogenation of nitriles to amines under very mild con-
ditions (0.1 MPa H2, 208C) was reported by Otsuka and co-
workers in the late 1970s but has fallen into disregard.[16]
[a] J.-H. Choi, Dr. M. H. G. Prechtl
Department of Chemistry
University of Cologne
Greinstr. 6, 50939 Cologne (Germany)
Fax: (+49)221-470-1788
E-mail: martin.prechtl@uni-koeln.de
Homepage: www.catalysislab.de
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201403047.
Scheme 2. Hydrogenation of nitriles to amines or imines catalysed by 1 and
2.
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thermore, the selective catalytic coupling of nitriles with vari-
ous amines to secondary imines was achieved by Milstein and
co-workers using a tridentate PNN Ru pincer complex under
mild conditions at 708C and 0.4 MPa H2.[23]
Until now many groups have maintained the control of the
reaction selectively (90–99%) into amines or imines, but to the
best of our knowledge, no homogenous catalyst has been re-
ported yet that can be used intentionally to hydrogenate ni-
triles selectively into both amines and imines by simple param-
eter changes. Herein, we present the first Ru pincer complex
system in which one catalyst can be used for the synthesis of
secondary imines or primary amines (Scheme 2). Our optimised
reaction conditions are very mild, in particular, low pressure
and temperature are used as well as a relatively low catalyst
loading, and generally high conversion and high selectivity of
the tested range of substrates are reported.
Figure 2. Catalytic hydrogenation of benzonitrile (2 mmol) to secondary
imine, secondary amine and primary amine by 1 in 3 mL toluene with a cata-
lyst loading of 0.5–1 mol%. Conversions and yields were determined by GC
with flame ionisation detection (FID).
Results and Discussion
Catalyst screening and reaction optimisation: Secondary
imines
served. Complex 1 is active to hydrogenate nitriles selectively
to secondary imines, which is best at 1008C and 0.4 MPa H2
within 10 h at a catalyst loading of 1 mol%. A lower catalyst
concentration at 0.5 mol% led to a lower conversion of the
benzonitrile (Figure 2, entries 7 and 8).
In recent reports, we described the formation of ruthenium
carbonyl hydrido complexes through the decarbonylation of
primary alcohols by ruthenium hydrides (Figure 1).[24,25] To test
the activity of 1a–4 and to optimise the hydrogenation reac-
tion of nitriles, benzonitrile was used as a model substrate.
Complex 2 shows a generally high activity at 708C (Figure 3,
entry 1) with almost full conversion, but only 58% of the sec-
ondary imine and 39% of the primary amine were obtained.
Furthermore, the results show that the selectivity towards sec-
ondary imines is temperature dependent. Increasing the tem-
perature lowered the selectivity of the secondary imine
(Figure 3, entries 2 and 3). The best selectivity of the secondary
imine was reached at 508C within 20 h with a catalyst loading
as low as 0.5 mol% (Figure 3, entries 5 and 6). Almost no con-
version was reached at room temperature, which indicates
that, despite the high activity of 2, a minimum temperature of
around 508C is required (Figure 3, entry 7).
Figure 1. Formation of ruthenium carbonyl hydrido complexes 2 and 4 by
the decarbonylation of alcohol by 1a and 3.[25]
The catalyst screening at 708C and 0.4 MPa H2 for 20 h with
a catalyst loading of 1 mol% showed high activity for 1 and 2.
Interestingly, the Me-PNP-based Ru complexes 3 and 4 were
basically not active (9% conversion with 3 and 0% with 4).
Complex 1 is predominantly present as trihydride 1a under Ar
and solely present as tetrahydride 1b under H2.[24] The non-
classical ruthenium hydride 1 was used for further evaluation,
and the results are summarised in Figure 2. Under the given re-
action conditions, 74% conversion selectively to the secondary
imine N-benzylidenebenzylamine was reached with 1 (Figure 2,
entry 1). As the reaction parameters were varied (Figure 2, en-
tries 2–6), a clear temperature and time dependence is ob-
Figure 3. Catalytic hydrogenation of benzonitrile (2 mmol) to secondary
imine, secondary amine and primary amine by 2 in 3 mL toluene with a cata-
lyst loading of 0.5–1 mol%. Conversions and yields were determined by GC–
FID.
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Hydrogenation of nitriles to secondary imines and the
cross-coupling of various amines
a yield of 83% of the imine was obtained with 1, whereas
moderate conversion and yield were obtained with 2 in a short-
er reaction time (Table 1, entry 3a/b). Without the addition of
amine, moderate conversion was obtained with both com-
plexes (Table 1, entry 4a/b). For 2, in addition to the imine
(32%), a significant yield (27%) of the secondary amine was
obtained.
We tested different nitrile substrates under the optimised reac-
tion conditions to favour the imine selectivity by 1 and 2. Fur-
thermore, we added various amines to the nitriles for cross-
coupling experiments (Scheme 3).
The full conversion of p-tolunitrile into imine was obtained
with good to excellent yields with both complexes (Table 1,
entry 5a/b). Unexpectedly, the addition of cyclohexylamine did
not lead to good yields with 1 (Table 1, entry 6a/b), despite
quantitative conversion as with the analogue benzonitrile
(Table 1, entry 2a). Moreover, 2 gave only 5% conversion and
yield (Table 1, entry 2b). If we added the smaller and less bulky
isobutylamine (Table 1, entry 7a), higher conversions with
1 were obtained but the selectivity remained low and gave
yields of only 25% of the coupled imine product and 51% of
the corresponding secondary imine of p-tolunitrile. If we used
2 (Table 1, entry 7b), almost full conversion was achieved, but
only 9% of the coupled imine and 90% of the primary amine
were obtained. With both complexes, the short-chained butyr-
onitrile was coupled with hexylamine to butylidenehexylamine
with excellent yields and almost full conversion (96%, Table 1,
entry 8a/b). The heptyl cyanide (Table 1, entry 9a) gave 97%
conversion with 1, but mostly the octylamine was obtained
with 94% yield. Complex 2 converted 54% of the heptyl cya-
nide to give only 36% imine and 17% octylamine (Table 1,
entry 9b). The addition of cyclohexylamine to heptyl cyanide
led to almost no conversion with 1 (Table 1, entry 10a), where-
as a good conversion of 71% was achieved with 2 accompa-
nied by a moderate selectivity (46% yield; Table 1, entry 10b).
4-Propoxybenzonitrile was hydrogenated in the presence of
hexylamine to give quantitative conversions with both com-
plexes (Table 1, entry 11a/b), and excellent yields of the cou-
pled imine were obtained (1, 98%; 2, 97%).
Scheme 3. Hydrogenation of nitriles to secondary imines by 1 and 2 and the
cross-coupling of amines.
The results are summarised in Table 1. Cross-coupling reac-
tions with benzonitrile and hexylamine or cyclohexylamine
gave quantitative conversions with good to excellent selectivi-
ty (Table 1, entries 1a/b and 2a/b). For the direct coupling of
p-bromobenzonitrile with hexylamine, 91% conversion and
Table 1. Hydrogenation of nitriles (2 mmol) to secondary imines by
1 (1 mol% at 1008C) and 2 (0.5 mol% at 508C) at 0.4 MPa H2 in 3 mL tol-
uene.
Entry Nitrile[a] Amine[a] Catalyst
t
Conversion[b] Imine yield[b,c]
[h] [%]
[%]
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
A
A
D
D
E
B
C
B
–
–
C
I
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
2
20 99
20 99
24 97
20 99
72 91
48 76
72 73
72 79
48 99
48 99
20 99
81
99
69
97
83
67
49
32
85
98
31
5
Optimisation and catalytic results of the selective hydroge-
nation of nitriles into primary amines
According to the results of the optimisation experiments for
the selectivity of secondary imines, 1 is only selective towards
secondary imines, whereas the total conversion increases at
higher temperatures (Figure 2, entries 1–5). Conversely, if 2 is
used, the amine ratio increases significantly at a higher tem-
perature of 908C up to 70% and decreases to under 50% at
1008C (Figure 3, entries 1–3). Varying the reaction parameters,
a higher H2 pressure (1.0 MPa) did not favour the selectivity to-
wards primary amines (Figure 4, entry 1). A longer reaction
time of 30 h at 908C with a catalyst loading of 1 mol% and
0.4 MPa H2 in toluene did not improve the product ratio, nor
E
20
5
E
20 98
20 99
20 96
20 99
20 97
20 54
25 (+51)[d]
9 (+90)
96
F
B
–
C
B
8b
9a
98
G
G
H
3 (+94)[e]
9b
10a
10b
11 a
11 b
36 (+17)[e]
24
2
1
46
98
97
24 71
24 99
24 99
[a] Substrates: benzonitrile (A), hexylamine (B), cyclohexylamine (C),
p-bromobenzonitrile (D), p-tolunitrile (E), butyronitrile (F), heptyl cyanide
(G), 4-propoxybenzonitrile (H), isobutylamine (I). [b] Conversions and
yields were determined by GC–FID. [c] Cross-coupled RÀR’ secondary
imines are given as yields, in the case of no additional primary amine, the
corresponding secondary imines to nitriles are given as yields. Other
products are the corresponding primary amines or secondary amines.
[d] Secondary imine. [e] Primary amine.
did
a higher catalytic loading of 1.5 mol% within 20 h
(Figure 4, entries 2 and 3). The use of a different solvent such
as THF did not favour the amine selectivity, in contrast to 2-
propanol, the use of which yielded 88% of the primary amine
(Figure 4, entries 3 and 4). Generally, the use of 2-propanol as
the solvent also led to a secondary acetone imine as a side-
product. N-(Isopropylidene)benzylamine is generated from the
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generated primary amine and acetone, all substrates give very
good to excellent yields. Different para substitutions on aro-
matics, such as that in p-bromobenzonitrile, p-tolunitrile or p-
proproxybenzonitrile, do not influence the reduction of the ni-
trile group (Table 2, entries 1–3), and even ortho-amine-func-
tionalised 2-aminebenzonitrile was hydrogenated to a diamine
(Table 2, entry 4). Furthermore, aliphatic heptyl cyanide was hy-
drogenated without any hindrance with high conversion
(Table 2, entry 5).
General aspects
The inactivity of 3 and 4 is thought to be because of the occu-
pied methyl group at the N atom of the PNP pincer backbone,
thus the metal–ligand cooperativity to accept a hydrogen pair
on the N site and Ru is necessary for the hydrogenation of ni-
triles. Complexes 1 and 2 are highly active in the reduction of
nitriles, in which the outer-sphere mechanism takes place.[19]
Contrary to aliphatic PNP pincer complexes, in particular the
non-classical hydride complex 1, a similar ruthenium hydride
complex with an aromatic PNP pincer backbone was reported
to reduce nitriles directly at the metal centre without the in-
volvement of the pincer backbone.[14,26] Under the given condi-
tions, the selectivity towards secondary imines is favoured with
1, with the exception of the bulky, long-chained heptyl cya-
nide, which was converted selectively into octyl amine
(Table 1, entry 9a). Conversely, 2 can be used exclusively for
the reduction of nitriles into primary amines by small parame-
ter changes, for which temperature is an important factor.[21,23]
Besides the influence of the catalyst loading and reaction time,
the use of a more protic, polar solvent, such as 2-propanol, is
crucial for the selectivity towards primary amines in such sys-
tems, possibly because of its protonating properties.[19] We
monitored the selectivity during the reaction and observed
that full conversion was reached in the first hour and the sec-
ondary imine was initially formed almost quantitatively
(Figure 5). In reported kinetic studies, it is proposed that the
Figure 4. Catalytic hydrogenation of benzonitrile (2 mmol) to primary amine
in 3 mL of the given solvent catalysed by 2 (0.5–1 mol%). Conversions and
yields were determined by GC–FID. Secondary acetone imine was obtained
as a side-product after the catalysis in 2-propanol.
side-reaction of the obtained primary amine and acetone
formed from 2-propanol after the release of the hydrogen
pressure. A decrease of the catalytic loading to 0.5 mol% led
to 55% of the primary amine and 27% of the side-product
(Figure 4, entry 6). At a lower temperature of 508C, the reac-
tion was improved significantly and a very good yield of pri-
mary amine (89%) was obtained, despite the slightly increased
amount of secondary imine (Figure 4, entry 7). A decrease of
the reaction time to 10 h with the given reaction conditions at
908C and a catalyst loading of 0.5 mol% led to the formation
of the secondary imine as the dominant species (Figure 4,
entry 8), which is different to 20 h reaction time (Figure 4,
entry 6). The solvent effect of 2-propanol that favoured the se-
lectivity of the primary amine was reported recently by Beller
et al. who used an Fe pincer complex for the hydrogenation of
various nitriles.[19] Moreover, their treatment of the reaction
medium with HCl after the hydrogenation possibly avoided
the formation of the secondary acetone imine by salting out
the amine.[19]
Under the reaction conditions given in Figure 4, entry 5, a va-
riety of nitriles was hydrogenated into primary amines
(Table 2). If we take into account the side-reaction between the
Table 2. Hydrogenation of nitriles (2 mmol) to primary amines by 2
(1 mol% at 908C, 0.4 MPa H2) in 3 mL 2-propanol.
Entry
Nitrile[a]
t
Conversion[b] Amine[b]
Secondary acetone
[h] [%]
yield [%] imine[b] yield [%]
1
2
3
4
5
D
E
H
J
22
20
24
24
22
99
99
99
81
93
86
80
92
81
56
11
16
7
–
29
G
Figure 5. Monitoring the selectivity of the hydrogenation of benzonitrile
into benzylamine in 2-propanol at 908C by 1 mol% 2, 0.4MPa (Note: the
yield of N-(isopropylidene)benzylamine was included in that of benzyl-
amine). Samples were taken at t=0.25, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4.5, 5.5, 7.5,
10 and 24 h.
[a] Substrates: p-bromobenzonitrile (D), p-tolunitrile (E), heptyl cyanide
(G), 4-propoxybenzonitrile (H), 2-aminebenzonitrile (J). [b] Conversions
and yields were determined by GC–FID.
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first hydrogenation to primary imine by the catalyst is the rate-
determining step. The following reactions to primary amines
or secondary imines are believed to be much faster
(Scheme 4).[27,28] The formation of the secondary imine is fav-
oured at the beginning of the reaction until saturation takes
place (Figure 5, t=0.5 h). A retrograde reaction occurs possibly
through the concentrated presence of generated ammonia
and the solvent influence (Scheme 4).[18,28]
a second catalytic cycle to the primary amine (Scheme 5).
Under the given conditions (508C, 0.4 MPa H2), the formation
of the gem-diamine intermediate occurs by the condensation
of the primary imine with an equivalent of primary amine. The
immediate elimination of ammonia gives the secondary imine.
We attempted to isolate the possible stable intermediate 2a
by adding an excess of benzonitrile to 2 following a similar
protocol to that reported by Sabo-Etienne and co-workers.[18]
As 2a is very soluble in common solvents, it was isolated only
as a muddy residue (Supporting Information). However, a clear
signal in the 31P NMR spectrum is visible at d=88.4 ppm along
2
with a change in the RuÀH signal from d=À20.87 (t, JPH
=
16.3 Hz) to À14.45 ppm (t, 2JPH =21.4 Hz). Routine 31P NMR
spectroscopy of the reaction contents after the catalytic hydro-
genation of benzonitrile shows the signal of 2a at d=
88.4 ppm (Figures S7 and S8). Liquid injection field desorption/
ionisation mass spectrometry (LIFDI-MS) shows only the frag-
ment of [RuH(CO)(PNP)] at m/z 491 caused by the loss of
PhÀCN under MS conditions. In the IR spectrum, the CꢀN band
of PhCꢀNÀRu is not visible, probably because of overlap with
the nCO signal at n˜ =1898 cmÀ1. The nCO and RuÀH stretching
bands are shifted towards higher frequencies by approximately
30–40 cmÀ1. Even though a back-bonding interaction between
PhÀCN and the metal centre can decrease the stretching fre-
quency of nCꢀN slightly, the range remains between n˜ =2300–
2100 cmÀ1, which is contrary to our results.[18,29–33] Moreover,
the in situ generation of a C=N ligand that exhibits a stretching
band at around n˜ =1400–1500 cmÀ1 is not likely as no hydro-
gen was added or generated in the preparation of 2a.[30] To
further evidence this consideration, a similar observation was
made by adding acetonitrile to 2. Despite partial conversion,
Scheme 4. Possible equilibrium reactions during the catalytic hydrogenation
of nitriles.
For complex 2, we assume the possible mechanism illustrat-
ed in Scheme 5 with the model substrate benzonitrile. The co-
ordination of the nitrile to the metal centre of 2 gives the co-
ordinated nitrile complex, which is assigned tentatively as 2a.
A non-classical hydride complex is formed if 2a accepts a hy-
drogen pair.[14,24] Intermediate 2b transfers one hydrogen atom
from the H2 to the coordinated nitrile via 2c to the primary
imine, and 2 is generated again. The primary imine undergoes
2
key signals in the same area at d=88.3 ppm (t, JPH =21.4 Hz)
1
in the 31P NMR spectrum and d=À14.41 ppm in the H NMR
spectrum were found (Figures S5 and S6). Apart from 2a, no
other complex intermediate could be assigned.
Conclusions
We report the hydrogenation of aromatic and aliphatic nitriles
by ruthenium pincer complexes 1 and 2 selectively to imines
and the cross-coupling with different amines to imines under
very mild conditions with generally high conversions and high
selectivity. The highlight of our report is the use of 2 not only
to reduce various nitriles selectively into imines but also selec-
tively into amines with full conversions and high selectivity by
simply changing reaction parameters and still under mild reac-
tion conditions.
Acknowledgements
We acknowledge the Ministerium fꢀr Innovation, Wissenschaft
und Forschung NRW (MIWF-NRW) for financial support within
the Energy Research Program for the Scientist Returnee Award
for M.H.G.P. (NRW-Rꢀckkehrerprogramm). M.H.G.P. acknowledges
the Ernst-Haage Foundation at the Max-Planck-Institute for
Chemical Energy Conversion for the Ernst-Haage Prize awarded
Scheme 5. Possible mechanism for the selective hydrogenation of nitriles by
2 with benzonitrile as a model substrate.
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for our research achievements. For access to LIFDI-MS analysis,
we gratefully acknowledge Prof. Dr. T. Braun, Dr. M. Ahrens and
J. Berger (Humboldt-University Berlin, Germany). J.-H.C. thanks M.
Springer and M. Fetzer for experimental support.
Keywords: amines · homogeneous catalysis · hydrogenation ·
reaction mechanisms · ruthenium
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Received: December 22, 2014
Revised: January 16, 2015
Published online on && &&, 0000
&
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
6
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPERS
Synthetic double punch: The selective
hydrogenation of nitriles into amines
and imines is described. The selectivity
towards amines or imines can be con-
trolled by using a ruthenium pincer
complex. The reactions are conducted
at low temperature and low pressure
without any additives at low catalytic
loadings of 0.5–1 mol%, which results in
quantitative conversions and high selec-
tivity.
J.-H. Choi, M. H. G. Prechtl*
&& – &&
Tuneable Hydrogenation of Nitriles
into Imines or Amines with
a Ruthenium Pincer Complex under
Mild Conditions
ChemCatChem 0000, 00, 0 – 0
www.chemcatchem.org
7
ꢀ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ