Ruthenium-catalyzed hydrogenation of nitriles: Insights into the mechanism

Article abstract of DOI:10.1021/ja102759z

Hydrogenation of benzonitrile into benzylamine is catalyzed under very mild conditions by the ruthenium bis(dihydrogen) complex RuH₂(H ₂)₂(PCyp₃)₂, incorporating two tricyclopentylphosphines. Two key intermediates have been isolated, resulting from the activation of benzonitrile at early stages of activation, i.e., either N-coordination through the nitrile function or first hydrogenation with benzylimine formation, followed by, thanks to C-H activation, coordination at ruthenium as an orthometalated ligand.

Full text of DOI:10.1021/ja102759z

Published on Web 05/19/2010
Ruthenium-Catalyzed Hydrogenation of Nitriles: Insights into the Mechanism
Rebeca Reguillo, Mary Grellier,* Nicolas Vautravers, Laure Vendier, and Sylviane Sabo-Etienne*
Laboratoire de Chimie de Coordination, CNRS, UPS, INPT, UniVersite´ de Toulouse, 205 Route de Narbonne,
F-31077 Toulouse, France
Received April 1, 2010; E-mail: sylviane.sabo@lcc-toulouse.fr
Table 1. Catalytic Hydrogenation of Benzonitrile (A) into
Benzylamine (C) and Dibenzylimine (D) with Complex 2, 3, or 5
(Scheme 2) at 22 °C and 3 bar H2
When considering the tremendous importance of adiponitrile as
a precursor to hexamethylenediamine, a key component for the
synthesis of the Nylon-6,6, it is rather surprising that the hydro-
genation of nitriles into primary amines has received little interest
among the organometallic community.1 One of the main problems
remains selectivity, as imines (B and D) or secondary amines (E)
are very often observed as side products (Scheme 1).
conversion of A (%)c
product ratio C:D (%)c
entry
cat.a
solvent
2 h
24 h
2 h
24 h
1
2
3
4
5
6
2
pentane
THF
THF
none
THF
THF
94
56
62
84
68
68
94
96
98
97
97
96
94:6
96:4
22:77
0.1:99.9
96:4
94:6
99:1
96:4
89:11
99:1
99:1
2
2b
2
Scheme 1. Hydrogenation of Nitriles (A) to Primary Amines (C)
and Secondary Reaction with the Intermediate Imines (B)
3
5
98:2
a Catalyst and benzonitrile in a 1:200 ratio unless otherwise stated.
b Catalyst and benzonitrile in a 1:500 ratio. c Determined by GC.
A typical plot of the change in concentrations over time is shown
in Figure 1. This plot reveals that dibenzylimine D is formed as an
intermediate product and then converted to benzylamine C.
The group of Beller has very recently addressed this problem
by using Ru(cod)(methylallyl)2 as a catalyst precursor and ben-
zonitrile as the model substrate.2 Indeed, ruthenium is a key metal
in hydrogenation,3 and in this area, the bis(dihydrogen) ruthenium
complex RuH2(H2)2(PCy3)2 (1) deserves specific attention.4 In 1996,
its use as a catalyst precursor for nitrile hydrogenation was disclosed
by Beatty and Paciello in a series of patents.5 The optimal conditions
involved 0.1 mol % of catalyst, a dihydrogen pressure in the range
of 50-70 bar, and a temperature in the range of 80-100 °C.
Recently, Morris et al. compared the performance of 1 with that
of hydride ruthenium complexes incorporating a tetradentate
ethP2(NH)2 ligand {ethP2(NH)2 ) [PPh2(o-C6H4)(CH2NHCH2)]2}.6
We have recently shown that a small variation in the cycloalk-
ylphosphine can induce dramatic changes in the properties of the
corresponding bis(dihydrogen) complex. In 2005, we prepared the
new complex RuH2(H2)2(PCyp3)2 (2), incorporating two tricyclo-
pentylphosphines (PCyp3).7 2 displays as the analogous PCy3
complex two labile dihydrogen ligands. However, it has shown very
different behavior, in particular regarding dehydrogenation pro-
cesses.8 Moreover, the superior activity of 2 as a catalyst precursor
for the Murai reaction and for H/D exchange pushed us to
investigate its activity for nitrile reduction. Herein, we describe the
results of a catalytic investigation of benzonitrile reduction by 2 at
ambient temperature and mild pressure and the first example of a
catalyst resting state resulting from C-H activation and trapping
of the intermediate imine B.
Figure 1. Hydrogenation of benzonitrile with 0.5% 2 in THF at 22 °C
and 3 bar H2.
We conducted a series of experiments at both the stoichiometric
and catalytic levels to gain information on the mechanism of the
1
reaction. Monitoring of the catalytic reaction by H and 31P{1H}
NMR was carried out using a THF-d8 solution in a Quick Pressure
Valve NMR tube (Supporting Information, Figures 1S and 2S).
Initially, the addition of 500 equiv of benzonitrile to 2 was
performed, and NMR spectra were recorded. At 223 K, the
disappearance of 2 was immediately observed, with concomitant
formation of two new species, characterized by two 31P signals at
1
43.6 ppm (3) and 65.3 ppm (4) and two H triplets in the hydride
The catalytic experiments were performed at ambient temperature
with 3 bar H2 in pentane or THF or in absence of any solvent,
with a catalyst:benzonitrile ratio of 1:200 or 1:500. The main results
are summarized in Table 1. Conversion of benzonitrile is given
after 2 or 24 h of reaction, with the corresponding selectivity in
benzylamine C and dibenzylimine D (Scheme 1, R ) Ph). The
formation of dibenzylamine E was never detected. The concentra-
tions of benzonitrile and the resulting hydrogenated products were
monitored over the course of the reaction by gas chromatography.
region at -11.36 ppm (3) and -14.68 ppm (4) (Scheme 2). We
then pressurized the tube with 3 bar H2 at ambient temperature.
New NMR spectra were recorded showing an increase of the signals
corresponding to 3 and detection of a new species 5 characterized
by a 31P NMR signal at 56.7 ppm and a broad hydride signal at
-8.70 ppm. Further monitoring after 60 and 105 min showed the
disappearance of 4. After 105 min, 3 was the only organometallic
species detected, together with a weak 31P NMR signal at 5 ppm
characteristic of free PCyp3.9 In view of these NMR data, we carried
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7854 J. AM. CHEM. SOC. 2010, 132, 7854–7855
10.1021/ja102759z  2010 American Chemical Society

C O M M U N I C A T I O N S
out stoichiometric reactions. The addition of 1 equiv of PhCN to 2
in THF-d8 at ambient temperature led to signals corresponding to
species 5 seen during the catalytic experiments. No other species
could be detected apart from 2 and 5. Complex 5 could be isolated
in good yield and fully characterized as a hydrido(dihydrogen)
cyclometalated species by multinuclear NMR, IR, and X-ray
diffraction (Supporting Information). It is remarkable that the
reaction leads to benzylimine formation (B), the first hydrogenation
intermediate in benzonitrile hydrogenation, and through C-H
activation gives rise to the formation of the orthometalated complex
5. The X-ray and spectroscopic data follow those reported for other
orthometalated complexes.10
and found, as shown in Table 1, entries 5 and 6, data very similar
to those reported in entry 2 when using 2 as a catalyst precursor
(see also Supporting Information, Figure 3S and Table 1S).
In summary, benzonitrile hydrogenation into benzylamine is
readily achieved under mild conditions by the bis(dihydrogen)
complex 2 incorporating tricyclopentylphosphines. A key event in
this system is ortho-directed C-H activation within the aryl group,
which induces fast cyclometalation and trapping of the intermediate
imine to generate 3, the catalyst resting state. With the coordination
and cyclometalation of arylimines previously observed for different
metals,10 the isolation of the cyclometalated imine adducts 3 and
5 from benzonitrile provides useful insight into the hydrogenation
mechanism. Theoretical studies and reactivity toward other nitriles
are the subject of ongoing research, and the role of ammonia in
promoting the formation of benzylamine will be especially analyzed.
Scheme 2. Benzonitrile Hydrogenation versus Coordination
Acknowledgment. We thank the ANR (Programme blanc
“HyBoCat” ANR-09-BLAN-0184) and the CNRS for support.
Supporting Information Available: Full details of the synthesis
and characterization of new compounds, standard catalytic procedures,
as well as X-ray data in CIF format for 3, 5, and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
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London, 2006; pp 79-113.
When 2 equiv of PhCN was added to 2, the signals corresponding
to 5 and 3 also seen in the catalytic experiments were observed,
prior to full conversion to 3. Complex 3 could also be isolated and
fully characterized by multinuclear NMR, IR and X-ray diffraction
(Figure 2 and Supporting Information). 3 displays a structure very
similar to that of 5, except with the dihydrogen ligand replaced by
a benzonitrile end-on coordinated to the metal through nitrogen. It
is remarkable that in 3, two different activation stages of benzonitrile
are present: a very early stage with one PhCN acting as a two-
electron donor ligand, and the first hydrogenation step with the
imine coordinated to the metal thanks to C-H activation.
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Figure 2. X-ray structure of 3.
It turned out to be impossible to isolate 4, the first species
observed when mixing 2 with benzonitrile in the absence of H2 at
223 K. Monitoring the mixture up to ambient temperature showed
the conversion of 4 into 3. We tentatively propose that 4 is the
dihydride complex RuH2(PhCN)2(PCyp3)2, resulting from the
substitution of the two dihydrogen ligands in 2 by two PhCN, end-
on coordinated through nitrogen to the ruthenium center.11 Having
established the identities of 3 and 5, we tested their catalytic activity
(11) We could isolate and fully characterize (X-ray structure) the analogous
bis(acetonitrile) complex RuH2(CH3CN)2(PCyp3)2 (6) (see Supporting
Information).
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