A highly active cyclometallated iridium catalyst for the hydrogenation of imines

Article abstract of DOI:10.1039/c3ob41150h

A cyclometallated iridium complex containing an imino ligand has been shown to catalyse the hydrogenation of imines. The catalyst is highly active and selective for imino bonds, with a wide variety of imines being hydrogenated in less than 1 hour at a substrate/catalyst (S/C) ratio of 2000 at 20 bar H ₂ pressure and 75 °C.

Full text of DOI:10.1039/c3ob41150h

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A highly active cyclometallated iridium catalyst for the
hydrogenation of imines†
Cite this: DOI: 10.1039/c3ob41150h
Barbara Villa-Marcos, Weijun Tang, Xiaofeng Wu and Jianliang Xiao*
Received 4th June 2013,
Accepted 18th July 2013
A cyclometallated iridium complex containing an imino ligand has been shown to catalyse the hydrogen-
ation of imines. The catalyst is highly active and selective for imino bonds, with a wide variety of imines
being hydrogenated in less than 1 hour at a substrate/catalyst (S/C) ratio of 2000 at 20 bar H2 pressure
and 75 °C.
DOI: 10.1039/c3ob41150h
www.rsc.org/obc
Introduction
Amines are highly valuable compounds in both laboratory and
industrial chemical syntheses. They are present in a large
number of bioactive compounds, with activities of relevance to
agrochemical and pharmaceutical industries.1 Therefore,
research on the production of amines has drawn a great deal
of interest. Among the methods developed, hydrogenation of
imines allows direct access to amines.1–3 However, in compari-
son with the situation of olefins and carbonyls, catalysts that
are highly efficient in imine hydrogenation are far fewer in
number.2
Scheme 1 Synthesis of cyclometallated complexes 1.
Recently, a series of cyclometallated iridium-imino com-
plexes, Iridicycles, by our group4 and cyclometallated ruthe-
nium, rhodium and iridium complexes by Davies5 and
We started with the optimisation of the catalytic hydrogen-
de Vries6 et al. were reported, which have been exploited in ation reaction by considering the model hydrogenation of 2a
transfer hydrogenation reactions of imines, ketones and alde- at room temperature under 5 bar of H2 (Table 1). Firstly, the
hydes and chemoselective reductive amination, including activity of catalyst 1a was compared in a series of solvents,
using formate as the hydrogen source in water.2e,4c,d We report which shows that trifluoroethanol (TFE) is the solvent of
herein that Iridicycles bearing electron-withdrawing groups choice (Table 1, entries 1–7). It is understood that the chloride
are also excellent catalysts for imine reduction under hydro- ligand needs to be dissociated in order to provide a vacant site
genation conditions. The use of H2 as the hydrogen source for the coordination of H2. TFE is acidic (pKa 12.5) and has
provides a cleaner, more scalable method.
high polarity and low nucleophilicity, thus facilitating the dis-
sociation and solvation of the chloride anion, while having
minimal interaction with the cationic complex.4f,8 No reaction
was observed with [IrCp*Cl2]2 (entry 8).
Results and discussion
One of the advantageous features of these cyclometallated
complexes is the modular nature of the ligand, which allows
the steric and electronic properties of the catalyst to be fine-
tuned with ease. The electronic effect of the substituents in
the ligand was subsequently studied (entries 9–16). The elec-
tron-deficient group at the para-position of the ketone ring
clearly enhances the catalytic activity (entries 9 and 13–14).
The position of the substituent has a significant effect on the
catalyst as well, as a NO2 group at the meta-position led to a
much lower conversion (entry 14 vs. 15). The para-MeO group
at the aniline ring also proves to be critical (entry 9 vs. 16). The
We synthesised a range of cyclometallated iridium-imino com-
plexes 1 by following a procedure developed by Davies et al.
(Scheme 1).5a According to mechanistic studies, this acetate-
assisted cyclometallation follows an electrophilic C–H acti-
vation pathway.5c,7
Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, UK.
E-mail: j.xiao@liverpool.ac.uk; Fax: +(44) (0)1517943588; Tel: +(44) (0)1517942937
†Electronic supplementary information (ESI) available: ESI features detailed
analytic data of catalysts and all products. See DOI: 10.1039/c3ob41150h
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Table 1 Optimisation of the hydrogenation of 2aa
Table 2 Hydrogenation of imines derived from aromatic ketones/aldehydes
with 1fa
Entry
1
Product
t (min)
Yieldb (%)
92
Entry
[Ir]
mol% [Ir]
Solvent
t (min)
Conv.b (%)
30
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1
1
1
1
1
1
1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.05
DMSO
DMF
DCM
Toluene
MeOH
EtOH
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
120
120
120
120
120
120
120
30
30
30
30
30
30
30
30
30
15
15
30
N.R.
N.R.
N.R.
N.R.
7
2
>99
1
74
9
21
51
54
87
26
37
76
95
>99
2
3
30
60
85
89
[IrCp*Cl2]2
9
1a
1b
1c
1d
1e
1f
1g
1h
1f
10
11
12
13
14
15
16
17
18c
19d
4
75
89
1f
1f
a Reaction conditions: 1 mmol of 2a, complex 1 (0.05–1 mol%), solvent
5
6
45
60
87
92
(1.5 mL),
5 bar of H2 and 25 °C unless otherwise indicated,
15–120 min. b Conversion was determined by 1H NMR of the crude
reaction mixture; N.R. = no reaction. c 20 bar of H2. d 20 bar of H2,
75 °C.
best result was obtained with complex 1f. Having identified
the most active catalyst, our attention turned to optimization
of other parameters. The increase of the hydrogen pressure
had a positive effect on the catalytic activity (entry 18). An
increase in temperature also led to higher activity, allowing for
almost complete hydrogenation of 2a in 30 min with only
0.05 mol% of 1f (entry 19). With these results in hand, the
optimal conditions chosen for investigating the substrate
scope were set at 20 bar of hydrogen, 75 °C and a low catalyst
loading of 0.05 mol%.
7
60
45
60
91
92
93
8
9
A series of imine substrates were hydrogenated to demon-
strate the high activity and versatility of this Iridicycle catalyst.
At an S/C ratio of 2000 a wide range of imines were efficiently
hydrogenated, with reaction times ranging from
75 minutes. Table 2 presents the results of the hydrogenation
of imines derived from aromatic ketones and an aldehyde. The
catalyst tolerates functional groups with different electronic 11c
properties on the ketone (entries 1–7), albeit requiring longer
reaction times for electron-deficient ketimines. The catalyst
5 to
10
120
5
95
91
12
60
78
also tolerates meta- (entry 7), di- (entry 6) or even more steri-
cally demanding ortho-substitution (entry 8). In this regard, it
is worth noting that the catalyst is effective for the synthesis of
sterically relatively hindered amines (entry 9). A cyclic imine
was also hydrogenated (entry 10). Aldimines are particularly
active under the conditions developed. Thus, even at an S/C of
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Table 2 (Contd.)
Table 3 Hydrogenation of imines derived from aliphatic ketones/aldehydesa
Entry
13
Product
t (min)
Yieldb (%)
81
Entry
1
Product
t (min)
Yieldb (%)
79
75
30
14
90
85
2
3
30
40
71
71
a Reaction conditions: 1 mmol of imine, 0.05 mol% 1f, 1.5 mL TFE, 20
bar of H2, 75 °C, 5–120 minutes. b Isolated yields. c 0.025 mol% 1f.
4
30
20
30
5
81
67
48
82
73
4000, amine 3k was isolated in 91% yield after only 5 minutes
(entry 11), resulting in a high TOF of 4.4 × 104 h−1
.
5
This catalyst is also effective for the hydrogenation of
N-benzyl imines (Table 2, entries 13 and 14). The corres-
ponding amines can then be easily debenzylated by hydro-
genolysis with Pd/C, providing a simple route for the synthesis
of primary amines. The catalyst is chemoselective of CvN
bonds over the benzyl group.
6
7c
8
Table 3 summarises the results of the hydrogenation of
imines derived from aliphatic ketones and aldehyde. Lower
yields than those in Table 2 were recorded, probably due to the
low stability of the aliphatic ketimines. Indeed, free aniline
was detected by NMR in the crude products of some of
these reactions. The catalyst tolerates electron-donating and
-withdrawing groups in the aniline ring (entries 1–4) as well as
sterically hindered anilines (entries 2 and 4). The excellent
chemoselectivity exhibited by the catalyst is worth noting.
Thus, CvN double bonds were reduced selectively in the pres-
ence of other reducible groups, such as internal and external
alkenes (entries 5 and 6, respectively). No reduction of the
60
a The reaction conditions were the same as those in Table 2. b Isolated
yields. c 0.025 mol% 1f.
1
CvC double bond was observed in the H NMR spectrum of
the crude product.
Preliminary mechanistic studies were undertaken in
order to gain some insight into the catalytic cycle. As men-
tioned before, the hydrogenation benefits from higher H2
pressure. Fig. 1 shows that there is almost a linear correlation
between the conversion and H2 pressure, indicating that H2
is probably involved in the rate-determining step of the
hydrogenation.
Fig. 1 Effect of H2 pressure on the conversion of 2a. Reaction conditions:
1 mmol of 2a, 0.2 mol% of 1f, 1.5 mL TFE, H2, 25 °C for 5 min. The conversion
was determined by 1H NMR.
The effect of the concentration of the imine substrate and
catalyst on the initial reaction rate has also been examined. As
shown in Fig. 2, there is negligible variation of the rate when
hydrogenation in question is probably rate-limited by the step
of hydride formation. Fig. 2 also reveals that little hydrogen-
ation takes place when the catalyst concentration is very low
(<0.17 mM). This presumably results from the catalyst being
poisoned by a small amount of impurities, e.g. O2, present in
the hydrogen gas or the system. The iridium-hydride inter-
mediate may be sensitive to oxygen.9
▲
the concentration of the substrate increases ( ). This suggests
that the reaction rate is zero order with respect to the imine
substrate. On the other hand, there is almost a linear depen-
dence of the initial rate on the concentration of the catalyst
■
( ). These results rule out the possibility of hydride transfer
controlling the catalytic turnover and suggest that the imine
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procedure for the hydrogenation makes the Iridicycles attrac-
tive for academic and industrial applications. Evidence
suggests that the hydrogenation is probably turnover-limited
by the step of hydride formation.
Experimental
General
Unless otherwise specified, all reagents were obtained com-
mercially and used without further purification. CH2Cl2 was
dried over CaH2 and distilled prior to use. 1H and 13C NMR
spectra were recorded at 400 (1H) and 100 (13C) MHz in ppm
with reference to TMS as an internal standard in CDCl3 at
ambient temperature unless otherwise stated. Mass spectra
were obtained by chemical ionization (CI). All compounds
were characterised by 1H and 13C NMR, MS, HRMS and micro-
analysis. Analytic data for new iridium complexes are given
Fig. 2 Effect of the substrate and catalyst concentrations on the initial reaction
rate of hydrogenation of 2a. Initial rates were calculated using conversions
(<10%) and based on the average of 3 reactions. Reaction conditions: 1.5 mL of
■
▲
TFE at 25 °C, 5 bar of H2; 0.33 mM of catalyst for ( ); 0.17 M substrate for ( ).
below. Imines were prepared according to
procedure.11
a literature
General procedure for the synthesis of cyclometallated
complexes4,5
An oven-dried Schlenk tube containing a stir bar was charged
with [IrCp*Cl2]2 (1 eq.), imine ligand (2 eq.) and NaOAc
(10 eq.). Following degassing with N2 three times, freshly dis-
tilled CH2Cl2 was then injected. The resulting mixture was
stirred at rt overnight. The reaction mixture was then filtered
through celite and concentrated in vacuo. The resulting solid
was washed with diethyl ether/hexane.
Typical procedure for imine hydrogenation with
cyclometallated iridium complexes
A glass liner containing a stir bar was charged with an imine
(1 mmol) and TFE was added (0.5 mL, except for the synthesis
of 3k and 3u where 1 mL was used). The mixture was stirred
until the imine was dissolved. 1 mL (0.5 mL for the products
3k and 3u) of a stock solution (5 mM) containing the catalyst
1f was then added. The glass liner was then placed in an auto-
clave followed by degassing with H2 three times. The hydro-
genation was carried out at 20 bar of H2 with stirring at 75 °C
for 5–120 min. The stirring was then stopped and the autoclave
allowed to cool down to rt. The hydrogen gas was then care-
fully released in the fumehood and the solution transferred to
a flask and concentrated in vacuo to afford the crude product.
Flash chromatography purification with a column of silica gel
eluted with petroleum ether–ethyl acetate (20 : 1 to 5 : 1)
yielded the desired amine product.
Scheme 2 Proposed catalytic cycle for the ionic hydrogenation of imines (S
may be a substrate molecule).
We propose that catalyst 1f promotes hydrogenation
through an ionic pathway which involves no imine coordi-
nation (Scheme 2).10,11 Following dissociation of the chloride
from 1f, which is likely to be promoted by TFE, the catalytic
cycle is made up of 3 steps: reversible coordination of H2, turn-
over-limiting heterolytic cleavage of H2 into a hydride and a
proton to be taken up by the imine, and subsequent transfer
of the hydride to the resulting iminium cation. Mechanistic
studies on the ionic pathway and the hydrogenation of imines
have been reported previously.12
Iridium complex 1b. The product was obtained as a bright
yellow solid according to the cyclometallation procedure in
1
17 h; H NMR (400 MHz, 253 K, CDCl3) δ 0.94 (d, J = 6.7 Hz,
Conclusions
3H), 0.97 (d, J = 6.5 Hz, 3H), 1.43 (s, 15H), 1.91–1.98 (m, 1H),
Cyclometallated iridium complexes have been shown to be 2.43 (s, 3H), 2.57–2.57 (m, 2H), 3.87 (s, 3H), 6.79 (d, J = 5.4 Hz,
highly active and chemoselective for the reduction of imines 1H), 6.84 (d, J = 7.5 Hz, 1H), 6.89 (d, J = 5.4 Hz, 1H), 6.98 (d, J =
under hydrogenation conditions. The combination of air-stabi- 6.7 Hz, 1H), 7.45 (d, J = 7.5 Hz, 1H), 7.60 (s, 1H), 7.80 (d, J =
lity and ease of synthesis of catalysts and a simple work-up 6.7 Hz, 1H); 13C NMR (100 MHz, 253 K, CDCl3) δ 8.8, 17.1,
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22.7, 23.0, 30.8, 46.1, 55.6, 88.9, 112.2, 114.8, 122.8, 123.5, the EPSRC UK National Mass Spectrometry Facility at Swansea
125.0, 128.3, 136.0, 144.2, 145.5, 146.0, 157.3, 167.7, 181.2; University.
Anal Calcd for C29H37ClIrNO: C, 54.15; H, 5.80; N, 2.18.
Found: C, 54.66; H, 5.90; N, 2.06.
Iridium complex 1e. The product was obtained as a bright
orange solid according to the cyclometallation procedure in
Notes and references
17 h; 1H NMR (400 MHz, 253 K, CDCl3) δ 1.43 (s, 15H), 2.43 (s,
3H), 3.88 (s, 3H), 6.82 (d, J = 8.4 Hz, 1H), 6.91 (d, J = 8.4 Hz,
1H), 7.00 (d, J = 8.3 Hz, 1H), 7.18 (dd, J = 8.2, 1.8 Hz, 1H),
7.39 (d, J = 8.3 Hz, 1H), 7.79 (d, J = 8.2 Hz, 1H), 7.91 (d, J =
1.8 Hz, 1H); 13C NMR (100 MHz, 253 K, CDCl3) δ 8.7,
17.3, 55.6, 89.4, 112.3, 114.9, 124.5, 127.5, 130.0, 137.2, 143.8,
145.1, 146.5, 152.6, 157.6, 170.1, 180.9; Anal Calcd
for C25H28BrIrNO: C, 45.08; H, 4.24; N, 2.10. Found: C, 44.98;
H, 4.25; N, 2.02.
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Iridium complex 1f. The product was obtained as a black
1
solid according to the cyclometallation procedure in 17 h; H
NMR (400 MHz, 253 K, CDCl3) δ 1.46 (s, 15H), 2.52 (s, 3H),
3.89 (s, 3H), 6.84–6.86 (m, 1H), 6.94–6.96 (m, 1H), 7.03 (d, J =
8.6 Hz, 1H), 7.64 (d, J = 8.5 Hz, 1H), 7.79 (d, J = 7.8 Hz, 1H),
7.89 (dd, J = 8.5, 2.2 Hz, 1H), 8.63 (d, J = 2.2 Hz, 1H); 13C NMR
(100 MHz, 253 K, CDCl3) δ 8.81, 55.7, 90.1, 112.5, 115.1, 117.1,
123.1, 124.4, 128.7, 129.2, 143.6, 148.8, 153.5, 157.9, 168.4,
180.5; HRMS for C25H28ClIrN2NaO3 [M + Na]+: m/z Calcd:
653.1292; Found: 653.1268; Anal Calcd for C25H15ClIrN2O3: C,
47.50; H, 4.46; N, 4.43. Found: C, 47.94; H, 4.51; N, 4.40.
Iridium complex 1g. The product was obtained as a red
solid according to the cyclometallation procedure in 17 h. It is
a mixture of two regioisomers in a ratio of 7 : 1, with the major
regioisomer having the NO2 group para to the iridium; 1H
NMR (400 MHz, 253 K, CDCl3) δ 1.41 (s, 1.9H), 1.44 (s, 13.1H),
2.35 (s, 0.4H), 2.55 (s, 2.6H), 3.86 (s, 0.4H), 3.88 (s, 2.6H), 6.80
(d, J = 8.6 Hz, 0.3H), 6.85 (d, J = 8.0 Hz, 0.8H), 6.95 (d, J = 8.8
Hz, 1.2H), 7.02 (d, J = 8.2 Hz, 0.8H), 7.66 (t, J = 8.0 Hz, 0.2H),
7.72 (d, J = 8.0 Hz, 0.8H), 7.96 (d, J = 8.4 Hz, 0.8H), 8.06 (dd, J =
8.4, 1.5 Hz, 0.8H), 8.35 (d, J = 1.5 Hz, 1H), 8.82 (s, 0.1H); 13C
NMR (100 MHz, 253 K, CDCl3) δ 8.8 (M), 9.0 (m), 17.4 (M),
17.7 (m), 55.6 (m), 55.7 (M), 90.2 (m), 90.6 (M), 112.5, 114.2,
115.2, 121.0, 122.2, 123.2, 123.3, 124.4, 125.1, 126.0, 129.6,
133.2, 135.7, 141.1, 142.7, 143.3, 143.5, 148.1, 148.6, 156.2,
157.9, 163.7, 173.9, 180.8, 181.1, 181.9; Anal Calcd for
C25H15ClIrN2O3: C, 47.50; H, 4.46; N, 4.43. Found: C, 47.44; H,
4.37; N, 4.41.
Iridium complex 1h. The product was obtained as a deep
red solid according to the cyclometallation procedure in 17 h;
1H NMR (400 MHz, 253 K, CDCl3) δ 1.42 (s, 15H), 2.49 (s, 3H),
6.88 (d, J = 7.6 Hz, 1H), 7.27–7.31 (m, 1H), 7.35 (d, J = 8.0 Hz,
1H), 7.45 (t, J = 7.6 Hz, 1H), 7.52 (t, J = 7.6 Hz, 1H), 7.59 (d, J =
8.0 Hz, 1H), 7.80 (d, J = 7.6 Hz, 1H), 8.06 (s, 1H); 13C NMR
(100 MHz, 253 K, CDCl3) δ 3.9, 12.8, 85.2, 109.6, 115.3, 117.3,
118.3, 120.5, 122.2, 123.2, 123.6, 125.6, 133.6, 145.4, 146.9,
162.9, 176.1; Anal Calcd for C25H26ClIrN2: C, 51.58; H, 4.50; N,
4.81. Found: C, 51.58; H, 4.44; N, 4.64.
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We thank AstraZeneca for a studentship (BVM), EPSRC for a
postdoctoral fellowship (WJT) and the University of Liverpool
for support (XFW). Mass spectrometry data were acquired at
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