Efficient hydrosilylation of imines using catalysts based on iridium(iii) metallacycles

Article abstract of DOI:10.1039/c4cy01233j

Ir(iii) metallacycles were applied as catalysts for the hydrosilylation of various ketimines and aldimines with sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate, NaBArF₂₄, as an additive. By using a slight excess of the organosilane reagent, the reactions proceeded rapidly and efficiently, at low catalyst loadings and at room temperature. Several examples of cationic Ir(iii) catalysts could be synthesised, characterized and tested. In situ-generated catalysts proved to be more active as compared to isolated ones and species with non-coordinating BArF₂₄ counterion gave the highest catalytic activities.

Full text of DOI:10.1039/c4cy01233j

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Efficient hydrosilylation of imines using catalysts
based on iridiumIJIII) metallacycles†
Cite this: Catal. Sci. Technol., 2015,
5, 1452
Y. Corre,ae W. Iali,c M. Hamdaoui,c X. Trivelli,d J.-P. Djukic,c
abe
F. Agbossou-Niedercornabe and C. Michon
*
IrIJIII) metallacycles were applied as catalysts for the hydrosilylation of various ketimines and aldimines with
sodium tetrakisij(3,5-trifluoromethyl)phenyl]borate, NaBArF24, as an additive. By using a slight excess of the
organosilane reagent, the reactions proceeded rapidly and efficiently, at low catalyst loadings and at room
temperature. Several examples of cationic IrIJIII) catalysts could be synthesised, characterized and tested.
In situ-generated catalysts proved to be more active as compared to isolated ones and species with
non-coordinating BArF24 counterion gave the highest catalytic activities.
Received 22nd September 2014,
Accepted 13th November 2014
DOI: 10.1039/c4cy01233j
www.rsc.org/catalysis
reports have stated the efficiency of such iridacycles for
hydrogenation reactions.2h,i,3e–h
Introduction
Amines are ubiquitous in natural products, building blocks
or targets for fine chemicals, farming-related chemicals and
biologically active compounds.1 Among the several synthetic
methods for amine, the transition-metal-catalysed reduction
of imines is a valuable pathway, which can be performed by
hydrogenation,2 hydrogen transfer3 or hydrosilylation.4 The
latter, which operates under mild reaction conditions without
any autoclave pressure, is an interesting alternative to hydro-
genation, provided that inexpensive and abundant hydro-
silanes are used. The stereoselective synthesis and reactivity
of metallacycles like iridacycle 1 and chromiumtricarbonyl-
bound iridacycle 2 have been studied by some of us for sev-
eral years (Scheme 1).5 Recently, complexes of type 2 were
found to display novel catalytic properties for the conversion of
terminal aromatic alkynes into racemic N-phenyl, 1-arylethyl-
amines by tandem hydroamination and hydrosilylation/
protodesilation reactions under mild “one pot” conditions.5a
The peculiar efficiency of such catalysts in promoting hydro-
silylation was assigned to the intervention of Ir–hydrido inter-
mediate 3,5a,e a key species for the transfer of the hydritic
H atom to the electrophilic imine substrate. In addition, other
Hence, considering the interest of some of us in the syn-
thesis and reactivity of iridacycles5 and the complementary
research of others on the synthesis of amines using hydro-
amination reaction,6 we investigated IrIJIII) metallacycles as
catalysts for the hydrosilylation of imines.
Results and discussion
Screening of catalysts, additives and reaction conditions was
performed on the hydrosilylation of N-phenylethylideneaniline
4a and we rapidly noticed that the use of an additive was criti-
cal to displace the iridium chloride ligand and afford an active
cationic catalyst (Table 1). Iridacycle 1 proved to be the most
active catalyst as compared to chromiumtricarbonyl-bound
iridacycle 2 (entries 1–4). Dichloromethane and 1,1′,2,2′-tetra-
chloroethane gave good yields and were the most interesting
solvents, considering their polarity and high boiling point
(entries 3–6). The use of toluene and tetrahydrofuran implied
lower yields (entries 7–9). By changing the couteranion of the
a Université Lille Nord de France, 59000 Lille, France.
E-mail: christophe.michon@ensc-lille.fr; Fax: +33 320436585
b CNRS, UCCS UMR 8181, 59655 Villeneuve d'Ascq, France
c Institut de Chimie de Strasbourg, UMR 7177, Université de Strasbourg, 4 rue
Blaise Pascal, F-67000 Strasbourg, France
d UGSF CNRS, UMR 8576, Université Lille Nord de France, 59655 Villeneuve
d'Ascq Cedex, France
e ENSCL, UCCS-CCM-CCCF UMR 8181, (Chimie-C7) CS 90108, 59652 Villeneuve
d'Ascq Cedex, France
† Electronic supplementary information (ESI) available: Experimental
procedures, analytical data, NMR spectra, and HRMS spectra. CCDC 1031536.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4cy01233j
Scheme 1 Studied iridacycles.
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Table Screening of catalysts, additives, solvents, times and
Paper
1
hundred-fold decrease. Interested by such an effect, we
studied in detail the activity of our catalytic system in order
to find the best loadings of catalyst 1 and NaBArF24 additive
(Table 2). When the catalyst loading was reduced from 1 to
0.5 mol%, 2 hours were needed to complete the hydro-
silylation of imine 4a at room temperature, but the catalyst
activity, i.e. turnover number (TON), increased (entries 1–4).
By operating at 0.1 mol% of 1, the yield of 5a was still high
within 2 hours and a 24 hour reaction time allowed quantita-
tive yields and TONs of 1000 (entries 5–10). When we
decreased further the catalyst loading, we noticed that a
higher concentration of the reaction medium could decrease
the yield by 10 to 20%, as well as the catalyst activity (entries
1, 2, 5, 6, 8, and 9). At 0.05 mol% loading, catalyst 1
remained quite active at room temperature, affording 88%
yield of 5a and a TON of 1760 within 24 hours (entries 11–13).
Finally, using a 0.01 mol% catalyst loading resulted in an
important decrease in 5a yield at room temperature (entry 14).
However, by increasing the reaction temperature to 40 °C
or even 60 °C, high yields and up to 9000 TON could be
achieved (entries 15, 16). To the best of our knowledge, spe-
cies 1 is among the most active IrIJIII) catalysts for hydro-
silylation reactions.7 Morever, by using 1.2 equivalents of the
reducing agent, iridacycle 1 appeared to be sustainable as
compared to catalysts requiring 3 equivalents of silane.
temperatures
Cat.
(mol%)
Additive
(mol%)
Time
(h)
T
(°C)
Yielda
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
1 (2.5)
2 (2.5)
1 (2.5)
2 (2.5)
1 (2.5)
1 (2.5)
1 (2.5)
1 (2.5)
1 (2.5)
1 (2.5)
1 (1)
—
—
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
TCEb
40
40
40
40
40
20
40
40
20
20
20
20
20
0.25
0.25
40
40
40
40
40
100
40
40
100
25
25
25
25
25
25
17
11
67
34
71
83
12
11
38
52
<5d
37
62
100
69
NH4BF4 (10)
NH4BF4 (10)
NH4BF4 (10)
NH4BF4 (10)
NH4BF4 (10)
NH4BF4 (10)
NH4BF4 (10)
NH4PF6 (10)
NaBF4 (2)
TCE
THFc
Toluene
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
9
10
11
12
13
14
15
1 (1)
1 (1)
1 (1)
2 (1)
NaPF6 (2)
NaSbF6 (2)
NaBArF24 (2)e
NaBArF24 (2)
a
b
Measured by 1H NMR after work-up.
TCE: 1,1′,2,2′-
c
d
tetrachloroethane. THF: tetrahydrofuran.
8% yield in 40 h.
NaBArF24: sodium tetrakisij(3,5-trifluoromethyl)phenyl]borate.
e
IrIJIII) catalyst, we confirmed that the less coordinating the
Various silane reagents could be used for the hydro-
silylation of 4a (Table 3). Whereas the triethoxysilane reagent
afforded 5a in a poor yield, triethylsilane proved to be an effi-
cient hydrosilylation agent for a moderate cost (entries 1–2).
The use of expensive phenyl-, diphenyl- and triphenylsilane
led also to good yields (entries 3–5). Sterically hindered
1,1,1,3,5,5,5-heptamethyltrisiloxane afforded 5a in a poor
yield and so did the electron-poor trichlorosilane (entries 6–7).
Finally, among the inexpensive and green hydrosilylation
anion was, the higher the yields were (entries 10–15). The
−
activity increased according to the following order: BF4
<
−
−
−
PF6 < SbF6 ≪ BArF24
.
Hence, sodium salts with specific anions were efficient
to displace chlorides and afford an active catalyst (entries 11–15).
Indeed, the use of tetrakisij(3,5-trifluoromethyl)phenyl]borate,
e.g. BArF24 anion, proved to be critical in drastically reducing
the reaction times from 20 to 0.25 hour, that is to say a
Table 2 Effects of catalyst and additive loadings
Entry
Cat. 1 (mol%)
NaBArF24 (mol%)
[4a] (mmol l−1
)
Time (h)
Temperature (°C)
Yielda (%)
TONb
1
2
3
4
5
6
7
8
1
1
2
2
1
1
0.077
0.039
0.039
0.039
0.39
0.39
0.39
0.77
0.77
0.77
0.77
0.77
0.77
0.77
0.77
0.77
0.5
0.5
0.5
2
0.5
2
24
0.5
2
24
0.5
2
25
25
25
25
25
25
25
25
25
25
25
25
25
25
40
60
100
100
84
100
30
78
100
18
58
100
37
56
88
100
100
168
200
300
780
1000
180
580
0.5
0.5
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.05
0.05
0.01
0.01
0.01
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.02
0.02
0.02
9
10
11
12
13
14
15
16
1000
740
1120
1760
3915
6826
9000
24
24
24
24
39
68
90
a
1
Measured by H NMR after work-up. b TON: turnover number (mol of product/mol of catalyst).
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Table 3 Screening of the silane reagent
Table 4 Hydrosilylation of ketimines catalysed by cycloiridated complex 1
Entry
Silane
Costa (€/mmol)
Time (h)
Yieldb (%)
Time
(h)
Yielda
Entry
Reagent
(isolated yield %)b
1
2
Et3SiH
(EtO)3SiH
0.41
0.79
0.25
0.25
17
0.25
0.25
0.25
0.25
17
100
8
1
2
3
4
5
6
0.25
0.25
0.25
7
0.25
2
5a 100 (87)
5b 100 (100)
5c 97 (95)
5d 100 (100)
5e 100 (100)
5f 90 (64)
16
64
100
100
6
3
4
5
6
Ph3SiH
Ph2SiH2
PhSiH3
IJMe3SiO)2SiHMe
1.08
1.55
1.64
0.38
7
7
Cl3SiH
0.42
0.25
17
0.25
0.25
17
8
16
100
5
ij(CH3)2SiH]2Oc
0.11
0.01
7
0.25
5g 100 (98) 4/5f
8
9
PMHSd
13
8
9
0.25
1
5h 100 (37)c
5i 100 (85)
a
b
According to ref. 4 and www.sigmaaldrich.com. Measured by 1H
c
d
NMR after work-up.
polymethylhydroxysiloxane.
1,1,3,3-Tetramethyldisiloxane.
PMHS:
10
11
1
5j 70 (68)
5k 34 (−)
reagents used, polymethylhydroxysiloxane (PMHS) showed
poor results, but we were glad to reach a quantitative yield
using the tetramethyldisiloxane reagent (entries 8–9).
24
We next studied the hydrosilylation of
a series of
ketimines. Iridacycle catalyst 1 was used under selected reac-
tion conditions in order to perform fast and straightforward
reduction reactions (Table 4). The hydrosilylation of
N-phenylethylideneaniline 4a and related para-substituted
ketimines 4b–c could be performed readily and afforded the
corresponding amines in high isolated yields (entries 1–3).
We noticed that the substrate reactivity could be influenced
by ortho-substituents as hydrosilylation of the brominated
reagent 4d required much more time to be completed (entry 4).
However, the ortho-alkylated substrate 4e reacted as well
as ketimines 4a–c (entry 5). Interestingly, halogenated sub-
strates 4c–d were reduced without any dehalogenation.
Whereas the hydrosilylation of triarylimine 4g was easily
performed (entry 7), the reduction of N-phenylpropyl-
ideneaniline 4f required 2 hours to be completed (entry 6).
Aliphatic imines, which are challenging substrates,2,2a–i,3,3a–h,4
could be also reduced efficiently. Cyclic reagent 4h reacted
easily (entry 8) and dialkylimines 4i–j required only one hour
of reaction to be reduced (entries 9, 10). As already noticed
with other catalysts,8 hydrosilylation of N-cyclohexyl imines
like 4k proved to be difficult, most likely because of
reagent steric hindrance (entry 11). Indeed, the switch to
N-butylimine 4l led, in our case, to a much more efficient
reduction (entry 12).
12
2
5l 100 (88)
a
b
Measured by 1H NMR after workup. Isolated yield after flash
chromatography. c Decomposition of product.
corresponding amines in high isolated yields independently
of the reagent substitution (entries 1–7). Indeed, haloge-
nated substrates 6b, d, and
f were reduced without
any dehalogenation. Regarding the hydrosilylation of
N-alkylaldimines 6h and 6i, the reactions were slower but
good yields were obtained after 1 to 2 hours of reaction (entries
8–9). Other aryl-, heteroaryl- and alkylaldimines 6j–l could be
reduced in high yields (entries 10–12). Finally, the hydro-
silylation of conjugated aldimine 6m gave 7m1 as the major
product through 1,2-reduction of the imine function. The
doubly reduced product 7m2 was isolated as a minor product
arising from the additional reduction of the CC bond
(entry 13). It was worth noting that the use of an excess of
triethylsilane allowed most of product 7m1 to be reduced to
7m2 within 18 hours (entry 14).
As a preliminary study regarding the possible active spe-
cies involved in the catalytic process and the mechanism of
such hydrosilylation reaction,9,10 we performed an analysis
by electrospray ionization mass spectroscopy on the reaction
mixture obtained from the hydrosilylation of ketimine 4b
The reaction scope was further studied with the hydro-
silylation of a series of aldimines. Iridacycle catalyst 1 and
selected conditions were applied for fast and straightforward
reactions (Table 5). The hydrosilylation of N-benzylidene
aniline 6a and related ortho- and para-substituted aldimines
6b–g could be performed readily and afforded the
1454 | Catal. Sci. Technol., 2015, 5, 1452–1458
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Table 5 Hydrosilylation of aldimines catalysed by cycloiridated complex
1
hydride complex like 3 to be observed, probably because of
the possible ionization of such species during the analysis
or because of the reaction with alcoholic solvents and air
atmosphere.
In order to compare in situ-generated and isolated cata-
lysts, cationic iridacycle 8a was isolated. Whereas 8a could be
characterized by elemental analysis, HRMS, H and 13C NMR
1
Time Yielda (isolated
(h)
spectra proved to suffer from strong dynamic effects. The lat-
ter were induced by the rapid switch of the pentamethyl-
cyclopentadienyl fragment from endo to exo positions with
respect to the 2-phenyl pyridine ligand. This phenomenon
was already observed on related iridacycles and predicted by
calculations by some of us.5b Complex 8b could also be syn-
thesized and its molecular structure was confirmed by X-ray
diffraction analysis, two iridacycle entities being bridged by a
chloride and with a single BArF24 anion (Table 6 and Fig. S1
in the ESI†). Iridacycles 8a–b were then tested on the hydro-
silylation of N-phenylethylideneaniline 4a (Table 6). Though
the in situ-prepared catalyst involving chlorinated iridacycle 1
and NaBArF24 afforded amine 5a quantitatively, moderate
yields of 27% and 39% were observed using catalysts 8a and
8b, respectively (entries 1–3). In addition, cationic acetonitrile
iridacycles 9a–d were prepared using a related procedure11
(see Table 6 and the ESI†). The coordination of acetonitrile to
iridium was confirmed unambiguously at the liquid state by
DOSY 1H and 2D-15N-HMBC NMR experiments (see Fig. S2
and S3 in the ESI†). Acetonitrile BArF24 complex 9a proved to
catalyse the hydrosilylation of ketimine 4a in a fair 82%
yield (Table 6, entry 4). However, complexes 9b–d bearing
SbF6, PF6 and BF4 anion, respectively, gave much lower
yields (Table 6, entries 5–8). Hence, we showed that in situ-
generated catalysts were more active as compared to isolated
cationic iridacycles. In both cases, the catalyst activity and
productivity strongly depended on the counterion and the
non-coordinating BArF24 anion proved to be the most suit-
able one. Interestingly, such an anion effect was already
observed for iridium-catalysed hydrogenation of alkenes and
imines12a,b and the nature of the cation was previously shown
to influence the catalytic activity of ruthenium-catalysed
hydrogenation of ketones.12c
Entry Reagent
yield %)b
1
2
3
4
5
6
7
0.25 7a 90 (90)
0.25 7b 100 (100)
0.25 7c 100 (100)
0.5
0.5
7d 97 (84)
7e 100 (89)
0.25 7f 100 (79)
0.25 7g 100 (95)
8
2
7h 100 (73)
7i 100 (62)
7j 100 (79)
9
1
10
0.5
11
12
0.25 7k 100 (97)
0.5
7l 100 (87)
13
14
7
100 (7m1 87 +
7m2 13)c
18
100 (7m1 20 +
7m2 80)d
Regarding the reaction mechanism and on the basis of
the bibliography, two pathways may be considered for the
hydrosilylation reaction catalysed by iridiumIJIII) (see Schemes
S1 and S2 in the ESI†). The first one is based on the Chalk–
Harrod mechanism which implies an iridium activation of
the silane reagent by oxidative addition.9
a
b
Measured by 1H NMR after workup. Isolated yield after flash
c
chromatography. 7m1 is obtained from imine reduction; 7m2 is the
doubly reduced product. d With 2.2 equivalents of triethylsilane.
The second reaction pathway considers the activation of
the substrate through
a silane–iridium adduct wherein
(Table
4).
We
could
unambiguously
characterize
another silane molecule participates in the hydride trans-
fer.10 When complex 8a was allowed to react with a stoichio-
metric amount of triethylsilane, the 1H NMR analysis of the
resulting mixture showed several mono- and dihydride spe-
cies without any coordinated hydrogen according T1 mea-
surements.13 Unfortunately, additional 29Si INEPT and
DEPT45 NMR experiments didn't provide significant informa-
tion (see Fig. S4–S7 in the ESI†). Hence, at this stage, further
dechlorinated IrIJIII) complex 1, i.e. cationic species 8a, along
with its BArF24 anion and the hydrolysed reaction product 5b
(Fig. 1 and the ESI†). Hence, though the reaction mixture
was not quenched, the simple use of methanol for solubiliz-
ing the mass analysis sample resulted in the hydrolysis of
the hydrosilylated reaction product and afforded amine 5b.
In addition, the mass analysis conditions didn't allow any
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Fig. 1 ESI(+)-MS analysis in methanol of the crude mixture resulting from the hydrosilylation of reagent 4b.
investigations on the reaction mechanism proved to be
difficult.
Table 6 Hydrosilylation of imine 4a by catalysts 8a–b, 9a–d
Conclusions
The combined use of an IrIJIII) metallacycle and NaBArF24 as a
catalyst was successfully applied for the hydrosilylation of
various ketimines and aldimines. By using a slight excess of
an affordable organosilane reagent, the reactions proceeded
rapidly and efficiently, at low catalyst loadings and under
very mild reaction conditions. Several examples of cationic
IrIJIII) catalysts could be synthesized, characterized and tested
for the hydrosilylation of imines. In situ-generated catalysts
proved to be more active as compared to isolated ones and
species with non-coordinating BArF24 counterion gave the
highest catalytic activities. Considering the high efficiency of
such cationic cyclometallated IrIJIII) catalysts, further applica-
tions and developments can be reasonably foreseen. Other
studies from our laboratories will be reported soon.
Entry Cat. (mol%) Additive (mol%) Time (h) T (°C) Yielda (%)
1
2
3
4
5
6
7
8
1 (1)
NaBArF24 (2)b
None
None
None
None
None
None
None
0.25
0.25
0.25
0.25
20
20
20
20
25
25
25
25
25
25
25
40
100
27
39
82
43
24
<5
24
8a (1)
8b (1)
9a (1)
9b (1)
9c (1)
9d (1)
9d (1)
Experimental section
General procedure for the catalysis
a
b
Measured by 1H NMR after work-up. NaBArF24
: sodium
In a glovebox, the imine reagent (0.15 mmol, 1 eq.), the
tetrakisij(3,5-trifluoromethyl)phenyl]borate.
1456 | Catal. Sci. Technol., 2015, 5, 1452–1458
selected iridiumIJIII) catalyst (x mol%) and the additive
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(2x mol%) were introduced in a Schlenk tube. Under nitro-
gen, the solvent (2 mL) was added followed by the silane
reagent (0.18 mmol, 1.2 eq.). The reaction mixture was then
heated at 25 °C under stirring. In order to follow the progress
of the reaction, aliquots (0.1 mL) were taken at defined times,
filtered through Celite with a CH2Cl2 wash (3 mL), evaporated
under vacuum and analysed by 1H NMR. At the end of the
reaction, the solvent was evaporated under vacuum and the
crude product was directly purified by flash chromatography
or by preparative TLC.
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The French Ministry of Research and Higher Education
and Région Nord-Pas de Calais are acknowledged for a PhD
fellowship (Y. C.). The University of Strasbourg is thanked for
a PhD funding (W. I.) and the LABEX Chimie des Systèmes
Complexes is thanked for a PhD funding (M. H). Support
from CNRS is warmly acknowledged. Partial fundings from
Région Nord-Pas de Calais and FEDER are also appreciated.
Ms Charlotte Denneulin is thanked for her help with some
experiments. Mrs C. Delabre and C. Méliet (UCCS) are
warmly thanked for GC, GC-MS and elemental analyses.
Lydia Brelot-Karmazin (Institut de Chimie Strasbourg) is
thanked for X-ray analysis of compound 8b. Mrs N. Duhal
and C. Lenglart (CUMA, Univ. of Lille) are thanked for HRMS
analyses.
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