Cyclopentadienyl N-heterocyclic carbene-nickel complexes as efficient pre-catalysts for the hydrosilylation of imines

Article abstract of DOI:10.1039/c3cy00514c

The in situ generated nickel hydride complex, [Ni(Mes₂NHC)HCp], and its cationic analogue, [Ni(Mes₂NHC)(NCMe)Cp](PF₆), are efficient and chemoselective pre-catalysts for the hydrosilylation of both aldimines and ketimines under mild conditions. The Royal Society of Chemistry.

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ARTIDCOIL: 10E.103T9/C3YCYP005E14C
Cyclopentadienyl N-heterocyclic carbene-nickel complexes as efficient
pre-catalysts for the hydrosilylation of imines
Linus P. Bheeter,a Mickaël Henrion,b Michael J. Chetcuti,b Christophe Darcel,a Vincent Ritleng*b and
Jean-Baptiste Sortais*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The in situ generated nickel hydride complex,
-
PF6
[Ni(Mes2NHC)HCp],
[Ni(Mes2NHC)(NCMe)Cp](PF6),
and
its
cationic
are efficient
analogue,
and
Ni+
Ni
N
Cl
10 chemoselective pre-catalysts for the hydrosilylation of both
C
N
N
H3C
N
N
aldimines and ketimines under mild conditions.
3
2
During the last decade, the development of catalytic reactions
based on inexpensive earth-abundant transition metals has
become an important area of research as the natural reserves of
15 precious metals decline and their prices increase tremendously. In
particular, considerable attention has been devoted to the use of
metals such as iron,1,2 zinc,3 titanium4 or copper5,6 for the
reduction of carbonyl derivatives via hydrosilylation. In contrast,
nickel, which is another attractive surrogate for precious metals in
20 terms of its abundance and low cost, has been much less studied
in this area.7 Therefore, the interest of some of us in the reactivity
of half-sandwich N-heterocyclic carbene (NHC) nickel
complexes,8-10 coupled with the complementary research of other
co-authors on iron-catalyzed hydrosilylation,11 notably with
25 analogous half-sandwich NHC-iron complexes,12 led us all to
demonstrate recently that the nickel hydride complex,
[Ni(Mes2NHC)HCp] 1 (Mes2NHC = 1,3-dimesitylimidazol-2-
ylidene, Cp = cyclopentadienyl) – generated in situ by reaction of
[Ni(Mes2NHC)ClCp] 2 and NaHBEt3 – is an efficient catalyst for
30 the hydrosilylation of both aldehydes and ketones at room
temperature.13,14
Transition-metal catalyzed hydrosilylation of aldimines and
ketimines is also an interesting target due to the significance and
omnipresence of the resulting amines in the field of natural
35 products, pharmaceutical and agronomical compounds.15
Compared to both the noble16-18 and most of the first-row
metals,19-24 nickel has again been very rarely employed for this
reduction reaction. We are indeed aware of only one example,
where 1:1 complexes formed in situ from [Ni(OAc)2] and O,N,S-
40 pincer type ligands were shown to reduce a small array of imines
via hydrosilylation.25,26 This scarcity, coupled with the recent
finding that, in addition of being an efficient pre-catalyst for the
hydrosilylation of carbonyl derivatives12a,b,d (as its nickel
analogues13,14), the cationic half-sandwich iron-NHC complex
50 Scheme 1 Selected cyclopentadienyl NHC-nickel(II) pre-catalysts.
these reactions (Scheme 1).
Initial studies focused on the hydrosilylation of N-benzylidene-
4-methoxyaniline 4 with one equivalent of Ph2SiH2 in THF in the
presence of catalytic amounts of complexes 2 and 3 under various
55 conditions, in order to optimize the reaction parameters (Table 1).
In the sole presence of the neutral complex 2 (5 mol%), 70 °C
were required to observe 50% conversion of the aldimine 4 to the
corresponding amine 5 after 24 h of reaction and a basic quench
(entries 1 and 2). Addition of 10 mol% of KPF6 as a chloride
60 scavenger led to full conversion under otherwise unchanged
Table 1 Optimization for the reduction of aldimines with 2 and 3a
OMe
OMe
(1) [Ni], additive, Ph2SiH2 (1 equiv.)
THF / 25-70 °C
HN
N
(2) 2 M NaOH, MeOH / 25 °C
5
4
Entry Pre-catalyst Additive
Temp. Time Conversion (%)b
(mol%)
2 (5)
2 (5)
2 (5)
3 (5)
2 (1)
3 (1)
3 (1)
3 (1)
2 (1)
-
(mol%)
(°C)
25
70
70
70
70
70
50
25
25
25
25
25
(h)
18
24
24
24
24
24
24
17
17
17
8
1
2
3
4
5
6
7
8
9
-
-
0
50
KPF6 (10)
> 98
> 98
40
> 98
> 98
10
> 98
0
90
-
KPF6 (2)
-
-
-
NaHBEt3 (2)
NaHBEt3 (2)
NaHBEt3 (2)
NaHBEt3 (1)
10
11
12
2 (1)
2 (0.5)
24
90
a
Typical procedure: activation of 2 with the additive in THF (4 mL) at
RT for 5 min or dissolution of 3 in THF (4 mL) at RT was followed by
the addition of 4 (1 mmol) and Ph2SiH2 (1 mmol), and the reaction
b
45 [Fe(Mes2NHC)(CO)2Cp]I was also an efficient pre-catalyst for 65 mixture was stirred at 25, 50 or 70 °C for 8 to 24 h. Conversions
determined by 1H NMR spectroscopy after methanolysis: 2M NaOH (2
mL), MeOH (2 mL), RT, 2 h.
the hydrosilylation of both aldimines and ketimines,12c prompted
us to investigate the catalytic activity of [Ni(Mes2NHC)ClCp] 226
and its cationic derivative [Ni(Mes2NHC)(NCMe)Cp](PF6) 310a in
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Catalysis Science & Technology
Page 2 of 6
Table 2 Scope of the reduction of adimines with 2-NaHBEt3 and 3a
conditions (entry 3). This promising result led us to use the well-
defined cationic complex 3 (5 mol%), and complete reduction
was also observed (entry 4). Reducing the catalytic loading to 1
mol% of 3 also permitted the reaction to reach full conversion
(entry 6), which contrasts with the result obtained with 1 mol% of
the in situ generated cationic complex from 2 and KPF6 (40%
conversion, entry 5).27 Interestingly, the temperature can be
decreased to 50 °C without loss of catalytic activity (entry 7).
However, further lowering of the reaction temperature to 25 °C
10 led to a dramatic drop in activity, as only 10% conversion was
detected after 17 h (entry 8). Nevertheless as stated above, we
have shown in a previous contribution that the nickel hydride
complex 1 generated by reaction of 2 equiv. of NaHBEt3 with the
(1) 2 (1 mol%), NaHBEt3 (2 mol%) / THF / 25 °C
View Article Online
or
DOI: 10.1039/C3CY00514C
Ar
Ar
3 (1 mol%) / THF / 50 °C
N
HN
+
Ph2SiH2
(2) 2M NaOH, MeOH / 25 °C
R
R
5
Entry
Substrate
Pre- Conv. Yield
cat.
(%)b
(%)c
1
2
3
4
R = Me
R = Me
R = OMe
R = Br
2
3
2
2
> 98
> 98
> 98
20
83
-
90
-
N
R
5
6
7
8
9
R = p-OMe
R = p-OMe
R = p-I
R = o-Me
R = o-Me
2
3
2
2
2
> 98
> 98
27
89
-
-
-
39
R
N
neutral complex
2 was a particularly active catalyst for
28
15 hydrosilylation of both aldehydes and ketones at room
temperature.13 Using 1 mol% of this in situ generated hydride
species allowed to observe a full conversion when performing the
reaction at 25 °C for 17 h (entry 9). Decreasing either the reaction
time to 8 h or else the catalytic loading to 0.5 mol% allowed us to
20 obtain 90% conversion (entries 11 and 12). Various other silanes
and solvents were also screened (see the Supporting Information),
but the combination of Ph2SiH2 and THF was found to be
optimal.
48d
10
11
12
13
14
15
16
17
18
R = p-OMe
R = p-OMe
R = p-NMe2
R = p-Cl
2
3
2
2
3
2
3
2
2
> 98
> 98
77
> 98
71
84
-
57
80
-
76
83
74
81
N
R = p-Cl
R = p-CO2Me
R = p-CO2Me
R = p-CN
95
> 98
94e
93
R
R = 3,4,5-OMe
With these optimized conditions in hand (1 equiv. of Ph2SiH2,
25 1 mol% of 2, 2 mol% of NaHBEt3, THF, 25 °C, 17 h or 1 equiv.
of Ph2SiH2, 1 mol% of 3, no additive, THF, 50 °C, 24 h), we then
explored the scope of the hydrosilylation of aldimines (Table 2).
Electronic effects at the para-position of the benzylidene or
aniline moiety were generally minor (entries 1-3, 5-6, and 10-20).
30 Thus, aldimines bearing an electron-donating group gave the
corresponding amines with good to excellent conversions (entries
1-3, 5-6, and 10-12). Interestingly, no dehalogenation occurred
with a chloro-substituted aldimine irrespective of the catalytic
system used, 2-NaHBEt3 or 3-no additive (entries 13 and 14), and
35 the corresponding amine was isolated in good yield (80%, entry
13). However, with bromo- and iodo-substituted aldimines, low
conversions were obtained, probably due to a rapid catalyst
deactivation, as previously observed for the hydrosilylation of
aldehydes and ketones (entries 4 and 7).13 Strikingly, functional
40 carbonyl groups such as esters and amides were not affected
under these catalytic conditions irrespective of the catalytic
system used, 2-NaHBEt3 or 3-no additive (entries 15-16 and 19-
20), and the corresponding secondary amines were isolated in
good yields (76 and 72%, entries 15 and 20). Moreover, although
45 5% of the fully reduced compound was detected in the crude
reaction mixture, the cyano functional group was also well
tolerated and the N-(4-cyanobenzyl)-p-toluidine resulting from
the selective reduction of the 4-cyanobenzylidene derivative was
isolated in 74% yield (entry 17). In contrast, only moderate
50 conversion was observed for the hydrosilylation of the 4-
methoxy-N-(4-nitrobenzilidene)aniline under forcing conditions,
and a mixture of products resulting from the reduction of the nitro
group was observed (entries 21 and 22).
OMe
19
20
21
22
R = NHAc
R = NHAc
R = NO2
2
2
2
2
70
90d
0
-
72
-
N
R = NO2
40f
-
R
OMe
23
24
2
2
20
-
57
N
80f
O
OMe
25
26
2
2
43
-
61
N
70f
N
OMe
27
28
29
2
2
2
20
60f
-
-
70
N
87f,g
N
N
30
2
> 98
85
Fe
60 a Typical procedure: activation of 2 (1 mol%) with NaHBEt2 (2 mol%) in
THF (4 mL) at RT for 5 min or dissolution of 3 (1 mol%) in THF (4 mL)
at RT was followed by the addition of aldimine (1 mmol) and Ph2SiH2 (1
mmol), and the reaction mixture was stirred at 25 °C for 17 h (2) or at 50
°C for 24 h (3). b Conversions determined by 1H NMR spectroscopy after
c
65 methanolysis: 2M NaOH (2 mL), MeOH (2 mL), RT, 2 h. Isolated
d
e
yields. 50 °C. 5% reduction of both the aldimine and the cyano group
was also observed. f 70 °C. g 2 (5 mol%), NaHBEt3 (10 mol%).
Substitution in ortho-position of the aniline moiety seems to
55 have an inhibiting effect, most probably for steric reasons, as
shown by the moderate conversion observed for the reduction of
benzylidene-o-methylaniline, even at 50 °C (entries 8 and 9).
Substitution at the meta-position of the benzylidene moiety seems
in contrast to have no notable effect (entry 18).
70
This reduction can also be conducted with heteroaromatic
aldimines such as 5-methylfur-2-yl-, pyridin-2-yl- and N-
methylpyrrol-2-yl-4-methoxyaniline, but at higher temperature
2 | Journal Name, [year], [vol], 00–00
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Page 3 of 6
Catalysis Science & Technology
45 Table 4 Scope of the reduction of ketimines with 2-NaHBEt3 and 3a
(50 °C or 70 °C), and the corresponding amines were isolated in
moderate yields (57-70%, entries 23-29). Finally, 4-methyl-N-
(ferrocenylmethylidene)aniline was totally reduced and led to the
corresponding amine in good yield (85%, entry 30).
Given the high activity of both catalytic systems for aldimines,
we then investigated their potential for the hydrosilylation of
ketimines, with N-[1-phenylethylidene]-4-methoxyaniline 6 as
the model substrate (Table 3). To obtain similar activities,
slightly harsher conditions had to be used, by either performing
(1) 2 (1 mol%), NaHBEt3 (2 mol%) / THF / 50 °C
View Article Online
or
DOI: 10.1039/C3CY00514C
Ar
Ar
3 (5 mol%) / THF / 50 °C
N
HN
R
+
Ph2SiH2
(2) 2M NaOH, MeOH / 25 °C
R
5
Entry
1
Substrate
Pre-cat. Conv. Yield
(%)b
(%)c
N
2
> 98
77
10 the reaction at higher temperature (with 2-NaHBEt3, entries 5-9)
or with higher pre-catalyst and Ph2SiH2 loadings (with 3, entries
3 and 4). Thus, to observe full conversion of 6 to the
corresponding amine 7 after a methanolysis step, 50 °C for 17 h
were required in the presence of 1 mol% of 2 and 2 mol% of
15 NaHBEt3 (entry 7), and 2 equiv. of Ph2SiH2 were required in the
presence of 5 mol% of 3 (entry 4). Notably, in the case of 2-
OMe
2
3
> 98
> 98
78
-
N
2
3
4
5
6
7
8
R = H
2
2
2
3
2
2
3
2
2
2
79
86
> 98
> 98
90
63
73
84
-
77
75
80
-
NaHBEt3, when the reaction was performed at
temperature or with a lower catalyst loading, the conversion
significantly decreased (entries 6 and 9).
a lower
R = Me
R = OMe
R = OMe
R = Cl
R = F
R = F
R = CF3
R = CF3
R = CF3
20 Table 3 Optimization for the reduction of ketimines with 2 and 3a
N
9
90
10
11
12
13
> 98
30
OMe
OMe
R
(1) [Ni], additive, Ph2SiH2 (1 equiv.)
52d
85d,e
-
69
THF / 25-70 °C
HN
N
(2) 2M NaOH, MeOH / 25 °C
7
6
Entry Pre-catalyst Additive
Temp. Time Conversion (%)b
N
14
2
90
59
(mol%)
2 (5)
2 (5)
3 (5)
3 (5)
2 (5)
2 (5)
2 (1)
2 (1)
2 (0.5)
(mol%)
(°C)
70
70
50
50
50
25
50
50
50
(h)
24
24
17
24
17
17
17
3
1
2
3
4c
5
6
7
8
9
-
10
60
70
> 98
> 98
50
> 98
80
KPF6 (10)
-
-
15
16
17
2
2
2
20
48d
-
-
66
N
NaHBEt3 (10)
NaHBEt3 (10)
NaHBEt3 (2)
NaHBEt3 (2)
NaHBEt3 (1)
80d,e
Fe
a Typical procedure: activation of 2 (1 mol%) with NaHBEt2 (2 mol%) in
THF (4 mL) at RT for 5 min or dissolution of 3 (5 mol%) in THF (4 mL)
at RT was followed by the addition of aldimine (1 mmol) and Ph2SiH2 (1
mmol (2) or 2 mmol (3)), and the reaction mixture was stirred at 50 °C for
17
65
a
Typical procedure: activation of 2 with the additive in THF (4 mL) at
RT for 5 min or dissolution of 3 in THF (4 mL) at RT was followed by
the addition of 6 (1 mmol) and Ph2SiH2 (1 mmol), and the reaction
b
1
50 17 h (2) or 24 h (3). Conversions determined by H NMR spectroscopy
after methanolysis: 2M NaOH (2 mL), MeOH (2 mL), RT, 2 h. c Isolated
yields. d 70 °C. e 2 (5 mol%), NaHBEt3 (10 mol%).
b
mixture was stirred at 25, 50 or 70 °C for 3 to 24 h. Conversions
25 determined by 1H NMR spectroscopy after methanolysis: 2M NaOH (2
mL), MeOH (2 mL), RT, 2 h. c Reaction run with 2 equiv. of Ph2SiH2.
(entries 11-13). Finally, the ferrocenyl imine derivative could also
55 be reduced to the corresponding amine with 80% conversion and
66% isolated yield by using the latter conditions (entry 17).
We demonstrated in our previous contribution which targeted
the hydrosilylation of carbonyl derivatives,13 that the nickel
hydride complex 1 resulting from the reaction of 2 and NaHBEt3
60 is most probably the true pre-catalyst with this catalytic system.
Simultaneously, Royo et al. demonstrated that an analogous Cp-
NHC tethered nickel hydride complex generated in situ by the
reaction of the corresponding alkoxide complex with PhSiH3 was
most probably the active species in a very similar hydrosilylation
65 process.14 We thus wondered if such hydride species was also
generated with the catalytic system composed of the sole cationic
complex 3 and Ph2SiH2.28
With these optimized conditions in hand (1 equiv. of Ph2SiH2,
1 mol% of 2, 2 mol% of NaHBEt3, THF, 50 °C, 17 h or 2 equiv.
30 of Ph2SiH2, 5 mol% of 3, no additive, THF, 50 °C, 24 h), the
scope of the hydrosilylation of ketimines was then explored. With
several ketimines derived from (substituted) acetophenones and
(4-substituted) anilines, the corresponding amines were obtained
with high conversions and good isolated yields (Table 4, entries
35 1-10). Notably, full conversion and good isolated yield were
obtained with N-(2-methylphenyl)ethylidene-aniline (entry 1),
which demonstrates that steric hindrance at the phenylethylidene
moiety does not inhibit the reaction. Similarly, good conversion
was obtained for the reduction of naphtylethylidene toluidine
40 (entry 14). As observed with aldimines, in the presence of a
strong electron withdrawing group such as a trifluoromethyl
group, the reaction was more difficult to carry out and harsher
conditions (5 mol% of 2, 10 mol% of NaHBEt3, 70 °C, 17 h)
were necessary to reach 85% conversion and 69% isolated yield
For that purpose, we reacted 3 with 0.5 or 1 equivalent of
Ph2SiH2 in THF-d8 at RT and 50 °C, and monitored the reactions
70 by 1H NMR spectroscopy. In all cases, we observed, after 5 min
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

Catalysis Science & Technology
Page 4 of 6
CNRS, Rennes Métropole, and the Ministère de l’Enseignement
Supérieur et de la Recherche for financial support. VR and MJC
are grateful to the Université de StrasbourDgOaI:n1d0.t1h0e39C/CN3RCSY0f0o5r14C
NaHBEt3
(2 equiv.)
View Article Online
Ni
Cl
N
their financial help. The Agence Nationale de la Recherche is also
50 acknowledged for its support to VR and the doctoral fellowship
of MH (ANR 2010 JCJC 716 1; SBA-15-NHC-NiCat).
quantitative
N
2
Ni
H
N
-
PF6
N
Notes and references
Ph2SiH2
(excess)
1
Ni+
a
“Institut des Sciences Chimiques de Rennes”, UMR 6226 CNRS,
N
C
N
H3C
Université de Rennes 1, Equipe “Organometallics: Materials and
55 Catalysis”, Centre for Catalysis and Green Chemistry, Campus de
Beaulieu, Bat 10C, Avenue du Général Leclerc, 35042 Rennes Cedex,
France; Fax: +33 2 2323 6939; Tel: +33 2 2323 6288; E-mail: jean-
baptiste.sortais@univ-rennes1.fr
< 10%
N
3
Scheme 2 Generation of the nickel hydride complex 1 from the neutral
and cationic complexes 2 and 3.
b Laboratoire de Chimie Organométallique Appliquée, UMR 7509 CNRS,
60 Université de Strasbourg, Ecole européenne de Chimie, Polymères et
Matériaux, 25 rue Becquerel, 67087 Strasbourg, France; Tel: + 33 3
6885 2797; E-mail: vritleng@unistra.fr
of reaction, the formation of a small amount (generally less than
10% with respect to the remaining amount of 3) of a nickel
hydride species, which we unambiguously identified as being 1
5
† Electronic Supplementary Information (ESI) available: full
1
by comparison with the H NMR spectrum of a pure sample in
experimental
details,
NMR
data
and
spectra.
See
65 DOI: 10.1039/b000000x/
THF-d8. Concomitantly, new signals started to appear in the
1
2
For representative reviews about Fe-catalyzed hydrosilylations, see:
(a) M. Zhang and A. Zhang, Appl. Organomet. Chem., 2010, 24, 751;
(b) K. Junge, K. Schröder and M. Beller, Chem. Commun., 2011, 47,
4849; (c) B. A. F. Le Bailly and S. P. Thomas, RSC Advances, 2011,
1, 1435.
For selected examples of Fe-catalyzed hydrosilylation of carbonyl
derivatives, see: (a) H. Nishiyama and A. Furuta, Chem. Commun.,
2007, 760; (b) N. S. Shaikh, K. Junge and M. Beller, Org. Lett.,
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Inagaki, L. T. Phong, A. Furuta, J.-I. Ito and H. Nishiyama, Chem.
Eur. J., 2010, 16, 3090.
For selected examples of Zn-catalyzed hydrosilylation of carbonyl
derivatives, see: (a) H. Mimoun, J. Y. de Saint Laumer, L. Giannini,
R. Scopelliti and C. Floriani, J. Am. Chem. Soc., 1999, 121, 6158; (b)
V. Bette, A. Mortreux, C. W. Lehmann and J.-F. Carpentier, Chem.
Commun., 2003, 332; (c) V. Bette, A. Mortreux, D. Savoia and J.-F.
Carpentier, Adv. Synth. Catal., 2005, 347, 289; (d) S. Das, D. Addis,
S. Zhou, K. Junge and M. Beller, J. Am. Chem. Soc., 2010, 132,
1770.
For selected examples of Ti-catalyzed hydrosilylation of carbonyl
derivatives, see: (a) S. Bower, K. Kreutzer and S. L. Buchwald,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1515; (b) K. Selvakumar and
J. F. Harrod, Angew. Chem. Int. Ed., 2001, 40, 2129; (c) S. Laval, W.
Dayoub, L. Pehlivan, E. Métay, A. Favre-Réguillon, D. Delbrayelle,
G. Mignani and M. Lemaire, Tetrahedron Lett., 2011, 52, 4072.
For representative reviews of Cu-catalyzed hydrosilylations, see: (a)
S. Rendler and M. Oestreich, Angew. Chem. Int. Ed., 2007, 46, 498;
(b) C. Deutsch, N. Krause and B. H. Lipshutz, Chem. Rev., 2008,
108, 2916; (c) B. H. Lipshutz, Synlett, 2009, 509.
For selected examples of Cu-catalyzed hydrosilylation of carbonyl
derivatives, see: (a) H. Brunner and W. Mehling, J. Organomet.
Chem., 1984, 275, C17; (b) B. H. Lipshutz, K. Noson and W.
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aromatic area (probably resulting from the oligomerization and/or
10 polymerization of Ph2SiH2), as well as a singlet at 1.94 ppm,
which we attribute to free CH3CN. The rest of the reaction
mixture mostly consisted in non-reacted 3 and Ph2SiH2 (see the
Supporting Information). It is noteworthy that in all cases we also
observed the immediate and steady evolution of a gas, which we
15 think is H2, as observed by Zargarian et al. in the reactions of
analogous nickel complexes of the type [Ni(PR3)Me(1-Me-
indenyl)] with PhSiH3.29 Finally, after a reaction time varying
from 20 min for the reactions conducted at 50 °C to 6-22 h for the
reactions conducted at RT (with 0.5 or 1 equiv. of Ph2SiH2), all
20 Ph2SiH2 was consumed, and the reaction medium consisted in a
complicated mixture of products with small remaining amounts
of complexes 1 and 3. In contrast, the neutral complex 2 gave
strictly no reaction with Ph2SiH2 (0.5 equiv.) in THF-d8 at RT,
even after 6 h, and required 70°C to produce traces amount of 1.
These results may explain the total absence of reduction of the
aldimine 4 when the reaction was performed in the sole presence
of 5 mol% of 2 at RT (Table 1, entry 1), as well as the moderate
conversion observed in the sole presence of 5 mol% of 2 at 70°C
(Table 1, entry 2) and the slightly harder conditions (50°C)
30 required with 3 to observe full reduction (Table 1, entries 7 and
8); only small amounts of 2 or 3 are converted to 1 by reaction
with Ph2SiH2,30 whereas all 2 is converted to 1 by reaction with 2
equiv. of NaHBEt3 (Scheme 2). Additionally, although another
true pre-catalyst (or active species) cannot be ruled out in the
35 absence of NaHBEt3, these results tend to confirm the necessity
to generate the nickel hydride complex 1 to observe a catalytic
activity.13
70
75
3
4
80
85
25
90
5
6
95
100
105
110
In summary, using the nickel hydride complex 1, generated in
situ from the neutral complex 2 and 2 equiv. of NaHBEt3, or the
40 cationic complex 3, an efficient and chemoselective
hydrosilylation of both aldimines and ketimines was carried out
at RT or 50 °C, leading the corresponding amines with moderate
to good yields.
7
Acknowledgements
45 JBS and CD are grateful to the Université de Rennes 1, the
4 | Journal Name, [year], [vol], 00–00
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Page 5 of 6
Catalysis Science & Technology
H. Hu, S. Li, X.-C. Zhang, A. S. C. Chan and J. Wu, Chem. Asian J.,
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90 27 The lower actvity of the in situ generated cationic species compared
to that of 3 is most probably due to the fact that it is generated in
THF instead of acetonitrile, and that THF does not have sufficient
coordinating power to stabilize such cationic species. Indeed no
cationic complex of [Ni(NHC)LCp]+ type could ever be isolated
95
when the halide was scavenged in a weakly coordinating solvent: V.
Ritleng, M. J. Chetcuti, unpublished results.
28 For other nickel hydride species generated by reaction with Ph2SiH2
or PhSiH3 as a hydride source, see: (a) D. Adhikari, M. Pink and D. J.
Mindiola, Organometallics, 2009, 28, 2072; (b) J. Breitenfeld, R.
Scopelliti and X. Hu, Organometallics, 2012, 31, 2128.
29 F.-G. Fontaine and D. Zargarian, Organometallics, 2002, 21, 401.
30 A control experiment revealed that the reaction of 3 with 20 equiv. of
Ph2SiH2 also generated less than 10% of 1.
12 (a) F. Jiang, D. Bézier, J.-B. Sortais and C. Darcel, Adv. Synth.
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35 13 L. P. Bheeter, M. Henrion, L. Brelot, C. Darcel, M. J. Chetcuti, J.-B.
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14 For a related example with a Cp-NHC tethered nickel-alkoxide
complex that was published simultaneously, see: L. Postigo and B.
Royo, Adv. Synth. Catal., 2012, 354, 2613.
40 15 (a) R. C. Larock, in Comprehensive Organic Transformations: A
Guide to Functional Group Preparation, Wiley-VHC, New York,
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45 16 For selected examples of Ru-catalyzed hydrosilylation of imines, see:
(a) Y. Nishibayashi, I. Takei, S. Uemura and M. Hidai,
Organometallics, 1998, 17, 3420; (b) H. Hashimoto, I. Aratani, C.
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50
55
60
(d) B. Li, J.-B. Sortais, C. Darcel and P. H. Dixneuf, ChemSusChem,
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1973, 14, 4865; (b) H. Brunner and R. Becker, Angew. Chem., Int.
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3468.
18 For selected examples of Ir-catalyzed hydrosilylation of imines, see:
(a) I. Takei, Y. Nishibayashi, Y. Arikawa, S. Uemura and M. Hidai,
Organometallics, 1999, 18, 2271; (b) L. D. Field, B. A. Messerle and
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19 For rare examples of Fe-catalyzed hydrosilylation of imines, see: (a)
ref. 2a; (b) 12c; (c) S. Zhou, K. Junge, D. Addis, S. Das and M.
Beller, Angew. Chem. Int. Ed., 2009, 48, 9507.
65 20 For selected examples of Zn-catalyzed hydrosilylation of imines, see:
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1029; (b) M. Bandini, M. Melucci, F. Piccinelli, R. Sinisi, S.
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3863.
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Journal Name, [year], [vol], 00–00 | 5

Catalysis Science & Technology
Page 6 of 6
View Article Online
DOI: 10.1039/C3CY00514C
The in situ generated nickel hydride
complex, [Ni(Mes2NHC)HCp], and its
cationic analogue,
[Ni(Mes2NHC)(NCMe)Cp](PF6), are efficient
and chemoselective pre-catalysts for the
hydrosilylation of imines under mild
conditions.