NHC-carbene cyclopentadienyl iron based catalyst for a general and efficient hydrosilylation of imines

Article abstract of DOI:10.1039/c1cc14403k

A general and efficient hydrosilylation of imines catalysed by a well defined NHC-carbene cyclopentadienyl iron complex has been developed. Both aldimines and ketimines are converted to the corresponding amines under mild conditions, and under visible light activation.

Full text of DOI:10.1039/c1cc14403k

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COMMUNICATION
NHC-carbene cyclopentadienyl iron based catalyst for a general and
efficient hydrosilylation of imineswz
Luis C. Misal Castro, Jean-Baptiste Sortais* and Christophe Darcel*
Received 20th July 2011, Accepted 13th October 2011
DOI: 10.1039/c1cc14403k
A general and efficient hydrosilylation of imines catalysed by a
well defined NHC-carbene cyclopentadienyl iron complex has
been developed. Both aldimines and ketimines are converted to
the corresponding amines under mild conditions, and under
visible light activation.
Scheme 1 General scheme for reduction of imines with well defined
iron complex 1.
The development of synthetic strategies towards amines has
hydrosilylation of aldehydes, ketones, and amides.14 Herein, we
attracted the interest of chemists due to the significance and
omnipresence of these compounds in the field of natural
describe the first general hydrosilylation of both aldimines and
products, pharmaceutical and agronomical compounds.1
ketimines involving this iron complex used as the pre-catalyst.
Among the direct and efficient synthesis of amines, imine
We started our investigation by the reduction of 4-methyl-
catalyzed-reduction methodologies2 (such as hydrogenation,2
hydrogen transfer3 or hydrosilylation)4 were developed using
N-(4-methylbenzylidene)aniline as the model substrate with
the iron complex 1 as the pre-catalyst (Scheme 1) under
transition metals such as ruthenium, iridium, rhodium and are
various conditions (Table 1 and Tables S1 and S2 in the ESIz).
some of the most ubiquitous protocols in organic synthesis.
Among a variety of silanes, optimal conversions were obtained
Besides, due to high natural abundance, benign environmental
with 2 equiv. of PhSiH3 under solvent-free conditions at 70 1C
for 16 h using 5 mol% of complex 1 (Table 1, entries 1–4). It is
impact, and low cost, iron has emerged as an interesting potential
surrogate in catalytic processes for precious transition metals.
noteworthy that the role of visible light irradiation is crucial to
Interestingly, the last decade has seen an impressive rise of its use
obtain a full conversion (entries 4 and 5). Notably, the
as the catalyst and efficient processes are now able to compete
temperature can be reduced to 30 1C and the pre-catalyst
loading to 2 mol%, and we found, eventually, that the
with other metal-catalyzed ones.5 In the reduction area,6 several
groups have described iron catalyzed hydrogenation6,7 or hydro-
reduction occurred with a full conversion at only 30 1C after
30 h with 2 mol% of the pre-catalyst 1 (entry 7).
gen transfer.8 For the reduction of ketimines, some very recent
impressive results were made in particular in asymmetric series.9
With an optimized catalytic system in hand, we then explored
In addition to these well established catalytic reductions with H2
the scope of the reaction with several aldimines (Table 2). We
or transfer hydrogen reagents, hydrosilanes are useful reducing
have found that the electronic effect at the 4 position on the
agents and iron catalyzed hydrosilylation of alkenes, aldehydes
and ketones were also intensively studied.10 On the other hand,
aniline moieties was limited (entries 1–4). However, for the p-iodo
more surprisingly, the apparently simple iron-catalyzed hydro-
Table 1 Optimisation study for the reduction of 4-methyl-N-
(4-methylbenzylidene)aniline with 1 as the pre-catalysta
silylation of aldimines and ketimines is still a challenging trans-
formation as to date, only few isolated examples of iron-catalyzed
hydrosilylation of imines were reported.11
Entry Silane
Solvent 1 [mol%] T/1C Time/h Conv. [%]b
During our recent studies on iron-catalyzed transformations,12
and particularly on hydrosilylation reactions,13 we have developed
an efficient well-defined complex, [CpFe(IMes)(CO)2]I, for catalytic
1
2
3
4
5c
6
7
8
PhSiH3 THF
5
5
5
5
5
2
2
2
70
70
70
70
70
30
30
30
16
16
16
16
16
16
30
60
51
44
71
95
24
60
497
23
PhSiH3 Toluene
PhSiH3 Dioxane
PhSiH3 Neat
PhSiH3 Neat
PhSiH3 Neat
PhSiH3 Neat
Ph2SiH2 Neat
UMR 6226 CNRS-UR1, ‘‘Sciences Chimiques de Rennes’’,
´
Team ‘‘Catalysis and Organometallics’’, Universite de Rennes 1,
Campus de Beaulieu, 35042 RENNES cedex, France.
E-mail: jean-baptiste.sortais@univ-rennes1.fr,
christophe.darcel@univ-rennes1.fr; Fax: +33 2 23 23 69 39
w Dedicated to Dr Christian Bruneau on the occasion of his 60th
birthday.
a
Typical conditions: aldimine (0.5 mmol), silane (2 equiv.), complex 1
and solvent were stirred upon visible light irradiation (using 24 watt
compact fluorescent lamp); the reaction is then hydrolysed with MeOH
b
1
and NaOH(aq.) 2 M. Determined by H NMR after extraction with
c
Et2O. No light activation.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc14403k
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 151–153 151

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Table 2 Scope of the reduction of aldiminesa
product resulting from the reduction of the ester group was
detected. Surprisingly, low conversions were observed for the
hydrosilylation of 4-nitro- and 4-cyanobenzylidene derivatives
but no reduction of the nitro or cyano15 group was observed,
and 80% of the starting material was recovered in the case of
the cyano derivative (entries 13 and 14). More interestingly,
heteroaromatic imines (entries 15–18), which are potentially good
bidentate ligands for the iron centre, were reduced with good to
excellent yields. Likewise, 4-methyl-N-(ferrocenylmethylidene)-
aniline was totally reduced and led to the corresponding amine
with good yield (entry 19). Finally, the reduction of 4-methyl-
N-cinnamylideneaniline afforded selectively the amine resulting
from the 1,2-reduction in 36% yield16 (entry 20).
Entry
Substrate
Isolated yield (%)
1
R = H
91
95
95
95
2
3
4b,c
R = Me
R = OMe
R = I
5
R = Me
95
95
95
92
95
71
30f
6d
7d
8d
9d
10e
11e
R = OMe
R = NMe2
R = Br
R = Cl
R = COCH3
R = CO2CH3
Given the very good activity of the catalytic system for
aldimines, we investigated its potential for reduction of ketimines,
with 4-methyl-N-[1-(4-methylphenyl)ethylidene]-aniline as the
model substrate. Compared with aldimines, 90% conversion
was reached under slightly harsher conditions, 24 h at 100 1C
under neat conditions with 5 mol% of pre-catalyst, and 2 equiv.
of PhSiH3 as the silane were needed (Table 3, entry 6). Notably,
when performing in THF, toluene or dioxane, or with lower
catalyst loading, the conversion dramatically decreased (Table 3,
entries 1–3, 7).
12b,c
94
13b,d
14e
R = CN
10f
15f
R = NO2
15b,d
95
The scope of the reaction was thus surveyed. With several
ketimine derivates from substituted acetophenone and 4-substituted
anilines (Table 4, entries 1–8), the corresponding amines where
obtained in good to high yields (83–95%). The reaction proceeds
well even with ortho-substituted phenylethylidene aniline or
naphthylethylidene toluidine (Table 4, entries 8 9, and 11), which
shows a low impact of the steric hindrance on to the reaction.
The ferrocenyl imine derivative was also converted into the
corresponding amine, but with a moderate yield (Table 4,
entry 10). More strikingly, the reaction could be performed
with N-alkylimines and aliphatic ketimines under the same
conditions with good yields (Table 4, entries 12 and 13).
In summary, we have developed the first general and efficient
catalytic system for the hydrosilylation of both aldimines and
ketimines using a well defined iron complex as the pre-catalyst
which gives access to various amines. Compared to other transition
metals used as catalysts in hydrosilylation reactions, the activity of
this iron catalyst is competitive for the reduction of aldimines.
Moreover, it is noteworthy that several functional groups easy to
reduce such as ketone, ester and alkene are tolerated with this
system. Further investigations into the reactivity of such complexes
16b,d
17
R = OMe
R = Br
95
53
18
63
83
19c
20c
36
26
21
a
Typical procedure: aldimine (0.5 mmol), PhSiH3 (1 mmol), and
complex 1 (2 mol%) were stirred at 30 1C without solvent upon visible
light irradiation for 30 h, the reaction is then hydrolysed with MeOH
b
c
and NaOH(aq.) 2 M. 50 1C, 24 h. 0.2 mL of toluene as the solvent.
0.1 mL of toluene as the solvent. Complex 1 (5 mol%), CH2Cl2
d
e
f
(1 mL). Conversion by H NMR.
1
Table
3 Optimisation study for the reduction of 4-methyl-
N-[1-(4-methylphenyl)ethylidene]aniline with 1 as the pre-catalysta
substituted derivative (entry 4), the reaction is complete at
50 1C after 24 h. It is noteworthy that, in some cases, due to
miscibility issues, a small amount of toluene was added in order
to solubilize all the reactants. When the benzylidene moieties
bear both electron donating or withdrawing substituents (entries
4–10), the corresponding amines were isolated with good to high
yields.
Entry Silane
Solvent
1 [mol%] T/1C Time/h Conv. [%]b
1
2
3
4
5
6
7
PhSiH3 THF
5
5
5
5
5
5
2
70
70
70
70
100
100
100
16
16
16
16
16
24
16
9
11
9
49
75
90
18
PhSiH3 Toluene
PhSiH3 Dioxane
PhSiH3 Neat
PhSiH3 Neat
PhSiH3 Neat
PhSiH3 Neat
Notably, no catalytic dehalogenation occurred during this
reaction (entries 4, 8 and 9). More strikingly, functional
carbonyl groups such as ketones and esters were not altered
under these catalytic conditions (entries 10 and 11), although
the conversion of the ester derivative was moderate (30%), no
a
Typical conditions: ketimine (0.5 mmol), silane (2 equiv.),
pre-catalyst 1 and solvent were stirred upon visible light irradiation;
the reaction is then hydrolysed with MeOH and NaOH(aq.) 2 M.
b
Determined by 1H NMR after extraction with Et2O.
c
152 Chem. Commun., 2012, 48, 151–153
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Table 4 Scope for the reduction of ketiminesa
4 (a) O. Riant, N. Mostefaı and J. Courmarcel, Synthesis, 2004,
¨
2943–2958; (b) J.-F. Carpentier and V. Bette, Curr. Org. Chem.,
2002, 6, 913–936.
Entry
Substrate
Isolated yield (%)
5 (a) C. Bolm, J. Legros, J. Le Paih and L. Zani, Chem. Rev., 2004,
104, 6217–6254; (b) B. Plietker, in Iron Catalysis in Organic
Chemistry, ed. Bernd Plietker, Wiley-VCH, Weinheim, 2008;
(c) S. Enthaler, K. Junge and M. Beller, Angew. Chem., Int. Ed.,
2008, 47, 3317–3321; (d) A. Correa, O. Garcia Mancheno and
C. Bolm, Chem. Soc. Rev., 2008, 37, 1108–1117; (e) B. D. Sherry
1b
2
R = H
R = Me
R = OMe
95
90
83
3c
4b
5b
R = Br
R = F
90
90
and A. Furstner, Acc. Chem. Res., 2008, 41, 1500–1511;
¨
(f) W. M. Czaplik, M. Mayer, J. Cvengros and A. Jacobi von
Wangelin, ChemSusChem, 2009, 2, 396–417; (g) C.-L. Sun, B.-J. Li
and Z.-J. Shi, Chem. Rev., 2011, 111, 1293–1314.
6b
7b
8b
R = CF3
91
93
91
6 Representative review on iron-catalyzed reduction: (a) K. Junge,
K. Schroder and M. Beller, Chem. Commun., 2011, 47, 4849–4859;
¨
(b) R. H. Morris, Chem. Soc. Rev., 2009, 38, 2282–2291.
7 Selected papers on iron-catalyzed hydrogenation: (a) C. Bianchini,
A. Meli, M. Peruzzini, P. Frediani, C. Bohanna, M. A. Esteruelas
and L. A. Oro, Organometallics, 1992, 11, 138–145; (b) S. Enthaler,
B. Hagemann, G. Erre, K. Junge and M. Beller, Chem.–Asian J.,
2006, 1, 598–604; (c) C. P. Casey and H. Guan, J. Am. Chem. Soc.,
2007, 129, 5816–5817; (d) C. Sui-Seng, F. Freutel, A. J. Lough and
R. H. Morris, Angew. Chem., Int. Ed., 2008, 47, 940–943.
8 Selected papers on iron catalyzed transfer hydrogenation:
(a) S. Enthaler, G. Erre, M. K. Tse, K. Junge and M. Beller,
Tetrahedron Lett., 2006, 47, 8095–8099; (b) C. Bianchini,
E. Farnetti, M. Graziani, M. Peruzzini and A. Polo, Organometallics,
1993, 12, 3753–3761; (c) A. Mikhailine, A. J. Louggh and
R. H. Morris, J. Am. Chem. Soc., 2009, 131, 1394–1395;
(d) Buchard, H. Heuclin, A. Auffrant, X. F. Le Goff and P. Le
Floch, Dalton Trans., 2009, 1659–1667; (e) N. Meyer, A. J. Lough
and R. H. Morris, Chem.–Eur. J., 2009, 15, 5605–5610; (f) A. Naik,
T. Maji and O. Reiser, Chem. Commun., 2010, 46, 4475–4477.
9 (a) S. Zhou, S. Fleischer, K. Junge and M. Beller, Angew. Chem., Int.
Ed., 2011, 50, 5120–5124; (b) S. Zhou, S. Fleischer, K. Junge, S. Das,
D. Addis and M. Beller, Angew. Chem., Int. Ed., 2010, 49, 8121–8125.
10 Representative examples of iron-catalyzed hydrosilylation:
(a) S. C. Bart, E. Lobkovsky and P. J. Chirik, J. Am. Chem.
Soc., 2004, 126, 13794–13807; (b) H. Nishiyama and A. Furuta,
Chem. Commun., 2007, 760–762; (c) N. S. Shaikh, K. Junge and
M. Beller, Org. Lett., 2007, 9, 5429–5432; (d) N. S. Shaikh,
S. Enthaler, K. Junge and M. Beller, Angew. Chem., Int. Ed.,
2008, 47, 2497–2501; (e) A. M. Tondreau, E. Lobkovsky and
P. J. Chirik, Org. Lett., 2008, 10, 2789–2792; (f) B. K. Langlotz,
H. Wadepohl and L. H. Gade, Angew. Chem., Int. Ed., 2008, 47,
4670–4674; (g) A. M. Tondreau, J. M. Darmon, B. M. Wile,
S. K. Floyd, E. Lobkovsky and P. J. Chirik, Organometallics,
2009, 28, 3928–3940; (h) T. Inagaki, L. T. Phong, A. Furuta,
J.-I. Ito and H. Nishiyama, Chem.–Eur. J., 2010, 16, 3090–3096.
11 Only two examples dealing with the hydrosilylation of the aldimine
PhCH = NPh were mentioned: (a) Ref. 10b; (b) S. Zhou, K. Junge,
D. Addis, S. Das and M. Beller, Angew. Chem., Int. Ed., 2009, 48,
9507–9510. Furthermore, an elegant iron-catalyzed reductive amination
of aldehyde and aniline derivatives using FeCl3 and PHMS was
reported: S. Enthaler, ChemCatChem, 2010, 2, 1411–1415.
9b
87
10b
11b
57
78
12b
13b
86
95
a
Typical procedure: ketimine (0.5 mmol), PhSiH3 (1 mmol), and
pre-catalyst 1 (5 mol%) were stirred at 100 1C without solvent upon
visible light irradiation for 24 h, the reaction is then hydrolysed with
b
c
MeOH and NaOH(aq.) 2M. 0.1 mL of toluene as solvent. 0.2 mL
of toluene as the solvent.
and especially directed towards the mechanism of this reaction are
currently underway in our laboratory.
CNRS, Rennes Me
l’Enseignement Superieur are acknowledged for financial
support, and the Ministere des affaires etrangeres and the
´
tropole, Ministere de la Recherche et
´
´
Fundacion Gran Mariscal de Ayacucho for a grant to L.C.M.C.
12 (a) X.-F. Wu and C. Darcel, Eur. J. Org. Chem., 2009, 1144–1147;
(b) X.-F. Wu, D. Bezier and C. Darcel, Adv. Synth. Catal., 2009,
351, 367–370; (c) X.-F. Wu, C. Vovard-Le Bray, L. Bechki and
C. Darcel, Tetrahedron, 2009, 65, 7380–7384; (d) X.-F. Wu and
C. Darcel, Eur. J. Org. Chem., 2009, 4753–4765.
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´
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Homogeneous Hydrogenation, ed. J. G. de Vries and C. J. Elservier,
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3 (a) S. Gladiali and E. Alberico, in Transition Metals for Organic
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13 L. C. Misal Castro, D. Be
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´
´
´
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¨
16 The conversion was 36% and only starting material and product
were detected in the crude H NMR spectrum.
1
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 151–153 153