Cyclopentadienyliron dicarbonyl dimer: A simple tool for the hydrosilylation of aldehydes and ketones under air

Article abstract of DOI:10.1016/j.catcom.2016.01.033

The readily available iron complex [CpFe(CO)₂]₂ (1) exhibits good catalytic activity in the hydrosilylation of aldehydes and ketones in the presence of diethoxymethylsilane. The procedure described is air-tolerant and applicable to a wide range of substrates.

Full text of DOI:10.1016/j.catcom.2016.01.033

Catalysis Communications 78 (2016) 52–54
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Cyclopentadienyliron dicarbonyl dimer: A simple tool for the
hydrosilylation of aldehydes and ketones under air
⁎
Thais Cordeiro Jung, Gilles Argouarch , Pierre van de Weghe
Université de Rennes 1, UMR CNRS 6226, Institut des Sciences Chimiques de Rennes, Equipe PNSCM, UFR des Sciences Biologiques et Pharmaceutiques, 2 Avenue du Professeur Léon Bernard,
35043 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The readily available iron complex [CpFe(CO)2]2 (1) exhibits good catalytic activity in the hydrosilylation of alde-
hydes and ketones in the presence of diethoxymethylsilane. The procedure described is air-tolerant and applica-
ble to a wide range of substrates.
Received 11 December 2015
Received in revised form 25 January 2016
Accepted 28 January 2016
Available online 3 February 2016
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Aldehydes
Hydrosilylation
Iron
Ketones
Reduction
1. Introduction
organometallic compounds, and is well known for its extensive use as a
cheap and common precursor to many iron carbonyl derivatives [23].
The replacement of precious metals with more abundant and less
toxic first-row transition metals in homogeneous catalysis is an impor-
tant goal of modern synthetic organic chemistry. Great achievements in
the field of iron-catalysed hydrosilylation of aldehydes and ketones
have been realised over the past decade [1]. This reduction reaction
consists of the addition of a hydrosilane to a carbonyl group under
mild conditions, forming a silyl ether intermediate whose hydrolysis
releases the corresponding primary or secondary alcohol. Several
iron-based catalyst systems have been developed for both asymmetric
[2–5] and non-asymmetric [6–22] hydrosilylation reactions. In particu-
lar, the groups of Guan [6], Li [7], Darcel [8], Tilley [9] and Chirik [10]
have successfully synthesized new iron-based catalysts that have
shown excellent activities in the non-asymmetric hydrosilylation of a
wide range of aldehydes and ketones. However, most of these catalyst
systems are elaborated well-defined iron complexes featuring various
stabilizing ligands. The preparation of such pre-catalysts often requires
considerable synthetic effort and resources, which offset the economic
advantages of an inexpensive metal centre, potentially to an impractical
and unsustainable degree. We became interested in identifying a readily
available and stable iron complex suitable for straightforward-
sustainable hydrosilylation.
On the other hand, when considering the ease of metal carbonyls to
be thermally or photochemically activated, there are relatively few re-
actions in which 1 serves directly as the effective pre-catalyst. Nonethe-
less, dehydrocoupling of amine–boranes [24], dehydration of amides to
nitriles [25], living radical polymerisation [26], and reductive amination
of unsaturated hydrocarbons [27], among others, have been previously
reported. Herein, we disclose our results concerning the hydrosilylation
of aldehydes and ketones catalysed by complex 1.
2. Results and discussion
After initial screening of several hydride sources1 and solvents using
4-bromobenzaldehyde (2a) as a model substrate, the relatively cheap
diethoxymethylsilane was identified as the best reducing agent and Tol-
uene as a solvent of choice. Thus, reaction of 2a in Toluene solution at
100 °C in the presence of (EtO)2MeSiH (1.5 equiv.) and 1 (2 mol%) for
16 h under an inert atmosphere gave alcohol 3a with a very good con-
version of 97% (Table 1, entry 1).
To our delight, increasing the catalyst loading to 5 mol% and the
amount of hydrosilane to 2 equiv. allowed a full conversion of the alde-
hyde (entries 2 and 3). We wondered if the excess of silane required for
this optimization could advantageously avoid the use of Toluene. In fact,
the result of the reaction carried out under solvent-free conditions was
The dinuclear iron(I) complex [CpFe(CO)2]2 (1) (Cp:
cyclopentadienyl_η5-C5H5) is one of the oldest and the most studied
1
⁎
Corresponding author.
Polymethylhydrosiloxane (PMHS) and triethylsilane were used in the optimization
E-mail address: gilles.argouarch@univ-rennes1.fr (G. Argouarch).
study with 2a (Air, 100 °C, 24 h, Toluene) but gave lower conversions.
http://dx.doi.org/10.1016/j.catcom.2016.01.033
1566-7367/© 2016 Elsevier B.V. All rights reserved.

T.C. Jung et al. / Catalysis Communications 78 (2016) 52–54
53
Table 1
electronic effects were encountered in the reduction of benzalde-
hyde derivatives (entries 2–9). High 1,2-selectivity was also ob-
served in the hydrosilylation of cinnamaldehyde (2n) leading to
cinnamyl alcohol (3n) with only traces of 3m present in the crude
product (entry 13). After purification, alcohols 3 were isolated in mod-
erate to fairly good yields (44–78%). This may be attributed to some ox-
idation of the starting aldehydes in the course of the reaction, owing to
its relatively harsh conditions.
Optimization study for the hydrosilylation of 2a with 1 as the pre-catalyst.
.
In order to assess this, we have in parallel investigated the scope of
this reaction with various aldehydes under an inert atmosphere (see
Supplementary data). Following the method depicted in entry 3 of
Table 1 (Argon, 100 °C, 16 h, Toluene), the average isolated yields thus
obtained were only 10% higher than those obtained under aerobic con-
ditions, demonstrating that air confers little detrimental effect. These
control experiments also revealed that the presence of oxygen was
not necessary to activate pre-catalyst 1.
We then examined hydrosilylation of a set of ketones using 1 as a
pre-catalyst under air (Table 3). For comparison purposes, most of
these reactions were also conducted under Argon according to condi-
tions described in entry 4 of Table 1 (Argon, 100 °C, 24 h, Neat). They
Entry 1 [mol%] Silane [equiv.] Conditions
Conv.a Yieldb
[%]
[%]
1
2
3
4
5
6
7
2
5
5
5
5
5
5
1.5
1.5
2
2
2
Argon, 100 °C, 16 h, Toluene 97
Argon, 100 °C, 16 h, Toluene 99
Argon, 100 °C, 16 h, Toluene N99
Argon, 100 °C, 24 h, Neat
Air, 100 °C, 16 h, Toluene
Air, 100 °C, 24 h, Toluene
Air, 100 °C, 24 h, Neat
81
83
84
91
77
77
77
N99
90
98
2
2
N99
a
Conversion determined by 1H NMR spectroscopy after basic hydrolysis.
Isolated yield after purification by column chromatography.
b
the same after 24 h (entry 4). We then probed the stability limits of the
iron pre-catalyst 1 by running the reduction under air. Gratifyingly, full
conversion of 2a was reached within 24 h under aerobic, solvent-free
conditions (entries 5–7). To the best of our knowledge, all iron-based,
homogeneous carbonyl hydrosilylation catalyst systems reported to
date require exclusion of moisture and oxygen [28]. Our methodology
addresses this limitation, greatly increasing the applicability of this
reaction. More generally, regardless the central metal, only a handful
of complexes allows for efficient hydrosilylation under aerobic condi-
tions [29–35].
Table 3
Hydrosilylation of ketones under air catalysed by 1.
Entry
Ketone
Conv.a [%]
Yieldb [%]
A series of representative aldehydes were then catalytically reduced
under air using complex 1 (Table 2). Overall, near-quantitative con-
versions were obtained for all (hetero)aromatic and non-aromatic
substrates. Of special interest is that no detrimental steric or
1
2
3
4
5
6
7
4a: X_H
4b: X_Ph
4c: X_Br
4d: X_OMe
4e: X_CN
4f: X_Cl
4g
96
69
78
79
N99
N99
85
N97
N99
N98
n.d.c
n.d.
93
Table 2
87
Hydrosilylation of aldehydes under air catalysed by 1.
8d
4h
92
85
9d
4i
4j
83
74
72
Entry
Aldehyde
Conv.a [%]
Yieldb [%]
10d
n.d.
1
2
3
4
5
6
7
8
2b: X_H
2c: X_Me
2d: X_OMe
2e: X_Cl
2f: X_CN
2g: X_F
2h: X_CF3
2i
N99
N99
N99
N99
N99
N99
N99
N99
76
71
78
n.d.c
66
73
76
73
11
12
4k
4l
N99
78
96
68e
13
4m
97
85
9
2j
N99
64
14
15
4n
4o
90
n.d.
83
10
11
2k
2l
N99
N99
n.d.
76
N99
12
13
2m
2n
N99
N99
44
72
a
Conversion determined by 1H NMR spectroscopy after basic hydrolysis.
Isolated yield after purification by column chromatography.
Not determined.
Reaction time was 48 h.
Endo/exo = 83/17.
b
c
Conversion determined by 1H NMR spectroscopy after basic hydrolysis.
Isolated yield after purification by column chromatography.
Not determined.
a
b
c
d
e

54
T.C. Jung et al. / Catalysis Communications 78 (2016) 52–54
showed only slight differences, which are emphasized below where
significant (see Supplementary data for details).
Supplementary data
Acetophenone (4a) and its para-substituted congeners were
efficiently converted to their corresponding secondary alcohols with
the exception of 4-methoxyacetophenone (4d) (entries 1–6). With
this strongly electron-donating substituent the conversion was only of
85% in air, but could be satisfyingly raised to 97% under Argon.
Propiophenone (4g) also underwent clean reduction (entry 7). For the
ketone series, some steric limitations were apparent; conversion was
incomplete (74–92%) for ortho-substituted acetophenones 4h,i or
cyclohexyl phenyl ketone (4j), even after extended reaction times
(entries 8–10). However, these results were far superior to those ob-
tained with the same substrates under Argon (48% average conver-
sion). When applying the present method to more challenging
carbonyl derivatives, excellent conversions were obtained with
dibenzyl ketone (4k), 2-norbornanone (4l), as well as for 9-fluorenone
(4m) (entries 11–13).
The reduction of benzophenone (4n) under air gave a good re-
sult with a conversion of 90%, which even exceeded 96% under an-
aerobic conditions. Finally, the activated ketone 4o was readily
converted to its corresponding alcohol (entry 15). Thus, the ability
of complex 1 to catalyse the hydrosilylation of less reactive ketones
(when compared to aldehydes) under aerobic conditions has been
clearly demonstrated.
Experimental procedures and analytical data of the products can be
found online at http://dx.doi.org/10.1016/j.catcom.2016.01.033.
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methodology are: (i) the iron-based pre-catalyst is a ready-to-use com-
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Acknowledgements
We thank Université de Rennes 1 and CNRS for financial support.