Efficient hydrosilylation of carbonyl compounds with the simple amide catalyst [Fe{N(SiMe3)2}2]

Article abstract of DOI:10.1002/anie.201005055

Keep it simple: A variety of ketones and two aldehydes underwent efficient hydrosilylation under mild conditions in the presence of the title complex (see scheme; R,R′=H, alkyl, aryl). In some cases, a catalyst loading of just 0.01-0.03 mol % was sufficie

Full text of DOI:10.1002/anie.201005055

Communications
DOI: 10.1002/anie.201005055
Iron Catalysis
Efficient Hydrosilylation of Carbonyl Compounds with the Simple
Amide Catalyst [Fe{N(SiMe3)2}2]**
Jian Yang and T. Don Tilley*
The hydrosilylation of carbonyl groups is an important
synthetic transformation that is widely employed in the
laboratory and in industry.[1] This reaction features high atom
economy and combines reduction of the carbonyl function-
ality with alcohol protection in a single step.[1] The alkoxy-
silane products are valuable synthetic intermediates and
monomers for organosilane polymers and materials. The most
common and active catalysts for this reaction are based on
precious metals, such as rhodium and ruthenium.[1] However,
the high cost of these metals and concerns about trace toxic
metal impurities in the organic products have focused
considerable attention on the development of alternative
catalysts.[2] In particular, catalysts based on iron are highly
attractive, since this metal is abundant, inexpensive, and
relatively nontoxic.[3] Recently, Nishiyama and coworkers[4a–c]
and Beller and co-workers[4d–f] reported the use of Fe(OAc)2
(5 mol%) combined with nitrogen-, phosphorus-, or sulfur-
based ligands for the hydrosilylation of carbonyl function-
alities[4] at 658C. Chirik and co-workers also demonstrated an
efficient hydrosilylation system based on bis(imino)pyridine
iron dialkyl complexes.[5] Although considerable savings can
be achieved by the use of inexpensive metals, specialized
ligands and activators may lead to substantial costs.[6,7]
During our attempts to develop new iron catalysts, we
obtained the unusual d-agostic iron h1-silane complex
[(MeQn2SiH)Fe{N(SiMe3)2}2] (Qn = 8-quinolyl).[8] When the
reactivity of this complex toward carbonyl compounds was
tested, it became apparent that the MeQn2SiH ligand readily
dissociated as a free species in solution, and further heating of
the reaction mixture at 608C produced the hydrosilylation
product.[8] Thus, we investigated the simple iron(II) silylamide
[Fe{N(SiMe3)2}2][9] as a reactive, low-coordination-number
catalyst precursor for transformations involving hydrosilanes.
Herein we report the use of [Fe{N(SiMe3)2}2] as a simple, cost-
effective, environmentally benign, and highly active catalyst
for the hydrosilylation of organic carbonyl groups. At mild
temperatures (ca. 238C), catalyst loadings as low as
0.01 mol% may be used.
The addition of acetophenone (37 equiv) and PhSiH3
(58 equiv) to [Fe{N(SiMe3)2}2] in C6D6 resulted in the rapid
consumption of acetophenone at room temperature [Eq. (1)].
Analysis of the products by NMR spectroscopy revealed an
approximately 1:6.5 ratio of PhSiH2(OCHMePh) and PhSiH-
(OCHMePh)2. Under similar conditions, benzaldehyde was
readily reduced to PhHSi(OCH2Ph)2 as the dominant product
within 3 h (ca. 90%).
Secondary silanes are also suitable substrates for hydro-
silylation catalyzed by [Fe{N(SiMe3)2}2]. With Ph2SiH2 as the
reductant, aldehydes and ketones underwent facile catalytic
hydrosilylation at 238C to cleanly afford the corresponding
silyl ethers Ph2SiHOCHRR’ (Table 1). Thus, rapid hydro-
silylation of benzaldehyde and acetophenone occurred at
238C, with a catalyst loading of 2.7 mol% (Table 1, entries 1
and 2). With 0.31 mol% of the catalyst, the hydrosilylation of
acetophenone proceeded to completion within 1.1 h (Table 1,
entry 3). Acetophenone underwent highly efficient hydro-
silylation with a catalyst loading of just 0.03 mol% (Table 1,
entry 4). Even in the presence of 0.01 mol% of [Fe{N-
(SiMe3)2}2], the complete hydrosilylation of p-methoxyaceto-
phenone (10000 turnovers) occurred in less than 17 h at 238C
(Table 1, entry 5).
The alkyl-substituted ketones 3-pentanone and cyclohex-
anone were rapidly reduced at 238C (Table 1, entries 6 and 7).
At a catalyst loading of 0.03 mol%, cyclohexanone under-
went rapid and clean hydrosilylation in 20 h at 238C (Table 1,
entry 8). Under similar conditions, the hydrosilylation of 3-
pentanone was initially very rapid, with a turnover rate of
> 40 minÀ1, and then slowed down, although 2200 turnovers
had been reached after 20 h (Table 1, entry 9).
[*] Dr. J. Yang, Prof. T. D. Tilley
Department of Chemistry
University of California
Notably, 5-hexen-2-one, cyclopropyl phenyl ketone,
p-bromoacetophenone, and p-dimethylaminobenzaldehyde
were conveniently hydrosilylated to the corresponding silyl-
Berkeley, California 94720 (USA)
Fax: (+1)510-642-8940
E-mail: tdtilley@berkeley.edu
=
ethers: the C C, cyclopropyl, bromoaryl, and dimethylamino
[**] Dow Corning is gratefully acknowledged for financial support of this
research. We thank the National Science Foundation for partial
financial support, and Richard Taylor, Binh Nguyen, and Misha Tzou
(Dow Corning) for valuable discussions. J.Y. thanks Dr. Robert J.
Wright for experimental assistance.
groups were not affected (Table 1, entries 10–13). p-Cyano-
acetophenone was viewed as a more challenging substrate, as
the nitrile functionality could potentially trap the unsaturated
iron center; however, this compound underwent hydrosilyl-
ation with a catalyst loading of 1.2 mol% in 17 h (Table 1,
entry 14).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005055.
10186
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Angew. Chem. Int. Ed. 2010, 49, 10186 –10188

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Chemie
Table 1: Hydrosilylation of ketones under the catalysis of
Supporting Information). This catalyst was found to be
equally effective.
[Fe{N(SiMe3)2}2].[a]
Entry Cat.
[mol%]
Substrate
t
[h]
Conv.[b] Product
[%]
1
2.7
2
>98
More sterically demanding silanes, such as (trip)PhSiH2
(trip = 2,4,6-iPr3C6H2), Ph3SiH, and Et3SiH, were not effec-
tive for this reaction. For example, under identical conditions
(238C, 2.7 mol% Fe loading), no catalysis was observed after
1 day. Heating of the reaction mixture of 3-pentanone and
Et3SiH to 608C resulted in the precipitation of insoluble Fe
species, and no catalytic hydrosilylation was observed.
The reaction solutions remained transparent throughout
the catalysis, and mercury had no effect on the reactions of
entries 2 and 8.[12] These observations are consistent with a
homogeneous catalytic system. The observation that no ring-
opening product formed during the hydrosilylation of cyclo-
propyl phenyl ketone is inconsistent with a radical mecha-
nism.[13] The stoichiometric reaction of [Fe{N(SiMe3)2}2] with
Ph2SiH2 (3 equiv) was slow at 238C (ca. 36% conversion after
21 h), whereas the addition of acetophenone resulted in rapid
consumption of [Fe{N(SiMe3)2}2] and the formation of HN-
(SiMe3)2. In situ monitoring of the operating catalytic system
by NMR spectroscopy also revealed the formation of HN-
(SiMe3)2 at early stages in the catalysis. The generated
HN(SiMe3)2 could serve as a labile ligand to support a
highly reactive iron complex, which may initiate hydrosilyla-
tion reactions. The mechanism of this catalysis remains largely
undefined, but future efforts will focus on obtaining more
mechanistic information.
Thus, [Fe{N(SiMe3)2}2], a simple iron complex known for
over two decades,[9] has proven to be a readily accessible
catalyst precursor for the efficient hydrosilylation of carbonyl
functionalities at room temperature. This [Fe{N(SiMe3)2}2]/
Ph2SiH2 system is one of the most active iron-based catalytic
systems for the hydrosilylation of ketones. Importantly, no
special neutral donor ligand or activator is required. Fur-
thermore, this system tolerates a variety of functional groups,
such as ether, olefin, cyclopropyl, bromoaryl, dimethylamino,
and even nitrile groups. The iron catalyst can be generated in
situ, and the reaction can be carried out without a solvent;
thus, this catalyst may provide a simple, economically and
environmentally attractive alternative to catalytic systems
currently employed for the hydrosilylation of ketones.
Furthermore, [Fe{N(SiMe3)2}2] appears to be a fundamentally
new type of iron-based hydrosilylation catalyst, and this
finding should stimulate further interest in the exploration of
simple metal amides[14] as catalyst precursors for transforma-
tions involving hydrosilanes. Future studies will address the
scope and mechanism of this catalytic hydrosilylation.
2
3
4
2.7
0.31
0.03
0.7 >98
1.1 >98
18
>98
2.2
17
41
>98
5
0.01
6
7
2.7
2.7
0.7 >98
0.7 >98
0.7
4
26
61
8
0.03
20
0.3
20
>98
26
67
9
0.03
1.0
1.2
5.5
88
95
10
9
28
70
96
11
12
0.23
0.76
1
>98
13
14
0.54
1.2
0.5 >98
17
>98
15[c] 2.7
0.7 >98
0.5 >98
16[d] 0.55
[a] Reaction conditions: substrate (0.49–1.47 mmol), Ph2SiH2
(1.6 equiv), 238C, C6D6. [b] Conversion was determined by NMR
spectroscopy on the basis of the consumption of the aldehyde/ketone,
and the identity of the product was confirmed by GC–MS. [c] PhMeSiH2
was used instead of Ph2SiH2. [d] The reaction was carried out without a
solvent.
PhMeSiH2 was also found to be effective for the hydro-
silylation of acetophenone (Table 1, entry 15). Interestingly,
these reactions occur readily in the absence of a solvent. Thus,
the solvent-free hydrosilylation of acetophenone was com-
plete in less than 0.5 h with a catalyst loading of 0.55 mol%
(Table 1, entry 16). A catalyst generated in situ from FeBr2
and KN(SiMe3)2 also proved effective [Eq. (2)].[10] Finally, in
light of recent studies on the effects of other metal contam-
inants in iron-catalyzed cross-coupling reactions,[11] we tested
a catalyst synthesized from FeBr2 of 99.999% purity (see the
Received: August 12, 2010
Published online: October 14, 2010
Angew. Chem. Int. Ed. 2010, 49, 10186 –10188
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www.angewandte.org 10187

Communications
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Keywords: carbonyl compounds · homogeneous catalysis ·
hydrosilylation · iron · metal amides
.
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