Hydrosilylation of ketones catalyzed with Mg-Al-O-t-Bu hydrotalcite

Article abstract of DOI:10.1080/00397911.2010.519849

In this article, a simple and efficient procedure of hydrosilylation of ketones catalyzed with Mg-Al-O-t-Bu hydrotalcite was described. Hydrosilylation of ketones with triethoxysilane was carried out smoothly at room temperature in the presence of Mg-Al- O-t-Bu hydrotalcite without solvent, and the conversions of ketones were more than 90%. The recyclability of Mg-Al-O-t-Bu hydrotalcite was also tested. Copyright Taylor & Francis Group, LLC.

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Hydrosilylation of Ketones Catalyzed
with Mg-Al-O-t-Bu Hydrotalcite
Yaoyun Chen a , Ying Bai a , Jiajian Peng a , Jiayun Li a & Guoqiao Lai
a
a Key Laboratory of Organosilicon Chemistry and Material Technology
of Ministry of Education , Hangzhou Normal University , Hangzhou ,
China
Accepted author version posted online: 30 Jun 2011.Published
online: 22 Aug 2011.
To cite this article: Yaoyun Chen , Ying Bai , Jiajian Peng , Jiayun Li & Guoqiao Lai (2011)
Hydrosilylation of Ketones Catalyzed with Mg-Al-O-t-Bu Hydrotalcite, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:24, 3689-3694,
DOI: 10.1080/00397911.2010.519849
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Synthetic Communications1, 41: 3689–3694, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.519849
HYDROSILYLATION OF KETONES CATALYZED
WITH Mg-Al-O-t-Bu HYDROTALCITE
Yaoyun Chen, Ying Bai, Jiajian Peng, Jiayun Li, and
Guoqiao Lai
Key Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, China
GRAPHICAL ABSTRACT
Abstract In this article, a simple and efficient procedure of hydrosilylation of ketones
catalyzed with Mg-Al-O-t-Bu hydrotalcite was described. Hydrosilylation of ketones with
triethoxysilane was carried out smoothly at room temperature in the presence of Mg-Al-
O-t-Bu hydrotalcite without solvent, and the conversions of ketones were more than 90%.
The recyclability of Mg-Al-O-t-Bu hydrotalcite was also tested.
Keywords Hydrosilylation; ketones; Mg-Al-O-t-Bu hydrotalcite; triethoxysilane
INTRODUCTION
Hydrosilylation of ketones provides a general route for their reduction to
alcohols. The metal-catalyzed reduction of carbonyl compounds to alcohols by
hydrosilylation, hydrogenation, or transfer hydrogenation has emerged as a powerful
method in organic synthesis and has found application in the commercial production
of fine chemicals, perfumes, and pharmaceuticals.[1] Catalytic hydrosilylation of
ketones is a valuable transformation because it generates protected alcohols in one
step.[2] Many transition-metal complexes, especially those based on ruthenium,[3,4]
rhodium,[5,6] iridium,[7] titanium,[8] zinc,[9,10] and copper,[11,12] are known to catalyze
hydrosilylation, but the metal catalysts are generally not recyclable. When metal
catalysts were used, there is a certain degree of harm to the environment. Low-cost
and highly efficient catalysts for hydrosilylation of ketones have become the state
of the art.
Layered double hydroxides (LDHs), also known as hydrotalcite-like com-
pounds, are naturally occurring anionic clays. LDHs are versatile lamellar compounds,
Received July 12, 2010.
Address correspondence to Jiajian Peng or Guoqiao Lai, Key Laboratory of Organosilicon
Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University,
Hangzhou 310012, China. E-mail: jjpeng@hznu.edu.cn; gqlai@hznu.edu.cn
3689

3690
Y. CHEN ET AL.
which can be made to incorporate a broad rang of trivalent and divalent metal cations.
LDHs can yield catalysts of varying basicities according to the activation procedure.
For example, Mg-Al-O-t-Bu hydrotalcite (Mg-Al-O-t-Bu-HT), which was produced
by exchange of tert-butoxide anion in LDHs, can catalyze many organic transforma-
tions, such as hydrogen transfer,[13] aldol,[14] Knoevenagel condensations and Michael
addition,[15] transesterification,[16] cyanoethylation of alcohols,[17] and epoxidation of
olefins.[18] On the other hand, hydrosilylation of ketones with triethoxysilane carried
out smoothly in toluene at room temperature in the present of potassium tert-butano-
late was reported by our group.[19] The efficiency of potassium tert-butanolate for
hydrosilylation of ketones prompted us to test the efficacy of Mg-Al-O-t-Bu-HT as
a heterogeneous catalyst for the hydrosilylation reaction.
EXPERIMENTAL
The Mg-Al hydrotalcite was prepared according to the reported procedures.[15]
Mg-Al-O-t-Bu-HT was prepared from Mg-Al hydrotalcite prepared previously
(1.214 g) by exchange with 0.1 M solution of potassium tert-butoxide in 100 mL tet-
rahydrofuran (THF) under stirring at room temperature for 24 h. The precipitate
(Mg-Al-O-t-Bu-HT) was filtered under nitrogen atmosphere and dried under
vacuum to obtain hydrotalcite (1.380 g).
In a 10-mL reaction tube, 1.0 mmol of ketones and 10 mg Mg-Al-O-t-Bu-HT
were stirred at room temperature for 5 min, and then triethoxysilane (2.0 mmol)
was added. The resulting mixture was stirred for 0.5 h. After consumption of the
starting ketone or no further conversion, the reaction mixture was treated with
10% NaOH solvent (2 mL), and the organic product was extracted with ether
(3 ꢀ 10 mL). The combined extract was washed with saturated NaHCO3 followed
by water, dried with anhydrous sodium sulfate, filtered, and concentrated in vacuum
to give the crude product. The conversion of ketone was determined by gas chroma-
tographic (GC) analysis. The residue was purified by column chromatography (5:1
petroleum ether=EtOAc) to afford the product.
1-Phenylethanol
1H NMR (CDCl3, 400 MHz) d (ppm): 1.39 (d, J ¼ 4.0 Hz, 3H), 2.10 (s, 1H),
4.76–4.81 (m, 1H), 7.16–7.21 (m, 5H).
1-(3-Nitrophenyl)ethanol
1H NMR (CDCl3, 400 MHz) d (ppm):1.53 (d, J ¼ 4.0 Hz, 3H), 2.37 (s, 1H),
4.99 (d, J ¼ 4.0 Hz, 1H), 7.50–7.54 (m, 1H), 7.70 (s, 1H), 8.10 (s, 1H), 8.24 (s, 1H).
1-(4-Bromophenyl)ethanol
1H NMR (CDCl3, 400 MHz) d (ppm): 1.46 (d, J ¼ 2.0 Hz, 3H), 1.87 (s, 1H),
4.83–4.88 (m, 1H), 7.23 (d, J ¼ 4.0 Hz, 2H), 7.46 (d, J ¼ 4.0 Hz, 2H).

HYDROSILYLATION OF KETONES
3691
Table 1. Effect of amounts of Mg-Al-O-t-Bu-HT used on the
hydrosilylation of acetophenone
Entry
Catalyst (g)
Conv. of acetophenone (%)
1
0.005
0.075
0.010
0.015
0.015
0.020
88
93
98
98
15
98
2
3
4
5a
6
Note. Acetophenone 1.0 mmol; HSi(OEt)3 2.0 mmol; 0.5 h; no
solvent; rt.
aThe second run.
RESULTS AND DISCUSSION
First, hydrosilylation of acetophenone with triethoxysilane (2.0 eq.) was taken as
a model reaction, and the effect of the amounts of Mg-Al-O-t-Bu-HT used on the
conversion of acetophenone was investigated. The results are listed in Table 1. As
can be seen from Table 1, for 1.0 mmol of acetophenone, conversion was increased
along with the amounts of Mg-Al-O-t-Bu-HT used, and when the Mg-Al-O-t-Bu-HT
was 0.01 g, 98% conversion of acetophenone was obtained. Further increasing the
amounts of Mg-Al-O-t-Bu-HT used did not enhance the conversion. Unfortunately,
the recyclability experiment results showed that Mg-Al-O-t-Bu-HT could not be
reused.
Second, the effect of solvent on the reaction was investigated. The experiments
show that the solvent has a great impact on the addition reaction between acetophe-
none and triethoxysilane. As can be seen from Table 2, when THF, dimethylforma-
mide (DMF), or acetontrile were used as the reaction solvent, excellent conversion of
acetophenone was obtained. Mg-Al-O-t-Bu-HT catalyst could also perform smoothly
under solvent-free conditions. However, low conversion was obtained when either
benzene or toluene were used as solvent. Additionally, according to the results listed
Table 2. Effect of solvent and temperature on the hydrosilylation of
acetophenone
Entry
Solvent
Temperature (ꢁC) Conv. of acetophenone (%)
1
2
THF
DMF
60
60
60
60
60
0
97
96
98
60
24
98
98
98
98
98
3
Acetonitrile
Benzene
Toluene
None
4
5
6
7
None
None
rt
8
40
60
80
9
10
None
None
Note. Acetophenone 1.0 mmol; HSi(OEt)3 2.0 mmol; 0.5 h; Mg-Al-
O-t-Bu-HT 0.01 g.

3692
Y. CHEN ET AL.
Table 3. Results of the hydrosilylation of ketones with triethoxysilane
Entry
Ketones
Conversion (%)
1
2
3
4
5
6
7
8
Acetophenone
3-Nitroacetophenone
4-Methylacetophenone
4-Bromoacetophenone
Butyrophenone
98
99
98
99
92
100
12
8
4-Methoxyacetophenone
2-Heptanone
Cyclohexanone
Note. Ketone 1 mmol; HSi(OEt)3 2.0 mmol; Mg-Al-O-t-Bu-HT
0.01 g; 0.5 h; no solvent; rt.
Scheme 1. Proposed mechanism of the hydrosilylation catalyzed with Mg-Al-O-t-Bu-HT.
in Table 2, hydrosilylation of acetophenone with triethoxysilane could be carried out
smoothly at 0 ꢁC or room temperature under solvent-free conditions.
To investigate the scope and limitation of Mg-Al-O-t-Bu-HT used as catalyst
for hydrosilylation of ketones, the hydrosilylation of substituted aromatic and
aliphatic ketones with triethoxysilane were tested. Results listed in Table 3 indicated
that substituent groups had a slight effect on the reaction when substituted aromatic
ketones were used as substrates. Mg-Al-O-t-Bu-HT had low catalytic activity in the
hydrosilylation of aliphatic ketones.
The proposed mechanism for Mg-Al-O-t-Bu-HT-catalyzed hydrosilylation of
ketone with triethoxylisane is depicted in Scheme 1. The activation of silane is the
key step at the beginning of reaction, and the positively charged bimetallic layer
results in the formation of intermediate 1.
CONCLUSION
The hydrosilylation of aromatic ketones was carried out smoothly at room
temperature in the presence of Mg-Al-O-t-Bu-HT under solvent-free conditions,
while Mg-Al-O-t-Bu-HT had low catalytic activity in the hydrosilylation of aliphatic
ketones. Mg-Al-O-t-Bu-HT as nontoxic and inexpensive solid base catalyst is a prac-
tical alternative for hydrosilylation of aromatic ketones.

HYDROSILYLATION OF KETONES
3693
ACKNOWLEDGMENT
We are grateful to the Program of Zhejiang Province (2008C14041) for financial
support.
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