Cesium carbonate catalyzed chemoselective hydrosilylation of aldehydes and ketones under solvent-free conditions

Article abstract of DOI:10.1002/chem.201403497

Cs₂CO₃ has been found to be an efficient and chemoselective catalyst for reduction of aldehydes and ketones to alcohols with one equivalent of Ph₂SiH₂ as the reductant under solvent-free conditions. Most of the aldehydes employed can be effectively hydrosilated quantitatively to give the corresponding silyl ethers in 2 h at room temperature, whereas the hydrosilylation of ketones proceeded smoothly at 80 °C. The catalyst system tolerates a number of functional groups including halogen, alkoxyl, olefin, ester, nitro, cyano, and heteroaromatic groups; the selective hydrosilylation of aldehydes in the presence of ketone can be effectively controlled by temperature; and hydrosilylation of α,β-unsaturated carbonyls resulted in the 1,2-addition products. The catalytic hydrosilylation of suitable dicarbonyls can be applied to the synthesis of poly(silyl ether)s with a high molecular weight and narrow molecular distribution. A general and practical protocol for the hydrosilylation of aldehydes and ketones under solvent free conditions by using a Cs ₂CO₃/Ph₂SiH₂ system is presented. Most of the aldehydes employed can be effectively hydrosilylated quantitatively to give the corresponding silyl ethers in 2 h at room temperature, whereas the reactions with ketones proceed smoothly at 80 °C.

Full text of DOI:10.1002/chem.201403497

DOI: 10.1002/chem.201403497
Communication
&
Hydrosilylation
Cesium Carbonate Catalyzed Chemoselective Hydrosilylation of
Aldehydes and Ketones under Solvent-Free Conditions
Mengdi Zhao, Weilong Xie, and Chunming Cui*[a]
shown that KOH and KOtBu are active catalysts for hydrosilyla-
Abstract: Cs2CO3 has been found to be an efficient and
tion of ketones and esters, but the reaction with the more
chemoselective catalyst for reduction of aldehydes and ke-
general catalyst KOtBu required an excess of hydrosilanes and
tones to alcohols with one equivalent of Ph2SiH2 as the re-
long reaction times.[9c] Despite the progress in this area, there
ductant under solvent-free conditions. Most of the alde-
is still a great demand for the development of practical and
hydes employed can be effectively hydrosilated quantita-
chemoselective alternatives.
tively to give the corresponding silyl ethers in 2 h at room
We recently reported on the efficient reduction of carboa-
temperature, whereas the hydrosilylation of ketones pro-
mides with PhSiH3 catalyzed by cesium carbonate under sol-
ceeded smoothly at 808C. The catalyst system tolerates
vent-free conditions.[10] We found that the catalyst is very sensi-
a number of functional groups including halogen, alkoxyl,
tive to the hydrosilanes that are employed for the reduction
olefin, ester, nitro, cyano, and heteroaromatic groups; the
reaction. To investigate further applications of this commercial-
selective hydrosilylation of aldehydes in the presence of
ly available and cheap source and its activation mode for hy-
ketone can be effectively controlled by temperature; and
drosilanes, we have studied the hydrosilylation of aldehydes
hydrosilylation of a,b-unsaturated carbonyls resulted in
and ketones with this catalyst. We envisioned that it is quite
the 1,2-addition products. The catalytic hydrosilylation of
possible to realize selective reduction of aldehydes and ke-
suitable dicarbonyls can be applied to the synthesis of
tones by careful choices of hydrosilanes. Herein, we report the
poly(silyl ether)s with a high molecular weight and narrow
results on cesium carbonate-catalyzed chemoselective reduc-
molecular distribution.
tion of aldehydes and ketones by using Ph2SiH2 as the reduc-
tant under solvent-free and relatively mild conditions [Eq. (1)].
This catalytic system tolerates a large range of functional
The hydrosilylation of carbonyl compounds is a valuable
method for the production of alcohols and silyl ether inter-
mediates for organosilane materials, and thus has been widely
employed in academia and industry.[1] Precious metal catalysts,
particularly Rh-, Ru-, and Ir-based catalysts, have been devel-
oped as the most widely employed protocols.[2] However, the
drawbacks of these catalysts are their high costs and toxicity,
which have become the main concerns in modern synthetic
chemistry. Recent studies have been focused on the explora-
tion of environmental benign and inexpensive alternatives.[3] In
this regard, abundant and inexpensive iron catalysts reported
by Beller, Nagashima, Chirik, and Tilley have attracted a great
deal of attention.[3f,4–7] These iron catalysts exhibit a high che-
moselectivity for hydrosilylation of aldehydes and ketones. On
the other hand, Lewis acid and base catalysts are also non-
toxic alternatives,[8,9] particularly B(C6F5)3 and KOtBu have been
successfully employed as catalysts for the hydrosilylation of
carbonyl compounds. These relatively simple catalysts feature
distinct mechanisms from transition-metal systems and some
of them exhibit a unique selectivity. Very recently, it has been
groups that are susceptible to reduction and the hydrosilyla-
tion of most of the aldehydes is complete in 2 h to give the
corresponding silyl ethers in quantitative yield. The system rep-
resents one of the most chemoselective catalytic systems and
practical protocols for the reduction of aldehydes and ketones.
The studies on the Cs2CO3-catalyzed reduction of aldehydes
and ketones were initiated by using the commercially available
Ph2SiH2 and (EtO)3SiH as the reductants. They are not reactive
enough for the reduction of carboamides with Cs2CO3 as the
catalyst. It was found that reduction of benzaldehyde and ace-
tophenone with Ph2SiH2 catalyzed by 5.0 mol% Cs2CO3 at
room temperature under solvent-free conditions led to a com-
plete reduction in 2 and 24 h, respectively (Table 1 and
Table 2). As shown in Table 1, Ph2SiH2 (entries 1–4) is much
more reactive for the reduction of benzaldehyde than
(EtO)3SiH (entries 5–7). The reduction of benzaldehyde under
the optimized conditions (entry 2) yielded Ph2SiH(OCH2Ph) and
Ph2Si(OCH2Ph)2 in quantitative yield. The results for the reduc-
tion of acetophenone are summarized in Table 2, which indi-
cated that Ph2SiH2 is also effective for the reduction of ketones,
albeit the relatively slow reduction reaction in comparison with
[a] M. Zhao, W. Xie, Prof. Dr. C. Cui
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P. R. China)
Fax: (+86)22-23503461
E-mail: cmcui@nankai.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403497.
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Communication
that of the aldehyde. Nevertheless, the reduction of the ketone
can be significantly accelerated at 808C; a high conversion
(94%) was achieved in 8 h under the optimized conditions
(Table 2, entry 4). These results demonstrated that hydrosilyla-
tion of aldehydes and ketones can be accomplished with
Ph2SiH2 as the reductant. Furthermore, this reduction system
may be employed for the selective reduction of aldehydes in
the presence of ketone functionalities at ambient temperature.
To investigate the scope of the Cs2CO3/Ph2SiH2 hydrosilyla-
tion system, various aldehydes and ketones were examined
under optimized conditions. The crude hydrosilylated products
Table 1. Cs2CO3-catalyzed hydrosilylation of benzaldehyde.[a]
Entry
Silane
Catalyst
t
Yield[b]
[%]
([equiv])
loading [mol%]
[h]
1
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (0.5)
(EtO)3SiH (1.0)
(EtO)3SiH (2.0)
(EtO)3SiH (2.0)
5.0
5.0
2.0
5.0
5.0
5.0
5.0
1
2
2
4
48
18
18
77
2[c]
3
100 (37:63)
1
8
12
22
47
4
5
6
7[d]
[a] Reaction conditions: benzaldehyde (3.0 mmol), room temperature.
[b] Yields were determined from the 1H NMR spectra of the crude prod-
ucts. [c] The value in parentheses refers to the molar ratio of Ph2SiH-
(OCH2Ph) and Ph2Si(OCH2Ph)2. [d] 808C.
1
were analyzed by H NMR spectroscopy for the determination
of the conversions and the ratios of mono- and disilylated
products. The results indicated that most of these substrates
can be converted to the corresponding silyl ethers in almost
quantitative yields. The alcohol products listed in Tables 3 and
4 were obtained by hydrolysis of the silylated products with
10% NaOH aqueous solution in methanol followed by separa-
tion and purifications. The results are summarized in Tables 3
and 4 for aldehydes and ketones, respectively.
Table 2. Cs2CO3-catalyzed hydrosilylation of acetophenone with
Ph2SiH2.[a]
Entry
Silane [equiv]
Temp. [oC]
t [h]
Yield[b][%]
The reduction of the most substituted aromatic aldehydes
(Table 3) went to completion at room temperature within 2 h
in the presence of 5 mol% Cs2CO3 under solvent-free condi-
tions. However, the 4-NMe2-substituted benzaldehyde (entry 6)
requires a much longer reaction time (24 h), whereas the sub-
strate with the CN group at the para position of the phenyl
ring (entry 8), which is insoluble in Ph2SiH2, required the addi-
tion of a small amount of THF as the solvent. It appeared that
electron-donating and electron-withdrawing groups on the
1
2
3
4
5
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (0.5)
r.t.
60
80
80
80
24
10
5
8
8
94
89
92
94
78
[a] Reaction conditions: acetophenone (3.0 mmol) with 5.0 mol% Cs2CO3
and Ph2SiH2. [b] Yields were determined from the 1H NMR spectra of the
crude reaction mixture.
[a]
Table 3. Cs2CO3-catalyzed hydrosilylation of aldehydes by Ph2SiH2.
Entry Substrate
t [h] Product
Yield [%][b] Entry Substrate
t [h] Product
Yield [%][b]
60
1
2
a
2
80
83
9
i
j
24
b
2
10
2
68
3
4
c
2
2
75
74
11[d]
12
k
l
2
2
100
84
d
5
e
2
73
13[e]
m
2
90
6
f
24
30
5
80
83
81
14
15
16
n
o
p
2
2
2
84
82
86
7
g
h
8[c]
[a] Reaction conditions: carbonyl compounds (3 mmol) with 5.0 mol% Cs2CO3 in Ph2SiH2 (3 mmol) at room temperature under solvent-free conditions.
[b] The values below the structures refer to the isolated yields (%). [c] 1.5 mL THF was added. [d] NMR yield. [e] 1.5 mmol of Ph2SiH2 was used.
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Table 4. Cs2CO3-catalyzed hydrosilylation of ketones by Ph2SiH2.[a]
Entry
1
Substrate
Product
Yield[b] [%]
77
q
Scheme 2. Hydrosilylation of 1,4-phthalaldehyde and p-acetylbenzaldehyde.
2
r
s
t
84
75
74
corresponding polymeric silyl ether A (Scheme 2) in an excel-
lent yield.
3
Silyl ether A has been characterized by GPC analysis (Mn =
6721) and could be converted to the corresponding alcohols
in good yield (84%) by the standard hydrolysis. The hydrosily-
lation of p-acetylbenzaldehyde at room temperature for 2 h
led to the selective hydrosilylation of the aldehyde group with
only 1/2 equivalent of the silane Ph2SiH2. The silylated product
B can be isolated in an excellent yield (90%, Scheme 2).
The ketones listed in Table 4 were effectively reduced with
Ph2SiH2 at 808C within 5 h at a catalyst loading of 5 mol%. Al-
though the hydrosilylation of ketones requires the relatively
harsh conditions, the C=C double bond, pyridine, and CF3
functionalities in the substrates (Table 4, entries 2, 4, and 5) re-
mained intact under the reaction conditions. The reaction
worked equally well for an aliphatic ketone (entry 3), and an
a,b-unsaturated ketone (entry 2) yielded the corresponding
secondary alcohols in good yields (75 and 84%). Similar to the
hydrosilylation of an a,b-unsaturated aldehyde with the
Cs2CO3/Ph2SiH2 system, the latter reaction led to the selective
1,2-hydrosilylation of the unsaturated ketone (Scheme 1). In
addition, it was found that the reaction of two equivalents of
acetophenone with only one equivalent of Ph2SiH2 and PhMe-
SiH2 also led to the complete SiÀH bond addition at 808C in
12 h. Likewise, reaction of PhSiH3 with three equivalents of ace-
tophenone under the same conditions resulted in the selective
formation of PhSi(OCHPhMe)3 in good yield. These results indi-
cated that the hydrosilyaltion is an atom-efficient protocol in
terms of the utilization of SiÀH bonds.
4[c]
5
u
83
[a] Reaction conditions: carbonyl compounds (3 mmol) with 5.0 mol%
Cs2CO3 in Ph2SiH2 (3 mmol) at 808C in 5 h under solvent-free conditons.
[b] Conversion of ketone, the values below the structures of products
refer to the isolated yields. [c] 1.5 mL THF was added.
phenyl ring did not have noticeable effects on the per-
formance of the catalyst except for the very strong electron-
donating NMe2 group. The a,b-unsaturated aldehydes (Table 3,
entries 7 and 10) can also be reduced under the mild condi-
tions, which selectively yielded the 1,2-addition products
(Scheme 1). This chemoselectivity for a,b-unsaturated alde-
Scheme 1. Hydrosilylation of a,b-unsaturated aldehydes and ketones.
hydes can also be achieved with a few other catalytic systems,
whereas most transition metal catalysts resulted in 1,4-addi-
tion. Aliphatic aldehydes (entries 9 and 11) can also be com-
pletely hydrosilylated at ambient temperature as indicated by
the NMR analysis of crude products. However, the isolation of
the hydrolyzed products led to the modest yields due to the
volatile nature of the resulting alcohols.
The initial mechanism for the Cs2CO3-catalyzed hydrosilyla-
tion of aldehydes and ketones might be very similar to that
proposed for the reduction of amides.[10] It is quite possible
that the interaction of Ph2SiH2 with the “naked” CO3 dianion
yielded a hypervalent hydrosilicate intermediate, which may
transfer the hydride donor to the carbonyl groups. The forma-
tion of hypervalent hydrosilicates by reactions of fluoride, alk-
oxide, and hydride ligands with hydrosilanes has been well-
documented in the literature.[9,11] The present results further
demonstrated that the Cs2CO3-catalyzed hydrosilane transfor-
mations are dependent on the hydrosilanes employed. We rea-
soned that Cs2CO3 combined with other hydrosilanes could be
applied to the selective reduction of other unsaturated sys-
tems.
Most importantly, the hydrosilylation of aldehydes with the
present catalytic system features a wide substrate scope and
the catalytic system tolerates a number of functional groups
including olefin, NO2, CN, ester, and ketone functionalities that
are susceptible to reduction and challenging ones for most of
the existing catalytic systems (Table 3). Furthermore, the hydro-
silylation of the heteroaromatic aldehydes (entries 15 and 16)
is also viable, and pyridine and thiophene carbaldehydes can
be converted to the corresponding alcohols in high yields (82
and 86%). It is noted that hydrosilylation of 1,4-phthalaldehyde
(entry 14) with one equivalent of Ph2SiH2 in THF yielded the
In conclusion, we have developed a general and practical
protocol for the hydrosilylation of aldehydes with the Cs2CO3/
Ph2SiH2 system. Both of the components are commercially
available, cost-effective, and non-toxic. The catalytic system is
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Communication
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very efficient and chemoselective for aldehydes and ketones
under solvent-free conditions in the presence of a variety of
functional groups. The hydrosilylation of a,b-unsaturated car-
bonyls led to 1,2-addition, and hydrosilyaltion of 1,4-phthalal-
dehyde led to the formation of the corresponding poly(silyl
ether). Further studies on the Cs2CO3-catalyzed hydrosilane
transformations and the mechanism for the reduction process
are currently in progress.
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Experimental Section
General procedure for the reduction of aldehydes: A 25 mL
oven-dried Schlenk tube containing a stir bar was loaded with
5.0 mol% Cs2CO3 (49 mg, 0.15 mmol). Subsequently, substrate
(3 mmol) and Ph2SiH2 (553 mg, 3 mmol) was added. The reaction
mixture was stirred at room temperature for 2 h. Longer reaction
times were required for the aldehydes f and i (24 h), g (30 h), and
h (5 h) (Table 3). For the substrate h, 1.5 mL THF was added.
General procedure for the reduction of ketones: A 25 mL oven-
dried Schlenk tube containing a stir bar was loaded with 5.0 mol%
Cs2CO3 (49 mg, 0.15 mmol). Subsequently, the substrate (3 mmol)
and Ph2SiH2 (553 mg, 3 mmol) was added. The reaction mixture
was stirred at 808C for 5 h.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China and the 973 Program (Grant No.
2012CB821600).
Keywords: alcohols
·
aldehydes
·
cesium carbonate
·
hydrosilylation · ketones · silyl ethers
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Received: May 12, 2014
Published online on July 2, 2014
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