Ruthenium complex catalyzed hydrosilylation of esters: A facile transformation of esters to alkyl silyl acetals and aldehydes

Article abstract of DOI:10.1016/S0040-4039(01)00094-6

Hydrosilylation of esters takes place in the presence of ruthenium catalysts to afford the corresponding alkyl silyl acetals in moderate to good yields, which can be converted into aldehydes by hydrolysis.

Full text of DOI:10.1016/S0040-4039(01)00094-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2149–2151
Ruthenium complex catalyzed hydrosilylation of esters: a facile
transformation of esters to alkyl silyl acetals and aldehydes
Mamoru Igarashi, Ryo Mizuno and Takamasa Fuchikami*
Sagami Chemical Research Center, 4-4-1 Nishi-Ohnuma, Sagamihara, Kanagawa 229-0012, Japan
Received 11 December 2000; revised 11 January 2001; accepted 12 January 2001
Abstract—Hydrosilylation of esters takes place in the presence of ruthenium catalysts to afford the corresponding alkyl silyl
acetals in moderate to good yields, which can be converted into aldehydes by hydrolysis. © 2001 Elsevier Science Ltd. All rights
reserved.
The conversion of ester groups to the corresponding
aldehyde derivatives is one of the most important trans-
formations in organic syntheses, and for a long time
diisobutylaluminum hydride (DIBALH) reduction has
been a sole efficient method for this purpose.1 However,
DIBALH reagent is difficult to handle in both labora-
torial and industrial scales, because of its flammable,
pyrophoric, moisture-sensitive, irritant, and toxic
nature. Many attempts toward this transformation have
been done using hydrosilanes as reducing agents, how-
ever, ethers2,3 or alcohols4–6 have been obtained as main
products. Ojima and Kumagai reported the Rh-cata-
lyzed 1,2-addition of hydrosilanes to conjugated esters,
in which 1,4-adducts were obtained as major products.7
Recently, Buckwald and co-workers reported Cp2Ti(p-
ClC6H4O)2–TBAF/alumina catalyzed reduction of lac-
tones to lactols using polymethylhydrosiloxane
(PMHS), where only five- and six-membered lactones
gave satisfactory results.8 In this paper, we wish to
describe efficient and general catalytic processes for the
conversion of esters to the corresponding alkyl silyl
acetals and aldehydes.
Re2(CO)10, Fe2(CO)9, Fe3(CO)12, Co2(CO)8 and
Rh6(CO)16 show no or little catalytic activity either in
the absence or presence of co-catalysts.
Optimized conditions are as follows: Method A: A
mixture of ethyl 2-methylpropanoate (1.0 mmol),
Et3SiH (1.5 mmol), and Ru3(CO)12 (0.0033 mmol) in
toluene (1.0 ml) was heated at 100°C for 16 h under an
Ar atmosphere. GLC analysis of the reaction mixture
showed the formation of 1-ethoxy-1-triethylsiloxy-2-
methylpropane in 78% yield. Method B: A mixture of
ethyl 2-methylpropanoate (1.0 mmol), Et3SiH (1.5
mmol), and [RuCl2(CO)3]2 (0.005 mmol), EtI (0.05
mmol), and Et2NH (0.05 mmol) in toluene (1.0 ml) was
heated at 100°C for 16 h under an Ar atmosphere. The
GLC analysis of the reaction mixture showed the for-
mation of 1-ethoxy-1-triethylsiloxy-2-methylpropane in
86% yield.
Representative results are summarized in Tables 1 and
2. Methyl, isopropyl, and phenyl esters afforded the
satisfactory results (entries 1, 2, and 3 in Table 1,
entries 1, 3, and 4 in Table 2). Sterically hindered esters
such as tert-butyl ester (entry 4 in Table 1) or 2,2-
dimethylpropionate (entry 2 in Table 2) gave moderate
yields of the products. Cyclohexanecarboxylate, ben-
zoate and phenylacetate also easily converted into the
corresponding methyl silyl acetals (entries 5, 6, and 7 in
Tables 1 and 2). In sharp contrast with the results
reported by Buchwald et al.,8 five-membered lactone
gave lower yield of the product (entry 8 in Tables 1 and
2) in the present catalytic systems.
Initially, we surveyed the active catalytic system in the
reaction of ethyl 2-methylpropanoate with triethylsi-
lane. We found that Ru3(CO)12 shows an effective
catalytic activity, and [RuCl2(CO)3]2 possesses a similar
activity in the presence of diethylamine and ethyl iodide
as co-catalysts. However, other transition-metal car-
bonyl complexes such as Cr(CO)6, W(CO)6, Mn2(CO)10,
Keywords: ruthenium and compounds; catalysts; silicon and com-
pounds; esters; acetals; aldehydes.
* Corresponding author. Fax: +81-42-749-7631; e-mail: scrc1@
sagami.or.jp
Instead of triethylsilane, both tert-butyldimethylsilane
and phenyldimethylsilane can be applicable in the
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00094-6

2150
M. Igarashi et al. / Tetrahedron Letters 42 (2001) 2149–2151
Table 1. Hydrosilylation of esters by triethylsilane using Ru3(CO)12 catalyst
Ru3(CO)12 ([Ru]=1mol%)
Ester
Et3SiH
(1.5eq)
Alkyl Silyl Acetal
+
PhMe
100°C, 16 h
Entry
Ester
Conv. of Ester(%)a)
Yield of Acetal(%)a,b)
O
1
2
3
4
100
89
57
94
74
45
OMe
O
OPh
O
100
100
O
O
O
O
90
61
5
6
7
8
OMe
O
51
81
93
57c)
84
Ph
OMe
O
Ph
OMe
O
36
O
a) Determined by GLC
b) Yields are conversion yields.
c) Determined by 1H-NMR
Table 2. Hydrosilylation of esters by triethylsilane using [RuCl2(CO)3]2–EtI–Et2NH catalyst
[RuCl2(CO)3]2 ([Ru]=1mol%)
EtI, Et2NH(5mol%)
Ester
Alkyl Silyl Acetal
Et3SiH
(1.5eq)
+
PhMe
100°C, 16 h
Entry
Ester
Conv. of Ester(%)a)
100
Yield of Acetal(%)a,b)
57
O
1
2
3
4
OMe
O
O
42
79
66
98
91
OMe
OPh
O
O
100
O
OMe
5
95
73
89
46
97
O
6
7
8
63c)
90
Ph
OMe
OMe
O
O
Ph
44
O
a) Determined by GLC
b) Yields are conversion yields.
c) Determined by 1H-NMR.

M. Igarashi et al. / Tetrahedron Letters 42 (2001) 2149–2151
Table 3. Hydrosilylation of ethyl 2-methylpropionate by hydrosilanes
2151
[RuCl2(CO)3]2
EtI, Et2NH
O
Ru3(CO)12
OSiR3
OEt
or
+
R3SiH
OEt
PhMe
100°C, 16 h
(1.5eq)
Entry
R3SiH
Cat.
Conv. of ester (%)a
Yield of acetal (%)a,b
1
2
3
4
5
6
Et3SiH
Et3SiH
Ru3(CO)12
90
100
35
94
54
78
86
79
[RuCl2(CO)3]2
c
tBuMe2SiH
tBuMe2SiH
PhMe2SiH
PhMe2SiH
Ru3(CO)12
c
c
[RuCl2(CO)3]2
87
c
Ru3(CO)12
87d
72d
[RuCl2(CO)3]2
81
a Determined by GLC.
b Yields are conversion yields.
c 5 mol% of [Ru] was used.
d Determined by 1H NMR.
O
O
OSiEt3
OMe
O
[RuCl2(CO)3]2
EtI, Et2NH
1N-HCl
OMe
OMe
H
H
+ Et3SiH
(1.5eq)
PhMe
100°C, 16 h
THF
r.t., 2 h
92%
98%
O
[RuCl2(CO)3]2
EtI, Et2NH
OSiEt3
OMe
1N-HCl
Ph
Ph
Ph
+ Et3SiH
(1.5eq)
PhMe
100°C, 16 h
THF
r.t., 2 h
71%
91%
Scheme 1.
References
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tert-butyldimethylsilyl group may be useful as the pro-
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Alkyl silyl acetals thus obtained can be easily converted
into the corresponding aldehydes in excellent yields by
the action of 1N HCl in tetrahydrofuran at room
temperature (Scheme 1).
In summary, we have developed facile and efficient
methods for the transformation of esters to aldehydes
via alkyl silyl acetals, which are obtained by Ru com-
plex-catalyzed hydrosilylation. Our method has the fol-
lowing advantages: All materials, starting hydrosilanes
and formed alkyl silyl acetals, are stable enough toward
air and moisture, and are easily handled without special
care. The obtained alkyl silyl acetals can be converted
into the corresponding aldehydes by acidic hydrolysis.
.