Rhenium-catalyzed reaction of carbonyl compounds with ketene silyl acetals

Article abstract of DOI:10.1016/j.tetlet.2009.11.109

It was found that rhenium complex is an effective catalyst for the reaction of carbonyl compounds with ketene silyl acetals. A wide range of β-silyloxy esters is obtained by the treatment of carbonyl compounds with ketene silyl acetals in the presence of

Full text of DOI:10.1016/j.tetlet.2009.11.109

Tetrahedron Letters 51 (2010) 793–795
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Rhenium-catalyzed reaction of carbonyl compounds with ketene silyl acetals
*
Yutaka Nishiyama , Kenta Kaiba, Rui Umeda
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
It was found that rhenium complex is an effective catalyst for the reaction of carbonyl compounds with
ketene silyl acetals. A wide range of b-silyloxy esters is obtained by the treatment of carbonyl compounds
with ketene silyl acetals in the presence of a catalytic amount of ReBr(CO)5 in moderate to good yields.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 25 September 2009
Revised 20 November 2009
Accepted 26 November 2009
Available online 11 December 2009
The reaction of carbonyl compounds with silicon nucleophiles,
such as enol silyl ethers, allyl silanes, and ketene silyl acetals, is
one of the useful methods for constructing carbon–carbon bond
in organic synthesis.1 There are many reports on these reactions.
These reactions are usually attained with the aid of a Lewis acid
promoter or catalyst. Therefore, several of these methods suffer
from drawbacks such as strictly anhydrous and strongly acidic
conditions.
(14%) (entry 3). In the case of 1-(tert-butoxy)-1-(tert-butyldimethyl-
silyloxy)ethene (2b), having a bulky tert-butyldimethylsilyloxy and
tert-butyl groups, some complicated by-products were formed and
the coupling products were not confirmed (entry 8). For the reaction
of 1-(tert-butyldimethylsilyloxy)-1-methoxyethene (2c), the cou-
pling reaction proceeded to give the methyl3-(tert-butyldimethylsi-
lyloxy)-3-phenylpropanoate (5c) in 95% yield with a high selectivity
(entry 9).6
Recently, we have shown the hitherto unknown capacity of
ReBr(CO)5, which is an air-stable and a water-tolerant compound,
as an efficient catalyst for the reaction of carbonyl compounds with
silicon and tin nucleophiles.2,3 We now find that the coupling of
carbonyl compounds with ketene silyl acetals occurred smoothly
using the rhenium-complex catalyst under nearly neutral condi-
tions to produce the corresponding b-silyloxy esters in moderate
to good yields (Scheme 1).4,5
To investigate the optimized reaction conditions, benzaldehyde
(1a) was reacted with ketene silyl acetals 2a–c in the presence of a
catalytic amount of ReBr(CO)5 under various reaction conditions
and these results are shown in Table 1. When 1-(tert-butoxy)-1-(tri-
methylsilyloxy)ethene (2a) was allowed to react with 1a in the pres-
ence of the ReBr(CO)5 catalyst in benzene solvent at 80 °C for 1 h, the
coupling products were obtained in 71% yield as a mixture of the
b-hydroxy ester 3 and carboxylic acid 4 (entry 3). At lower reaction
temperatures, the yields of the coupling products decreased (entries
1–2). Even when toluene, dichloroethane, and THF instead of ben-
zene were used as solvents, the coupling of 1a with 2a proceeded
smoothly to give the corresponding coupling products in good
yields, whereas the use of coordinating solvent such as acetonitrile
significantly decreased the yield of the products (entries 4–7). The
yields and selectivity of the coupling products, b-hydroxy ester 3,
b-hydroxy carboxylic acid 4, and b-silyloxy ester 5, were affected
by the ketene silyl acetal used. When 1-(tert-butoxy)-1-(trimethyl-
silyloxy)ethene (2a) was used, the coupling products were obtained
with a mixture of b-hydroxy ester 3a (57%) and carboxylic acid 4a
In order to determine the applicability of the preparation of var-
ious b-silyloxy esters, various carbonyl compounds were reacted
with 2c in the presence of a catalytic amount of ReBr(CO)5 at
80 °C for 1 h. These results are shown in Table 2. 4-Methyl-, 4-
methoxy-, and 4-chlorobenzaldehyde were also coupled with 2c
to give the b-silyloxy ester 5 in 85–91% yields (entries 1–3). When
4-nitrobenzaldehyde substituted with an electron-withdrawing
group was treated with 2c under the same reaction conditions as
that of benzaldehyde (1a), the yield of the coupling product de-
creased; however, the product yield was improved by extending
the reaction time (entry 4). Similarly, aliphatic aldehydes, such as
hexanal, isobutylaldehyde, and pivalaldehyde, were coupled with
2c to give the coupling products in good yields by extending the
reaction time (entries 5–7). Arylalkyl-, dialkyl-, and cyclic ketones
as well as aldehydes gave the corresponding b-silyloxy esters 5 in
good yields for longer reaction times (entries 8–10).
For the coupling reaction of benzaldehyde (1a) with (E)-1-(tert-
butoxy)-1-(trimethylsilyloxy)propene (2d), the anti-b-hydroxy es-
ter was preferentially formed in 68% yield (anti/syn = 79/21)
(Scheme 2). Similarly, 1a was coupled with (Z)-1-(tert-butoxy)-1-
(trimethylsilyloxy)propene (2e) with anti selectivity to give the
coupling products as a mixture of the b-hydroxy ester (45%, anti/
syn = 72/28) and carboxylic acid (22%, anti/syn = 67/33). 1-Meth-
oxy-2-methyl-1-(trimethylsilyloxy)propene (2f) was also coupled
R3Si
cat. ReBr(CO)5
OSiR3
OR4
O
O
O
+
R1
R2
R1
OR4
R2
* Corresponding author. Tel.: +81 6 6368 1121; fax: +81 6 6339 4026.
E-mail address: nishiya@kansai-u.ac.jp (Y. Nishiyama).
Scheme 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.109

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Y. Nishiyama et al. / Tetrahedron Letters 51 (2010) 793–795
Table 1
Rhenium-catalyzed coupling reaction of benzaldehyde (1a) with ketene silyl acetals
2a–ca
entry R1
R2
temp (°C) solvent
yield / %b)
3
4
5
Scheme 2. Rhenium-catalyzed coupling reaction of benzaldehyde (1a) with (E)-
and (Z)-1-(tert-butoxy)-1-(trimethylsilyloxy)propene.
1
2
3
4
5
6
Me tBu (2a)
Me tBu (2a)
Me tBu (2a)
Me tBu (2a)
Me tBu (2a)
Me tBu (2a)
Me tBu (2a)
25
60
80
80
80
80
80
benzene
benzene
benzene
toluene
trace trace trace
22
57
35
6
trace
trace
trace
trace
14
15
8
ClCH2CH2Cl 48
THF
47
trace
—
trace trace
7
CH3CN
benzene
benzene
0
—
0
trace
—
8c
9d
tBu tBu (2b) 80
tBu Me (2c)
80
0
95 (74)
a
Reaction conditions: 1a (0.3 mmol), 2 (0.4 mmol), ReBr(CO)5 (0.025 mmol) and
solvent (3 mL) for 1 h.
b
1H-NMR yield based on 1a. The number in parenthesis shows the isolated yield.
Some complicated by-products were formed.
After the reaction, desilylation with HCl aq. was not carried out.
c
d
Scheme 3. Rhenium-catalyzed coupling reaction of benzaldehyde (1a) with 1-
methoxy-2-methyl-1-(trimethylsilyloxy)propene (2f).
with 1a to give 3-hydroxy-2,2-dimethyl-3-phenylpropanoate in
74% yield (Scheme 3).
It has already been reported that the coordinative unsaturated
16-electron complex, ReBr(CO)4, was generated by the dissociation
of carbon monoxide from ReBr(CO)5 under toluene reflux condi-
tions.7 Based on this information, we suggested that the coordina-
tive unsaturated rhenium species would act as a Lewis acid
catalyst during the coupling reaction.
For the reaction of (E)- and (Z)-1-(tert-butoxy)-1-(trimethylsi-
lyloxy)propenes, 2d and 2e, it was found that the anti-addition
products were preferentially formed and the selectivity was not
dependent on the geometry of the ketene silyl acetal. Although,
at the present time, we cannot get a clear reason why the anti-iso-
mer was preferentially formed on these reactions, a plausible cause
is as follows. The steric interactions on intermediates 7a and 7b are
greater than those of intermediates 6a and 6b due to the steric
repulsion of CH3 and ReBr(CO)4 coordinated with the carbonyl
group (Scheme 4).8 Therefore, it is suggested that the anti-products
were preferentially formed.
In summary, we found that rhenium-catalyzed reaction of var-
ious carbonyl compounds such as not only aromatic and aliphatic
aldehydes but also ketones with ketene silyl acetal 2c gave the cor-
responding b-silyloxy esters in moderate to good yields.
Table 2
Rhenium-catalyzed coupling reaction of carbonyl compounds with 1-(tert-butyldi-
methylsilyloxy)-1-methoxyethene (2c)a
entry
R1
R2
yield / %b)
1
2
4-CH3C6H4
4-CH3OC6H4
4-ClC6H4
4-NO2C6H4
nC5H11
H
H
H
H
H
H
H
CH3
C2H5
91 (80)
85 (76)
85 (70)
97 (81)
83 (73)
87 (66)
87 (67)
75 (63)
72 (50)e)
94 (67)
3
4c
5
6c
7c
8c
9d
10c
iC3H7
tC4H9
C6H5
C2H5
–(CH2)5–
a
Reaction conditions: 1 (0.3 mmol), 2c (0.4 mmol), ReBr(CO)5 (0.025 mmol) and
benzene (3 mL) at 80 °C for 1 h.
b
1H-NMR yield based on 1. The numbers in parenthesis show the isolated yields.
For 6 h.
For 15 h.
c
d
e
A
small amount of bis(tert-butyldimethylsilyl)ether (0.02 mmol) was also
obtained as inseparatable mixture.
Scheme 4. The formation of anti-isomer as a main product.

Y. Nishiyama et al. / Tetrahedron Letters 51 (2010) 793–795
795
4. The coupling reaction of carbonyl compounds with enol silyl ether or ketene
silyl acetals was generally called the Mukaiyama Aldol reaction. See: (a)
Mukaiyama, T.; Narasaka, K.; Banno, K. Chem. Lett. 1973, 1011; (b) Mukaiyama,
T.; Banno, K.; Narasaka, K. J. Am. Chem. Soc. 1974, 96, 7503; (c) Saigo, K.; Osaki,
M.; Mukaiyama, T. Chem. Lett. 1975, 989.
Acknowledgments
This research was supported by a Grant-in-Aid for Science Re-
search, and Strategic Project to Support the Formation of Research
Bases at Private Universities from the Ministry of Education, Cul-
ture Sports, Science and Technology of Japan.
5. (a) Kobayashi, S.; Matsui, S.; Mukaiyama, T. Chem. Lett. 1988, 1491; (b)
Mukaiyama, T.; Matsui, S.; Kashiwagi, K. Chem. Lett. 1989, 993; (c) Hara, R.;
Mukaiyama, T. Chem. Lett. 1989, 1909; (d) Ooi, T.; Tayama, E.; Takahashi, M.;
Maruoka, K. Tetrahedron Lett. 1997, 38, 7403; (e) Chen, J.; Sakamoto, K.; Orita, A.;
Otera, J. Tetrahedron 1998, 54, 8411; (f) Giuseppone, N.; Weghe, P. V.; Mellah,
M.; Collin, J. Tetrahedron 1998, 54, 13129; (g) Chen, J.; Sakamoto, K.; Orita, A.;
Otera, J. J. Org. Chem. 1998, 63, 9739; (h) Chen, J.; Otera, J. Tetrahedron Lett. 1998,
39, 1767; (i) Doucet, H.; Parrain, J.-L.; Santelli, M. Synlett 2000, 6, 871; (j)
Cesarotti, E.; Araneo, S.; Rimoldi, I.; Tassi, S. J. Mol. Catal. A: Chem. 2003, 204–205,
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A.; Khaksar, S.; Sheykhan, M.; Tajbakhsh, M. J. Mol. Catal. A: Chem. 2008, 287, 5;
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.11.109.
References and notes
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6. Typical procedure: A benzene (3.0 ml) solution of benzaldehyde (1a) (0.3 mmol),
1-(tert-butyldimethylsilyloxy)-1-methoxyethene (2c) (0.4 mmol) and ReBr(CO)5
(0.025 mmol) was stirred under an atmosphere of nitrogen at 80 °C for 1 h. After
the reaction was complete, H2O was added to the reaction mixture and
extracted with diisopropylether. The organic layer was dried with MgSO4. The
resulting mixture was filtered, and the filtrate was concentrated. Purification of
the residue by silica gel column chromatography (hexane/EtOAc = 7/1) afforded
the corresponding b-silyloxy ester in 74% yield. The structures of the products
were assigned by their 1H and 13C-NMR, and mass spectra.
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