Remarkable access to fluoroalkylated trisubstituted alkenes via highly stereoselective cobalt-catalyzed hydrosilylation reaction of fluoroalkylated alkynes

Article abstract of DOI:10.1039/b819476a

Hydrosilylation reaction of various fluoroalkylated alkynes with Et ₃SiH in the presence of a catalytic amount of Co₂(CO) ₈ was investigated. The hydrosilylation of the alkynes having fluoroalkyl and aryl groups took place

Full text of DOI:10.1039/b819476a

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Remarkable access to fluoroalkylated trisubstituted alkenes via highly
stereoselective cobalt-catalyzed hydrosilylation reaction of fluoroalkylated
alkynes†
Tsutomu Konno,* Ken-ichi Taku, Shigeyuki Yamada, Kazuki Moriyasu and Takashi Ishihara
Received 4th November 2008, Accepted 15th December 2008
First published as an Advance Article on the web 29th January 2009
DOI: 10.1039/b819476a
Hydrosilylation reaction of various fluoroalkylated alkynes with Et3SiH in the presence of a catalytic
amount of Co2(CO)8 was investigated. The hydrosilylation of the alkynes having fluoroalkyl and aryl
groups took place smoothly with good regioselectivity (ca. 80:20). In sharp contrast, the reaction of the
alkynes having a fluoroalkyl group and a benzyl-type substituent, or various propargyl alcohols gave
the corresponding vinylsilanes in an excellent regio- and stereoselective manner. Treatment of the
vinylsilanes with various aldehydes in the presence of Zn(OTf)2 and TBAF afforded the coupling
products in good yields.
Table 1 Investigation of the reaction conditions
Introduction
The transition-metal-catalyzed hydrosilylation reaction of alkynes
represents one of the most important and straightforward methods
for the preparation of vinylsilanes, which can be easily converted
into variously substituted ethenes with retention of configuration
through the Hiyama coupling reaction (Scheme 1).1,2 Further-
more, the high chemoselectivity as well as mild reactivity of
organosilicon compounds, as compared with other organometallic
reagents, such as lithium, magnesium reagents, and so on, make the
hydrosilylation and the subsequent reactions extremely valuable
and applicable for preparing a wide variety of substances.
Entry
Si-H
Yielda/% of 2+3
Ratioa (2:3)
1
2
3
4
5
6
7
Et3SiH (a)
Et3SiHb
quant. (95)
83:17
80:20
75:25
—
42:58
—
70
12
0
84
0
Et3SiHc
(i-Pr)3SiH
Me2PhSiH
Ph3SiH
(EtO)3SiH
81
29:71
a Determined by 19F NMR. Value in parentheses is of isolated yield. b In the
presence of 3 mol% of Co2(CO)8. c In the presence of 1 mol% of Co2(CO)8.
Scheme 1
the desired vinylsilanes 2 and 3 quantitatively in a ratio of 83:17
(Entry 1). As shown in Entries 2 and 3, the use of less than 5 mol%
of catalyst led to a decrease of the yield and regioselectivity. We
also investigated the reaction using various silane compounds,
such as triisopropylsilane, dimethylphenylsilane, triphenylsilane,
and triethoxysilane, as described in Entries 4–7. As a result,
the reaction with triisopropylsilane and triphenylsilane did not
proceed at all, and the starting alkyne was recovered quantitatively.
In sharp contrast, the use of dimethylphenylsilane and triethoxysi-
lane caused a smooth reaction, giving the corresponding adducts
in high yields, though the regioselectivity decreased significantly,
as compared with that of the reaction with Et3SiH.
Although the hydrosilylation of various alkynes has been
developed and studied extensively in the nonfluorinated chemistry,
the hydrosilylation of fluorine-containing alkynes has attracted
little attention so far.3 Herein we wish to describe the highly regio-
and stereoselective hydrosilylation reaction of fluoroalkylated
internal acetylene derivatives in the presence of a catalytic amount
of Co2(CO)8.4
Results and discussion
Initially, the addition of Et3SiH to the trifluoromethylated internal
alkyne 1a5 was examined in the presence of 5 mol% of Co2(CO)8,
as described in Table 1. Thus, treatment of 1a with 1.2 equiv. of
Et3SiH in 1,2-dichloroethane at the reflux temperature for 3 h gave
Next, our attention was directed toward the hydrosilylation of
various alkynes. The results are summarized in Table 2.
As shown in Entries 1–4, changing the aromatic substituent
of the alkynes from p-ClC6H4 to p-MeOC6H4, p-MeC6H4,
p-EtO2CC6H4 did not significantly affect the yield and the re-
gioselectivity, though a strongly electron-withdrawing group, such
as a nitro group, disturbed the reaction, giving the corresponding
product in only 16% yield (Entry 5). Additionally, a large influence
of the position of the substituent on the benzene ring was not
Department of Chemistry and Materials Technology, Kyoto Institute of
Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan. E-mail:
konno@chem.kit.ac.jp; Fax: +81-75-724-7580; Tel: +81-75-724-7573
† Electronic supplementary information (ESI) available: General ex-
perimental procedure for the hydrosilylation and the reaction of the
hydrosilylation products with various aldehydes, and characterization data
for all new compounds. See DOI: 10.1039/b819476a
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Table 2 The hydrosilylation of various fluorinated alkynes
Table 3 The hydrosilylation of various fluoroalkylated propargyl
alcohols
Entry Rf
R
Yielda/% of 2+3 Ratioa (2:3)
Entry Rf
R1 R2
Yielda/% of 5+6 Ratioa (5:6)
1
2
3
4
5
6
7
8
9
CF3
p-ClC6H4 (a)
p-MeOC6H4 (b)
p-MeC6H4 (c)
p-EtO2CC6H4 (d)
p-O2NC6H4
m-ClC6H4 (e)
o-ClC6H4 (f)
Ph(CH2)3 (g)
p-ClC6H4CH2 (h)
p-MeOC6H4CH2 (i)
(j)
quant. (95)
(64)
62 (45)
80 (61)
16
77 (67)
94 (83)
(75)
89 (83)
89 (84)
(86)
83:17
84:16
77:23
81:19
N.D.b
82:18
71:29
88:12
98:2
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
1
2
3
4
5
6
7
8
9
CF3
CF3
CF3
CF3
CF3
CF3
CF3
HCF2
H(CF2)3
H
H
H
H
H
H
Ph (a)
82 (79)
99:1
99:1
p-MeOC6H4 (b) quant. (97)
Ph(CH2)2 (c)
Et2CH (d)
Cy (e)
81 (80)
82 (79)
quant. (97)
81 (80)
0
76:24
100:0
99:1
100:0
—
t-Bu (f)
Me Ph
H
H
p-MeOC6H4 (g) 84 (78)
p-MeOC6H4 (h) (62)
96:4
10
11
96:4
100:0
90:10
a Determined by 19F NMR. Values in parentheses are of isolated yields.
12
CF3
(k)
(74)
97:3
Stereochemistry
13
14
15
16
17
CF3
CF3
HCF2
HCF2
EtO2C
(EtO)2P(O)
p-ClC6H4 (l)
37
0
43:57
—
97:3
95:5
80:20
The stereochemistry of the hydrosilylation products was deter-
mined as follows (Scheme 2). Thus, 19F NMR spectra of the major
isomer 2a in the reaction of the alkyne 1a with Et3SiH, showed
a singlet peak, suggesting strongly that a triethylsilyl group is
attached to a carbon bearing a trifluoromethyl group. On the
contrary, 19F NMR spectra of the minor isomer 3a showed a
doublet peak, which means that a Et3Si group is attached to a
benzylic carbon.
quant. (87)
p-MeOC6H4CH2 (m) (66)
H(CF2)3 p-MeOC6H4CH2 (n) (71)
a Determined by 19F NMR. Values in parentheses are of isolated yield.
b Not determined.
observed (Entries 1, 6, 7), though the use of o-substituted phenyl
group as R caused a slight decrease of the regioselectivity. The use
of the alkyl group as R did not substantially change the yield
and the regioselectivity (Entry 8). It is noted that the benzyl-
type substituent brought about the high regioselectivity. Thus, the
reaction with the alkynes having p-ClC6H4CH2, p-MeOC6H4CH2
group, and so on, proceeded smoothly to give the desired adducts
in a highly regioselective manner (2:3 = >96:<4) (Entries 9~12).
Unfortunately, the alkynes having a CO2Et or P(O)(OEt)2 group
did not react with Et3SiH smoothly (Entries 13 and 14).
We also examined the reaction of the alkynes having various
fluoroalkyl groups. In the case of the hydrosilylation of difluo-
romethylated alkynes, the high regioselectivity as well as the high
yield were obtained even when the p-ClC6H4 group was used as
R, in addition to the benzyl-type substituent (Entries 15 and 16).
Switching a fluoroalkyl group from a CF3 or CHF2 group to a
H(CF2)3 group also led to the smooth reaction, however, a slight
decrease of the regioselectivity was observed (Entry 17).
We next investigated the hydrosilylation using various types of g-
fluoroalkylated propargyl alcohols 4.6 The results are summarized
in Table 3. As shown in Entries 1, 2, 4–6, propargyl alcohols having
various substituents R2, such as phenyl, 1-ethylpropyl, cyclohexyl,
t-Bu and so on, gave the corresponding adducts 5 and 6 in high
yields with high regio- and stereoselectivity. In the case of R1 and
R2 π H, the reaction did not take place and the starting alkyne was
recovered quantitatively (Entry 7). Changing the fluoroalkyl group
from a CF3 to a HCF2 group did not bring about any significant
influence on the yield or the regioselectivity (Entry 8). However,
the use of a hexafluoropropyl group as Rf led to a slight decrease
of the regioselectivity (Entry 9).
Scheme 2
The mixture of 2a and 3a was subjected to tetrabutylammonium
fluoride (TBAF) in THF/H2O to afford the disubstituted alkene
7 as a sole product. The analysis of the H NMR of 7 showed
the coupling constant of Ha and Hb to be 12.8 Hz, indicating
that 2a and 3a have the E configuration.7 The stereochemistry of
other hydrosilylation products was determined on the basis of the
comparison with 19F NMR spectra of 2a and 3a.
Accordingly, it was revealed that the present hydrosilylation
proceeded in a highly cis-selective manner.
1
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Mechanism
A plausible reaction mechanism of the reaction is outlined in
Schemes 3 and 4.8 Thus, Et3SiH may react with Co2(CO)8 even
at ambient temperature to give HCo(CO)4 and Et3SiCo(CO)4.
HCo(CO)4 may react with Et3SiH again, affording Et3SiCo(CO)4
together with the generation of hydrogen. Elimination of CO from
Et3SiCo(CO)4 affords Et3SiCo(CO)3, which may coordinate with
fluoroalkylated alkynes, giving the corresponding cobalt-acetylene
complex Int-1. The subsequent silylmetalation reaction followed
by the oxidative addition of Et3SiH into Co gives Int-2. Finally, the
reductive elimination of Et3Co(CO)3 gives the desired vinylsilane.
Scheme 3
Scheme 5
Summary
In conclusion, we have demonstrated the hydrosilylation reaction
of various fluoroalkylated alkynes with Et3SiH in the presence
of a catalytic amount of Co2(CO)8 at the reflux temperature of
dichloroethane for 3 h. The reaction proceeded smoothly to give
the corresponding adducts in good to high yields. Especially,
the reaction with the alkynes having benzyl-type substituents
or various propargyl alcohols took place in a highly regio- and
stereoselective manner. Additionally, it was revealed that the
MOM ethers derived from the hydrosilylation products reacted
with various aldehydes in the presence of TBAF/Zn(OTf)2,
affording the various allylic alcohols in good yields.
Scheme 4
Experimental
Synthetic application
General experimental procedure for the hydrosilylation and the
reaction of the hydrosilylation products with various aldehydes,
and characterization data for all new compounds are available via
the supplementary information.†
Finally, we attempted the coupling reaction of the hydrosilylation
product with various aldehydes (Scheme 5).9 Thus, treatment
of the propargyl alcohol 4b with 1.2 equiv. of Me2PhSiH in
the presence of 5 mol% of Co2(CO)8 at the reflux temperature
for 3 h gave the corresponding hydrosilylation product 5i in
86% yield, along with 7% of regioisomer. Subsequently, the
protection of the hydroxy group of 5i afforded the MOM
ether 8i in 85% yield. After several attempts at the coupling
reaction of 8i with benzaldehyde, we found that the treatment
of 8i with 1.5 equiv. each of benzaldehyde and zinc triflate
in the presence of 20 mol% of TBAF in NMP at 80 ◦C for
20 h gave the corresponding allylic alcohol 9a in 59% yield
as a 1:1 diastereomeric mixture. Similarly, the reaction with
various aldehydes, such as p-tolualdehyde, p-chlorobenzaldehyde,
p-trifluoromethylbenzaldehyde, n-butyraldeyde, took place
smoothly to provide the corresponding adducts in 44 ~ 72% yield
as a 1:1 diastereomeric mixture.
References
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3 To our best of knowledge, the hydrosilylation of fluoroalkylated alkyens
has not been reported thus far. For other hydrometalation reaction,
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7 It has been reported that the coupling constant of the E and Z isomers
is 16.5 and 12.5 Hz, respectively. See ref. 3c.
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1170 | Org. Biomol. Chem., 2009, 7, 1167–1170
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