Stereoselective synthesis of either (E)- or (Z)-silyl enol ether from the same acyclic α,β-unsaturated ketone using cationic rhodium complex-catalyzed 1,4-hydrosilylation

Article abstract of DOI:10.1016/j.tetlet.2013.10.085

The stereoselective synthesis of either (E)- or (Z)-silyl enol ether from the same acyclic α,β-unsaturated ketone is reported. Highly (Z)-selective conditions were the use of [Rh(cod)₂]BF ₄/DPPE at room temperature with no solvent, whereas (E)-selective conditions were the use of [Rh(cod)₂]BF₄/P(1-Nap) ₃ (1-Nap = 1-naphthyl) under refluxing dichloromethane.

Full text of DOI:10.1016/j.tetlet.2013.10.085

Tetrahedron Letters 55 (2014) 310–313
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Tetrahedron Letters
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Stereoselective synthesis of either (E)- or (Z)-silyl enol ether
from the same acyclic
a,b-unsaturated ketone using cationic
rhodium complex-catalyzed 1,4-hydrosilylation
à
⇑
Gen Onodera , Ryosuke Hachisuka, Tomomi Noguchi, Hiroki Miura , Toru Hashimoto, Ryo Takeuchi
Department of Chemistry and Biological Science, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The stereoselective synthesis of either (E)- or (Z)-silyl enol ether from the same acyclic
a
,b-unsaturated
Received 12 August 2013
Revised 14 October 2013
Accepted 18 October 2013
Available online 21 November 2013
ketone is reported. Highly (Z)-selective conditions were the use of [Rh(cod)2]BF4/DPPE at room temper-
ature with no solvent, whereas (E)-selective conditions were the use of [Rh(cod)2]BF4/P(1-Nap)3 (1-
Nap = 1-naphthyl) under refluxing dichloromethane.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
1,4-Hydrosilylation
Rhodium
Silyl enol ether
a,b-Unsaturated ketone
The enolate anion is one of the most important active species in
The 1,4-hydrosilylation of acyclic a,b-unsaturated ketones gives
carbon–carbon bond-forming reactions. The stereoselectivity of
the carbon–carbon double bond generated with enolate formation
is primarily associated with the structures of the parent carbonyl
compounds and bases used for deprotonation.1 The stereoselectiv-
ity of some enolate reactions depends on whether a E- or Z-enolate
is involved, even if the orientation and facial bias of the reaction
are controlled. The diastereoselectivity of the aldol reaction with
a boron enolate is known to be determined by the stereochemistry
of the boron enolate used.2 It would be highly desirable to be able
to selectively form either a E- or Z-metal enolate from the same
carbonyl compound at will.3
Silyl enol ether is especially important among metal enolates
because it offers numerous opportunities for regio- and stereose-
lective carbon–carbon bond formation.4 Silyl enol ether is gener-
ally obtained by the generation of an enolate anion followed by a
reaction with chlorosilane. This method is always accompanied
by the formation of a metal salt as waste. A more atom-economical
method that does not form a metal salt as a by-product is the tran-
a mixture of (E)- and (Z)-silyl enol ether.5a,b,d,f The stereoselectivity
of the resulting carbon–carbon double bond is not high enough for
the stereoselective synthesis of silyl enol ether. For example, the
[Rh(cod)OH]2-catalyzed hydrosilylation of (E)-3-nonen-2-one has
been reported to give a 35:65 mixture of (E)- and (Z)-silyl enol
ether.5d The highly stereoselective hydrosilylation of acyclic
a,b-
unsaturated ketones has not been reported. Furthermore, the
highly stereoselective formation of either (E)- or (Z)-silyl enol
ether, at will, from the same acyclic
been desired.
a,b-unsaturated ketone has
We have been studying cationic rhodium complex-catalyzed
hydrosilylation.6 We found that either (E)- or (Z)-silyl enol ether
could be obtained with high selectivity from the same acyclic
a
,b-unsaturated ketone through proper selection of the reaction
conditions (Scheme 1). In this Letter, we wish to report the highly
stereoselective hydrosilylation of acyclic ,b-unsaturated ketones.
a
We optimized Z-selective conditions for the reaction of (E)-3-
nonen-2-one (1a) with Et3SiH (Table 1). First, we examined the ef-
fect of the rhodium complex and ligand on the reaction. When PPh3
sition metal-catalyzed hydrosilylation of
a,b-unsaturated carbonyl
compounds.5
cat.[Rh (cod)2]BF4
cat. ligand
OSiEt3
⇑
Corresponding author. Tel.: +81 42 759 6231; fax: +81 42 759 6493.
E-mail address: takeuchi@chem.aoyama.ac.jp (R. Takeuchi).
O
R2
Et3SiO
R1
R2
1
+ HSiEt3
+
R
R1
R3
R3
Present address: Division of Chemistry and Material Science, Graduate School of
Engineering, Nagasaki University, 1-14 Bunkyo, Nagasaki 852-8521, Japan.
R3
R2
2
(Z)-
1
à
(E)-2
Present address: Department of Applied Chemistry, Faculty of Urban Environ-
mental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo
192-0397, Japan.
Scheme 1. Hydrosilylation of a,b-unsaturated ketones.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.085

G. Onodera et al. / Tetrahedron Letters 55 (2014) 310–313
311
Table 1
product was obtained as a by-product (entry 4). A phenethyl group
at the (E)-b-position slightly decreased the Z-selectivity. The reac-
tion of 1e gave product 2e in 77% yield with 88% Z-selectivity (en-
try 5). The effect of steric hindrance by isopropyl, cyclohexyl, and
1-ethylpentyl groups at the (E)-b-position was examined. These
ketones 1f–h gave good results with regard to both yield and Z-
selectivity (entries 6–8). b,b-Disubstituted ketone 1i was smoothly
converted to silyl enol ether 2i in 84% yield with 98% Z-selectivity
(entry 9). (E)-1,3-Diphenyl-2-propen-1-one (1j) was converted to
2j in 84% yield with 88% Z-selectivity (entry 10).
Optimization of Z-selective conditionsa
Entry Rh complex
Ligand Solvent Time (h) Yieldb (%) E/Zc of 2a
1d
2e,f
3e
4g
5h
6
7
8
9
10
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)2]BF4 PPh3
[Rh(cod)2]BF4 DPPE
[Rh(cod)2]BF4 DPPM
[Rh(cod)2]BF4 DPPP
[Rh(cod)2]BF4 none
[Rh(cod)2]BF4 DPPE
[Rh(cod)2]BF4 DPPE
PPh3
DPPE
None
Neat
Neat
Neat
Neat
Neat
Neat
Neat
Neat
DCEi
THFj
24
2
24
24
1
24
48
3
24
24
55
85
Trace
41
95
45
4
86
64
81
58/42
34/66
—
56/44
8/92
12/88
50/50
31/69
11/89
16/84
Encouraged by the fact that we were able to successfully opti-
mize Z-selective conditions, we next tried to optimize E-selective
conditions. The results are summarized in Table 3. We focused
on the use of a monodentate ligand to optimize E-selective condi-
tions, since bidentate ligands favored the formation of Z-silyl enol
ether, as described in Table 1. When PPh3 was used, the reaction at
room temperature gave 2a in 41% yield (Table 1, entry 4). We per-
formed the reaction at 85 °C to improve the yield. The effect of
monodentate phosphine ligand on the reaction was examined. Al-
most the same level of E-selectivity was observed when the ligand
was PPh3 or PPh2Cy (entries 1 and 2). An electron-withdrawing li-
gand such as PPh2C6F5 decreased the E-selectivity to 53% (entry 3).
A more-hindered phosphine ligand improved the yield and E-selec-
tivity. Tris(1-naphthyl)phosphine ligand gave 2a in 86% yield with
85% E-selectivity (entry 4). To improve the E-selectivity, we exam-
ined other reaction variables using tris(1-naphthyl)phosphine. We
next examined the effect of the solvent on E-selectivity. In contrast
to the reaction with PPh3, the reaction with tris(1-naphthyl)phos-
phine gave the product in high yield even at room temperature
(entry 5). The reaction with THF at room temperature gave 2a in
80% yield with 87% E-selectivity (entry 6). DCM and 1,2-dichloro-
ethane gave better results than THF (entries 7 and 9). When the
reaction was performed under refluxing DCM, the yield increased
from 85% to 90% with 93% E-selectivity (entry 8). Thus, the optimal
E-selective conditions were determined to be under refluxing DCM
with [Rh(cod)2]BF4/tris(1-naphthyl)phosphine (P/Rh = 2).
a
A mixture of 1a (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), and
ligand (0.5 mol %) was stirred at rt under Ar.
b
Isolated yield.
c
Determined by 1H NMR.
d
0.25 mol % of [Rh(cod)Cl]2 and 1 mol % of PPh3 were used.
0.25 mol % of [Rh(cod)Cl]2 was used.
At 70 °C.
1 mol % of PPh3 was used.
e
f
g
h
A mixture of 1a (5 mmol), HSiEt3 (5.5 mmol), [Rh(cod)2]BF4 (0.5 mol %), and
DPPE (0.5 mol %) was stirred at rt under Ar.
i
DCE = 1,2-dichloroethane (5 mL).
THF (5 mL).
j
(P/Rh = 2) was used as a ligand, [Rh(cod)Cl]2 and [Rh(cod)2]BF4
gave almost equal amounts of (E)- and (Z)-silyl enol ether 2a (en-
tries 1 and 4). The reaction with DPPE was more Z-selective than
that with PPh3 (entries 2 and 5). [Rh(cod)2]BF4/DPPE gave the best
results with regard to both yield and Z-selectivity (entry 5). The
yield and stereoselectivity were considerably decreased with
DPPM and DPPP (entries 6 and 7). [Rh(cod)2]BF4/DPPE was more
selective than [Rh(cod)2]BF4 alone (entry 8). The above results
were obtained in the absence of solvent. The use of solvent resulted
in a decrease in both yield and Z-stereoselectivity (entries 9 and
10). Thus, the optimal Z-selective conditions were no solvent at
room temperature with [Rh(cod)2]BF4/DPPE.
The 1,4-hydrosilylation of various a,b-unsaturated ketones was
The 1,4-hydrosilylation of various a,b-unsaturated ketones was
carried out under the optimized E-selective conditions described
above (Table 4). The effect of the substituent on ketone was exam-
ined, and the results showed that the E-selectivity was sensitive to
the substituent on ketone. Methyl ketone 1a was converted to silyl
enol ether 2a in 90% yield with 93% E-selectivity (entry 1). On the
other hand, isopropyl ketone 1b and phenyl ketone 1c were less E-
selective substrates than methyl ketone 1a. Isopropyl ketone 1b
was converted to 2b in 87% yield with 44% E-selectivity (entry 2).
Phenyl ketone 1c was converted to 2c in 96% yield with 77% E-
carried out under the optimized Z-selective conditions mentioned
above. A wide range of ketones could be used in the reaction, and
the results are summarized in Table 2. Methyl ketone (1a) was con-
verted to 2a in 95% yield with 92% Z-selectivity (entry 1). The reac-
tion of isopropyl ketone 1b and phenyl ketone 1c gave 2b and 2c in
respective yields of 77% and 80% (entries 2 and 3). The Z-selectiv-
ities were 90% and 94%. The effect of the b-substituent of methyl
ketone was examined. The reaction of 4-phenyl-3-buten-2-one
(1d) gave 2d in 68% yield with 73% Z-selectivity. 1,2-Addition
Table 2
Z-Selective hydrosilylation of 1a–ja
Entry
1
R1
R2
R3
Time (h)
2
Yieldb (%)
E/Zc of 2
1d
2e
3e
4f,g
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
Me
i-Pr
Ph
Me
Me
Me
Me
Me
Me
Ph
H
H
H
H
H
H
H
H
Me
H
n-Pentyl
n-Pentyl
n-Pentyl
Ph
2-Phenylethyl
i-Pr
1
2a
2b
2c
2d
2e
2f
2g
2h
2i
95
77
80
68
77
79
80
82
84
84
8/92
10/90
6/94
27/73
12/88
3/97
3/97
2/98
2/98
12/88
24
24
20
4
1
5
5
6
4
Cy
1-Ethylpentyl
Me
Ph
9
10f
1j
2j
a
b
c
d
e
f
A mixture of 1 (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), DPPE (0.5 mol %), and solvent (5 mL) was stirred at rt under Ar.
Isolated yield.
Determined by 1H NMR.
A mixture of 1a (5 mmol), HSiEt3 (5.5 mmol), [Rh(cod)2]BF4 (0.5 mol %), and DPPE (0.5 mol %) was stirred at rt under Ar.
At 30 °C.
At 50 °C.
g
1,2-Addition product 2d0 was obtained in 17% yield.

312
G. Onodera et al. / Tetrahedron Letters 55 (2014) 310–313
Table 3
Optimization of E-selective conditionsa
Entry
Ligand
Solvent
Temperature
Time (h)
Yieldb (%)
E/Zc of 2a
1
2
3
4
5
6
7
8
9
PPh3
PPh2Cy
PPh2(C6F5)
P(1-Nap)3
P(1-Nap)3
P(1-Nap)3
P(1-Nap)3
P(1-Nap)3
P(1-Nap)3
Neat
Neat
Neat
Neat
Neat
THF
CH2Cl2
CH2Cl2
DCEe
85 °C
85 °C
85 °C
85 °C
rt
rt
rt
Reflux
rt
2
1
2
2
2
3
3
1
1
63
76
70
86
86
80
85
90
89
75/25
77/23
53/47
85/15
84/16
87/13
93/7
d
d
d
d
d
d
93/7
91/9
a
b
c
A mixture of 1a (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), ligand (1 mol %), and solvent (5 mL) was stirred under Ar.
Isolated yield.
Determined by 1H NMR.
1-Nap = 1-naphthyl.
DCE = 1,2-dichloroethane.
d
e
Table 4
E-Selective hydrosilylation of 1a–ja
Entry
1
R1
R2
R3
Time (h)
2
Yieldb (%)
E/Zc of 2
1
2
1a
1b
1c
1d
1e
1f
1g
1h
1i
Me
i-Pr
Ph
Me
Me
Me
Me
Me
Me
Ph
H
H
H
H
H
H
H
H
Me
H
n-Pentyl
n-Pentyl
n-Pentyl
Ph
2-Phenylethyl
i-Pr
1
2a
2b
2c
2d
2e
2f
2g
2h
2i
90
87
96
77
87
68
79
74
63
90
93/7
24
24
24
1
2
5
24
24
24
44/56
77/23
86/14
94/6
93/7
93/7
91/9
92/8
72/28
3
4d
5
6
7
Cy
8e
9f
10
1-Ethylpentyl
Me
Ph
1j
2j
a
b
c
d
e
f
A mixture of 1 (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), P(1-Nap)3 (1 mol %, 1-Nap = 1-naphthyl), and CH2Cl2 (5 mL) was stirred at reflux under Ar.
Isolated yield.
Determined by 1H NMR.
1,2-Addition product 2d0 was obtained in 9% yield.
1 mol % of [Rh(cod)2]BF4 and 2 mol % of P(1-Nap)3 (1-Nap = 1-naphthyl) were used.
A mixture of 1i (5 mmol), HSiEt3 (5.5 mmol), [Rh(cod)2]BF4 (0.5 mol %), P(1-Nap)3 (1 mol %, 1-Nap = 1-naphthyl), and CH2Cl2 (8 mL) was stirred at reflux under Ar.
selectivity (entry 3). We next examined the effect of the (E)-b-sub-
stituent on E-selectivity. (E)-4-Phenyl-3-buten-2-one (1d) was
converted to 2d in 77% yield with 86% E-selectivity (entry 4). (E)-
4-Phenyl-3-buten-2-one (1d) was a less (E)-selective substrate
than (E)-3-nonen-2-one (1a). Methyl ketones with (E)-b-primary
and secondary alkyl substituents gave good results with regard
to both yield and E-selectivity. Ketone 1e was converted to silyl
enol ether 2e in 87% yield with 94% E-selectivity (entry 5). The
reaction of 1f-h gave the products in 68–79% yields with 91–93%
E-selectivity (entries 6–8). b,b-Disubstituted ketone 1i was con-
verted to 2i in 63% yield with 92% E-selectivity (entry 9). (E)-1,3-
Diphenyl-2-propen-1-one (1j) was converted to 2j in 90% yield
with 72% E-selectivity (entry 10).
O
H
H
H
R'
R'
R
SiEt3
+
+
+
LnRh
LnRh
1
RhLn SiEt3
R'
O
R'
R'
OSiEt3
3
3
silylrhodation
1 R
4
R
HSiEt3
LnRh+
OSiEt3
R
2
(Z)-
H
H
+
+
LnRh
LnRh
OSiEt3
R'
OSiEt3
3
3
reductive elimination
1
1
π-allyl Rh 5
6
σ-allyl Rh
R
R
OSiEt3
H
The reversal of stereoselectivity observed here can be explained
based on mechanistic considerations (Scheme 2). The catalytic cy-
+
R'
R
LnRh
2
(E)-
R'
OSiEt3
3
cle of the reaction involves a
p-allyl rhodium complex as a key
1
reductive elimination
intermediate. The 1,4-hydrosilylation of enones begins with the
oxidative addition of hydrosilane to a rhodium center to give
Rh(III) species 3. Enone coordinates to 3 in an s-cis form.7 Silylrho-
dation of the carbon-oxygen double bond gives intermediate 4.5h,i,8
Coordination of the carbon–carbon double bond to the rhodium
σ-allyl Rh 7
syn-anti
R
isomerization
OSiEt3
OSiEt3
H
1
1
+
3
LnRh
3
OSiEt3
R'
R
R'
+ R
+
1
LnRh
LnRh
center gives
merizes to a
p
r
-allyl intermediate 5. This
-allyl intermediate when reductive elimination oc-
p-allyl intermediate iso-
R'
R
3
H
H
π-allyl Rh 8
9
σ-allyl Rh
curs. To avoid steric repulsion between the triethylsiloxy moiety
and the rhodium moiety, reductive elimination from -allyl inter-
mediate 6 occurs at the C-3 position to give (Z)-silyl enol ether 2.
Syn–anti isomerization of
-allyl intermediate9 5 plays a decisive
role in providing (E)-silyl enol ether 2. Syn–anti isomerization of
-allyl intermediate 5 to 8 followed by reductive elimination gives
r
Scheme 2. Plausible reaction pathway.
p
(E)-silyl enol ether 2. The relative rates of syn–anti isomerization of
-allyl intermediate 5 via a process and reductive elimina-
p
p–r–p
p

G. Onodera et al. / Tetrahedron Letters 55 (2014) 310–313
313
tion from 5 determine the stereoselectivity. This proposed mecha-
A. Supplementary data
nism is similar to that for the hydrosilylation of 1,3-dienes to give
allylsilanes.10
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013
10.085.
The proposed mechanism can explain the results described
above. Z-Selective conditions require the use of DPPE and no sol-
vent. (Z)-Silyl enol ether 2 is obtained when reductive elimination
from 5 is faster than the syn–anti isomerization of 5 to 8. It is well
known that organic groups that are to be eliminated from a metal
center have to take a cis-geometry.11 A strongly cis-chelating
bidentate diphosphine such as DPPE would be more suitable than
a monodentate ligand for a cis-geometry leading to rapid reductive
elimination. Another important factor for high Z-selectivity is the
absence of solvent. It has been suggested that reductive elimina-
tion to give a hydrosilylation product is a bimolecular process with
an incoming hydrosilane.12 Thus, reductive elimination requires
the participation of another molecule of hydrosilane. Therefore, a
high concentration of hydrosilane accelerates reductive elimina-
tion from 5 to increase the Z-selectivity. In contrast, dilution by a
solvent slows reductive elimination from 5 to decrease the Z-
selectivity.
References and notes
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E-Selective conditions require the use of P(1-Nap)3 and a sol-
vent. (E)-Silyl enol ether 2 is obtained when the syn–anti isomeri-
zation of 5 to 8 is faster than reductive elimination from 5. The
steric bulkiness of P(1-Nap)3 promotes the syn–anti isomerization
of
p-allyl intermediate 5. Silylrhodation of the carbon–oxygen dou-
ble bond of enone coordinated in an s-cis form gives
p
-allyl rho-
dium intermediate 5, in which a triethylsiloxy substituent is
located at an anti position with respect to the central hydrogen.
Steric repulsion between an anti triethylsiloxy substituent and
the P(1-Nap)3-ligated rhodium moiety promotes the syn-anti isom-
erization of 5 to 8 via a p–r–p process to the more stable p-allyl
rhodium intermediate 8 in which the triethylsiloxy substituent is
located at a syn position.13 In addition, dilution by a solvent slows
reductive elimination from 5 to increase the E-selectivity.
The ketone substituent affected the stereoselectivity only under
E-selective conditions (Table 4, entries 1–3). In contrast, the ketone
substituent had no effect under Z-selective conditions (Table 2, en-
tries 1–3). This difference can be explained by the proposed mech-
anism. When the ketone substituent is bulky, such as an isopropyl
or phenyl group, the difference in the steric bulkiness of the ketone
substituent and the triethylsiloxy substituent is small. With
intermediate 8, the location of the triethylsiloxy substituent at the
syn position does not always lead to a more stable -allyl interme-
diate. Syn–anti isomerization in such cases is not promoted as
much as it is with a methyl ketone. Consequently, E-selectivity de-
creases. Under Z-selective conditions, the reductive elimination of
5 is faster than syn–anti isomerization from 5 to 8. Consequently,
the ketone substituent has no effect on Z-selectivity.
6. (a) Takeuchi, R.; Ebata, I. Organometallics 1997, 16, 3707; (b) Takeuchi, R.;
Yasue, H. Organometallics 1996, 15, 2098; (c) Takeuchi, R.; Nitta, S.; Watanabe,
D. J. Org. Chem. 1995, 60, 3045; (d) Takeuchi, R.; Nitta, S.; Watanabe, D. J. Chem.
Soc., Chem. Commun. 1994, 1777; (e) Takeuchi, R.; Tanouchi, N. J. Chem. Soc.,
Perkin Trans. 1 1994, 2909; (f) Takeuchi, R.; Tanouchi, N. J. Chem. Soc., Chem.
Commun. 1993, 1319.
7.
g
4-Coordination of enones to transition metal complexes, see: (a) Moulton, B.
E.; Duhme-Klair, A. K.; Fairlamb, I. J. S.; Lynam, J. M.; Whitewood, A. C.
Organometallics 2007, 26, 6354; (b) Kanaya, S.; Imai, Y.; Komine, N.; Hirano, M.;
Komiya, S. Organometallics 2005, 24, 1059; (c) Marcuzzi, A.; Linden, A.; von
Philipsborn, W. Helv. Chim. Acta 1993, 76, 976;
g
4-Coordination of 1,3-diene to
Rh, see: (d) Nelson, S. M.; Sloan, M.; Drew, M. G. B. J. Chem. Soc., Dalton Trans.
1973, 2195.
p-allyl
8. (a) Riener, K.; Högerl, M. P.; Gigler, P.; Kühn, F. E. ACS Catal. 2012, 2, 613; (b)
Reyes, C.; Prock, A.; Gierring, W. P. Organometallics 2002, 21, 546.
9. (a) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to Catalysis;
University Science Books: Sausalito, 2010. p 104; (b) Trost, B. M.; Van Vranken,
D. L. Chem. Rev. 1996, 96, 395; (c) Consiglio, G.; Waymouth, R. M. Chem. Rev.
1989, 89, 257.
p
10. Silylmetallation of 1,3-dienes gives anti-p-allyl intermediate, see: Wu, J. Y.;
Stanzl, B. N.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 13214.
11. (a) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1981, 54,
1868; (b) Ozawa, F.; Ito, T.; Yamamoto, A. J. Am. Chem. Soc. 1980, 102, 6457; (c)
Gillie, A.; Stille, J. K. J. Am. Chem. Soc. 1980, 102, 4933.
In summary, we could obtain either (E)- or (Z)-silyl enol ether
with high selectivity from the same acyclic a,b-unsaturated ketone
by choosing the conditions for the 1,4-hydrosilylation reaction.
Further examinations of the scope of the reaction and mechanistic
studies are underway.
12. In the case of rhodium-catalyzed hydrosilylation of, 1-alkynes with
triethylsilane, reductive elimination to give
a vinylsilane product is
suggested to be a bimolecular process with an incoming triethylsilane, see:
Ojima, I.; Clos, N.; Donovan, R. J.; Ingalina, P. Organometallics 1990, 9, 3127.
13. Syn isomer is more stable than anti isomer in
Fryzuk, M. D. Inorg. Chem. 1982, 21, 2134.
g
3-(1-Me-C3H4)rhodium, see:
Acknowledgments
We thank Mr. Naoki Tomita for the assistance of experiments.
We thank Prof. Nishibayashi and Dr. Nakajima for measuring
NMR and HRMS spectra.