Rhodium-catalyzed acylation of vinylsilanes with acid anhydrides: Application to the transformation of α-acyloxy vinylsilanes to unsymmetrical 1,2-diketones

Article abstract of DOI:10.1246/cl.2004.424

Acylation of vinylsilanes with acid anhydrides is catalyzed by [RhCl(CO)₂]₂ to give α,β-unsaturated ketones. The application of this catalytic system to the acylation of α-acyloxy vinylsilanes affords α-acyloxy enones in good yields, which are converted to unsymmetrical 1,2-diketones or their mono acetals.

Full text of DOI:10.1246/cl.2004.424

424
Chemistry Letters Vol.33, No.4 (2004)
Rhodium-Catalyzed Acylation of Vinylsilanes with Acid Anhydrides:
Application to the Transformation of ꢀ-Acyloxy Vinylsilanes to Unsymmetrical 1,2-Diketones
Motoki Yamane, Kazuyoshi Uera, and Koichi NarasakaÃ
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received February 13, 2004; CL-040168)
Acylation of vinylsilanes with acid anhydrides is catalyzed
RhIX(CO)n
R"
R
R"
by [RhCl(CO)2]2 to give ꢀ,ꢁ-unsaturated ketones. The applica-
tion of this catalytic system to the acylation of ꢀ-acyloxy vinyl-
silanes affords ꢀ-acyloxy enones in good yields, which are con-
verted to unsymmetrical 1,2-diketones or their mono acetals.
SiMe2R'
R"
O
1
X
SiMe2R'
III
R"
RhX(CO)n
RhI(CO)n
O
R
A
Acylation of vinylsilanes has provided a regioselective prep-
aration of ꢀ,ꢁ-unsaturated ketones, and is generally performed
by the treatment with acid halides or anhydrides in the presence
of Lewis acid such as AlCl3.1 However, the use of more than a
stoichiometric amount of Lewis acid is required in this transfor-
mation and the development of the catalytic processes is strongly
desired in the synthetic point of view.
X = Cl or OCOR
(RCO)2O
Scheme 1. Rh(I)-catalyzed acylation of vinylsilane 1.
Table 1. Rh(I)-catalyzed acylation of ꢁ-alkyl vinylsilane 1.a
5 mol%
[RhCl(CO)2]2
We have found that the rhodium(I) carbonyl complex,
[RhCl(CO)2]2, is available for the transmetalation of acylsilanes
to generate acylrhodium intermediates in the intramolecular cyc-
lization reaction of ꢂ-alkynoylsilanes.2 Because vinylsilane also
bears an sp2 carbon–silicon bond like acylsilane, it was expected
that transmetalation between the Rh(I) complex and vinylsilane
would occur to afford vinylrhodium intermediate. Accordingly,
ꢁ-alkyl vinylsilane 1a was allowed to react with an equimolar
amount of [RhCl(CO)2]2 in toluene-d8 at 80 ꢀC and the reaction
Ph(CH2)2
R
Ph(CH2)2
2
(RCO)2O
SiMe2R'
O
+
1,4-Dioxane
1a R' = Ph
1b R' = Me
Ph(CH2)2
R
90 °C
3
(RCO)2O
R
Entry
1
Time / h
Products / %b
1
2
3
4
5
6
1a
1a
1a
1a
1b
1b
Me
Et
7
2a 84
3a
3b
3c
3d
3a
3d
–
1
18
18
18
18
24
2b 67
2c 72
2d 74
2a 82
2d 76
–
–
5
–
6
was monitored by H NMR. The formation of chlorodimethyl-
i-Pr
Ph
Me
Ph
phenylsilylane was observed as the result of metal exchange
(Eq 1). For the transmetalation between transition metal com-
plexes and vinylsilanes, the use of fluoride ion or the introduc-
tion of alkoxy or hydroxy group on the silyl group is essential
to activate vinylsilanes, as being reported in the transition met-
al-catalyzed arylation of vinylsilanes with aryl halides to prepare
vinyl arenes.3–5 It is noteworthy that the transmetalation pro-
ceeded without any activating reagents in the present metal ex-
change between [RhCl(CO)2]2 and vinylsilane 1a.
aVinylsilane 1 : [RhCl(CO)2]2 : (RCO)2O = 1.0 : 0.05 : 3.0.
bIsolated yield based on vinylsilane 1.
was obtained in 84% yield (Entry 1). Propionic anhydride and
isobutyric anhydride also reacted smoothly with 1a (Entries 2
and 3). In the reaction with benzoic anhydride, the desired phen-
yl vinyl ketone 2d was obtained in 74% yield with 5% yield of
phenylated product 3d (Entry 4). Even trimethylvinylsilane 1b
reacted with acid anhydrides under the same reaction conditions
to give the corresponding enones 2 in good yields (Entries 5 and
6).
1/2[RhCl(CO) ]
2 2
Ph(CH )
2 2
SiMe Ph
2
toluene-d , 80 °C
8
1a
(1)
R"
I
+
ClSiMe Ph
2
Rh (CO)
n
1
HNMR
δ 0.43 (s, CH )
3
R" = (CH ) Ph
2 2
The catalytic acylation of vinylsilane with acid anhydride
was supposed to proceed by applying this transmetalation. That
is, the generation of vinylrhodium intermediate A would be fol-
lowed by sequential oxidative addition of acid anhydride and re-
ductive elimination of ꢀ,ꢁ-unsaturated ketone along with the re-
generation of Rh(I) catalyst (Scheme 1).
In fact, ꢁ-alkyl vinylsilanes 1a and 1b reacted with various
acid anhydrides in the presence of a catalytic amount of
[RhCl(CO)2]2 to give enones, as being summarized in Table 1.6
The acylation of dimethylphenylvinylsilane 1a with acetic
anhydride proceeded at 90 ꢀC, and ꢀ,ꢁ-unsaturated ketone 2a
It was not confirmed whether this acylation reaction initiated
by the transmetalation or by the oxidative addition of acid anhy-
dride to the Rh(I) complex. To have insight into the initial step, a
stoichiometric amount of [RhCl(CO)2]2 was allowed to react
with acetic anhydride at 80 ꢀC, but no reaction occurred. This re-
sult supports the hypothesis of the reaction mechanism shown in
Scheme 1.7
The rhodium catalyzed acylation was applied to the synthe-
sis of 1,2-diketones and their derivatives from ꢀ-acyloxy vinyl-
silanes 5, which were easily prepared stereoselectively in E-form
by the O-acylation of lithium enolate of the corresponding acyl-
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.4 (2004)
425
silane 4 (Eq 2).8
This work was supported by the Grant-in-Aid for The 21st
Century COE program for Frontiers in Fundamental Chemistry
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
O
OCPh
1) LDA, HMPA, THF
O
–78 °C, 2 h
Ph(CH )
(2)
3
2 2
Ph(CH )
2 2
SiMe
3 2) (PhCO) O
2
SiMe
4
5 86%
–78°C tort
References and Notes
As summarized in Table 2, ꢀ-acyloxy vinylsilane 5 exhibit-
ed an sufficient reactivity in the acylation reaction.6 Both of pri-
mary and secondary carboxylic acid anhydrides gave acylation
products 6a–c in excellent yields (Entries 1–4),9 whereas the
acylation with pivaric anhydride gave only 7% yield of product
6d (Entry 5). In the reaction with benzoic anhydride, the benzo-
ylation product 6e was obtained in 66% yield with 24% yield of
decarbonylative coupling product 7e (Entry 6). Trifluoroacetyla-
tion proceeded as well and the corresponding diketone 8 was ob-
tained as a hydrate form in 79% yield, after purification by silica
gel column chromatography (Entry 7).
1
For Lewis acid mediated electerophilic acylation of vinylsi-
lanes, see: AlCl3; I. Fleming and A. Pearce, Chem. Com-
mun., 1975, 633; W. E. Fristad, D. S. Dime, T. R. Bailey,
L. A. Paquette, Tetrahedron Lett., 1979, 1999; TiCl4; A.
Tubul and M. Santelli, Tetrahedron, 44, 3975 (1988); SnCl4;
F. Cooke, R. Moerck, J. Schwindeman, and P. Magnus, J.
Org. Chem., 45, 1046 (1980); ZnCl2; T. Sasaki, A.
Nakanishi, and M. Ohno, J. Org. Chem., 47, 3219 (1982).
M. Yamane, T. Amemiya, and K. Narasaka, Chem. Lett.,
2001, 1210.
2
3
4
T. Hiyama and E. Shirakawa, Top. Curr. Chem., 219, 61
(2002) and references therein.
Table 2. Rh(I)-catalyzed acylation of ꢀ-benzoyloxy vinylsi-
lane 5.a
For fluoride ion-promoted transmetalation between transi-
tion metals and vinylsilanes, see: S. E. Denmark and J. Y.
Choi, J. Am. Chem. Soc., 121, 5821 (1999); S. E. Denmark
and D. Wehrli, Org. Lett., 2, 565 (2000); K. Itami, T.
Nokami, Y. Ishimura, K. Matsudo, T. Kamei, and J. Yoshida,
J. Am. Chem. Soc., 123, 11577 (2001); T. Koike, X. Du, T.
Sanada, Y. Danda, and A. Mori, Angew. Chem., Int. Ed.,
42, 89 (2003) and references therein.
For transmetalation between transition metals and alkoxy or
hydroxy vinylsilanes, see: S. Oi, Y. Honma, and Y. Inoue,
Org. Lett., 4, 667 (2002); S. E. Denmark and R. F. Sweis,
J. Am. Chem. Soc., 123, 6439 (2001); K. Hirabayashi, A.
Mori, J. Kawashima, M. Suguro, Y. Nishihara, and T.
Hiyama, J. Org. Chem., 65, 5342 (2000) and references
therein.
General procedure is as follows. A mixture of vinylsilane
(0.2 mmol), [RhCl(CO)2]2 (0.01 mmol), and acid anhydride
(0.6 mmol) in 1,4-dioxane or toluene (2 mL) was heated at
a temperature for a period given in the Tables.
For oxidative addition of acid anhydride to rhodium(I) spe-
cies, see: C. G. Frost and K. J. Wadsworth, Chem. Commun.,
2001, 2316; K. Oguma, M. Miura, T. Satoh, and M. Nomura,
J. Organomet. Chem., 648, 297 (2002).
For stereoselective generation of enolate of acylsilane by
using HMPA, see: M. Honda, W. Oguchi, M. Segi, and T.
Nakajima, Tetarahedron, 58, 6815 (2002).
Lewis acid mediated electerophilic acylation of ꢀ-oxygenat-
ed vinylsilane has never been reported. In fact, in the reac-
tion of ꢀ-benzoyloxy vinylsilane 5 with acetyl chloride in
the presence of AlCl3, the desired ꢀ-benzoyloxy ꢀ,ꢁ-unsat-
urated ketone was not obtained but the Friedel–Crafts acyla-
tion of phenyl group proceeded to give 11 in 74% yield (Eq
4).
O
OCPh
Ph(CH2)2
R
5 mol%
[RhCl(CO)2]2
O
OCPh
6
O
O
(RCO)2O
Ph(CH2)2
+
SiMe3
OCPh
Toluene
5
Ph(CH2)2
R
5
7
Temp.
/ °C
(RCO)2O
R
Entry
Time / h
Products / %b
1
2c
Me
Me
Et
80
80
80
80
80
80
50
4
12
2
6a 98
6a 87
7a
–
7a
7b
7c
7d
–
–
–
–
3
6b 99
6c 97
4
5d
i-Pr
t-Bu
Ph
3
6
7
24
5
6d
6e 66
6f (79)e 7f
7
6
7e 24
7
CF3
4
–
aVinylsilane 5 : [RhCl(CO)2]2:(RCO)2O = 1.0:0.05:3.0.
bIsolated yield based on vinylsilane 5.
O
c1.2 molar amounts of Ac2O was used.
CF3
d
Ph(CH2)3
Vinylsilane 5 was recovered in 8% yield.
eHydrate 8 was obtained in 79% yield.
8
HO OH
8
9
Thus obtained ꢀ-benzoyloxy ketone 6a could be converted
to ꢀ-dimethoxy ketone 9 in 83% yield by the treatment with
K2CO3 in methanol and neutral work up. While, acidic work
up gave 1,2-diketone 10 in 85% yield (Eq 3). On the whole,
the present rhodium-catalyzed reaction of ꢀ-acyloxy vinylsi-
lanes 5 provided a novel method for the preparation of unsym-
metrical 1,2-diketones 10 and their derivatives 6a and 9, which
are regarded as regioselectively masked forms at each of carbon-
yl group of 1,2-diketone 10.
O
OCPh
AcCl (2.0 equiv.)
AlCl3 (2.0 equiv.)
O
OCPh
O
pH 7
Buffer
Ac
Me
Ph(CH )
Ph(CH2)2
2 3
O
(4)
SiMe3
SiMe3
MeO OMe
83%
CH2Cl2, rt, 24 h
OCPh
5
11 74%
K CO
2
3
9
Ph(CH )
Me
2 2
(3)
MeOH
0 °C
15 min
O
O
6a
Me
Ph(CH )
1M HCl
2 3
O
10 85%
Published on the web (Advance View) March 6, 2004; DOI 10.1246/cl.2004.424