1168
A. Mori, T. Kato
LETTER
Table 3 Rhodium-Catalyzed Hydrosilylation with Various , -Un-
trialkylsilane, HSiEt3, also affected the reaction in excel-
lent yields. Disiloxane, HSiMe2OSiMe3 and alkoxysi-
lanes, HSiMe2(OEt), HSiMe(OEt)2, and HSi(OEt)3 were
found similarly effective. The hydrosilylation with cyclic
tetrasiloxanes (HSiMeO)4 and polysiloxane (HSiMeO)n
(PMHS; n = ca. 45–50)7 also afforded the corresponding
enolsilanes although it took longer reaction periods at 60
°C.
saturated Carobonyl Compoundsa
Substrate
Temp.,
°C
Time,
min
% Yield
(ratio)b
2-cyclopenten-1-one
r.t.
r.t.
10
10
86
(E)-n-C5H11CH=CHCOMe
93c,d
(E/Z=35:65)
Me2C=CHCOMe
60
30
87
Table 2 Hydrosilylation of 2-Cyclohexen-1-one with Various Ter-
tiary Silanesa
(E/Z=26:74)
r.t.
12 h
95d
(E/Z=21:79)
Silane
Solvent
THF
none
THF
THF
none
THF
THF
none
THF
THF
Temp., °C Time
%Yield
>99
>99
>99
82
HSiEt3
60
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
60
4 h
CH2=C(Me)COOMe
r.t.
r.t.
10
10
87d
3 h
(E)-MeCH=CHCHO
80c,d
(E/Z=19:81)
HSiMe2OSiMe3
HSiMe2OEt
10 min
30 min
10 min
30 min
13 h
a Unless otherwise noted the reaction was carried out with , -unsat-
urated carobonyl compound (1.0 mmol) and phenyldimethylsilane
(1.1 mmol) in the presence of [Rh(OH)(cod)]2 (0.15 mol %) in 3 mL
of THF.
>99
92
b The isomer ratio was determined by 1H NMR analysis.
c The rhodium catalyst loaded was 0.005 mol %.
d The reaction was carried out without solvent.
HSiMe(OEt)2
HSi(OEt)3
>99
>99
>99
85
3 h
On the other hand, 1,2-hydrosilylation with secondary si-
lanes (R2SiH2) was found to be less selective when
[Rh(OH)(cod)]2 was employed as a catalyst. When the re-
action was carried out with a secondary silane, H2SiPhMe,
and twice molar amounts of 2-cycohexen-1-one (2) in the
presence of 0.15 mol% of 1, alkoxyenolsilane 5 was ob-
tained in 85% yield as shown in Scheme suggesting to af-
ford 1:1 mixture of 1,2- and 1,4-hydrosilylation products,
respectively. The result suggests that the secondary silane
would undergo the 1,2-hydrosilylation leading to a possi-
ble intermediate A, which is a tertiary silane. Hence, A
would then affect 1,4-hydrosilylation of 2 to afford 5. This
would be due to the relatively slower 1,2-hydrosilylation
toward 1,4-reaction with [Rh(OH)(cod)]2 as a catalyst,
which enables the rapid 1,4-reaction of A. In the reaction
with RhH(PPh3)4 or RhCl(PPh3)3, by contrast, sluggish
hydrosilylation of the formed tertiary silane that corre-
sponds to the intermediate A would result in the 1,2-selec-
tive reaction.3 Consequently, the reaction with the
hydroxorhodium catalyst afforded the mixture of 1,2- and
1,4-products despite high selectivity of both 1,2-hydrosi-
lylation with a secondary silane and 1,4-reaction of the
formed tertiary silane.
(HSiMeO)4
(HSiMeO)n
4 h
6 days
a The reaction was carried out with 2-cyclohexen-1-one (1.0 mmol)
and a tertiary silane (1.1 mmol) in the presence of [Rh(OH)(cod)]2
(0.15 mol %) in 3 mL of THF or neat. The yield was estimated by
1H NMR analysis using trichloroethylene as an internal standard.
The reaction was examined with further smaller catalyst
loading. Hydrosilylation of 2-cyclohexen-1-one (2) with
ethoxydimethylsilane was carried out in the presence of
0.005 mol% of the rhodium catalyst 1 at room temperature
without solvent. After stirring for 20 min the correspond-
ing enolsilane was produced quantitatively (confirmed by
1H NMR) and isolated in 97% yield by bulb-to-bulb dis-
tillation.8
Next, hydrosilylation with various , -unsaturated carbo-
nyl compounds was carried out. The results are summa-
rized in Table 3. The five-membered cyclic ketone, 2-
cyclopenten-1-one, and 3 underwent the hydrosilylation
similarly to give the enolsilane in 86% isolated yield. The
reaction
with
an
acyclic
enone,
(E)-n-
C5H11CH=CHCOMe also proceeded although the ob-
tained product was a mixture of stereoisomers. A trisub-
stituted enone, Me2C=CHCOMe could affect the reaction
at a slightly higher reaction temperature or a longer reac-
tion period. In addition to ketones, , -unsaturated ester
and aldehyde such as methyl methacrylate,
CH2=C(Me)COOMe
MeCH=CHCHO afforded the corresponding enolsilanes
in excellent yields.
In conclusion, we have shown that a rhodium complex,
[Rh(OH)(cod)]2 is a highly efficient catalyst for the syn-
thesis of enolsilanes from , -unsaturated carbonyl com-
pounds. Since the reaction can be carried out under mild
conditions without solvent and with a small amount
(0.005–0.15 mol%) of the catalyst loading, the product
could be isolated by direct distillation of the reaction mix-
ture. The present process, therefore, would be a practical
synthetic method for the precursor of Mukaiyama aldol
reactions.
and
crotonaldehyde,
(E)-
Synlett 2002, No. 7, 1167–1169 ISSN 0936-5214 © Thieme Stuttgart · New York