Platinum-catalyzed generation of silylenes from hydrodisilanes and their addition to α,β-unsaturated ketones

Article abstract of DOI:10.1246/cl.2008.108

The reaction of hydrodisilanes with α,β-unsaturated ketones in the presence of a platinum catalyst gave high yields of oxasilacyclopentenes, which are probably formed by the addition of silylenes to the enones. Treatment of the oxasilacyclopentenes with methyllimium followed by hydrolysis gave the corresponding β-silyl ketones. Copyright

Full text of DOI:10.1246/cl.2008.108

108
Chemistry Letters Vol.37, No.1 (2008)
Platinum-catalyzed Generation of Silylenes from Hydrodisilanes
and Their Addition to ꢀ,ꢁ-Unsaturated Ketones
Kazuhiro Okamoto and Tamio Hayashiꢀ
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502
(Received November 6, 2007; CL-071225; E-mail: thayashi@kuchem.kyoto-u.ac.jp)
The reaction of hydrodisilanes with ꢀ,ꢁ-unsaturated ke-
ꢂ0:08) corresponding to the nonequivalent methyls on the sili-
tones in the presence of a platinum catalyst gave high yields
of oxasilacyclopentenes, which are probably formed by the addi-
tion of silylenes to the enones. Treatment of the oxasilacyclo-
pentenes with methyllithium followed by hydrolysis gave the
corresponding ꢁ-silyl ketones.
con atom are consistent with the five-membered ring system of
the silylene addition product. The yield of 4am was much lower
with a platinum complex coordinated with a phosphine ligand
(Entry 2). Palladium complex, PdCl2(cod), also catalyzed the
present silylene reaction to some extent (Entry 3), but the
reaction was not catalyzed by phosphine- or cod-complexes
of nickel, gold, rhodium, and iridium under the same reaction
conditions (Entries 5–8).
The reaction of silylenes (divalent silicon species, R2Si:)
with ꢀ,ꢁ-unsaturated carbonyl compounds is known to give
[4 + 1] cycloaddition products.1,2 Of several methods reported
for generation of silylenes, we became most interested in transi-
tion-metal-catalyzed decomposition of hydrodisilanes leading to
silylenes together with hydrosilanes,3 because of their mild reac-
tion conditions and the ready availability of the hydrodisilanes.
Here, we wish to report that silylenes generated from hydrodisi-
lanes under the catalysis by a platinum complex is incorporated
into ꢀ,ꢁ-unsaturated ketones giving [4 + 1] cycloaddition prod-
ucts.4
In the first set of experiments, several transition-metal com-
plexes were examined for their catalytic activity and selectivity
for the reaction of 1,3-diphenyl-2-propenone (1a) with pentam-
ethyldisilane (2m). Of the complexes examined (Table 1),
PtCl2(cod) (cod = 1,5-cyclooctadiene) was found to be most ac-
tive for the addition of dimethylsilylene giving oxasilacyclopen-
tene 3am. Thus, enone 1a was allowed to react with hydrodisi-
lane 2m (1.2 equiv. to 1a) in the presence of 3 mol % of
PtCl2(cod) in toluene at 40 ꢁC for 12 h. Treatment of the reaction
mixture with methyllithium in ether followed by hydrolysis gave
62% yield of 1,3-diphenyl-3-trimethylsilylpropan-1-one (4am)
(Entry 1). Formation of oxasilacyclopentene 3am before treat-
ment with methyllithium was confirmed by 1H NMR of the reac-
tion mixture. Two doublets with an identical coupling constant
(ꢂ 5.74 and 3.23, J ¼ 3:4 Hz) and two singlets (ꢂ 0.22 and
Pentaphenyldisilane (2n) was found to be a better silylene
source for the present platinum-catalyzed reaction with enone
1a, giving a higher yield of the corresponding oxasilacyclopen-
tene 3an,5 which is incorporated with diphenylsilylene. Treat-
ment of the reaction mixture containing 3an with methyllithium
followed by hydrolysis gave ꢁ-diphenyl(methyl)silyl ketone 4an
in 89% yield (Table 2, Entry 2). The reaction of 1,2-diphenyl-
(trimethyl)disilane (2o), which is expected to generate methyl-
(phenyl)silylene by the platinum catalysis, gave, after methyl-
ation and hydrolysis, 46% of ꢁ-dimethyl(phenyl)silyl ketone
4ao together with a minor amount (24%) of ꢁ-trimethylsilyl
ketone 4am (Entry 3). The formation of 4am indicates that the
alkyl scrambling took place on the platinum catalyst between
dialkylsilylene and trialkylsilyl group.6
Table 3 summarizes the results obtained for the platinum-
catalyzed reaction of pentaphenyldisilane (2n) with several
types of ꢀ,ꢁ-unsaturated ketones. ꢁ-Phenyl ketones 1b–1d are
good substrates for the silylene addition giving high yields of
the ꢁ-silyl ketones (Entries 2–4).7 Cyclic enones 1e and 1f also
underwent the silylene addition to give the corresponding ꢁ-silyl
ketones where silyl and acetyl groups are cis (Entries 5 and 6).
Table 2. Hydrodisilanes for platinum-catalyzed silylene addi-
tion
R
Si
O
R
PtCl2(cod) (3 mol %)
O
+
HSiR2SiR3
Ph
Ph
Table 1. Catalyst screening for silylene addition
Toluene, 40 °C, 12 h
Ph
Ph
Me
Si
1a
2m–2o
3a
Me
Ph
O
O
O
Catalyst (3 mol %)
R2MeSi
Ph
O
3am: SiR2 = SiMe2
3an: SiR2 = SiPh2
Ph 3ao: SiR2 = SiMePh
H2O
MeLi in Et2O
+
HSiMe2SiMe3
Toluene, 40 °C, 12 h
Ph
Ph
Ph
Ph
0 °C, 1 h
1a
2m
3am
4a
Me
Ph
3Si
H2O
Yielda
/%
MeLi in Et2O
Entry
Hydrodisilane
Product
0 °C, 1 h
4am
Me3Si
Ph
O
HSiMe2SiMe3 2m
Entry
Catalyst
Yielda/% Entry
Catalyst
Yielda/%
1
2
3
62
4am
4an
4ao
Ph
Ph
Ph
1
2
3
4
PtCl2(cod)
62
12
34
9
5
6
7
8
NiCl2(PEt3)2
AuCl(PPh3)
[RhCl(cod)]2
[IrCl(cod)]2
0
0
<5
<5
Ph2MeSi
O
PtCl2(PEt3)2
PdCl2(cod)
PdCl2(PPh3)2
HSiPh2SiPh3
2n
2o
89
Ph
PhMe2Si
O
46b
HSiMePhSiMe2Ph
Ph
aThe yields were determined by 1H NMR of the crude products
(using MeNO2 as an internal standard).
aIsolated yield. bProduct 4am (24%) was also obtained.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.1 (2008)
109
Table 3. Platinum-catalyzed addition of diphenylsilylene to
ꢀ,ꢁ-unsaturated ketones
H
R
Si
HSiR2SiR
3 2
R
O
[Pt]
HSiR3
H
R3Si
1) PtCl2(cod) (3 mol %)
Toluene, 40 °C, 12 h
R1
O
Ph2MeSi
R1
R4
O
[Pt]
R2
A
R3
R3SiR2Si
R
+
HSiPh2SiPh3
3
R2
R4
R
1 R2
R4
2) MeLi in Et2O
0 °C, 1 h
B
R
R
R3
R3
–
H
Si
[Pt]
1
2n
3) H2O
4n
+
R1
O
H
R3Si
Yielda
/%
[Pt] Si
Entry
Enone
Product
R2
R4
R2
R1
O
R3Si
R
R3
D
C
O
Ph2MeSi
O
O
O
R4
1
2
89
73
1a
1b
4an
R3
1
Ph
Ph
Ph
Ph2MeSi
Ph
O
O
Scheme 1. A proposed catalytic cycle.
4bn
Ph
Ph
Me
Ph
Ph2MeSi
Me
enol pivalates 5 with perfect Z geometry from both linear and cy-
clic enones (eqs 1 and 2).
3b
71c
Ph
1c
1d
4cn
4dn
1) PtCl2(cod) (3 mol %)
HSiPh2SiPh3 (2n)
O
Ph2MeSi OCOtBu
Ph2MeSi
Me
O
O
O
Toluene, 40 °C, 12 h
4
5
67
(1)
(2)
Me
Ph
Ph2MeSi
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2) MeLi in Et2O
0 °C, 1 h
3) t-BuCOCl, rt, 3 h
1a
5a: 92% yield
O
55d
4en
4fn
Me
Me
1e
1f
Ph2MeSi
O
OCOtBu
Me
O
Ph2MeSi
O
Me
6
55d
Me
Me
O
1f
5f: 56% yield
O
Ph2MeSi
4gn
4hn
7
8
65e
56
1g
1h
K. O. thanks the Japan Society for the Promotion of Science
for the award of a fellowship for graduate students.
Ph
Ph
Ph
Ph
Ph2MeSi
Ph
O
O
Ph
Ph
Ph
References and Notes
aIsolated yield. bThe reaction was carried at 100 ꢁC. cRatio of the
diastereoisomers is 6:1. dCis isomer with >99% selectivity.
eMeMgBr in Et2O/THF was used instead of MeLi in Et2O.
1
2
H. Ottosson, P. G. Steel, Chem.—Eur. J. 2006, 12, 1576.
For examples of the reactions of conjugate enones or enoates with
silylenes, see: a) M. Ishikawa, K. Nakagawa, M. Kumada, J.
Organomet. Chem. 1977, 135, C45. b) J. Heinicke, B. Gehrhus,
J. Organomet. Chem. 1992, 423, 13. c) J. Heinicke, S. Meinel, J.
Organomet. Chem. 1998, 561, 121. d) S. A. Calad, K. A. Woerpel,
J. Am. Chem. Soc. 2005, 127, 2046.
The cis-geometry is accounted for by protonation of the lithium
enolates from the other side of silyl group. In the reaction of 2,4-
dienone 1g, the silylation took place on the ꢁ-carbon with per-
fect regioselectivity (Entry 7). The reaction of 1,4-dien-3-one
1h resulted in the exclusive formation of mono-silylated ketone
4hn (Entry 8) because the silylene addition intermediate, which
is an alkenyl-substituted oxasilacyclopentene, does not undergo
the second silylation.
3
a) K. Yamamoto, H. Okinoshima, M. Kumada, J. Organomet.
Chem. 1970, 23, C7. b) K. Yamamoto, H. Okinoshima, M.
Kumada, J. Organomet. Chem. 1971, 27, C31. c) H. Okinoshima,
K. Yamamoto, M. Kumada, J. Am. Chem. Soc. 1972, 94, 9263. d)
H. Okinoshima, K. Yamamoto, M. Kumada, J. Organomet. Chem.
1975, 86, C27.
4
5
6
7
We have reported another type of silylene reaction giving oxasila-
cyclohexenes; K. Okamoto, T. Hayashi, Org. Lett. 2007, 9, 5067.
A considerable amount (>90% yield) of triphenylsilane was also
formed.
For example of the alkyl-scrambling on a iridium(III) complex,
see: M. Okazaki, H. Tobita, H. Ogino, Chem. Lett. 1997, 437.
It is interesting that PhCOCH=C(Me)Ph (1d) produces the oxasi-
lacyclopentene, while PhCOCH=CMe2 produces an oxasilacyclo-
hexene under similar reaction conditions (see ref. 4).
a) H. Ogino, Chem. Rec. 2002, 2, 291. b) J. D. Feldman, G. P.
Mitchell, J.-O. Nolte, T. D. Tilley, Can. J. Chem. 2003, 81, 1127.
W. Ando, K. Hagiwara, A. Sekiguchi, Organometallics 1987, 6,
2270.
A catalytic cycle proposed for the present silylation giving
oxasilacyclopentene 3 is shown in Scheme 1. Generation of di-
alkylsilylenes from hydrodisilanes has been reported to take
place on transition-metal complexes including platinum.8 It is
most probable that the reaction proceeds through nucleophilic
attack of the carbonyl oxygen of enone 1 onto a dialkylsilylene
C stabilized by coordination to platinum, which is generated
from hydrodisilane 2 and a platinum complex A by way of B.
Cyclization on zwitterionic species D9 produces oxasilacyclo-
pentene 3 leaving A.
Because the oxasilacyclopentenes 3 formed by the present
silylation have a cyclic structure, the lithium enolates generated
by treatment with methyllithium should have exclusive Z geom-
etry.10 The (Z)-lithium enolates can be trapped as (Z)-enol esters
by O-acylation. Thus, successive treatment of the oxasilacyclo-
pentenes 3 with methyllithium and then pivaloyl chloride gave
8
9
10 For example of the selective formation of ꢁ-silyl enolates, see: R.
A. N. C. Crump, I. Fleming, J. H. M. Hill, D. Parker, N. L. Reddy,
D. Waterson, J. Chem. Soc., Perkin Trans. 1 1992, 3277.
11 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) December 15, 2007; doi:10.1246/cl.2008.108