New method for introduction of a silyl group into a,β-enones using a disilane catalyzed by a copper(I) salt [8]

Full text of DOI:10.1021/ja9822557

11196
J. Am. Chem. Soc. 1998, 120, 11196-11197
New Method for Introduction of a Silyl Group into
Scheme 1. Conjugate Silylation of an R,â-Unsaturated
Carbonyl Compound and a Disilane catalyzed by a Copper(I)
Salt
r,â-Enones Using a Disilane Catalyzed by a
Copper(I) Salt1
Hajime Ito, Tomoko Ishizuka, Jun-ichi Tateiwa,
Motohiro Sonoda, and Akira Hosomi*
Department of Chemistry, Graduate School of Chemistry
UniVersity of Tsukuba, Tsukuba
Ibaraki 305-8571, Japan
ReceiVed June 29, 1998
Introduction of silyl functionalities into organic compounds
along with the formation of the Si-C bond is of great interest in
organic synthesis.2 Conjugate silylation of R,â-unsaturated car-
bonyl compounds is an important tool, and this reaction is usually
achieved by the use of silyl nucleophiles.3 In the hitherto known
methods, stoichiometric preparation of a silyl anionic species,
relatively inaccessible, has been indispensable for the generation
of silyl nucleophilic reagents. Although the Pd-catalyzed reactions
of a disilane and R,â-unsaturated carbonyl compounds are
comparable with the method using silyl nucleophiles, there are
Table 1. Reaction of 1,2-Diphenyltetramethyldilsilane (1a) and
2-Cyclohexen-1-one (2a) in the presence of a Cu(I) Catalyst under
Various Conditions
some limitations to both the silyl groups and substrates available
disilane Cu(I)/ Bu
3
P/
temp/ time/
yield
of 3a/%
for these reactions.4 We wish to report herein, as a partial solution
entry
a
1a/equiv equiv equiv solvent
°C
h
b
to these problems, an unprecedented cleavage reaction of the
silicon-silicon bond in a disilane with a Cu(I) salt to generate a
silyl nucleophile and its new 1,4-addition reaction toward R,â-
enones in the presence of a catalytic amount of a Cu(I) salt.
1
2
3
4
5
1.0
1.0
1.2
1.2
1.2
1.0
0.1
0.1
0.1
0.1
DMI
DMI
r.t.
24
24
21
4
20
33
88
91
77
100
100
100
0.11 DMI
0.11 DMF
0.11 Diglyme 100
21
(Scheme 1).
a
A mixture of a disilane (1.0-1.2 mmol), an R,â-unsaturated
We have recently reported exchange reactions of a silyl group
compound (1.0 mmol), (CuOTf)
tylphosphine (0-0.11 mmol) was stirred in a solvent (1.0 ml). Isolated
yield.
2
6 6
‚C H (0.05-1.0 mmol) and tribu-
of alkynyl-, aryl- and hydrosilanes with a Cu(I) salt in an aprotic
polar solvent such as 1,3-dimethyl-2-imidazolidinone (DMI) and
various synthetic procedures using organocopper(I) reagents based
on this exchange reaction.5 Although there are many reports of
the cleavage reaction of a Si-Si bond in disilanes to generate a
b
analogous to the reaction between hydrosilanes and CuCl,
previously reported.
For the purpose of trapping a reactive intermediate of this
reaction generated in situ, the reaction using 1,1,2,2-tetramethyl-
5a
6
3b
silyl nucleophile with alkali metal, alkyllithium, metal alco-
holate, metal hydride, fluoride ion,9 and transition metal
complex,1 to our knowledge, the cleavage reaction of disilanes
with a Cu(I) salt is unknown. We started our investigation of
the reaction between a disilane and a copper(I) salt in DMI
7
8
0,11
12
1
,2-diphenyldisilane 1a and (CuOTf)
2
‚C
6
H
6
in DMI was ex-
amined in detail in the presence of 2-cyclohexen-1-one as an
electrophile, after trials using several combinations between a Cu-
(I) salt and a solvent (Table 1). In entry 1, a mixture of 1,1,2,2-
(
1) Studies on Organosilicon Chemistry. No. 145.
(
2) (a) Fleming, I. Organocopper Reagents: A Practical Approach; Taylor,
tetramethyl-1,2-diphenyldisilane (1.0 mmol), (CuOTf)
6 6
‚C H (1.0
2
R. J. K., Ed.; Oxford University Press: Oxford, 1994; pp 257-292. (b) Colvin,
E. W. Silicon Reagents in Organic Synthesis; Academic Press: London, 1988;
pp 51-55.
mmol) and 2-cyclohexen-1-one 2a (1.0 mmol) in DMI (1.0 mL)
was stirred for 24 h at room temperature.13 After acidic workup,
a conjugate silylation product 3a was obtained in 20% yield. To
confirm the synthetic feasibility of this reaction, we explored
reaction conditions for a catalytic use of the Cu(I) salt. However,
the use of 10 mol % of the copper(I) salt as a catalyst for the
conjugate silylation of R,â-enones resulted in low yield at 100
(
3) (a) Lipshutz, B. H.; Sclafani, J. A.; Takanami, T. J. Am. Chem. Soc.
1
998, 120, 4021-4022. (b) Still, W. C. J. Org. Chem. 1976, 41, 3063-3064.
(
c) Ager, D. J.; Fleming, I.; Patel, S. K. J. Chem. Soc., Perkin. Trans. 1 1981,
2
1
520-2526. (d) Lipshutz, B. H.; Reuter, D. C.; Ellsworth, E. L. J. Org. Chem.
989, 54, 4975-4977. (e) Crump, R. A. N. C.; Fleming, I.; Urch, C. J. J.
Chem. Soc., Perkin. Trans. 1 1994, 701-706. (f) Smith, J. G.; Quinn, N. R.;
Viswanathan, M. Synth. Commun. 1983, 13, 1-5. (g) Kawachi, A.; Tamao,
K. Bull. Chem. Soc. Jpn. 1997, 70, 945-955.
°
C together with unidentified side products (entry 2). Under these
(
4) For the Pd-catalyzed 1,4-bis-silylation of R,â-unsaturated ketones,
conditions, the precipitation of Cu(0) was observed. Then, we
found that the addition of a catalytic amount of tributylphosphine
dramatically improved this reaction (entry 3). The silylation
product 3a was obtained in good yield even with 5 mol % (10
mol % for Cu(I)) of (CuOTf)
tylphosphine (11 mol %) (entries 3-5). DMI and DMF are both
good solvents for this reaction. The rate of the silylation reaction
see: (a) Tamao, K.; Okazaki, S.; Kumada, M. J. Organomet. Chem. 1978,
1
1
46, 87-93. (b) Hayashi, T.; Matsumoto, Y.; Ito, Y. J. Am. Chem. Soc. 1988,
10, 5579-5581.
(
5) (a) Ito, H.; Ishizuka, T.; Arimoto, K.; Miura, K.; Hosomi, A.
Tetrahedron Lett. 1997, 38, 8887-8890. (b) Ito, H.; Arimoto, K.; Sensui, H.;
Hosomi, A. Tetrahedron Lett. 1997, 38, 3977-3980. (c) Ito, H.; Sensui, H.;
Arimoto, K.; Miura, K.; Hosomi, A. Chem. Lett. 1997, 639-640. (d) See
also: Nishihara, Y.; Ikegashira, K.; Mori, A.; Hiyama, T. Tetrahedron Lett.
2
6 6
‚C H in the presence of tribu-
1
998, 39, 4075-4078.
(
6) Gilman, H.; Lichtenwalter, G. D. J. Am. Chem. Soc. 1958, 80, 608-
(11) Palladium-catalyzed reaction of hexamethyldisilane, see: (a) Ito, Y.;
Suginome, M.; Murakami, M. J. Org. Chem. 1991, 56, 1948-1951. (b)
Yamashita, H.; Reddy, N. P.; Tanaka, M. Chem. Lett. 1993, 315-318. (c)
Yamashita, H.; Reddy, N. P.; Tanaka, M. Macromolecules 1993, 26, 2143-
2144. (d) Obora, Y.; Tsuji, Y.; Kawamura, T.; J. Am. Chem. Soc. 1995, 117,
9814-9821.
6
11.
(
7) (a) Sakurai, H.; Okada, A.; Kira, M.; Yonezawa, K. Tetrahedron Lett.
1
1
971, 12, 1511-1514. (b) Dervan, P. B.; Shippey, M. A. J. Am. Chem. Soc.
976, 98, 1265-1267.
(
8) Corriu, R. J. P.; Guerin, C. J. Chem. Soc., Chem. Commun. 1980, 168-
1
69.
(12) (CuOTf)
2
6 6
‚C H is highly sensitive for both moisture and air. It should
(
9) Hiyama, T.; Obayashi, M.; Mori, I.; Nozaki, H. J. Org. Chem. 1983,
be weighed in a glovebox under nitrogen (see Supporting Information). Cohen,
T.; Ruffner, R. J.; Shull, D. W.; Fogel, E. R.; Flack, J. R. Org. Synth. 1980,
59, 202-210.
4
8, 912-914.
(10) For recent reviews with respect to transition metal-mediated reactions
using a Si-Si bond, see: (a) Sharma, H. K.; Pannell, K. H. Chem. ReV. 1995,
9
2
(13) In the absence of 2a, a mixture of recovered 1a (38%) and PhMe -
5, 1351-1374. (b) Horn, K. A. Chem. ReV. 1995, 95, 1317-1350.
2
SiOSiMe Ph (55%) was obtained.
1
0.1021/ja9822557 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/16/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 43, 1998 11197
Scheme 2. Proposed Mechanism for the Conjugate Silylation
of an a,b-Unsaturated Carbonyl Compound and a Disilane in
the Presence of a Copper(I) Catalyst
for the introduction of a trimethylsilyl group to R,â-enones, since
1
4
the preparation of trimethylsilylcopper is not easily accessible.
Interestingly, the trimethylsilyl group was mainly introduced
into the substrate when using unsymmetrical disilane 1c (eq 1).
This result provides a clear contrast with the case of palladium-
catalyzed 1,4-disilylation of R,â-unsaturated ketones with 1,1-
dichrolo-1-phenyl-2,2,2-trimethyldisilane.15
A plausible catalytic cycle is shown in Scheme 2. Reaction
between a disilane and a copper(I) salt forms reactive intermediate
A, and conjugate addition to R,â-enones takes place. The copper-
(
(
I) salt is regenerated by the reaction between the resultant copper-
I) enolate and silyl triflate, which is formed at the first stage of
this cycle. Tributylphosphine coordinates to copper(I) and
presumably plays an important role in the stabilization of the
intermediate A and copper(I) enolate B. After the reaction was
completed, acidic workup gave 3.
In conclusion, we have found an unprecedented and exclusive
cleavage reaction of disilanes followed by the conjugate silylation
1
6
to R,â-enones with a catalytic amount of a Cu(I) salt. The
present procedure provides a convenient and synthetically useful
route for introduction of a silyl group into R,â-enones without
prior stoichiometric preparation of silylmetals. Further work on
the precise mechanism for these reactions and on the structure of
the active species is actively underway.
Acknowledgment. Financial support for this work is partly provided
by Grants-in-Aid for Scientific Research on Priority Areas from the
Ministry of Education, Japan, and Pfizer Pharmaceuticals, Inc. We thank
Dow Corning Toray Silicone Co., Ltd., Chisso Co., Ltd., and Shin-Etsu
Chemical Co., Ltd. for a gift of organosilicon compounds.
of 3a was increased in DMF. Diglyme can also be used as a
solvent (entry 5).
This catalytic reaction was applied to 3-silylation of various
R,â-unsaturated carbonyl compounds (Scheme 1). Representative
results are listed in Table 2. Optimized reaction time and solvent
depend on the substrates. The conjugate silylation product of a
dimethylphenylsilyl group was obtained in good yield for various
R,â-unsaturated ketones (entries 1-5). Without acidic workup,
the corresponding silyl enol ether 4 was isolated (entry 4). An
R,â-unsaturated aldehyde 2f can be also used as a good acceptor
Supporting Information Available: Experimental details for the
1
13
general procedure of the catalytic reactions, H, C NMR spectra of all
products (7 pages, print/PDF). See any current masthead page for
ordering information and Web access instructions.
JA9822557
(
entry 6). However, 3-methyl substitution of the substrate 2
retarded this reaction, presumably due to the steric effect (entry
). The products were obtained in good yield even with
hexamethyldisilane 1b, which is relatively unreactive for the Si-
(14) In the hitherto known methods for the preparation of trimethylsilylmetal
species, HMPA (hexamethylphosphoramide), known as a strong carcinogen,
is needed as the solvent.
7
(
15) (a) Hayashi, T.; Matsumoto, Y.; Ito, Y. Tetrahedron Lett. 1988, 29,
4147-4150. (b) Matsumoto, Y.; Hayashi, T.; Ito, Y. Tetrahedron 1994, 50,
Si cleavage reaction (entries 8-11).7
,11
For increasing the
335-346.
(
16) Although R,â-unsaturated esters instead of R,â-enones gave a silylated
solubility of hexamethyldisilane, a mixed solvent of DMF and
diglyme (1:1) was employed (entry 8). This method is useful
product in poor yield (at most, ca. 20%) under the same conditions, efforts to
explore better conditions are now under way.