Regio- and stereoselective approach to 1,2-di- and 1,1,2-trisilylethenes by cobalt-mediated reaction of silyl-substituted dibromomethanes with silylmethylmagnesium reagents

Article abstract of DOI:10.1002/anie.200500576

(Chemical Equation Presented) Functional ethenes: Treatment of dibromo(silyl)methanes 2 a with silylmethyl Grignard reagents 1 and catalytic CoCl₂ affords (E)-1,2-disilylethenes 3 a in excellent yields. Dibromodisilylmethanes 2 b undergo a simi

Full text of DOI:10.1002/anie.200500576

Communications
Silyl Ethenes
Regio- and Stereoselective Approach to 1,2-Di-
and 1,1,2-Trisilylethenes by Cobalt-Mediated
Reaction of Silyl-Substituted Dibromomethanes
with Silylmethylmagnesium Reagents**
Hirohisa Ohmiya, Hideki Yorimitsu, and
Koichiro Oshima*
Vinylsilanes are useful organometallic reagents in organic
2
À
synthesis because the C(sp ) Si bonds undergo numerous
transformations.[1] Multiply silylated ethenes are thus likely to
represent platforms for a variety of highly substituted ethenes.
Moreover, multiply silylated ethenes themselves attract
considerable attention from the viewpoint of structural
organic chemistry.[2] Despite their importance, there is a
limited number of access routes to multiply silylated ethenes.
Hydrosilylation of silylacetylenes[3] and bissilylation of ace-
tylenes[4] are most convenient procedures.[5] Scheme 1 (top)
Scheme 1. Conventional and novel approaches toward 1,2-di- and
1,1,2-trisilylethenes.
shows representative approaches, for instance, to trisilyl-
ethenes with three different silyl groups. However, difficulties
are often encountered in such synthetic strategies in terms of
regioselectivity and the occurrence of several side reactions.
During the course of our studies into cobalt-mediated
reactions of organic halides with Grignard reagents,[6] we
[*] H. Ohmiya, Dr. H. Yorimitsu, Prof. Dr. K. Oshima
Department of Material Chemistry
Graduate School of Engineering
Kyoto University
Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2438
E-mail: oshima@orgrxn.mbox.media.kyoto-u.ac.jp
[**] This work was supported by Grants-in-Aid for Scientific Research for
Young Scientists and COE Research from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3488
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200500576
Angew. Chem. Int. Ed. 2005, 44, 3488 –3490

Angewandte
Chemie
Table 2: Synthesis of 1,1,2-trisilylethenes 2.
serendipitously found that the reaction of dibromomethane
with dimethyl(phenyl)silylmethylmagnesium chloride in the
presence
of
a
cobalt
salt
provided
dimethyl-
(phenyl)vinylsilane in excellent yield [Eq. (1)]. This observa-
Entry R1 Si
R2 Si
R3 Si
2
Yield [%] E/Z
3
3
3
1
2
3
4
5
6
7
Me3Si
Me3Si
Me3Si
Me3Si
Me3Si
Me2PhSi
a
b
c
d
e
f
75
73
55
58
48
54
53
–
–
–
tion encouraged us to explore the potential of silylated
dibromomethanes[7] as precursors for multiply silylated
ethenes (Scheme 1, bottom). Herein we report an inherently
regioselective and, fortunately, stereoselective preparation of
1,2-di- and 1,1,2-trisilylethenes by means of a cobalt salt, thus
MePh2Si MePh2Si Me3Si
tBuMe2Si Me3Si
MePh2Si Me3Si
MePh2Si Me3Si
MePh2Si Me3Si
Me3Si
(iPrO)Me2Si
Me2PhSi
100:0[a]
90:10[a]
94:6[a]
94:6[a]
=
(CH2
g
CHCH2)Me2Si
creating
a novel approach toward multiply silylated
8
MePh2Si (Bu3Sn) Me3Si
h
51
8:92[b]
ethenes.[8,9]
[a] Determined by NOE experiments. [b] Judged by JSn-H
.
After optimization of reaction conditions, the synthesis of
1,2-disilylethene proved to require cobalt(ii) chloride
(10 mol%) and a Grignard reagent (3 equiv). THF was the
solvent of choice, while ether, dioxane, and HMPA were far
inferior. The best results were found at a reaction temper-
ature of 208C. Table 1 summarizes the syntheses of various
modisilylmethanes under similar conditions yielded (E)-2e–
2g with three different silyl groups stereoselectively (Table 2,
entries 5–7). The reaction of a dibromosilylstannylmethane
furnished the corresponding (Z)-1,2-disilyl-1-stannylethene
2h with good stereoselectivity (Table 2, entry 8). There are
few facile methods for the stereo- and regioselective synthesis
of ethenes with different Group 14 metal substituents.
Unfortunately, the attempted synthesis of tetrasilylethene
from (R3Si)2CBr2 and (R3Si)2CHMgCl did not succeed.
We propose a mechanism for the stoichiometric reaction
as shown in Scheme 2. Halogen–cobalt exchange initially
takes place to produce intermediate 3. One of the silylmethyl
groups on the cobalt center migrates to generate 4 with
concomitant liberation of bromide.[12] b-Hydride elimination
finalizes the formation of 1,1,2-trisilylethene 2. The major
E stereoisomers in Table 2 would originate from the more
stable eclipsed conformer upon b-hydride elimination. The
Table 1: Synthesis of (E)-1,2-disilylethenes 1.
Entry
R3Si
R’Si
1
Yield [%]
1
2
3
4
5
6
7
Me3Si
Me3Si
Me3Si
Me2PhSi
Me2PhSi
(iPrO)Me2Si
Me2PhSi
MePh2Si
tBuMe2Si
Me3Si
Me2PhSi
Me2PhSi
Me2PhSi
a
b
c
a
d
e
f
87
86
70
90
78
88
79
=
(CH2 CHCH2)Me2Si
1
2
=
formation of the by-product (R Si)(R Si)C CH2 can stem
3
3
1,2-disilylethenes. All the reactions resulted in the exclusive
formation of (E)-1,2-disilylethenes 1 in high yields. The steric
hindrance of the silyl groups such as MePh2Si and tBuMe2Si
had virtually no adverse influence on the synthesis. Interest-
ingly, isopropoxy- and allyl-substituted silylmethyl Grignard
reagents participated in this reaction.
from b-silyl elimination.[13]
The high efficiency of this method prompted us to
examine dibromodisilylmethanes as starting materials. Con-
trary to our expectation, the catalytic conditions did not give
satisfactory results. Instead, stoichiometric use of a cobaltate
reagent [(R3 SiCH2)4Co(MgCl)2],[10,11] prepared from Coii
3
chloride and a Grignard reagent (4 equiv), allowed the
efficient synthesis of 1,1,2-trisilylethenes (Table 2). The
reactions of (Me3Si)2CBr2 proceeded smoothly to afford 2a
and 2b in good yields (Table 2, entries 1 and 2). The bulkier
MePh2Si- and tBuMe2Si-substituted precursors were also
converted into 2 in reasonable yields (Table 2, entries 3–7).
The reactions were clean, and the main by-products were
(R1 Si)(R2 Si)C(H)Br, (R1 Si)(R2 Si)CH2, and (R1 Si)-
3
3
3
3
3
2
=
(R Si)C CH2, which were readily separated from the desired
3
Scheme 2. Proposed mechanism for the formation of 1,1,2-trisilyl-
ethenes in the presence of stoichiometric amounts of the cobaltate
complex.
products by size-exclusion chromatography (see below).
Gratifyingly, treatment of unsymmetrically substituted dibro-
Angew. Chem. Int. Ed. 2005, 44, 3488 –3490
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3489

Communications
Voronkov, J. Organomet. Chem. Libr. 1977, 5, 1 – 179; c) I. Ojima
in The Chemistry of Organic Silicon Compounds (Eds.: S. Patai,
Z. Rappoport), Wiley, Chichester, 1989, pp. 1479 – 1526;
d) Comprehensive Handbook on Hydrosilylation (Ed.: B. Mar-
ciniec), Pergamon, Oxford, 1992; e) T. Hiyama, T. Kusumoto in
Comprehensive Organic Chemistry, Vol. 8 (Eds.: B. M. Trost, I.
Fleming), Pergamon, Oxford, 1991, chap. 3.12; f) K. A. Horn,
Chem. Rev. 1995, 95, 1317 – 1350.
In summary, we have developed a novel method for the
synthesis of 1,2-di- and 1,1,2-trisilylethenes in a regio- and
stereoselective manner. The products are not only useful as
precursors for various alkenes but are also structurally
interesting. Further improvement of this strategy will allow
access to a wide range of ethenes that are multifariously
substituted with Group 14 metals.
[4] a) M. Suginome, Y. Ito, Chem. Rev. 2000, 100, 3221 – 3256; b) J.
Hibino, S. Nakatsukasa, K. Fugami, S. Matsubara, K. Oshima, H.
Nozaki, J. Am. Chem. Soc. 1985, 107, 6416 – 6417; c) H.
Watanabe, M. Kobayashi, M. Saito, Y. Nagai, J. Organomet.
Chem. 1981, 216, 149 – 157.
[5] For the disproportionation of vinylsilane, see: a) B. Marciniec, E.
Walczuk-Gusciora, P. Blazejewska-Chadyniak, J. Mol. Catal. A
2000, 160, 165 – 171; b) B. Marciniec, J. Guliꢁski, J. Organomet.
Chem. 1984, 266, C19-C21; for Ru-catalyzed dehydrogenative
silylation of vinylsilanes, see: c) Y. Seki, K. Takeshita, K.
Kawamoto, J. Organomet. Chem. 1989, 369, 117 – 123.
[6] a) K. Wakabayashi, H. Yorimitsu, K. Oshima, J. Am. Chem. Soc.
2001, 123, 5374 – 5375; b) Y. Ikeda, T. Nakamura, H. Yorimitsu,
K. Oshima, J. Am. Chem. Soc. 2002, 124, 6514 – 6515; c) T. Tsuji,
H. Yorimitsu, K. Oshima, Angew. Chem. 2002, 114, 4311 – 4313;
Angew. Chem. Int. Ed. 2002, 41, 4137 – 4139; ; d) T. Fujioka, T.
Nakamura, H. Yorimitsu, K. Oshima, Org. Lett. 2002, 4, 2257 –
2259; e) K. Mizutani, H. Shinokubo, K. Oshima, Org. Lett. 2003,
5, 3959 – 3961; f) K. Mizutani, H. Yorimitsu, K. Oshima, Chem.
Lett. 2004, 33, 832 – 833; g) T. Nishikawa, H. Yorimitsu, K.
Oshima, Synlett 2004, 1573 – 1574; h) H. Ohmiya, H. Yorimitsu,
K. Oshima, Chem. Lett. 2004, 33, 1240 – 1241; i) H. Ohmiya, T.
Tsuji, H. Yorimitsu, K. Oshima, Chem. Eur. J. 2004, 10, 5640 –
5648; j) Y. Ikeda, H. Yorimitsu, H. Shinokubo, K. Oshima, Adv.
Synth. Catal. 2004, 346, 1631 – 1634; k) H. Shinokubo, K.
Oshima, Eur. J. Org. Chem. 2004, 2081 – 2091.
Experimental Section
General procedure (1a): Anhydrous cobalt(ii) chloride (6.5 mg,
0.05 mmol) was placed in a 20-mL reaction flask and heated with a
hair dryer in vacuo for 2 min. After the cobalt salt turned blue,
anhydrous THF (3.0 mL) was added under argon. The mixture was
stirred for 3 min at room temperature. Dibromo(dimethyl(phenyl)-
silyl)methane (154 mg, 0.50 mmol) and a solution of trimethylsilyl-
methylmagnesium chloride in diethyl ether (1.0m; 1.5 mL, 1.5 mmol)
were successively added dropwise to the reaction mixture at 08C.
While the Grignard reagent was being added, the mixture turned
brown. After being stirred for 1 h at 208C, the reaction mixture was
poured into water. The product was extracted with hexane (2 ꢀ
20 mL). The combined organic layer was dried over sodium sulfate
and concentrated. Purification of the crude oil by silica-gel column
chromatography (hexane) provided the corresponding (E)-1,2-di-
silylethene 1a (102 mg, 0.43 mmol) in 87% yield.
2b: Anhydrous cobalt(ii) chloride (97.5 mg, 0.75 mmol) was
placed in a 30-mL reaction flask and dried in vacuo for 2 min.
Anhydrous THF (5.0 mL) was added under argon, and the mixture
was stirred for 3 min at room temperature. A solution of dimethyl-
(phenyl)silylmethylmagnesium chloride in diethyl ether (0.95m;
3.16 mL, 3.0 mmol) was added dropwise to the reaction mixture at
À208C. After the mixture was stirred for 15 min at À208C,
dibromobis(trimethylsilyl)methane (159 mg, 0.50 mmol) was added
dropwise to the reaction mixture at À208C. After being stirred for an
additional 1 h at À208C, the reaction mixture was poured into water.
The product was extracted with hexane (2 ꢀ 20 mL). The combined
organic layer was dried over sodium sulfate and concentrated. Silica-
gel column chromatography (hexane) followed by gel-permeation
chromatography (toluene, to remove by-products described above)
provided the corresponding 1,1,2-trisilylethene 2b (111 mg,
0.36 mmol) in 73% yield.
[7] A. Inoue, J. Kondo, H. Shinokubo, K. Oshima, Chem. Lett. 2001,
956 – 957, and references therein.
[8] For the cobalt-mediated reaction of dibromocyclopropane with
methyl and butyl Grignard reagents, see: Y. Nishii, K. Wakasugi,
Y. Tanabe, Synlett 1998, 67 – 69.
[9] We previously reported the manganese-catalyzed reaction of
dibromo(silyl)methane with alkyl Grignard reagents to form 1-
silyl-1-alkene, and only one example of the synthesis of 1,2-
disilylethene was described. However, the reaction is not very
=
efficient (tBuMe2SiCH CHSiMe3, 57% yield by using a stoi-
chiometric amount of manganate complex
Received: February 16, 2005
Published online: April 28, 2005
a
[(Me3SiCH2)3MnMgCl]): a) H. Kakiya, R. Inoue, H. Shinokubo,
K. Oshima, Tetrahedron Lett. 1997, 38, 3275 – 3278; b) H.
Kakiya, H. Shinokubo, K. Oshima, Bull. Chem. Soc. Jpn. 2000,
73, 2139 – 2147.
Keywords: alkenes · cobalt · Grignard reagents · silicon ·
synthetic methods
.
[10] The exact structure of the species [(R3 SiCH2)4Co(MgCl)2] in
3
solution is not clear. For convenience, we use this expression.
Several organocobaltate(ii) compounds were characterized by
X-ray crystallographic analysis: R. S. Hay-Motherwell, G. Wil-
kinson, B. Hussain, M. B. Hursthouse, Polyhedron 1990, 9, 931 –
937.
[1] a) K. Oshima in Science of Synthesis (Houben-Weyl), Vol. 4 (Ed.:
I. Fleming), Georg Thieme, Stuttgart, 2002, chap. 4.4.34; b) K.
Miura, A. Hosomi in Main Group Metals in Organic Synthesis,
Vol. 2 (Eds.: H. Yamamoto, K. Oshima), Wiley-VCH, Wein-
heim, 2004, chap. 10.4; c) J. S. Panek in Comprehensive Organic
Chemistry, Vol. 1 (Eds.: B. M. Trost, I. Fleming), Pergamon,
Oxford, 1991, chap. 2.5; d) M. A. Brook, Silicon in Organic,
Organometallic, and Polymer Chemistry, Wiley-VCH, New
York, 2000.
[2] a) H. Sakurai, K. Ebata, K. Sakamoto, Y. Nakadaira, C. Kabuto,
Chem. Lett. 1988, 965 – 968; b) H. Schmidbaur, J. Ebenhoech, Z.
Naturforsch. B 1987, 42, 1543 – 1548; c) M. Kira, H. Nakazawa,
H. Sakurai, J. Am. Chem. Soc. 1983, 105, 6983 – 6987; d) H.
Sakurai, Y. Nakadaira, H. Tobita, T. Ito, K. Toriumi, H. Ito, J.
Am. Chem. Soc. 1982, 104, 300 – 302.
[11] For a review on cobaltate complexes, see: T. Kauffmann, Angew.
Chem. 1996, 108, 401 – 418; Angew. Chem. Int. Ed. Engl. 1996,
35, 386 – 403.
[12] For examples of halogen–metal exchange between gem-diha-
loalkane and ate complexes followed by alkyl migration, see: A.
Inoue, J. Kondo, H. Shinokubo, K. Oshima, Chem. Eur. J. 2002,
8, 1730 – 1740, and references therein.
[13] a) F. Kakiuchi, A. Yamada, N. Chatani, S. Murai, N. Furukawa,
Y. Seki, Organometallics 1999, 18, 2033 – 2036; b) Y. Wakatsuki,
H. Yamazaki, M. Nakano, Y. Yamamoto, J. Chem. Soc. Chem.
Commun. 1991, 703 – 704; see also reference [5].
[3] a) T. Suzuki, P. Y. Lo, J. Organomet. Chem. 1990, 391, 19 – 25;
b) E. Lukevics, Z. V. Belyakova, M. G. Pomerantseva, M. G.
3490
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 3488 –3490