A facile synthesis of 1,1-disilylethenes via Me3MgLi-induced monomethylation of dibromodisilylmethanes

Article abstract of DOI:10.1246/cl.2001.956

Lithium trimethylmagnesate (Me₃MgLi) induces monomethylation of dibromodisilylmethanes in excellent yields. Subsequent dehydrobromination of the resulting 1-bromo-1,1-disilylethanes with DBU affords 1,1-disilylethenes in good yields.

Full text of DOI:10.1246/cl.2001.956

956
Chemistry Letters 2001
A Facile Synthesis of 1,1-Disilylethenes via Me3MgLi-Induced
Monomethylation of Dibromodisilylmethanes
Atsushi Inoue, Junichi Kondo, Hiroshi Shinokubo, and Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501
(Received June 12, 2001; CL-010551)
Lithium trimethylmagnesate (Me3MgLi) induces
monomethylation of dibromodisilylmethanes in excellent
yields. Subsequent dehydrobromination of the resulting 1-
bromo-1,1-disilylethanes with DBU affords 1,1-disilylethenes
in good yields.
The vinylsilane chemistry has been well-established
because of its great importance in organic synthesis, and a num-
ber of preparative methods and widespread application of vinyl-
silanes have been reported.1 In contrast to vinylsilanes, howev-
er, the utility of 1,1-disilylalkenes remains still unexplored,
although one can expect that 1,1-disilylalkenes possess similar
versatility to vinylsilanes.2,3 1,1-Bis(trimethylsilyl)ethene is
easily prepared by the reaction of tris(trimethylsilyl)methyllithi-
um and paraformaldehyde.2a,b However, this method lacks gen-
erality due to the difficulty in the preparation of tris(trialkylsi-
lyl)methyllithium, and development of alternative routes is
hence required. We have undertaken a new preparative route to
1,1-disilylethenes as outlined in eq 1. We have chosen dibro-
modisilylmethanes, which can be easily prepared from com-
mercially available materials in one step, as starting substrates
(vide infra). The introduction of the methyl group and the sub-
sequent dehydrobromination reaction with a base would lead to
1,1-disilylethenes.
Metal–halogen exchange of one of the two bromines and
the subsequent addition of methyl iodide did not give satisfacto-
ry results presumably because of the steric hinderance of silyl
groups. During our study on the reaction of organomagnesium
ate complexes with gem-dibromo compounds,4 we have found
that monomethylation of dibromomethylsilanes (R3SiCHBr2)5a
occurred by the action of trimethylmagnesate reagent
(Me3MgLi).6 This procedure proved to be applicable for
monomethylation of dibromodisilylmethanes.
Starting gem-dibromo compounds are prepared by several
procedures (methods A, B, C, and D) shown in Scheme 1.5 To
a THF solution of lithium trimethylmagnesate (Me3MgLi), pre-
pared by mixing methylmagnesium bromide and methyllithium
in a 1:2 ratio, was added gem-dibromo compounds 1 at –78 °C
and the mixture was stirred for 0.5 h. Aqueous workup fol-
lowed by purification afforded monomethylation products 2 in
good to excellent yields (Table 1).7
Substrates such as dibromomethylsilane (entry 1), 1,1-
dibromoethylsilane (entry 2) and dibromodisilylmethanes
(entries 3–7) can be converted into the corresponding
monomethylated compounds in good yields regardless of the
bulkiness of silyl substituents. The use of other reagents such
as nBu3MgLi did not give the monoalkylated products.4
Although the reaction pathway of this methylation is not clear
at present, we assume the following mechanism involving the
formation of the ate-type carbenoid species 3 via bromine–mag-
nesium exchange reaction8 (Scheme 2). The 1,2-migration of
one of the methyl groups on magnesium provides 4 with con-
comitant elimination of the bromide anion. One of methyl
groups in Me3MgLi was transferred to 4 to form the ate com-
plex 5, which is more reactive than Me3MgLi for the
bromine–magnesium exchange reaction. The complex 5
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
957
abstracts the bromine atom from 1 to yield the monobromo
compound 2 and the carbenoid 3.
Dunoguès, R. Smithers, Org. React. (N. Y.), 39, 57 (1989).
For preparations of 1,1-disilylalkenes, see: a) B. -Th.
Gröbel and D. Seebach, Chem. Ber., 110, 852 (1977). b) G.
Fritz and J. Grobe, Z. Anorg. Allg. Chem., 309, 77 (1961).
c) I. Fleming and C. D. Floyd, J. Chem. Soc., Perkin Trans.
1, 1981, 969. d) I. Fleming and U. Ghosh, J. Chem. Soc.,
Perkin Trans. 1, 1994, 257. e) K. Narasaka, N. Saito, Y.
Hayashi, and H. Ichida, Chem. Lett., 1990, 1411. f) D. M.
Hodgson, P. J. Comina, and M. G. B. Drew, J. Chem. Soc.,
Perkin Trans. 1, 1997, 2279. g) E. Negishi, L. D.
Boardman, H. Sawada, V. Bagheri, A. T. Stoll, J. M. Tour,
and C. L. Rand, J. Am. Chem. Soc., 110, 5383 (1988). h) C.
Flann, T. C. Malone, and L. E. Overman, J. Am. Chem.
Soc., 109, 6097 (1987).
2
3
For reaction of 1,1-disilylalkenes, see: a) B. -Th. Gröbel
and D. Seebach, Angew. Chem., 86, 102 (1974); Angew.
Chem., Int. Ed. Engl., 13, 83 (1974). b) D. Seebach, R.
Bürstinghaus, B. -Th. Gröbel, and M. Kolb, Liebigs Ann.
Chem., 1977, 830. c) B. -Th. Gröbel and D. Seebach,
Chem. Ber., 110, 867 (1977). d) M. Kira, T. Hino, Y.
Kubota, N. Matsuyama, and H. Sakurai, Tetrahedron Lett.,
29, 6939 (1988). See also ref. 2f–h.
We then investigated the dehydrobromination reaction of
2. After several attempts, we found that treatment of the
methylation products 2 with 2.0 equiv of DBU (1,8-diazabicy-
clo[5.4.0]undec-7-ene) in DMF at 90 °C for 8 h provided 1,1-
disilylethenes 6.9 Table 2 summarizes the results. In each case,
the desired product is obtained in good yield. The reaction of 2
with nBuLi or iPrMgBr also provided 6 in moderate yields. The
use of BuOK as a base caused decomposition of substrates 2i
to yield methyldiphenylsilanol (Ph2MeSiOH) and 1,3-dimethyl-
1,1,3,3-tetraphenyldisiloxane (Ph2MeSi OSiMePh2).
4
5
J. Kondo, A. Inoue, H. Shinokubo, and K. Oshima, Angew.
Chem., 113, 2146 (2001); Angew. Chem. Int. Ed., 40, 2085
(2001).
a) C. Bacquet, D. Masure, and J. F. Normant, Bull. Soc.
Chim. Fr., 1975, 1797. b) K. Yoon and D. Y. Son, J.
Organomet. Chem., 545–546, 185 (1997). c) C. Eaborn, W.
Clegg, P. B. Hitchcock, M. Hopman, K. Izod, P. N.
O’Shaughnessy, and J. D. Smith, Organometallics, 16,
4728 (1997).
t
6
7
a) G. Wittig, F. J. Meyer, and G. Lange, Liebigs Ann.
Chem., 571, 167 (1951). b) T. Greiser, J. Kopf, D.
Thoennes, and E. Weiss, Chem. Ber., 114, 209 (1981). c)
E. C. Ashby, L. -C. Chao, and J. Laemmle, J. Org. Chem.,
39, 3258 (1974).
To a solution of 1i (5.66 g, 10 mmol) in THF (50 mL) was
added a solution of lithium trimethylmagnesate, prepared
by mixing methylmagnesium bromide (10.8 mL, 0.93 M
solution in THF, 10 mmol) and methyllithium (17.5 mL,
1.14 M solution in Et2O, 20 mmol) in THF (15 mL), at –78
°C under argon atmosphere. After stirring for 0.5 h at –78
°C, the mixture was poured into 1 M HCl carefully and
extracted with AcOEt. The combined organic layers were
dried over anhydrous Na2SO4 and concentrated in vacuo.
Washing the residual colorless solid with pentane provided
1-bromo-1,1-bis(methyldiphenylsilyl)ethane (2i, 4.51 g,
9.0 mmol) in 90% yield.
In summary, we have developed a new efficient route to
1,1-disilylethenes via Me3MgLi-induced monomethylation of
dibromodisilylmethanes and the subsequent dehydrobromina-
tion reaction. This new facile method provided us with a syn-
thetic route to various 1,1-disilylethenes.
This work was supported by a Grant-in-Aid for Scientific
Research (A) (No. 12305058) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. A.I. is grate-
ful to Research Fellowships of the Japan Society for the
Promotion of Science for Young Scientists.
8
9
a) K. Kitagawa, A. Inoue, H. Shinokubo, and K. Oshima,
Angew. Chem., 112, 2594 (2000); Angew. Chem. Int. Ed.,
39, 2481 (2000). b) A. Inoue, K. Kitagawa, H. Shinokubo,
and K. Oshima, J. Org. Chem., 66, 4333 (2001).
A solution of 2i (4.51 g, 9.0 mmol) and DBU (2.7 mL, 18.0
mmol) in DMF (40 mL) was stirred for 8 h at 90 °C. The
mixture was poured into 1 M HCl carefully and extracted
with AcOEt. The combined organic layers were dried over
anhydrous Na2SO4 and concentrated in vacuo. Washing
the residual colorless solid with cold hexane provided 1,1-
bis(methyldiphenylsilyl)ethene (6i, 3.37 g, 8.0 mmol) in
89% yield.
Dedicated to Prof. Hideki Sakurai on the occasion of his
70th birthday.
References and Notes
1
a) E. W. Colvin, “Silicon Reagents in Organic Synthesis,”
Academic Press, London (1988), p 7. b) I. Fleming, J.