Allylsilanes

Scheme 11 From Allylic Halides by Electrochemical Synthesis [60±62]
Br
Cl
EtO
F
18
F
Ph
TMSCl, 2 e
Br
67%
SiMe3 Cl
TMSCl, 2 e
94%
TMSCl, 2 e
49%
Me3Si
EtO
19
14 n = 2
(2-Ethoxy-1,1-difluoro-3-phenylallyl)trimethylsilane (19); Typical Procedure: [62]
The reaction was conducted in a cylindrical undivided cell, fitted with a Mg rod as anode
and a Ni foam cathode under argon. Enol ether 18 (3.95 g, 17 mmol) and freshlydistilled
TMSCl (8.7 mL, 68 mmol) were added to a soln of DMF (50 mL) containing TBAB (100 mg,
0.3 mmol). The electrolysis was performed under a constant current (i = 0.06 A) until no
more starting material was left. After extraction with Et 2O, the organic phases were
washed with 1 M HCl and brine and dried (MgSO 4). Solvents were removed under reduced
pressure, and after filtration (silica gel, pentane/Et 2O 95:5) of the residue, pure silane 19
was obtained as a colorless liquid; yield: 4.36 g (94%).
4.4.40.5 Method 5:
By Silylation of Metalated Alkenes
Allylsilanes are available bydirect metalation of alkenes, followed bysilylation with trialkylhalosilanes.
Two procedures are followed: in one butyllithium is used, often in the
presence of a chelating amine, for example, N,N,N¢,N¢-tetramethylethylenediamine, to
generate allyllithium species, while, in the other, Schlosser s base (butyllithium and potassium
tert-butoxide) is used to generate allylpotassium intermediates for subsequent
silylation.
4.4.40.5.1 Variation 1:
From Lithiated Alkenes
FOR PERSONAL USE ONLY
4.4.40 Allylsilanes 843
Silylation of lithiated alkenes generated by deprotonation of alkenes with butyllithium in
the presence of N,N,N¢,N¢-tetramethylethylenediamine gives allylsilanes. The regioselectivityof
this reaction depends on the deprotonation site of the alkene and the selectivity
of attack of the allyllithium species on the halosilane. For methylenecyclobutane, a mixture
of allylsilanes, for example, 20 and 21, forms, with the latter slightlyfavored
(Scheme 12). [64] This method is suited for the preparation of a varietyof terpenic allylsilanes,
for example, silane 22. [65]
Alkenes containing a hydroxy group are also amenable to this procedure. [66±70] For example,
allylsilane 24 (n = 1) was prepared from alcohol 23 (n = 1) where a large excess of
butyllithium was needed for success; here alkoxide coordination at allyllithium appears
to be essential for facile C-silylation, which does not occur when n = 2 or 3. [66,67]
F
F
Ph
Br
17
SiMe 3
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG
for references see p 920