Allylsilanes

Scheme 12 Allylsilanes from Lithiated Alkenes [64±67]
H
OH
( ) n
H
23
1. BuLi, TMEDA, hexane, rt
2. TMSCl, −78 oC, 4 h
1. BuLi, TMEDA
2. TMSCl
58%
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844 Science of Synthesis 4.4 Silicon Compounds
1. BuLi, TMEDA
2. TMSCl
3. MeOH, H2O
60%
22
SiMe3
H
20
SiMe3
OH
( )n
H
24
+
40:60
SiMe3
SiMe3 21
(Cyclobut-1-enylmethyl)trimethylsilane (20) and
Trimethyl(2-methylenecyclobutyl)silane (21): [64]
Methylenecyclobutane (1.36 mL, 14.7 mmol) was added to a stirred soln of TMEDA (0.86 g,
1.2 mL, 7.4 mmol) and BuLi (7.4 mmol) in hexane (3 mL) under argon. After the soln had
stirred at rt overnight, hexane (2±3 mL) was added.
The alkenyllithium (118 mmol), prepared as above, was cooled to ±788C, and TMSCl
(distilled from Bu 3N; 21.6 mL, 146 mmol) was added rapidly. The soln was stirred for 4 h,
then poured into H 2O, and extracted with Et 2O. The Et 2O layer was washed with H 2O (2 ”),
dried (MgSO 4), filtered, and concentrated in vacuo to yield a pale green oil, which on distillation
(bp 1308C/760 Torr) gave 20 and 21 (20/21 2:3); total yield: 12.68 g (77%, based on
alkenyllithium).
4.4.40.5.2 Variation 2:
From Alkenylpotassium Compounds
Z-Allylsilanes can be prepared by the deprotonation of an alk-1-ene or alk-2-ene with
Schlosser s base, and subsequent treatment of the allylmetal intermediate with a halosilane.
[71±77] Allylsilanes 12 (R 1 = t-Bu; R 2 = Me) and (Z)- and (E)-25 have been prepared bythis
protocol (Scheme 13). [72,73] Of note is complete or predominant retention of alkene configuration
in products (Z)- and (E)-25. Bytrapping of the allylpotassium intermediate with a
halosilane immediatelyafter its formation, it was shown that (Z)-but-2-ene leads to an
endo-alkyl-substituted allylpotassium intermediate, while the exo-alkyl intermediate results
from (E)-but-2-ene. [73±75] However, the allylpotassium intermediate may undergo torsional
isomerization under thermal or catalytic conditions, so that the thermodynamicallystronglyfavored
endo intermediate forms, no matter whether an alk-1-ene, a Z-orE-alk-
2-ene, or even a mixture of such isomers are used as starting materials, and silylation then
results in Z-allylsilanes. [71,72,74,76] In this way, a range of terminal allylsilanes, for example,
allylsilane 26, has been prepared with more than 95% Z selectivity. [72] The presence of a
free hydroxy group on the carbon backbone is compatible with this procedure; in such a
case, 2 equivalents of the superbasic reagent are needed, as well as pent-1-ene as catalyst,
to effect stereohomogenization of the allylpotassium intermediate; an example of this is
the preparation of silane 27. [76] The reason for endo selectivityis not clear at present. [74] A
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG