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