Allylsilanes
Allylsilanes
Allylsilanes
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
through a double-ended needle. The color of the ylide disappeared instantly. The mixture<br />
was stirred for another 1±1.5 h, poured onto crushed ice, and extracted with pentane<br />
(4 ” 25 mL). The organic phase was washed with H 2O (25 mL) and brine (25 mL), dried<br />
(MgSO 4), and filtered, and the solvent was removed in vacuo (ca. 208C, 15 Torr). The residue<br />
was purified byflash chromatography(silica gel, Et 2O/pentane 1:49) to give the pure<br />
allylsilane. [In the case of relatively unstable and highly volatile á-silyl aldehydes, the corresponding<br />
á-silyl SAMP (or RAMP) hydrazone was cleaved by ozonolysis in dry pentane at<br />
±78 8C and the mixture was used directlyfor the Wittig alkenation after the solvent was<br />
removed under reduced pressure (15 Torr) at ca. 0 8C.]<br />
(S)-tert-Butyl(1-hexylallyl)dimethylsilane [(S)-40, R 1 = H]; yield: 80%.<br />
4.4.40.9 Method 9:<br />
From Carbonyl Compounds and Trialkyl[2-(trimethylsilyl)ethylidene]phosphoranes<br />
by a Wittig Reaction<br />
Terminally substituted allylsilanes have been prepared by Wittig alkenation of carbonyl<br />
compounds with triphenyl[2-(trimethylsilyl)ethylidene]phosphorane (41, Ar 1 = Ph), generated<br />
bydeprotonation of its phosphonium salt (Scheme 20). [96±98] The limitations of the<br />
method are the lack of E/Z selectivity, the difficulty in extending this protocol to the preparation<br />
of allylsilanes with a substituent on C1, as there is no easy way to make the requisite<br />
ylides, and the formation of byproduct 42 bysilyl-group rearrangement when the<br />
reaction is run under more or less diluted conditions. [1] Replacement of alkylidenephosphorane<br />
41 (Ar 1 = Ph) bythe stericallymore congested 41 (Ar 1 = 2-Tol) in these reactions<br />
improves stereoselectivityin favor of the Z-isomer, as well as yields, by suppressing the<br />
formation of ether 42. [99] If ylide 41 (Ar 1 = Ph) is generated with sodium silazide, Z selectivityis<br />
also improved, albeit with reduced yields. [100]<br />
Scheme 20 <strong>Allylsilanes</strong> by a Wittig Reaction [96±99]<br />
R 1<br />
H<br />
O<br />
+<br />
SiMe3<br />
Ar1 1<br />
3P 2<br />
41 Ar1 = Ph, 2-Tol<br />
R 1 SiMe 3<br />
H<br />
+<br />
R 1<br />
OSiMe3<br />
Two variations of this method have been reported, particularlywith ylide 41 (Ar 1 = Ph) as<br />
reagent; in one reaction the ylide is generated from a preformed phosphonium salt, while<br />
in the other the phosphonium salt is generated in situ. It works particularlywell for aldehydes<br />
and some reactive ketones, such as cyclohexanone and acetophenone, but gives<br />
poor yields with cyclopentanone and when there is alkyl substitution on C2 of the ylide. [1]<br />
4.4.40.9.1 Variation 1:<br />
Via a Preformed â-Silylated Phosphonium Salt<br />
<strong>Allylsilanes</strong> are accessible by alkenation of carbonyl compounds with triphenyl(â-silylalkylidene)phosphorane<br />
41 (Ar 1 = Ph), prepared in two steps: reaction of methylidenetriphenylphosphorane<br />
with (iodomethyl)trimethylsilane, followed by deprotonation of the<br />
resulting crystalline salt 43. This method is illustrated in the preparation of 44 (Scheme<br />
21). [96,97]<br />
Scheme 21 <strong>Allylsilanes</strong> by Wittig Reaction with a Preformed â-Silylated Phosphonium<br />
Salt [96,97]<br />
+<br />
Ph3P 43<br />
SiMe 3<br />
I −<br />
FOR PERSONAL USE ONLY<br />
850 Science of Synthesis 4.4 Silicon Compounds<br />
PhLi<br />
Ph 3P<br />
SiMe 3<br />
41 Ar 1 = Ph 44 (E/Z) 1.7:1<br />
42<br />
Me(CH2)5CHO<br />
( )<br />
71% 5<br />
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG<br />
SiMe 3