Allylsilanes

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