Allylsilanes

pressure. Flash chromatography(silica gel, pentane/Et 2O 20:1) gave the product as a colorless
oil; yield: 5.33 g (97%); bp 134±1358C/0.22 Torr.
4.4.40.1.2 Variation 2:
Silylation of an In Situ Generated Grignard Reagent
The addition of an allyl halide to a suspension of magnesium metal and chlorotrimethylsilane
in tetrahydrofuran yields the corresponding allylsilane, for example, in the preparation
of 13 and 14 (Scheme 8). [56,57] This reaction is believed to involve the in situ generation
of the allylmagnesium halide, which reacts with the trialkylhalosilane by displacement.
Scheme 8 Silylation of an In Situ Generated Grignard Reagent [56,57]
Cl
( )n
X
TMSCl, Mg
Cl Me3Si SiMe3
TMSCl, Mg, THF
X = Cl; n = 1 94%
X = Br; n = 2 54%
FOR PERSONAL USE ONLY
4.4.40 Allylsilanes 841
Cyclopent-2-enyltrimethylsilane (14, n = 1); Typical Procedure: [56]
A mixture of TMSCl (55.33 g, 0.51 mol), Mg (18.5 g, 0.77 mol), and THF (250 mL) was cooled
to 58C in an ice bath. An addition funnel was charged with a soln of 3-chlorocyclopentene
(52.27 g, 0.51 mol) in THF (500 mL), and cooled with a dry-ice jacket. The resulting cooled
soln was added dropwise to the stirred mixture over several hours. The mixture was allowed
to warm to rt, and then stirred overnight. It was washed with H 2O (5 ” 100 mL),
and the aqueous washings were back-extracted with pentane (50 mL). The combined organic
phases were washed with brine (100 mL), dried, and concentrated under reduced
pressure; yield: 67.1 g (94%); bp 140±1468C/760 Torr.
( ) n
14
13
SiMe 3
4.4.40.2 Method 2:
From Allylmagnesium Halides by Transition-Metal-Catalyzed Coupling
with Hydrosilanes
E-But-2-enylsilanes are available by nickel-catalyzed coupling of but-2-enylmagnesium
bromide with hydrosilanes. The most effective catalyst for this reaction is [1,1¢-bis(diphenylphosphino)ferrocene]dichloronickel(II)
[NiCl 2(dppf)], which is used in the preparation
of 15 (Scheme 9). [1,58] Of note is the requirement of a large excess of the Grignard reagent
for the yield to be good and for the regio- and stereoselectivity to be high. For some
simple allylsilanes, such as allyltris(4-methoxyphenyl)silane, the yield of product by this
protocol is higher than bythe Grignard method. [55]
Scheme 9 Preparation of E-But-2-enylsilanes [1,58]
R 1 R 2 2SiH
MgBr SiR
15
1R2 NiCl2(dppf) (cat.)
R
2
1 = Me; R2 = Ph 85%; (E/Z) 98:2
R1 = Ph; R2 = Me 72%; (E/Z) 98:2
[(E)-But-2-enyl]dimethylphenylsilane (15,R 1 = Ph; R 2 = Me); Typical Procedure: [1,58]
[NiCl 2(dppf)] (13.6 mg, 0.02 mmol) was placed in a two-necked flask equipped with a magnetic
stirring bar and a three-waystopcock. After evacuation, the flask was filled with N 2
and was cooled to 08C. To it was added a soln of 0.54 M but-2-enylmagnesium bromide in
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG
for references see p 920