Allylsilanes

TBDPSO
b216
SiMe3 TBDPSO
aq HI, benzene
rt, 12 h
CO2Me
79%
217
CO 2Me
trans-1-Phenyl-4-[(E)-(prop-1-enyl)]cyclohexane (215); Typical Procedure: [21,147]
Allylsilane 86B (200 mg, 0.60 mmol) and the BF 3 ·2AcOH complex (40% BF 3 in AcOH;
0.08 mL) were stirred in CH 2Cl 2 (5 mL) at 08C for 20 min and then at 208C for 40 min. Aq
NaHCO 3 was added, the aqueous layer was extracted with hexane, the organic layers were
combined and dried (MgSO 4), and the solvent was removed under reduced pressure. The
residue was crystallized from MeOH to give alkene 215; yield: 110 mg (92%); mp 43±44 8C.
Methyl [2-Allyl-3-(tert-butyldiphenylsiloxy)cyclopentyl]acetate (217); Typical Procedure: [214]
To a stirred soln of ester 216 (450 mg, 0.89 mmol) in drybenzene (12 mL) was added 57%
aq HI (0.113 ìL), and the mixture was stirred for 12 h at rt. The organic layer was washed
with H 2O and brine, dried (Na 2SO 4), and concentrated. Preparative TLC (silica gel, pentane/
EtOAc 98:2) gave the product as a colorless thick oil; yield: 305 mg (79%).
4.4.40.56 Method 56:
Allylation of Reactive Alkyl Halides
Allylsilanes react with reactive alkyl halides such as tert-butyl chloride, benzyl bromide,
and so forth, in the presence of a Lewis acid to provide alkenes. [20,21,30] Examples include
the preparation of alkene 218 (Scheme 82). [98] Allylsilane 46 was prepared bythe method
described in Section 4.4.40.9.2.
Scheme 82 Allylation of tert-Butyl Chloride [98]
46
SiMe3
t-BuCl, TiCl4, CH2Cl2 −78 oC, 1 h
98%
An analogous reaction was used for the accurate determination of the degree of anti stereospecificityin
the S E2¢ reaction of allylsilanes, as shown in Scheme 83. [170] The Z-product
(S,Z)-219 was enantiomericallypure, but the accompanying isomer (R,E)-219 showed
some erosion of the anti stereospecificity. In this application, practically enantiopure allylsilane
(E)-9 was needed, preparation of which was based on the highlydiastereoselective
aldol reaction of â-silyl enolates and stereospecific decarboxylative elimination described
in Scheme 45 (Section 4.4.40.26).
Scheme 83 Accurate Determination of the Degree of Stereospecificity in an S E2¢ Reaction
of Allylsilanes [170]
SiMe3
(E)-9 >99.9% ee
>99.95% E
+
FOR PERSONAL USE ONLY
902 Science of Synthesis 4.4 Silicon Compounds
Cl
TiCl 4, CH 2Cl 2
−78 o C, 30 min
218
Bu t
(S,Z)-219 >98% ee
>99.95% Z
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG
+
60:40 (R,E)-219 80% ee
>99.95% E