Allylsilanes

Et 2O (37 mL, 20 mmol) followed byMe 2PhSiH (136 mg, 1.0 mmol). The mixture was stirred
at rt for 69 h and then hydrolyzed with 10% aq HCl. The organic layer and the Et 2O extracts
from the aqueous layer were combined, washed with sat. aq NaHCO 3 and H 2O, and
dried (MgSO 4). Removal of the solvent under reduced pressure followed bybulb-to-bulb
distillation of the residue gave 15 (R 1 = Ph; R 2 = Me) containing 2% Z-isomer; yield:
137 mg (72%).
4.4.40.3 Method 3:
From Allyl Chlorides by Zinc-Mediated Silylation
Allylsilanes have been prepared by the treatment of alkyltrichlorosilanes with an equimolar
amount of allyl chloride in the presence of zinc powder in 1,3-dimethyl-2-imidazolidinone
(DMI) as solvent, for example, in the formation of allylchlorosilanes 16 (Scheme
10). [59] Use of at least 2 equivalents of 1,3-dimethyl-2-imidazolidinone is recommended,
otherwise the yields are lower. In some cases, such as allylation of trichlorohexylsilane
or trichlorophenylsilane, no more than 2 equivalents of 1,3-dimethyl-2-imidazolidinone
should be used, as excess 1,3-dimethyl-2-imidazolidinone is rather difficult to separate
from the product. The zinc-mediated allylation reaction can also be applied to dichlorodimethylsilane
and chlorotrimethylsilane. [59] A notable feature of this protocol is the selective
formation of dichloro(methyl)(1-methylallyl)silane 16 (R 1 =R 2 = Me) from (E)-1-chlorobut-2-ene,
since silylation of but-2-enyl Grignard reagents invariably gives a regio- and
stereoisomeric mixture of allylsilanes [53] (see Section 4.4.40.1).
Scheme 10 From Allyl Chlorides by Zinc-Mediated Silylation [59]
R 1
Cl
FOR PERSONAL USE ONLY
842 Science of Synthesis 4.4 Silicon Compounds
R 2 SiCl 3, Zn, DMI
R1 = H; R2 = Me 69%
R1 = H; R2 = iPr 60%
R1 = R2 = Me 69%
R 1
16
SiR 2 Cl 2
Allyl(dichloro)methylsilane (16, R 1 =H;R 2 = Me); Typical Procedure: [59]
A mixture of DMI (100 mL), Zn powder (13.0 g, 0.2 mol), and MeSiCl 3 (29.4 g, 0.2 mol) was
placed in a 200-mL three-necked flask. The mixture was subjected to a N 2 purge and heated
to 70 8C. Allyl chloride (15.0 g, 0.2 mol) was added dropwise to the mixture over 5 min.
After the addition, Et 2O was added and the precipitated salts were filtered off. The mixture
was then heated for another 1 h, and the product was isolated bydistillation under
reduced pressure; yield: 21.4 g (69%).
4.4.40.4 Method 4:
From Allyl Halides by Electroreductive Synthesis
Allyl halides undergo electrochemical reduction in the presence of a silylating agent in a
solution of tetraalkylammonium salt in dimethylformamide to give allylsilanes, for
example, the preparation of cyclohex-2-enyltrimethylsilane (14, n = 2; Scheme 11). [60]
The regiochemical outcome of these reactions depends both on the steric properties of
the silylating agent as well as the electronic properties of the allyl moiety. [61] This method
tolerates the presence of some reactive groups in the substrate, as in, for example, the
selective formation of (4-bromocinnamyl)trimethylsilane (17). [61] Additionally, it also allows
the synthesis of certain functionalized (difluoroallyl)silanes, for example, silane 19
from the corresponding chlorodifluoromethyl-substituted enol ether 18; note that this reaction
succeeds in spite of the low labilityof a chlorine atom in a chlorodifluoromethyl
moiety. [62] In addition to halides, an acetoxygroup also serves as a leaving group for this
reaction, [61] but onlyin the presence of tetrakis(triphenylphosphine)palladium(0) as catalyst.
[63]
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG