Allylsilanes

equivalent of sodium hydride, followed by alkylation with (iodomethyl)trimethylsilane.
This procedure has been used for the preparation of 49 [R 1 = Me, 2-(CH 2) 5OTHP] (Scheme
23). [101±103] Stereoselectivityof this one-pot process is low. In addition, poor yields have
been obtained with aldehydes containing a benzyloxy substituent at the 4-position. [104]
The alkenation reaction can also be carried out with preformed phosphonate 48; this
leads to improved Z selectivity. [105,106]
Scheme 23 Synthesis of 2-(Alkoxycarbonyl)allylsilanes by the Horner±Wadsworth±Emmons
Reaction [101±103]
EtO
O
P CO2Et EtO
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852 Science of Synthesis 4.4 Silicon Compounds
1. NaH, DME
2. Me3SiCH2I EtO
O
P CO2Et EtO
47 48
1. NaH
2. R1CHO SiMe3
R 1
CO2Et
SiMe 3
49 R 1 = Me 49%; (E/Z) 1:4
R 1 = 79%; (E/Z) 1:2
Ethyl 8-(Tetrahydropyran-2-yloxy)-2-[(trimethylsilyl)methyl]oct-2-enoate
[49,R 1 = 2-(CH 2) 5OTHP]; Typical Procedure: [103]
After 60% NaH in mineral oil (248 mg, 6.20 mmol) had been placed in a 50-mL two-necked
flask under argon, the dispersion was washed with dryhexane (3 ”) to remove the mineral
oil. The flask was cooled in an ice bath, and dryDME (5 mL), followed bya soln of phosphonate
47 (1.12 mL, 5.65 mmol) in DME (3 mL) were added dropwise. After the mixture
had stirred for 30 min at rt, a soln of Me 3SiCH 2I (1.0 mL, 6.7 mmol) in DME (2 mL) was
added, and the mixture was warmed to 708C for 4 h. It was cooled to 08C again, and a second
portion of NaH (203 mg, 5.08 mmol) was added. After the mixture had stirred at rt for
1.5 h, a soln of 6-(tetrahydropyran-2-yloxy)hexanal (802 mg, 3.75 mmol) in DME (5 mL) was
added at 08C, and the mixture was stirred at rt overnight. Aq NH 4Cl was added to quench
the mixture, which was extracted with Et 2O. The Et 2O soln was dried (MgSO 4) and concentrated,
and the crude product was chromatographed (silica gel, hexane/Et 2O 19:1), to afford
the product as an oil [(E/Z) 1:2]; yield: 1.09 g (79%).
4.4.40.11 Method 11:
From Carbonyl Compounds and â-Silyl Thioacetals by
Titanium(II)-Promoted Reductive Alkenation
Allylsilanes are available from carbonyl compounds by titanium(II)-promoted reductive
alkenation of â-silylthioacetals. In this protocol, the low-valency titanium species 50
(Scheme 24), generated in situ from dichlorobis(cyclopentadienyl)titanium(IV) and magnesium
in the presence of triethyl phosphite, reduces the â-silylthioacetal, and the follow-up
treatment with an aldehyde or a ketone produces the allylsilane, for example, 52
(Scheme 24). [107] In spite of moderate stereoselectivity, this method has an advantage over
the other carbonyl alkenation reaction (Section 4.4.40.9), in that ã-heteroatom-substituted
allylsilanes can be prepared from carboxylic acid derivatives, as in the preparation of
allylsilane 53. The mechanism of this reaction probablyinvolves a titanium±alkylidene
( )
5 O
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG
O