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