Allylsilanes

Scheme 18 Allylsilanes from Vinyl Trifluoromethanesulfonates [89]
EtO2C
37
OTf
OTf
Al(CH2SiMe3)3
Pd(PPh3)4 (cat.), 23 oC, 2 h
81%
Al(CH2SiMe3)3
Pd(PPh3)4 (cat.)
EtO2C EtO 2C
39
EtO2C
{[4-(Ethoxycarbonyl)cyclohex-1-enyl]methyl}trimethylsilane (38); Typical Procedure: [89]
To a magneticallystirred suspension of AlCl 3 (1.13 g, 8.5 mmol) in dry1,2-dichloroethane
(50 mL) under argon was added, over 10 min, 1 M Me 3SiCH 2Li in pentane (26 mL,
26 mmol). The resulting mixture was stirred at rt for 30 min, and then treated rapidly, by
cannula transfer, with a soln of trifluoromethanesulfonate 37 and the catalyst Pd(PPh 3) 4,
which was prepared in a separate flask as follows: A soln of Pd(OAc) 2 (130 mg, 0.58 mmol)
and Ph 3P (610 mg, 2.33 mmol) in drybenzene (25 mL) was treated under argon with 2.5 M
BuLi in hexane (0.5 mL, 1.25 mmol). Trifluoromethanesulfonate 37 (1.8 g, 6 mmol) was
added 5 min later, either as a neat liquid or dissolved in drybenzene (10 mL). This soln
was immediatelytransferred bycannula as described above, and the resulting mixture
was stirred for 2 h at 238C. The workup consisted of dilution with CH 2Cl 2 and washing
with 0.2 M aq HCl, H 2O, and brine, followed bydrying (MgSO 4). Purification byflash chromatography(silica
gel, CH 2Cl 2/hexane) gave the pure product as a colorless liquid; yield:
1.17 g (81%).
SiMe3
4.4.40.8 Method 8:
From á-Silyl Aldehydes and Alkylidinetriphenylphosphoranes
by a Wittig Reaction
Optically active 1-substituted or 1,3-disubstituted prop-2-enylsilanes, for example, allylsilanes
(S)-40 (Scheme 19), can be synthesized from the homochiral á-silyl aldehydes by
Wittig alkenation with alkylidenetriphenylphosphoranes. [94] Enantiomericallyenriched
á-silyl aldehydes are available from simple aldehydes and silylating agents employing the
(S)-(±)- or (R)-(+)-1-amino-2-(methoxymethyl)pyrrolidine (SAMP or RAMP) hydrazone method.
[95] For generation of alkylidenetriphenylphosphoranes, use of butyllithium is recommended,
since sodium amide leads to substantial racemization of silyl aldehydes.
Scheme 19 Homochiral Allylsilanes from a Chiral á-Silyl Aldehyde [94]
O
H
SiMe2Bu t
( ) 5
FOR PERSONAL USE ONLY
4.4.40 Allylsilanes 849
R1CH2PPh3 X −
+
BuLi, THF
R1 = H 80%
R1 = Me 80%; (E/Z) 97:3
R 1
38
H ( ) 5
SiMe2Bu t
(S)-40 R1 = H > 98% ee
R1 = Me 98% ee
Enantiomerically Enriched Allylsilanes from Chiral á-Silyl Aldehydes;
General Procedure: [94]
At ±78 8C, under argon, 1.6 M BuLi in hexane (6.2 mL, 10 mmol) was added over ca. 20 min
to a well-stirred suspension of the alkyltriphenylphosphonium halide (16 mmol) in dry
THF (50 mL). The mixture was allowed to warm to rt (ca. 208C) over 2 h. In a separate flask,
the chiral á-silyl aldehyde (10 mmol) was dissolved, with stirring, in dry THF (20 mL) and
was cooled to ±788C. The pregenerated ylide was then added slowly to the aldehyde
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG
−
>−
SiMe 3
for references see p 920