Allylsilanes
Allylsilanes
Allylsilanes
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1. Me3Si SiMe3 2. Pd(dba)2<br />
THF, rt<br />
92%<br />
OCOCF3 69<br />
70<br />
OCOCF 3<br />
1. Me3Si SiMe3 2. Pd(dba)2<br />
THF, rt<br />
89%<br />
Allylic acetates can replace the corresponding trifluoroacetates in these reactions only<br />
when an added chloride salt, for example, lithium chloride, is present in the reaction medium.<br />
However, the reactions proceed onlyat higher temperatures and are accompanied<br />
byreduced yields. [136]<br />
Configurational inversion is normally observed with cycloalkenyl trifluoroacetates.<br />
For example, methyl cis-5-trifluoroacetoxycyclohex-3-ene carboxylate (71) gives trans-72<br />
in the tetrahydrofuran/acetonitrile mixed solvent. Note that this reaction is essentially<br />
solvent dependent, since use of pure tetrahydrofuran leads to a 1:1 mixture of trans- and<br />
cis-72 (Scheme 33). [136]<br />
46<br />
SiMe 3<br />
Scheme 33 An Allylsilane from an Allyltrifluoroacetate with Stereochemical<br />
Inversion or Randomization [136]<br />
CO 2Me<br />
71<br />
OCOCF3<br />
FOR PERSONAL USE ONLY<br />
860 Science of Synthesis 4.4 Silicon Compounds<br />
Pd(dba)2 (cat.)<br />
Me3Si SiMe3<br />
THF, MeCN<br />
89%<br />
Pd(dba)2 (cat.)<br />
Me3Si SiMe3 THF<br />
85%<br />
CO 2Me<br />
trans-72<br />
CO 2Me<br />
trans-72<br />
SiMe3<br />
SiMe3<br />
+<br />
1:1<br />
CO 2Me<br />
(2-Cyclohexylideneethyl)trimethylsilane (46); Typical Procedure: [136]<br />
A 20-mL flask was charged with Pd(dba) 2 (17 mg, 0.030 mmol) and THF (5.5 mL) under an<br />
argon atmosphere. The mixture was stirred, and the palladium complex dissolved, affording<br />
a deep-purple soln. Then Me 3SiSiMe 3 (293 mg, 2.0 mmol) and ester 69 (222 mg,<br />
1.0 mmol) were added in this order. The mixture turned pale yellow, and the reaction<br />
was allowed to proceed at rt for 12 h. The mixture was then diluted with Et 2O (20 mL),<br />
washed with sat. aq NaHCO 3 (20 mL), dried (MgSO 4), and concentrated. Kugelrohr distillation<br />
afforded silane 46; yield: 168 mg (92%); bp 80 8C/1 Torr.<br />
4.4.40.19.2 Variation 2:<br />
Using Samarium(II) Iodide and Chlorotrimethylsilane<br />
An alternative procedure for the preparation of terminallysubstituted allylsilanes involves<br />
palladium-catalyzed reductive metalation of allylic phosphates with the samarium(II)<br />
iodide±hexamethylphosphoric triamide complex. Thus, the E-allylsilane (E)-74 is<br />
available in good yield from (E)-73 (Scheme 34). [137] In general, low temperature is recom-<br />
cis-72<br />
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG<br />
SiMe3