Allylsilanes
Allylsilanes
Allylsilanes
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OAc<br />
Ph<br />
(Z)-81<br />
OAc<br />
Li2[Cu(CN)(SiMe2Ph)2]<br />
THF, Et2O<br />
Li[Cu(SiMe2Ph)2]<br />
THF, 0 oC to rt<br />
93%<br />
PhMe2Si<br />
Ph<br />
(Z)-82<br />
84<br />
+<br />
18:82<br />
SiMe 2Ph<br />
Ph<br />
83<br />
SiMe2Ph<br />
The substitution reactions of allylic acetates take place with anti stereospecificity; examples<br />
include the respective preparations of silanes 85 [143] and 86 [147,148] (Scheme 38). From a<br />
mechanistic point of view, no general information is available as to how the regioselectivities<br />
of the reactions should be rationalized. However, it is probable that these reactions<br />
involve incompletelyequilibrating ó-allyl±copper(III) intermediates, presumably generated<br />
by anti oxidative cuprate addition to allyl esters, which then undergo fast reductive<br />
elimination. [144,152]<br />
Scheme 38 <strong>Allylsilanes</strong> from Allyl Esters with Stereochemical Inversion [143,147,148]<br />
Ph<br />
Li2[Cu(CN)(SiMe2Ph) 2]<br />
THF, Et2O OBz PhMe2Si 85<br />
OAc Li2[Cu(CN)(SiMe2Ph) 2]<br />
THF, Et2O Ph<br />
86<br />
SiMe2Ph<br />
(2-Cyclopentylideneethyl)dimethylphenylsilane (84); Typical Procedure: [146]<br />
1-Vinylcyclopentyl acetate (2.3 g, 15 mmol) in THF (20 mL) was added to a stirred soln of<br />
Li[Cu(SiMe 2Ph) 2] (20 mmol) in THF (140 mL) at 08C under a N 2 atmosphere; subsequently,<br />
the mixture was kept at 08C for 1 h and at rt overnight. The product was obtained byaqueous<br />
workup, extraction with pentane, column chromatography(silica gel, petroleum<br />
ether), and distillation; yield: 3.2 g (93%); bp 102±1048C/0.5 Torr.<br />
4.4.40.22.2 Variation 2:<br />
From Allyl Carbamates<br />
FOR PERSONAL USE ONLY<br />
864 Science of Synthesis 4.4 Silicon Compounds<br />
An improved synthesis of regio- and stereodefined allylsilanes from secondary allyl alcohols<br />
makes use of the corresponding carbamate derivatives. For example, the regioisomeric<br />
cis allyl carbamates give the regioisomeric allylsilanes 83 and (E)-82 (Scheme<br />
39); [142±144] note that in the alternative procedure (Scheme 37, Section 4.4.40.22.1), the corresponding<br />
acetate (Z)-81 gives a mixture of regioisomeric allylsilanes 82 and 83, with the<br />
latter predominating. The variation here involves generation of a N-bound mixed silylcuprate,<br />
formed byinitial deprotonation of the carbamates and successive exposure to<br />
copper(I) iodide and (dimethylphenylsilyl)lithium, from which the silyl group is delivered<br />
internallyto the ã-carbon. [144,152] Mechanistically, it is believed that the N-bound mixed cuprate<br />
undergoes a cyclic intramolecular oxidative addition of the ã-carbon to copper, to<br />
give a copper(III)±ó-allyl complex which undergoes reductive elimination to give syn-ã-silylated<br />
products. [144,152]<br />
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG