_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main Groups IV and V
arsenic ylides, however, the reactions can take another course leading to oxiranes
(epoxides) or their rearrangement products. In this they resemble sulphonium or
oxosulphonium ylides.
These differences can be discussed in terms of the two step mechanism.
A major factor is the strength of the E=O bond, which provides the main driving
force for path (a) in the case of phosphorus.
The carbanion [Me 3 Si-CH 2 r is isoelectronic with the phosphorus ylide
+ -
Me 3 P-CH 2 • The lithium derivative of the anion Me 3 SiCH; can be prepared
by treating tetramethylsilane with butyllithium/TMEDA at - 78°C in
tetrahydrofuran.
BunLi
Me,Si-CH, ------> Me,Si-CH 2 Li
TMEDA
These a-silyl carbanions react with carbonyl compounds to give intermediates
which readily eliminate the Me 3 Si0- group either spontaneously or on treatment
with acids or bases.
Normally a mixture of cis and trans olefins results.
R Z
"c=o+ Li-<
R'/ S'M
1 e 3
(The Peterson reaction)·
The by-product, hexamethyldisiloxane, is volatile and is sometimes easier to
remove from the olefinic product than solid Ph 3 PO which is formed in the Wittig
reaction.
140