Studies on the use of alpha-silyl substituted carbanions in organic synthesis.
The stereochemical outcome of the Peterson olefination
reaction was studied using o(-trimethylsilylbenzyl carbanion
and a series of N-aliphatic/aromatic azomethines.
All reactions led stereoselectively to trans-olefins in
moderate to low yields. The Peterson intermediate was
isolated when neopentylideneaniline was used at room
temperature. This intermediate, on treatment with base
or acid, gives the corresponding trans-alkene and trimethylsilylphenylmethane,
thus providing unambiguous
evidence for the reversibility of the first step in the
Silyl-substituted aziridines were synthesised using thermolytic
and photolytic reactions between - chloromethyltrimethylsilyl
carbanion and N-benzylidenepropylamine or
N-benzylideneaniline. The former system gave cis-l-propyl-
2-trimethylsilyl-3-phenylaziridine in moderate yield and
the latter gave trans- and cis-1-phenyl-2-trimethylsilyl-
3-phenylaziridine (1: 1) in good yield.
These silylaziridines were shown to undergo ring-preserving
reactions on alkylation, protonation or desilylation.
Further, on reacting with nucleophiles such as hydrogen
halides and trimethylsilyl reagents, ring-opened products
are formed. The silylaziridines react analogously to
silyl epoxides with hydrogen halides; these involve an
Sy2 mechanism and lead to the corresponding ring-opened
adduct which reacts further to give the protonated derivative.
Silylaziridines react with trimethylsilyl iodide to give
the trans-ß-silylenamine via an SN1-type process and do
not react with trimethylsilyl- chloride, bromide or pseudohalides.
Trans- and cis-l-phenyl-2-trimethylsilyl epoxides react
with trimethylsilyl halides via an S2 mechanism to give
F-haloolefins whereas 1,1-diphenyl-2-trimethylsilyl
epoxide reacts with trimethylsilyl halides or pseudohalides
to give 1,1-diphenyl-2-trimethylsilylenol ether in
quantitative yields via an SN1 process. However, 2-tri"-
ýmethylsilyl-l-oxa&piro[2,5]octane gives the f-pseudohaloolefin
via an SN2 mechanism on reaction with trimethylsilyl
pseudohalides; tran -/ci-l-phenyl-2-trimethylsilylepoxides
give the corresponding olefin with trimethysilyl
azide and the corresponding adduct with trimethylsilyl
The impure isomers, obtained from the reaction between
(+)-1,1'-binaphthalene-2,2'-diyl hydrogen phosphate were
separated by fractional crystallisation and shown to be