Studies on the use of alpha-silyl substituted carbanions in organic synthesis

Soobramanien, Marie-Claire Chantal (1989). Studies on the use of alpha-silyl substituted carbanions in organic synthesis. PhD thesis The Open University.



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 olefination process.

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 cyanide. The impure isomers, obtained from the reaction between c-l-propyl-2-trimethylsilyl-3-phenylaziridine and (+)-1,1'-binaphthalene-2,2'-diyl hydrogen phosphate were separated by fractional crystallisation and shown to be optically active.

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