Trimethylsilyl-substituted oxirane

Upon treatment with a fluoride ion of trimethylsilyl-substituted oxiranes (144 and 146), and -aziridine (148) generate, the corresponding naked anionic species, which react with aldehydes to give the corresponding adducts (145,147, and 149) with retention of configuration (Scheme 5.37) [54]. 

Block et al.194 examined the effects of trimethylsilyl substitution on the first vertical ionization potentials by photoelectron and Penning ionization electron spectroscopy studies of a range of cyclic and noncyclic sulfides and ethers. It was shown that substitution of oxirane 218 with a trimethylsilyl substituent as in 219 lowered the ionization potential by 0.90 eV (20.8 kcal/mol), while similar substitution of dimethyl ether 220 in 221 lowered the ionization potential by 0.64 eV (14.8 kcal/mol). By comparison, the effects of silyl substitution on sulfur lone-pair ionization potentials was found to be smaller thus the ionization potential of dimethyl sulfide 222 is lowered by 0.37 eV upon trimethylsilyl substitution in 223, and the trimethylsilyl-substituted thiirane 225 is lowered by 0.59 eV relative to thiirane 224. The raising of the energy of the sulfur lone-pair electrons in the thiirane 225 is also apparent from its UV spectrum, where there is a bathochromic shift in the absorption maximum compared to the parent 224. 

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

A comparison of the configuration of the substrates and reaction products shows that the oxiranyl anions arc configurationally stable under the reaction conditions. Only one example is known in which isomerization was observed. When the ci.v-tm-butyl-substituted epoxysilane27 was metalated and quenched with 2-cyclohexenone, addition product 27 was obtained under inversion of the anionic center. Presumably the strain created in forcing the ter/-butyl and the trimethylsilyl group cis on the oxirane ring facilitates the isomerization process13. 

The Michael-type reaction of an anion (generated from compound 77) with ethyl crotonate yielded the corresponding ester 78 in 82% yield (Scheme 19). Alkylation of compound 77 with benzyl bromide afforded derivative 79 in 85% yield. The attempted reactions of the anion with oxiranes and trimethylsilyl chloride did not lead to the expected substitution products and the starting oxadiazoles were recovered in 70-80% yields <2001ARK101>. 

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