The regiochemistry of alkenylsilyl, alkenyldisilanyl and alkenylsilyloxy radical cyclizations
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The regiochemistry of intramolecular cyclization of some alkenylsilyl and alkenyldisilanyl radicals has been examined. It has been demonstrated that the silyl radicals produced by hydrogen abstraction from a pentenylsilane, a butenylsilane, a butenyldisilane, an allyldisilane, and a butenyloxysilane all cyclize in an endo-fashion. This is in complete contrast to what is known for the analogous carbon-centered radicals. No cyclic products are observed for chain lengths shorter than five atoms. Also, when alkenyldisiloxanyl and butadienyl-type silyl radicals are generated by pyrolytic cleavage of silicon-allyl bonds, endo-closure is found to be the exclusive mode of cyclization. However, alkenyldisilanyl radicals undergo disproportionation to silenes when generated by pyrolysis of allyldisilanes. The silenes, formed by what is formally a retroene elimination of propene, afford 1,3-disilacyclics via silene to silylene rearrangement;The pyrolysis of 3-butenylsilanes was found to give (alpha)-silyl radicals via carbon-allyl homolysis of the butenyl groups. A 1,2-hydrogen migration from silicon to a carbon-centered radical has been observed for dimethylsilylcarbinyl radical. The radical was produced from 3-butenyldimethylsilane and afforded the first example of this type 1,2-hydrogen shift. Intramolecular endo-cyclization has also been observed to occur for (alpha)-silyl radicals possessing an alkenyl substituent;The pyrolysis of allyloxysilanes was also examined in a search for precursors to alkenylsilyloxy radicals. From this study, allyloxy-allyldimethylsilane was found to be an excellent dimethylsilanone precursor; the initial step being oxygen-allyl cleavage to the corresponding silyloxy radical.