SILAHETEROCYCLES .26. FACILE SYNTHESIS OF SILICON DICHLORO SUBSTITUTED 3-VINYL-1-SILACYCLOBUTANES FROM SILENE BUTADIENE [2- MODEL COMPOUNDSFOR VINYLSILACYCLOBUTANE -] SILACYCLOHEXENE REARRANGEMENTS(2] CYCLOADDITION REACTIONS )

The reaction between in situ formed 1,1-dichloro-2-neopentylsilene, Cl 2Si=CHCH2-t-Bu (1), and 1,3-butadienes leads almost solely to the form ation of monosilacyclobutanes (2, 5, 6, 10, 14, 15, 16). NMR spectrosc opic investigations of the products prove the periselective addition o f the silene unit to the dienes. The reaction does, however, not proce ed stereospecifically: the neopentyl and vinyl groups on the carbon sk eleton of the silacyclobutane give rise to E/Z isomerism. On thermolys is the E configured 3-vinylsilacyclobutanes undergo ring expansion to the [4 + 2] cycloadducts; the Z isomers undergo retro ene reactions in volving the neopentyl and vinyl groups which lead to cleavage of the s ilacyclobutane rings and give allylvinylsilanes. Appropriate substitue nts of a cycloadduct (14) disfavor the retro ene reaction and lead to the unexpected formation of nearly only the Diels-Alder products. The results obtained from cycloaddition reactions of 1 characterized the s ilene to have a high polarity and a lower HOMO as compared to diorgano silenes: both properties lead us to suggest a stepwise, strongly dipol ar reaction pathway in the formation of the silacyclobutanes. Zwitteri onic species are involved in their thermolysis reactions leading to th e [4 + 2] cycloadducts. The retro ene reaction proceeds via a differen t reaction pathway: this can be deduced from the ratios of isomers bef ore and after thermolysis reactions.