Silaheterocycles. 26. Facile Synthesis of Silicon Dichloro Substituted 3-Vinyl-l-silacyclobutanes from Silene/Butadiene [2 + 2] Cycloaddition Reactions: Model Compounds for Vinylsilacyclobutane → Silacyclohexene Rearrangements

The reaction between in situ formed 1,1dichloro-2-neopentylsilene, Cl₂Si=CHCH₂-t-Bu (1), and 1,3-butadienes leads almost solely to the formation of monosilacyclobutanes (2,5,6,10,14, 15,16). NMR spectroscopic investigations of the products prove the periselective addition of the silene unit to the dienes. The reaction does, however, not proceed stereospecifically: the neopentyl and vinyl groups on the carbon skeleton of the silacyclobutane give rise to E/Z isomerism. On thermolysis the E configured 3-vinylsilacyclobutanes undergo ring expansion to the [4 + 2] cycloadducts; the Z isomers undergo retro ene reactions involving the neopentyl and vinyl groups which lead to cleavage of the silacyclobutane rings and give allylvinylsilanes. Appropriate substituents 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 silene to have a high polarity and a lower HOMO as compared to diorganosilenes: both properties lead us to suggest a stepwise, strongly dipolar reaction pathway in the formation of the silacyclobutanes. Zwitterionic species are involved in their thermolysis reactions leading to the [4 + 2] cycloadducts. The retro ene reaction proceeds via a different reaction pathway: this can be deduced from the ratios of isomers before and after thermolysis reactions.