Symmetry allowed π4s+π2s additions silacyclopentadienes

π⁴s+π²s cycloaddition of hexaphenylsilacyclopentadiene with ethyl acrylate gives rise to a bicyclosilaheptene derivative. High resolution NMR studies have shown that the ester group in this adduct is in the endo position and that the ester methylene protons are magnetically inequivalent. Similar cycloadditions of hexaphenylsilacyclopentadiene, 1,1-dimethyl-2,3,4,5-tetraphenylsilacyclopentadiene, 1-methyl-1,2,3,4,5-pentaphenylsilacyclopentadiene, and 1-methyl-1-vinyl-2,3,4,5-tetra-phenylsilacyclopentadiene with dienophiles like acrylonitrile, maleic anhydride and dibenzoylacetylene have been carried out. The stereochemistry of these adducts has been deduced on the basis of NMR studies. Mass spectral fragmentation studies of some of the silabicycloheptenes have been carried out.