The mechanism of [2+1] and [2+2] cycloaddition reactions of 1-phenylseleno-2-(trimethylsilyl)ethene: an isotopic labelling study

The reactions of deuterio-labelled 1-seleno-2-silylethenes with trimethyl 2 -phosphonoacrylate 2 and methyl vinyl ketone 4 in the presence of SnCl4 gav e deuterio-substituted cyclopropanes with 1,2-silicon migration. The reacti on of deuterio-labelled 1-seleno-2-silylethenes with dimethyl 2,2-dicyanoet hene-1,1-dicarboxylate 6 in the presence of SnCl4 and ZnBr2 gave deuterium- substituted cyclobutanes without silicon migration. This labelling study st rongly confirms the 1,2-silicon migration for [2+1] cycloadditions of 1 and non-1,2-silicon migration for the [2+2] cycloaddition reactions, respectiv ely.