A mechanistic study of the addition of alkynes to Brook silenes

The addition of the alkyne-containing mechanistic probes (trans-2-phenylcyclopropyl)ethyne, (trans, trans-2-methoxy-3-phenylcyclopropyl)ethyne, and (trans, trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne (1a-1c) to a Brook silene 2-(1-adamantyl)-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silene (14) was examined. When alkyne 1a was added to the silene, an ene adduct was observed; however, addition of alkyne 1c to 14 gave a mixture of silacyclobutenes and silacycloheptenes. The regiochemistry of the phenyl and methoxy substituents on the seven-membered ring of the silacycloheptenes provides convincing evidence for the formation of a biradical intermediate along the reaction pathway.