Addition of Cyclopropyl Alkynes to a Brook Silene: Definitive Evidence for a Biradical Intermediate

The addition of three newly developed mechanistic probes, (trans-2-phenylcyclopropyl)ethyne,
(trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1a−c, to a Brook silene, 2-tert-butyl-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silene, 10, was
examined. When alkyne 1a was added to silene 10 products derived from a formal ene reaction were
obtained. When alkynes 1b−c were added to silene 10, in addition to the typical silacyclobutenes, a variety
of silacycloheptenes were obtained in which the cyclopropyl ring had clearly opened. Formal ene-addition
products were also produced from the addition of 1b to 10. Based on the relative positions of the phenyl
and methoxy substituents within the seven-membered ring of the silacycloheptenes and the known behavior
of the alkyne probes under both radical and ionic conditions, it was concluded that a biradical intermediate
was formed during the addition of alkynes 1b−c to silene 10. In the addition of alkynes 1a−b to silene 10,
the ene products are most likely formed by a competitive pericyclic reaction. We also present a
straightforward method for the unambiguous determination of the regiochemistry of silacyclobutenes derived
from the cycloaddition of terminal alkynes to silenes.