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.
Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7.
Kaarina K
KK
Milnes
Michael C
MC
Jennings
Kim M
KM
Baines