title: Silylboranes as new sources of silyl radicals for chain-transfer reactions.
abstract: Various silylboranes, which were outfitted with a catecholborane moiety at one end and a (Me(3)Si)(3)Si moiety at the other end of a carbon chain, were prepared through the hydroboration of the corresponding unsaturated silanes. The C-centered radical species generated from these silylboranes efficiently cyclized to provide, through a 5-exo intramolecular homolytic substitution at the silicon center, the corresponding silacycle and a Me(3)Si radical that was subsequently trapped by sulfonyl acceptors. These cyclizations proceeded at unprecedented rates, due, in part, to a strong gem-dialkyl effect that was attributable to the presence of bulky substituents on a quaternary center located on the chain. In parallel, we designed arylsilylboranes that produced silyl radicals through a 1,5-hydrogen transfer. Such silyl radicals may be valuable radical chain carriers, for instance, in oximation reactions of alkyl halides. Finally, computational studies allowed calculation of activation barriers of the homolytic substitution step and additionally illustrated that the overall reaction mechanism involved a transition state in which the attacking carbon center, the central silicon atom, and the Me(3)Si leaving group were collinear.
authors.affiliation: Université de Bordeaux, Institut des Sciences Moléculaires, UMR-CNRS 5255, 351, Cours de la libération 33405 Talence Cedex, France.
authors.forename: Guy
authors.initials: G
authors.lastname: Rouquet
authors.forename: Frédéric
authors.initials: F
authors.lastname: Robert
authors.forename: Raphaël
authors.initials: R
authors.lastname: Méreau
authors.forename: Frédéric
authors.initials: F
authors.lastname: Castet
authors.forename: Philippe
authors.initials: P
authors.lastname: Renaud
authors.forename: Yannick
authors.initials: Y
authors.lastname: Landais