Mechanistic study of the ring-enlargement reaction of (3-oxa-2-silacyclopentyl)methyl radicals into 4-oxa-3-silacyclohexyl radicals. Evidence for a pentavalent silicon-bridging radical transition state in 1,2-rearrangement reactions of beta-silyl radicals

A mechanistic study was performed on a novel radical ring-enlargement react ion of (3-oxa-2-silacyclopentyl)methyl radicals into 4-oxa-3-silacyclohexyl radicals. Two pathways, one via a pentavalent silicon bridging radical tra nsition state (or intermediate), the other via beta-elimination to give a r ing-opened silyl radical, can be postulated. The radical reactions of 1 and 2, which are precursors for a (3-oxa-2-silacyclopentyl)methyl radical C' a nd a 4-oxa-3-silacyclohexyl radical D', respectively, showed that the ring- enlargement rearrangement of C' into D' is irreversible. H-1 NMR analysis o f the radical reactions of 8a and 8b, which have an asymmetric center at si licon, indicated that the configuration at the silicon atom is retained via a pentavalent silicon-bridging radical transition state (or intermediate) during the ring-enlargement reaction. Furthermore, examination of the radic al ring-enlargement reaction with a deuterium-labeled substrate 12D showed that the ring-enlargement reaction did not involve beta-elimination to give a ring-opened silyl radical. Based on these results, we conclude that the ring-enlargement reaction occurs via a pentavalent silicon-bridging radical transition state (or intermediate). This is the first experimental evidenc e for such a pentavalent silicon radical, which has been previously postula ted to understand radical reactions of organic silicon compounds.