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 β-Silyl Radicals
journal contributionposted on 04.02.2000, 00:00 by Satoshi Shuto, Isamu Sugimoto, Hiroshi Abe, Akira Matsuda
A mechanistic study was performed on a novel radical ring-enlargement reaction of (3-oxa-2-silacyclopentyl)methyl radicals into 4-oxa-3-silacyclohexyl radicals. Two pathways, one via a pentavalent silicon-bridging radical transition state (or intermediate), the other via β-elimination to give a ring-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‘ and a 4-oxa-3-silacyclohexyl radical D‘, respectively, showed that the ring-enlargement rearrangement of C‘ into D‘ is irreversible. 1H NMR analysis of the radical reactions of 8a and 8b, which have an asymmetric center at silicon, 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 radical ring-enlargement reaction with a deuterium-labeled substrate 12D showed that the ring-enlargement reaction did not involve β-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 evidence for such a pentavalent silicon radical, which has been previously postulated to understand radical reactions of organic silicon compounds.