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

Abstract

A mechanistic study was performed on a novel radical ring-enlargement reaction of (3-oxa-2silacyclopentyl)methyl radicals into 4-oxa-3-silacyclohexyl radicals. Two pathways, one via a pentavalent siliconbridging 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. 1 H 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.

Tethered 1,2-Si-Group Migrations in Radical-Mediated Ring Enlargements of Cyclic Alkoxysilanes: An EPR Spectroscopic and Computational Investigation

“…Treatment of 5 with high concentrations of Bu 3 SnH furnished the expected indano-3-oxa-2-silacyclopentanes 11 . However, when low tin hydride concentrations were employed a mixture of 11 , together with the ring-enlarged 12 , was obtained . By way of contrast, tin hydride treatment of 6 yielded only 4-oxa-3-silacyclohexanes 12 .…”

“…An alternative possibility was a 1,2-migration of the Si-group via a transition state (TS) such as 10 (route ii). Deuterium substitution experiments confirmed the basic details of the ring enlargement. , …”

Highly Functionalized and Potent Antiviral Cyclopentane Derivatives Formed by a Tandem Process Consisting of Organometallic, Transition‐Metal‐Catalyzed, and Radical Reaction Steps

“…A similar tandem reaction with silylated substrate 10 allows the preparation of vinylcyclopentane 11 in the absence of 8 (Scheme 4). The stability of b-silyl radicals to fragmentation [18] and the necessity to employ two equivalents of 7 indicates that two SET oxidation steps are involved in the sequence. The ability to access vinyl-substituted cyclopentanes 11 opens many pathways for further functionalization of this group.…”

Metal‐Free Synthesis of Aryl Selenocyanates and Selenaheterocycles with Elemental Selenium

“…Over the past decade, aryl selenocyanates (ArSeCN) have attracted considerable attention since they displayed biological activity [2] and found wide applications in organic synthesis. For example, aryl selenocyanates have appeared as an important organoselenium reagent to construct various organic selenium compounds including monoselenides, [3a–e] diselenides, [3f,g] trifluoromethyl (difluoromethyl) selenides, [3h,i] selenoesters [3j] and selenium‐containing ligands [3k] . In some cases, aryl selenocyanates could serve as a useful bifunctional reagent for the direct cyanoselenenylation of unsaturated C−C bonds [4]…”