Homolytic Substitution Reaction at a Silicon Atom

Abstract

Tris(trimethy1silyl)silane (2) is an efficient hydrosilylating agent of 1,6-dienes 1 operating via a free-radical chain mechanism. The intermediate alkyl radical 3 attacks the second olefinic bond of the diene 1 and forms cyclopentylalkyl radical 4. Using a low concentration of the hydrogen donor 2, a radical substitution reaction at the Si-center of cis-4 occurs and yields the bicyclic silane 6. The rate of this homolytic substitution at the Si-atom is 2.4 . lo5 s -~( S O O ) .

SHi Reactions at Silicon as Unimolecular Chain Transfer Steps in the Formation of Cyclic Alkoxysilanes

“…In addition, the cyclic silanes formed in these UMCT steps could be transformed into the corresponding alcohol derivatives by Tamao ± Fleming oxidation. [9] e) P. J. Dandliker, R. E. Holmlin, J. K. [6]). We suppose that the closest G is the initially oxidized base.…”

“…[11] Acidification after standard workup provided lactone 23, which was isolated in good yields (71, 72 %). (6) In summary, it has been shown that S H i reactions at silicon by C-centered radicals in g-position of stannylated silyl ethers are efficient UMCT reactions providing cyclic alkoxysilanes, which can easily be converted into the corresponding diol derivatives. In addition, excellent 1,2 stereoinduction was obtained in the cyclization reaction of chiral silyl ethers.…”

“…[5] These S H i reactions can be used for the formation of siliconor tin-containing heterocycles. Giese et al [6] and Utimoto et al [7] independently reported intramolecular homolytic substitutions at tris(trimethylsilyl)-substituted Si atoms by Ccentered radicals. It was concluded that the trimethylsilyl radical generated during the intramolecular substitution acts as the chain carrier (Scheme 1).…”

“…The desilylation product 12 was not seen in these tin hydride experiments. By competition experiments with (TMS) 3 SiH, [6] a rate constant of 1.2 Â 10 6 s À1 at 808 was estimated for the cyclization of radical 14 leading to 10. In the case of iodide 13, obtained by silylation ((TMS) 3 SiI) of 3- chloro-1-phenylpropan-1-ol followed by Finkelstein reaction (32 % overall), no UMCT reaction could be observed under atom transfer conditions with Bu 3 SnSnBu 3 (0.1 equiv) as the additive.…”

Intramolecular Homolytic Substitutions in Synthesis

“…The first examples of cyclization reactions involving substitution at silicon were reported by Giese and coworkers 60 and Utimoto et al . 61 almost 20 years ago; these were included in the 1996 review.…”