Silicon-centered radicals formation

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

In the case of poly(di-n-hexylsilane) (PDHS), an early radical with a g value of 2.0046 was observed with a Si hyperfine splitting of 75G. This g value is consistent with those previously reported for persistent silicon-centered radicals (43), and the low value for the Si hyperfine splitting is reasonable for a polysilylated silyl radical. Continued irradiation produced another persistent silicon-centered radical with no proton splittings and with a Si hyperfine coupling of 56G. Irradiation of PDHS in deuterated pentane produced no photodegraded materials that showed a Si-D stretch in the IR spectrum. On the other hand, irradiation at 248 nm of PDHS that had been fully deuterated in the a-carbon positions resulted in the formation of both Si-H and Si-D bonds. 

The reactions of atoms or radicals with silicon hydrides, germanium hydrides, and tin hydrides are the key steps in formation of the metal-centered radicals [Eq. (1)]. Silyl radicals play a strategic role in diverse areas of science, from the production of silicon-containing ceramics to applications in polymers and organic synthesis.1 Tin hydrides have been widely applied in synthesis in radical chain reactions that were well established decades ago.2,3 Germanium hydrides have been less commonly employed but provide some attractive features for organic synthesis. 

The reaction of carbon-centered radicals with silicon hydrides is of great importance in chemical transformations under reducing conditions where an appropriate silane is either the reducing agent or the mediator for the formation of new bonds.23... 

Triaminosilanes have also been prepared from NHC-diaminosilylenes and cyclohexylisocyanide [195]. The reactivity of Af-heterocyclic silylene-NHC complexes with muonium has been explored by Percival et al. According to muon spin spectroscopy and theoretical calculations, radical formation at the exocyclic methylene group or the silicon center is the most likely result [1%]. 

A new view on this problem arose after the diamagnetic centers with two-coordinate silicon atom were observed at the surface of mechanically activated silica [70]. The subsequent studies of the optical properties of these centers [71] have shown that they are close to those centers that were observed earlier in the bulk of quartz glass but referred to the oxygen vacancies. It has become evident that, apart from the combination reaction of two radicals (=Si-0)3Si and (=Si-0)3Si resulting in the oxygen vacancy, an alternative reaction of their disproportionation with the formation of a center containing two-coordinate silicon and the restoration of one of the siloxane bonds is possible ... 

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