Silyl radical stable

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

The chemistry of the heavy analogs of organic free radicals, that is, radicals centered on the Si, Ge, Sn, and Pb atoms, has been thoroughly reviewed several times, particularly that of the silyl radicals. Therefore, in the present review we will briefly bring together the most important discoveries in this field with particular attention paid to the most recent developments and progress, especially in the synthesis of the stable representatives of heavier group 14 elements centered radicals. ... 

Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si radical contrary to the stable Ph3C radical is mentioned above. The decay of the trimesitylsilyl radical at 63°C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-h Ay groups to decrease pyramidalization... 

The most persistent trialkyl-substituted silyl radical is [(Me3Si)2CH]3Si , which at 20 °C follows a first-order decay with a half-life of 480 s [36]. An exceptionally stable diradical was isolated by reaction of l,l-dilithio-2,3,4,5-tetraphenylsilole with l,l-dichloro-2,3-diphenylcyclopropene, for which the structure 13 was suggested on the basis of EPR data and theoretical calculations... 

The addition of silyl radicals to various 2,6-disubstituted quinones takes place at two different sites, i.e., at the less hindered C=0 and at the C=C double bond, the former being ca 4 times slower (Scheme 5.5) [48]. However, kinetic studies showed that radical adducts 27 are prone to rearrange to the thermodynamically more stable isomers 28 via a four-membered transition... 

Radical hydrosilylation takes place according to a usual free-radical mechanism with silyl radicals as chain carriers. Products are formed predominantly through the most stable radical intermediate. Even highly hindered alkenes undergo radical hydrosilylation. This process, however, is not stereoselective, and alkenes that are prone to free-radical polymerization may form polymers. 

The proposed intermediate is the pentacoordinate silyl radical, which eliminates the most stable alkyl radical to give the corresponding fluorosilane (equation 1 and Table 2). 

Photolyses of some highly silyl-substituted disilanes were found to result in exclusively Si—Si homolysis leading to stable silyl radicals, studied by ESR techniques, as shown in... 

Tris(trimethylsilyl)silyl radical is relatively stable and can therefore serve as a radical leaving group. This reaction has been extended to the radical-initiated allylation of organic halides202 203. Thus, thermolyses of bromides a to a carbonyl substituent 144 or of simple iodides with allyltris(silyl)silane in the presence of a radical initiator gives the corresponding allylation products (equation 112). 

Silyl radicals have also been observed during /-irradiation of solid polysilanes. Tagawa and coworkers examined the EPR spectrum observed upon irradiation of solid poly-(dimethylsilane) and concluded that the spectrum corresponded to silyl radicals generated by homolysis of the silicon skeleton in the polysilane (equation 5)18. Indeed, the EPR spectrum of the poly(dimethylsilane) radical (13), with hyperfine splitting constants H) and a(y-1 H) of 0.813 and 0.046 mT respectively, corresponded remarkably well to that published for the dimethyl(trimethylsilyl)silyl radical [a( -1H) = 0.821 mT a(/-1H) = 0.047 mT]1. Radical (13) appears to be very stable in solid poly(dimethyl-silane), since the EPR signal was strong and clearly observable at room temperature. 

The photochemistry of trisilanes 16 and 19 has been investigated in some detail (Schemes 1 and 2)21. Upon irradiation of compound 16 only the Si—SiMe2Ph bond is broken and the initially formed silyl radical 17 undergoes a rearrangement to the more stable silacyclobutenyl radical 18 whose EPR spectrum has been recorded (Scheme l)21a. Irradiation of trisilane 19 with a medium pressure mercury lamp resulted in the formation of hexamethyldisilane, 2-(trimethylsilyl)thiophene and 20, with 20 dominating (Scheme 2)21b. In the presence of carbon tetrachloride, a significant yield (19.2%) of the... 

Anodic oxidation of tetraalkylsilanes in the presence of fluoride ions provides the corresponding fluorosilanes derived from cleavage of the C-Si bond, as in Eq. (20) [54]. The proposed intermediates are the pentacoordinate silyl radicals, which eliminate the most stable alkyl radical to give the corresponding fluorosilane. Becker and Shakkour found that anodic oxidation of cyclic peralkylsilanes results in the formation of a, )-difluorosi-lanes via Si-Si bond cleavage, as in Eq, (21) [55]. 

Silicone Polymers - Laser flash photolysis studies on poly(silylenes) generates radical cations along with silyl radicals and polysiloxane composities for the space shuttle have been found to be stable to far UV light exposure. Linear polysiloxanes have been found to be more unstable than branched or crosslinked polymers while the transparency of poly(methylphenylsilane) increases with light exposure. Photooxidised polysiloxanes doped with iodine are converted into semiconductors. ... 

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