Radicals silicon-centered

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals. fhe radicals have nucleophilic character. 

The mechanism of the thermal conversion of allyltrimethylsilane to trimethylvinylsilane at 525°C would involve homolytic cleavage of the silicon-methyl bond (rather than that of the silicon-allyl bond expected) followed by cyclization of the silyl radical to the (Cy 3) radical in the Cy3/Cy4 case, followed by the jff-scission process.  

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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. 

Therefore, by using the appropriate molecules, one can generate various low-molecular radicals r (H, D, OH, OD, CH3, etc.) and subsequently obtain the silicon-centered radicals containing chemically different substituents. The EPR parameters of some of the PCs obtained by this method are presented in Table 7.7. 

Silicon-Centered Radicals (RaSi ). Silyl radical chemistry is not nearly as developed as its carbon counterpart. In striking contrast to its... 

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. 

Several dimerization rates of alkyl-substituted silyl radicals were measured earlier [8]. However, the dimerization rates of silyl-substituted silicon-centered radicals have not previously been determined. In this study we have measured, using EPR spectroscopy, the rate constants for the recombination of four silyl radicals (lb, 2b, 3b, and 4b), to produce the corresponding disilane dimers of type a (i.e. la, 2a, 3a, and 4a respectively). This dimerization reaction is shown as the backward reaction of Eq. 2 in Scheme 1. Radicals lb, 2b, and 3b were generated photochemically fiom the corresponding disilane dimers of type a (Scheme 1, Eq. 2), while radical 4b was generated photochemically from the corresponding silylmercury compound 4c (Scheme 1, Eq. 1). 

Silicon-centered radicals and related radical ions... 

En ective Nuclear Potential of Silicon Centers Radical Cation State Comparison of the Isovaient Moiecuies HaSiSiHs and H3CCH3... 

The species [H2BCH2] and [H2BSIH2] were included in a study of the effect of substituents on the stabilization of carbon- and silicon-centered radicals. The process [H2BXH2] + XH4- H2BXH3 + [XHa] (X = C or Si) is endotherm. Delocalization of the odd electron into the vacant p orbital on boron is responsible for stabilizing [H2BCH2] by 9.7 kcal/mol and [H2BSiH2] by 11.3 kcal/mol [14]. 

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