Radicals silicon

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes.  

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The radical-based functionalization of silicon surfaces is a growing area because of the potential practical applications. Although further knowledge is needed, the scope, limitations, and mechanism of these reachons are sufficiently well understood that they can be used predictably and reliably in the modification of hydrogen-terminated silicon surfaces. The radical chemistry of (TMSlsSiH has frequently served as a model in reactions of both hydrogen-terminated porous and flat silicon surfaces. We trust that the survey presented here will serve as a platform to expand silicon radical chemistry with new and exciting discoveries. 

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. 

Linford and coworkers have shown that the attachment of alkenes to H-terminated silicon surfaces can also be initiated by direct mechanical scribing, in a process termed chemomechanical functionalization [145-147]. The reaction of 1-alkenes (as well as 1-alkynes) leads to attachment of the molecule to the surface through two new Si—C bonds. The proposed mechanism is the mechanical cleavage of Si—H and Si—Si bonds, leading to silicon radicals that then react with the reactive liquid. Interestingly, Linford and coworkers have also extended this work to show that chemomechanical functionalization can be carried out not only on H-terminated Si, but also on sihcon covered with oxide, and have shown that the process works with a variety of halides, alcohols, and epoxides in both the liquid and gas phase [146]. 

All the reactions may involve attack of a phenylsilyl radical formed upon irradiation on C6o to give the radical intermediate as shown in Scheme 53. There is probably a rapid equilibrium between this radical and the cyclohexadienyl radical. The product ratio depends on the relative rates of recombination with the other silicon radical produced initially. Bulky phenylalkylsilyl radicals react preferentially at the 16-position of the C6o poly cycle in the initially formed radical. In contrast, trimethylsilyl and silylsilyl radicals prefer the cyclohexadienyl ring. 

Just as the oxygen vacancy and the SC can be considered as a product of the addition or disproportionation reaction of two silicon radicals (fundamental sources of the oxygen deficiency in Si02), so can the peroxide bridge and the DOSG be considered as products of the respective reactions between two oxy radicals (=Si-0-)3Si-0 (fundamental defects containing excess oxygen in silica) ... 

Fig. 5. Redox cycle for dehydrocoupling by silicon radicals. One of three variations proposed by Harrod.3d,6° 81,83...

Davidson, I. M. T. Some aspects of silicon radical chemistry. Quart. Rev. (London) 25, 111-33(1971). 

Returning to the introductory sentence of this section, it may be noted that hypervalent silicon hydrides have been prepared as anions in the gas phase and their ion chemistry has been investigated by Squires and co-workers [238]. Thus, the synthetic methods for the precursor anions are known and it is perhaps only a question of time for pentavalent silicon radicals to be generated by NRMS as transient species in the gas phase. 

Radicals are less important in silicon than in carbon chemistry. However, silicon radicals have been detected in solution by esr and have been isolated in matrices. They are made by hydrogen abstraction with r-butoxy and other radicals generated photochemically, for example,... 

Since the TMS monomer gas contains both carbon and silicon, the TMS signal could originate from either carbon or silicon radicals. A comparison of the TMS signals with carbon-based signals obtained by methane plasma deposition disclosed some... 

The stability and inertness of diradical 4 are quite unprecedented for silicon radicals. 4 does not react at room temperature with water, alcohols, or chloroform, and only slowly with oxygen. The diradical is not indefinitely stable, but the half-time for its disappearance at room temperature is more than one year. The surprising, indeed amazing, inertness of 4 can be explained partly by steric effects, but probably also by specific stabilization due to the silole ring. The results of this study suggest that silole free radicals may form a previously unrecognized class of stable radical species. 

The reduction current on p-Si is small in the dark because it is limited by reaction (6.14), which requires electrons. On the active surface the reaction scheme is more complex due to the interaction between silicon radical and hydrogen peroxide generating a -OH radical ... 

The -OH then injects a hole into the valence band, which is responsible for tbe reduction current observed on an active surface ofp-Si in the dark. At anodic potentials, reaction (6.16) competes favorably with the electron injection into the conduction band from the silicon radical because when H2O2 is present the anodic current peak on n-Si (due to the electron injection) disappears. 

Robertson. R., and Gallagher, A., Mono- and di-silicon radicals in silane-argon DC discharges. 7. Appl. Phys. 59, 3402-3411 (1986). 

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