Silyl radicals reactions, importance

The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X are centred at carbon, nitrogen, oxygen, or sulfur atoms... 

Whatever the initial step of formation of surface silyl radicals, the mechanism for the oxidation of silicon surfaces by O2 is expected to be similar to the proposed Scheme 8.10. This proposal is also in agreement with the various spectroscopic measurements that provided evidence for a peroxyl radical species on the surface of silicon [53] during thermal oxidation (see also references cited in [50]). The reaction being a surface radical chain oxidation, it is obvious that temperature, efficiency of radical initiation, surface precursor and oxygen concentration will play important roles in the acceleration of the surface oxidation and outcome of oxidation. 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

Similar extrusion reactions have been observed for acyclic polysilanes (55). The isolation of hydrogen-terminated silanes containing fewer silicon atoms than the starting materials was taken as evidence for the intermediacy of silyl radicals, and the importance of chain scission seemed to increase with increasing catenation. 

Organosilicon compounds receive attack of nucleophilic, electrophilic, and homolytic reagents. The latter is related to the formation of radicals and divalent species. Organosilicon reactive intermediates corresponding to free radicals, carbenium ions, carbanions, and carbenes play important roles in these reactions. These are silyl radicals, silylium... 

As mentioned above, a crucial requirement in the photoacoustic experiment is that the net reaction 2c must be fast when compared to the response of the tranducers. The rate constants for the slower reaction, 2b, were available for most of the silanes studied and allowed the concentrations of those substrates to be calculated so as to meet the above condition. Another important issue concerns the possible addition of silyl radicals to solvent molecules. Since this reaction is predicted to be exothermic by ca. 60 kJ/mol, its occurence would have a significant impact on the results. However, parallel experiments involving triethylsilane, carried out in benzene and also using the silane as solvent, led to similar AH values, indicating that the addition to benzene was irrelevant on the timescale of the photoacoustic experiment. 

Stereoselectivity of radical addition is not limited to sulfonyl radicals. The (rani-addition has also been observed for tin, bromine, chlorine, and silyl radicals. Varying degrees of selectivity has been observed for addition of carbon-centered radicals, depending on the substituents size and effect on the inversion barrier. Because the importance of negative hyperconjugation decreases for radicals in comparison to the anions, the barriers for inversion decrease in parallel. As a result, the selectivity can erode under conditions when trapping of the radical is slower than the equilibration, e.g. in the thiol-yne click reaction, which often provide a mixture of E and Z-vinyl sulfides. ... 

Nitrogen-based Functional Groups. Important efforts have been devoted to (bromomethyl)dimethylsilyl radical reactions featuring nitrogen atoms. Renaud introduced a powerful new terminating step for the Nishiyama-Stork cyclization. After conversion of the silyl ether to the corresponding iodide, cycliza-tion in the presence of phenylsulfonyl azide allowed azidation of the cyclized radical (eq 13). 

Suda and coworkers described the anodic oxidation of 2-silyl-l,3-dithianes which have two sulfur atoms on the carbon adjacent to silicon [42], In this case, however, the C Si bond is not cleaved, but the C-S bonds are cleaved to give the corresponding acylsilanes (Scheme 12). Although the detailed mechanism has not been clarified as yet, the difference in the anode material seems to be responsible for the different pathway of the reaction. In fact, a platinum plate anode is used in this reaction, although a carbon anode is usually used for the oxidative cleavage of the C-Si bond. In the anodic oxidation of 2-silyl-l,3-dithianes the use of a carbon anode results in a significant decrease in the yield of acylsilanes. The effects of the nature of the solvent and the supporting electrolyte may also be important for the fate of the initially formed cation radical intermediate. Since various 2-alkyl-2-silyl-l,3-dithianes can be readily synthesized, this reaction provides a convenient route to acylsilanes. 

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