Pentamethyldisilanyl radical

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

The low stereoselectivity here is consistent with ESR data which had indicated a more planar structure for the pentamethyldisilanyl radical Me3SiSiMe2 (20). 

The low stereoselectivity here is consistent with ESR data, which indicated a more planar structure for the pentamethyldisilanyl radical Me3Si-SiMe286. Nevertheless, the pyramidal silyl radical exhibits significant configurational stability. The inversion of the pyramidal species, is generally slow enough to allow reactions in which the configuration at silicon is retained. 

When pentamethyldisilanyl radicals were produced by hydrogen atom abstraction in a room-temperature solution, they also underwent disproportionation and recombination reactions238. No evidence was found for fragmentation to dimethylsilylene and a trimethylsilyl radical. 

Since photon energies are insufficient for simultaneous cleavage of two SiSi bonds to afford two silyl radicals and a silylene, Path C is an unlikely mechanism. The stepwise mechanism (Path A) could also be excluded because thermal a-elimination from pentamethyldisilanyl radical (i.e. equation 5) does not occur at room temperature. The concertedness of the photochemical silylene extrusion from a peralkyltrisilane is corroborated by the high stereoselectivity. Thus, irradiation of E-and Z-l,3-diphenyl-l,2,2,3-tetramethyl-l,2,3-trisilacycloheptane gave, respectively, E-and Z-l,2-diphenyl-l,2-dimethyl-l,2-disilacyclohexane, in a highly stereospecific manner, together with dimethylsilylene (equations 8 and 9). 

Decomposition of benzoyl peroxide in hexamethyldisilane at 80° C gives, as major products, benzene, benzoic acid, l,2-bis(pentamethyldisilanyl)-ethane and benzylpentamethyldisilane (151). The reaction of hexamethyldisilane in carbon tetrachloride with benzoyl peroxide (at reflux temperature) and with di-tert-butyl peroxide (in a sealed tube at 129° C) gives (chloro-methyl)pentamethyldisilane as the main product arising from the silane (150). In no case are rearrangement products formed. Therefore, in solution at relatively low temperature, the pentamethyldisilanylmethyl radical does not undergo rearrangement as in the thermolysis. The main fate of this free radical is dimerization in the absence of solvent or chlorine atom abstraction when carbon tetrachloride is present. 

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