Silyl radical photochemical generation

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

The reaction of thermally and photochemically generated tcrt-butoxyl radicals with silicon hydrides (Reactions 3.13 and 3.14) has been extensively used for the generation of silyl radicals in EPR studies, time-resolved optical techniques, and organic synthesis. 

Ando demonstrated that the photolysis of partially tert -butyl-substituted disilanes 45 in the presence of Cgo results in the formation of 1,16-adducts 46 in 54-62% yield (Scheme 8)37,38. Interestingly, unusual adducts similar to 49 were obtained as by-products with disilanes having trimethylsilyl substituents (Scheme 9). The authors explain these observations by a mechanism involving the intermediacy of silyl radicals which are generated photochemically by Si—Si bond homolysis. Initial free-radical addition of silyl... 

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). 

Coupling of photochemically-generated silyl radicals is a useful method for synthesis of branched oligosilanes. An example is given in equation 18. 

A different approach must be used for the photochemical hydrophosphination of electron-poor olefins, and this involves a PET reaction. Silyl phosphites (e.g., 30) were used as electron donors, whereas conjugated ketones have the double role of electron acceptors and absorbing species. Thus, the irradiation of a mixture containing 2-cydohexenone and 30 generated an ion pair. The phosphoniumyl radical cations decomposed to give trimethylsilyl cations (which in turn were trapped by the enone radical anion) and phosphonyl radicals. A radical-radical combination afforded the 4-phosphonylated ketones in yields ranging from 78% to 92% (Scheme 3.20) [49]. This reaction was exploited for the preparation of substituted phosphonates, which serve as key intermediates in the synthesis of a class of biologically active compounds. 

Hydrosilylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro silylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

The gas-phase chemistry of Si-containing radicals forms one important sub-mechanism which is attracting considerable attention. The two most important Si radicals generated in either discharge, pyrolysis, or photochemical CVD sources are silylene, SiH2, and silyl, SiHs. 

Studies of SET-induced excited-state reactions of phthalimides with a-trialkylsilyl substituted ethers, thioethers, and amines have shown that cation radical desilylation is the foundation of interesting photochemical reactions that proceed by pathways involving the generation and coupling of radical intermediates. In an early effort, we observed that simple, a-silyl-substituted ethers, thioethers, amines, and amides undergo efficient photoadditions to phthalimide and its N-methyl derivative. One example in this series of closely related reactions is the photoaddition of silylmethylpropylthioether 4 to N-methylphthalimide, which produces the product 5 (Scheme 4). In the mechanistic route for this process, thermodynamically/kinetically driven SET from 4 to the excited phthalimide leads to formation of the ion radical pair 6. Solvent (MeOH)-promoted desilylation of the cation radical and protonation of the phthalimide anion radical then provides radical pair 7, the direct precursor of adduct 5. 

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