Silylene photochemical extrusion

Another interesting ring-contraction method is the photochemical extrusion of dimethyl-silylene from cyclic oligosilanes. Ando et al. used this method to cut out two Mc2Si units from dodecamethylhexasilacyclooctyne to yield octamethyltetrasilacyclohexyne (Scheme 8-9) [32, 33]. 

On the basis of the results described, there seem to be at least three processes which are responsible for the molecular weight reduction in substituted silane high polymers upon irradiation in solution (i) Chain abridgement by silylene extrusion which occurs only at short wavelengths (ii) chain scission by silicon-silicon bond homolysis and (iii) chain scission by 1,1-photochemical reductive elimination. 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

Whereas 2,2-diphenylhexamethyltrisilane (8) is a well-known photochemical precursor of diphenyl si ly Icnc72,155 156, a 1,3-silyl migration is usually a major side reaction in solution at room temperature. In the presence of excess ethanol, the irradiation of 8 in hexane gives 9 via diphenylsilylene extrusion and 10 (an isomeric mixture) via 1,3-silyl migration in 50 and 37%, respectively (Scheme 3)157. Since the product ratio does not depend on the solvent polarity, both reactions, silylene extrusion and 1,3-silyl migration, do not occur via the ICT state but via the nonpolar excited state158. However, the excited... 

There is a dramatic kinetic stabilization by bulky substituents of the three-membered ring products from silylene addition to jr-bonds. Addition of t-Bu2Si to ethylene led to the first silirane with no substituents on its ring carbon atoms as a distillable liquid 164. Interestingly, l,l-di(terf-butyl)silirane does not undergo photochemical or thermal silylene extrusion, but instead polymerizes. A distillable silirane was also reported from addition of t-Bu2Si to 2-methylstyrene165. 

Clearly, synthetic alternatives to the reductive elimination are missing. It is noteworthy that the application of photochemical methods, common for the synthesis of transient silylenes, have not been described for the generation of l,3,2(A2)-diazasilacyclopentenes and -pentanes. A potential alternative to the described reduction chemistry could be the formal disproportionation reaction of a l-halo-2-alkyl disilane with extrusion of a silylene as shown in Scheme 13. The scope and the limitation of this chemistry has however not been tested. 

Systems which contain cr(E-E)-7r conjugated systems (E = Si or Ge) have been the subject of many photochemical investigations. For the most part investigations have focussed on silicon systems, that first identified carbon-silicon double-bonded species as photoproducts . Later studies also indicated that arylsilanes undergo 1,3-sigmatropic shift reactions as well as extrusion processes yielding silylenes . ESR studies have... 

Polysilane derivatives constitute a new class of radiation-sensitive materials with interesting physical and electronic properties. The photochemical decomposition of polymers containing disilanyl units seems adequately explained by silicon-silicon bond homolysis and subsequent radical reactions. The solution photochemistry of longer silicon catenates results in the extrusion of substituted monomeric silylenes, as well as the formation of silyl radicals produced by chain homolysis. Recent studies indicating that the... 

Three distinct photochemical processes have been identified as shown in Scheme 7 (1) chain abridgement by silylene extrusion, (2) chain cleavage by silylene elimination, and (3) chain cleavage by homolytic scission (Miller and Michl, 1989). 

In the case where the silacyclopropane was derived from cyclooctene, there is also evidence of photochemically induced extrusion of the silylene from the ring, leading to the reformation of cyclooctene19 (equation 10). 

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). 

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