Photochemical extrusion, dimethylsilylen

Linear permethylpolysilanes Me(Me2Si) Me with n = 4-8 also undergo photolysis when their cyclohexane solutions are irradiated with a low-pressure mercury lamp (32,33). Octamethyltrisilane is always produced as the major volatile product, in addition to 1-hydroheptamethyltrisilane and polymeric substances arising from polymerization of dimethylsilyl-ene. Octamethyltrisilane (Xmax 215.0 nm) is photochemically rather stable under the conditions used, but prolonged irradiation of its dilute cyclohexane solution causes skeletal contraction, giving hexamethyldisilane with extrusion of dimethylsilylene. 

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). 

Dimethylsilylene extrusion shortens the silicon backbone of per-methylated oligosilanes by one Me2Si unit and is known (27) to be the dominant photochemical process. Starting with racked hexa-silane l[m,6] and octasilane l[m,8], all Si chain lengths from n = 4—8 become available in useful yields. As the extended silicon... 

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