Silylene thermal extrusions

Silylene extrusion from siliranes in the presence of alkynes, notably bis(trimethyl-siiyl)acetylene, gives the silirene (35) in good yield (Scheme 41) (76JA6382). Compound (35) is more stable thermally than hexamethylsilirane and shows 2 Si NMR absorptions for the ring atom at 5 = 106.2 p.p.m., some 50 p.p.m. downfieid from those of silacyclopropanes, and about 100 p.p.m. downfieid from normal cyclic and acyclic tetraalkylsilanes. Notable reactions include alcoholysis and the insertion of aldehydes and ketones, dimethylsilylene... 

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

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. 

Interestingly, in the case of the reaction of silylene 85 with halocarbons the thermal rearrangement of the disilane 132 to the 1 1 addition product 133 with extrusion of the silylene was found to be a general follow-up reaction (Scheme 13) <2005JCD2945>. 

Photolysis of acyclic trisilanes leads in a first step to the extrusion of a silylene which, in the presence of sufficient shielding by its substituents, undergoes dimerization to furnish a thermally stable disilene (equation 1). This strategy led not only to the first isolated disilene1, but is still the most frequently used method to prepare compounds of this type. 

Ohshita et al. studied the rate of thermal silylene extrusion of silacyclopropenes 62a-d. Substitutents at the ring silicon and carbon atoms (Scheme 13) affect this rate. The origin of the effects has not been unambiguously explained yet <2003OM2436>. 

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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