Silylene extrusion reaction

On keeping, this product converted via formal silylene extrusion reaction into the hydride bridged complex 17 ... 

The reversibility of most silylene addition reactions allows the cycloadduct of a silylene to 1,3-diene to be employed as a sUylene source. Extrusion of a silylene from l-silacyclopent-3-ene (3) has been achieved by thermolysis in the gas phase and also by photolysis in solution (Scheme 14.4). 

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... 

In relation to the reactions of CT excited states of aryldisilanes, photochemistry of LE and ICT states of the rigid, p-cyano-substituted styryldisilane 7 was investigated by Steinmetz and coworkers154. Although no 1,3-silyl shift is observed in this sytem, 7 affords a major product attributable to addition of alcohol across the Si—Si bond in the CT state. The roles of LE and CT states in the formation of additional minor products of silylene extrusion and homolytic Si—Si cleavage have also been elucidated. 

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... 

The preparative application of photolytic ring contraction reactions of cyclotetrasi-lanes, which also lead to the formation of cyclotrisilanes, is largely impeded by the limited stability of the cyclotrisilane moiety. The photolysis of octaisopropylcyclote-trasilane, for instance, affords the corresponding cyclotrisilane only as an intermediate, which can be detected by UV spectroscopy. Further silylene extrusion gives rise to the... 

The pyrolysis of polysilanes played an important role in the discovery of silylene reactions1-3,5 and is still widely used for the generation of silylenes in the gas phase. Many such reactions are concerted silylene extrusions in which a substituent migrates from the incipient divalent silicon atom during Si—Si bond cleavage (equation 1). 

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). 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

Metal-catalyzed silylene extrusion from hexamethylsilirane 20 was also used to generate transition metal silylenoids [equation (7.2)] 47 In the presence of a metallo-porphyrin, [Os(TTP)]2 (where TTP = the dianion of meso-tetra-p-tolylporphyrin), hexamethylsilirane 20 extruded dimethylsilylene to produce the THF-stabilized osmium silylenoid complex 21. Despite the coordinated THF molecule, crystallographic analysis of the silylosmium complex 21 revealed the sp2 nature of the silicon atom (Os-C-Si bond angles 121° and 117°). Osmium silylenoid 21 was also synthesized from the reaction of [Os(TPP)]2- and dimethyldichlorosilane.47... 

One involves photorearrangement giving the silylalkene mentioned above and the other, the main route, is silylene extrusion, the reverse of the reaction that leads to silacyclopropane formation (52). 

During the 1960s and 1970s, clean methods were developed for the generation of silylenes [5]. It required many years, however, for woricers in the field to recognize the connection between the most useful reactions for the production of silylenes, extrusion from disilanes and siliranes, and reactions that efficiently consume them, insertions into cr-bonds and addition to r-bonds. Yet staring us in the face was a phenomenon quite different from carbene chemistry nearly all silylene reactions are reversible [6]. This provides a compact framework within which to present a selection of the reactions that lead to silylenes and those reactions that consume them, as shown in Scheme 1. 

Cage polysilanes undergo only two chemical reactions. Photolysis of Mcj Sig leads to silylene extrusion and the formation of the 2.2.1-heptasilabicyclo-heptane ... 

Silylene complexes have been prepared by coordination of free silylene, extrusion of dihydrogen from a dihydrosilane, a-hydrogen elimination from a hydrosilyl complex, extraction of hydride, halide, or pseudo-haUde from a metal-silyl complex, and photoextrusion of silylene from a disilanyl complex. Examples of these reactions are shown in Equations 13.39-13.45. 

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