Silylene dimethylsilylene

Photochemical behaviour of compounds 83-86 [33] in the gas phase has been reported, in order to distinguish between silyl radical and silylene formation. Photolysis of the noncyclic precursors 83 and 84 gave products derived from silyl radicals, which come from a direct Si—Si bond homolysis, with a little evidence of silylene formation. In contrast, dimethylsilylene (Mc2Si ) was observed as a direct photoproduct from the cyclic precursors 85 and 86. The reaction sequence including a Sni step shown in Scheme 6.18 for the formation of dimethylsilylene was proposed to explain the different observations for cyclic and noncyclic systems. 

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacety-lene, Atwell and Weynberg obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1). ... 

In spite of its long-assumed intermediacy in several reactions, no carbon-substituted silylene was directly observed for many years. In 1979, however, Drahnak et al. detected a broad ultraviolet (UV) absorption band (3-niax = 450 nm) after the photolysis of dodecamethylcyclohexasilane (6) in 3-methylpentane. This band was assigned to dimethylsilylene (5). Many different approaches to this intermediate, either photochemicaUy or thermally, were examined (Scheme 14.6). ... 

Addition to Carbonyi and Thiocarbonyi Compounds. Thermal generation of dimethylsilylene in the presence of carbonyl compounds leads to the addition of the silylene to the C=0 bond with the formation of oxasiliranes... 

Silylene 132 showed absorption maxima at 260 and 440 nm in its UV-vis spectra, the latter of which is assignable to the (Si)-3p(Si) transition and close to those observed for dimethylsilylene (453 nm) and l-silacyclopentane-l,l-diyl (436 nm). Thus, the electronic structure of silylene 132 is not much per-... 

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... 

The first 1,2-disilacyclobutene (82) was prepared in 1973 by the gas phase reaction of dimethylsilylene and 2-butyne (73JOM(52)C21). It probably results through silylene insertion into the intermediate silacyclopropene (Section 1.20.3.4), but silylene dimerization followed by addition to the alkyne is also suggested (76JA7746), since (82) is formed in good yield if the disilene is generated directly (Scheme 127) (78JOM(162)C43). 

Whereas UV spectroscopic data are available for base complexes of a variety of matrix-isolated silylenes (Table IV), solution data are reported almost exclusively for complexes of dimethylsilylene (13) (Table V). Again, the complexation of this silylene is accompanied by a significant blue shift of the absorption maximum. Probably due to matrix effects of unknown nature, the Amax values of dimethylsilylene complexes in solution are shifted to wavelengths shorter than those of the corresponding matrix-isolated complexes. 

According to the reactivity-selectivity principle, reduced reactivity of a compound is expected to be accompanied by increased selectivity of its reactions. Exactly this was observed in early experiments by Steele and Weber37 when the insertion reaction of dimethylsilylene (13) into the O-H bond of alcohols was performed in different solvents In a more polar solvent such as THF the reaction of the silylene with the... 

Levin et al.55 observed, in line with the qualitative results of Gillette et al. 6 that less bulkily substituted silylene 13 is stabilized when coordinated by Lewis bases. Dimethylsilylene 13 was generated photochemically from 32 in cyclohexane solutions at 296 K, and it was shown by means of UV spectroscopy that the lifetime of this silylene is 0.4 fjs under the conditions... 

Maier and coworkers have shown that it is possible to induce by irridation using the appropriate absorption band, a 1,2-hydrogen shift in silenes and in silylenes29,158. Thus, it is possible to switch photochemically between silylenes and silenes. Michl, West and coworkers have used this approach to isomerize dimethylsilylene 285 and 1-methylsilene 26 several times (equation 70). Due to the clean formation of 285 from the diazido precursor 286153 it was possible to measure the IR transition moment directions for both 26 and 285156. 

Silylenes are produced by several photochemical routes. Dimethylsilylene, Me2Sit, is produced during the photolysis at 254 nm of dodecamethylcyclohexasilane, (Me2Si)6, but the cyclopentasilane and lower homologues are by-products of this approach, which... 

The earlier controversy about whether silene-silylene interconversions could occur was settled by studies of West, Michl and coworkers158. They generated dimethylsilylene Me2Si from several precursors in glasses at temperatures from 10 to 77 K and characterized it by its UV and IR spectra. This yellow species was bleached by irradiation... 

Regioselective hydrosilylation of poly[(dimethylsilylene)but-l,3-diyne] (296a) with HSiEt3 using Rhg(CO)i6 as the catalyst at 40 °C gives poly[2-(triethylsilyl)(dimethyl-silylene)but-l-en-3-yne-l,4-diyl]294. In a similar manner, the reactions of poly[(silylene)... 

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... 

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