Silylene forms

One novel and interesting method of generating a silacarbonyl ylide occurred through the addition of a carbonyl species with a silylene formed under photolytic conditions. Komatsu and co-workers (177) found that photolysis of trisilane (315) in solution with a bulky carbonyl species led initially to the formation of a silacarbonyl ylide followed by a dipolar cycloaddition of an olefinic or carbonyl substrate. Reaction of simple, nonbulky aldehydes led to only moderate yields of cycloadduct, the siladioxolane. One lone ketone example was given, but the cycloadduct from the reaction was prepared in very low yield (Scheme 4.89). 

The direct reaction of methyl chloride with silicon metal is the foundation stone of the worldwide silicone industry352. In corporate laboratories over the years, the reaction has been carefully engineered to provide the maximum amount of the desired product, dimethyldichlorosilane. Despite the industrial importance of the direct reaction, and the great amount of research devoted to it, its mechanism is still obscure353. Recently, however, a model has been suggested in which silicon in silylene form provides the crucial intermediate. 

The stable silylene forms an adduct with B(C6F5)3 that slowly (20 days) decomposes via transfer of a -C6F5 group to silicon, forming a novel silylborane (Entry 45).164 Only partial adduct formation is observed when weaker Lewis acids such as BPh(CgF5)2 or C1B(C6F5)2 are reacted with the silylene. 

Methylene insertion into a silicon sihcon bond is also possible for a reactive Si-Si bond (equation 36). Insertion reactions of divalent silicon species, silylene, also occur to the same substrate. Silylene forms a new silicon-carbon bond with a variety of substrates, which will be described later. 

Because H-Si-SiHs is nearly isoenergetic with H2Si = SiH2, it is not obvious which structure would be favored for hexasilabenzene, SieHe. Whereas proponents of aromaticity may take heart from the theoretical calculations that show the hexasilabenzene structure to be 34 kcal/mol more stable than the tris(silylene) form, they will be reminded of how little aromaticity has to do with the magic of six tt electrons by the revelation that the hexasilaprismane is calculated to be 14 kcal/mol more stable yet. 

Key questions regarding the reactivity of silylene complexes concern their potential role in metal-catalyzed transformations. For the participation of intermediate silylene complexes in a catalytic cycle, low-energy chemical pathways must exist for the conversion of simple silanes to silylene ligands via activation processes at the metal center. Most probably, a key step in such silylene-forming processes would be the a-migration of a group from silicon to the metal. In search of such a reaction, we prepared the methyl silyl complex shown in Eq. 3. This complex is quite... 

The ability of PECVD Si02 to act as a flow layer is unusual, but is readily related to the reaction chemistry involved. The complete oxidation of silane with hydrogen peroxide is thermodynamically quite favorable. It is proposed that the first reaction step involves the hydroxylation of silylene formed from the cracking of silane (Eq. 5.11). 

By warming solutions of ( Bu3Si)2SiBrLi to room temperature, the silanide — as in Scheme 3 — eliminates LiBr and the silylene formed thereby stabilizes itself by insertion of the divalent silicon into a C-H bond of a tBu-group [1]. 

Silylenes formed by the thermal decomposition processes are all likely to be in the singlet state as reasoned by Skell, Ring, Purnell, Walsh, and other... 

The relative insertion rates of various silylenes, formed by the thermal decomposition of halosilanes, into the Si-H bonds of SiH3Et were also examined by Ring and coworkers ... 

Among the insertion reactions, the Si-H, Si-0, and O-H insertions are the three which have been experimentally employed as means to trap silylenes formed by various kinds of methods. 

It is also prepared by a decomposition reaction with heat or light. For example, the silylenes are obtained by light irradiation of the cyclic compounds [9,12]. In contrast, silylenes form cyclic compounds as described below,... 

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