Silylenes disproportionation

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

The 1,3-silyl shift in aryl disilanes is suppressed when the aromatic ring is ortho-substituted144. An attempted silylene synthesis from 1,3-dimesitylhexamethyltrisilane 259, however, led to low yields of silylene trapping products (ca 30% generation of Me2S ). The major pathway is the homolytic cleavage of the trisilane, followed by disproportionation of the radicals 260 and 261 to the silene 262 and the disilane 263 (equation 65). 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

Clearly, synthetic alternatives to the reductive elimination are missing. It is noteworthy that the application of photochemical methods, common for the synthesis of transient silylenes, have not been described for the generation of l,3,2(A2)-diazasilacyclopentenes and -pentanes. A potential alternative to the described reduction chemistry could be the formal disproportionation reaction of a l-halo-2-alkyl disilane with extrusion of a silylene as shown in Scheme 13. The scope and the limitation of this chemistry has however not been tested. 

The C NMR spectra confirm these conclusions. It is interesting to note that the corresponding inethoxy compound was isolated by Atwell et al. [3] after disproportionation of tetramethoxy-dimethyldisilane. The formation of the branched silane was considered as a proof for a silylene reaction mechanism which is also discussed for our system. 

In Sect. 2.3, generation of silylene complexes of transition metals was discussed on the basis of the reactivity of disilanyl-transition-metal complexes. The formation of silylene species in the presence of a catalytic amount of transition metals is also involved in the reactions of hydrodisilanes, which may readily form disilanyl complexes through oxidative addition of the Si-H bond prior to the activation of the Si-Si bond. Platinum-catalyzed disproportionation of hydrodisilanes affords a mixture of oligosilanes 89 up to hexasilane (Eq.45) [83]. The involvement of silylene-platinum intermediate was proven by the formation of a l,4-disila-2,5-cyclohexadiene derivative in the reaction of the hydrodisilane in the presence of diphenylacetylene. 

Studies by Curtis group have offered evidence for the participation of silylene and silanone (R2Si=0) ligands in other catalyzed redistributions. These reactions effect the disproportionation of HMe2SiOSiMe2H into Me2SiH2 and linear polysiloxanes, poss-... 

These thermally induced reactions are thought to proceed via silylene intermediates. Trapping products of silylenes, e.g., with diphenylacetylene, could be observed. More recent publications describe the alkoxylate catalyzed disproportionation reaction to proceed via silanide anions (MeO)2MeSi or (MeO)Me2Sr [62]. 

As mentioned in Section III.6, disproportionation between silicon and SiX4 molecules at high temperatures generates halogenated silylenes, SiX2. In practice, this type of reaction has been extensively employed in the formation of SiFj, as shown in equation (16). The chemistry of the difluorosilylene thus formed likely represents the most comprehensive study of all the known silylenes. 

Evidence for a silylene intermediate in the disproportionation of methylsilicon hydrides, catalysed by /ra j-[PtCl2(PEt3)2], has been obtained by trapping the silylene with diphenylacetylene, giving (55). ... 

The base catalyzed disproportionation of chlorinated disUanes was studied and found to give oligosilanes with up to five silicon atoms. The reaction is believed to involve base stabilized silylenes which insert into Si-Cl bcaids (Scheme 25) [259]. 

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