Perphenylated cyclosilanes

The most important process so far has been the reductive elimination of halogens with the formation of Si-Si bonds. Kipping used this reaction and discovered the first perphenylated cyclosilanes, yielding polysilanes as a by-product [8]. Similarly dodecamethylcyclohexasilane was found using dimethyldichlorosilane as a starting material for this reaction by Burkhardt in 1949, but 90% of the yield appeared as poly silane by-products [9]. 

The oldest known cyclosilanes are the phenyl derivatives. Perphenylated cyclosilanes (Si< ) with n = 4,5 and 6 are known. Their high molecular weight, chemical stability and low solubility led to some mistaken conclusions regarding ring sizes and structures206- 274- 682>, but today the picture seems clearer. 

The triflic acid derivatives were prepared from the perphenylated cyclosilanes by dearylation with triflic acid (Eq. 3). The subsequent reaction of Si5Phg(CF3S03)2 with lithium aluminium hydride and with hydrogen chloride in a second step led to l,3-dihydrooctachlorocyclo-pentasilane(Eq. 4). 

The reaction of perphenylated cyclosilanes with hydrogen halides under pressure or with aluminum halides as catalysts allows the preparation of (SiCl2)s and (SiCl2)6 (Eq. 5) as well as the preparation of SisPhsIs (Eq. 6). 

In the case of higher polysilanes, similar cleavage reactions have been observed however, in view of the presence of additional Si—Si bonds, reaction control is usually necessary to obtain the desired products. Thus, in the perphenylated cyclosilane series, octaphenylcyclotetrasilane 14) and decaphenylcyclopentasilane 14, 58) have been reported to react with halogens, and the order of halogen reactivity appears to parallel that observed with the hexaalkyldisilanes. On the other hand, dodecaphenylcyclohexa-silane 14,101) does not react with bromine or iodine and reacts with chlorine only under forced conditions. With dodecamethylcyclohexasilane, cleavage with chlorine 20), bromine 128), and iodine 50) has been observed. 

The purpose of this chapter is to discuss the structure, properties, preparation, and reactions of the cyclosilanes in the light of recent investigations of the phenyl and methyl series, and to correct some misconceptions concerning the perphenylated cyclosilanes. Because reactions of these cyclosilanes invariably lead to polysilanes, a section has been included to list these derivatives, many of them new compounds, in tabular form. 

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