Cyclosilanes silanes

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 reaction of cyclic silane 23 with lithium metal in THF opens an unexpected way to c-(MenSi6)Li (24). Whereas in tri-, tetra- and penta-cyclosilanes a Si—Si bond is cleaved under these conditions (Section VLB.3), 23 reacts with cleavage of a exocyclic Si—C bond (equation 35)85. The reasons for the different behavior of 23 toward lithium remain unclear. It was, however, suggested that the facile formation of 24 is due to its thermodynamic stabilization by electron delocalization into the silicon framework85. 

Cyclic peralkylsilanes exhibit unique behavior which distinguishes these compounds from saturated catenates of carbon. In some ways, the properties of the cyclosilanes resemble these of poly-unsaturated or aromatic hydrocarbons. As shown in Table 12, five-membered permethyl cyclic silane shows a higher oxidation potential compared with... 

Hexakis(2,6-dimethylphenyl)cyclotrisilane, the first known stable three-membered cyclosilane, was prepared in 1982 by Masamune and coworkers from dichlorobis(2,6-dimethylphenyl)silane by the reductive dechlorination with lithium naphthalenide in low yield34. Other cyclotrisilanes bearing substituents of comparable steric bulk have been synthesized analogously. The chemistry of the cyclotrisilanes presently known has recently been reviewed35. 

V. POLYCYCLIC SILANES AND CAGES A. Linearly Connected Cyclosilanes... 

Difunctional cyclosilanes, which are useful monomers for the synthesis of oligocyclic and cage like silanes, are obtained after chlorination of dodecamethylcyclohexasilane with SbCls in CCI4 [1, 2] or with Me3SiCl/AlCl3 [3], respectively. 

A well established method for the preparation of metalated silanes is the cleavage of Si-Si a-bonds by alkali metals [1]. The reaction of arylsubstituted cyclosilanes yields the corresponding a,(0-dilithiated oligosilanes. Whereas the reaction of cyclopenta- and cyclotetrasilanes with lithium is well known [2, 3], a metal mediated cleavage of a cyclotrisilane was not described up to now. Here we report the reaction of cyclotrisilane 1 with lithium, which affords, depending on the conditions, either 1,3-dilithiotrisilane 2-Li or l,2-dilithiodisilane3-Li. 

To elucidate the question whether or not cyclosilane structures are essential for the color and fluorescence of the polymers, linear and cyclic silanes with silicon halogen or -triflate functions were prepared and hydrolyzed to siloxene-like structures. 

Cyclosilanes can be synthesized according to the methods outlined above in the sections on saturated and unsaturated organylsilanes. In the following we will discuss the various classes of cyclic silanes in the order of increasing ring size. 

The feature of the reaction with potassium alkoxide not to cleave inner Si-Si bonds made us optimistic that with larger silanes multiply metalated products could be obtained. Such compounds have been prepared before by cleavage of strained cyclosilanes with alkali metals (Eq. 12). ... 

Apparently all organopolysilanes are stable to spontaneous decomposition, and the majority of them melt without decomposition. In the cyclosilane series, octaphenylcyclotetrasilane 14), octa-/>-tolylcyclotetra-silane 14), decaphenylcyclopentasilane 14, 58), and dodecaphenylcyclo-hexasilane 14,58,101) all decompose to some extent at their melting points. Although dodecamethylcyclohexasilane 14) appears to be stable toward decomposition, it is reported 102) to undergo an irreversible change in crystalline form when heated to temperatures greater than 74° C. 

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. 

Cyclosilane ring opening Acoxysilanes from silanes... 

The first examples of well-defined cyclosilanes, also reported by Kipping, were products obtained from the reaction of sodium with dichlorodiphenyl-silane in boiling toluene. Three distinct crystalline compounds were isolated they were designated as Compound A, Compound and Compound C. Other amorphous solids and intractable glues were also obtained, although now they appear to have been polymeric in nature. Compound A was also prepared in comparable yield from chloro-phenoxydiphenylsilane and sodium. 

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