Cyclosilane

In any event, between 1951 and 1975, no papers appeared on polysilane high polymers. However, linear permethylpolysilanes of the type MelSiMezhiMe were prepared and studied, especially by Kumada and his students,(5) and cyclic polysilanes were being investigated in several laboratories.(6,7) Studies of the permethyl-cyclosilanes, (Me2Si)n where n = 4 to 7, showed that these compounds exhibit remarkable delocalization of the ring sigma electrons, and so have electronic properties somewhat like those of aromatic hydrocarbons.(6)... 

Although homonuclear cyclosilanes have been known for a long time, it is only recently that the smallest members of this series, the cyclotrisilanes, have become available [1], Cyclotrisilanes 1 are obtained by reductive halogen elimination from ort/zo-disubstituted diaryldichlorosilanes [2, 3] or, in particular cases, from hexaalkyl-l,3-dichlorotrisilanes [4]. 

We also were able to isolate the first transition metal-cyclosilane with different metal substituents... 

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]. 

Reductive dehalogenating coupling can be used for the preparation of other new and interesting compounds, and a brief consideration of the special field of polysilanes, the cyclosilanes, will show this. 

For the last few years we have tried to synthesize larger two-dimensional silicon frameworks. We wanted to find out how large a molecule must be in order to show delocalization effects of electrons as in elemental silicon. It is well known that radical anions are possible in simple cyclosilanes in which the additional electron is completely delocalized in the cycle [11]. 

We have now tried to make new larger cyclosilanes by the reductive dehalogenation of mixtures of methylchlorosilanes and methylchlorodisilanes. After a separation of the reaction product mixture by means of GC/MS, we found some new cyclosilanes (Fig. 1) [12]. 

Besides some new cyclosilanes with side chains and cage-like compounds, we were able to isolate the decalin analogue cyclosilane Si Me in a yield of about 20%. We characterized the compound by the usual spectroscopic methods, investigated the structure, and found that this compound is able to form a radical anion [12]. In former investigations we were not able to form such radical anions with linearly connected polycyclic systems [13]. 

Reductive dehalogenation cannot be completely controlled, and mostly complicated mixtures were formed which are difficult to separate. Salt elimination opens the possibility of a reaction aimed at polysilane formation. Some examples are shown in Fig. 2. The key compounds are the alkali metal cyclosilanes, which we have isolated via the mercury compounds by the action of sodium/potassium alloy and used for the first time [13]. 

Besides these alkali metal derivatives, partially halogenated cyclosilanes were necessary for these syntheses. The preparation of such compounds is sometimes very troublesome and a description of these routes would be too long for this review, but most of these syntheses have been published [13]. The separation of the isomers is sometimes very difficult and can be made via derivatives with large... 

The most intriguing difference between the chemical properties of cyclopolysilanes and those of cycloalkanes is the ability of the former to form either anion or cation radicals upon one-electron reduction or oxidation, respectively. For example, the cyclic pentamer (Mc2Si)5 is reduced to the corresponding radical anion by sodium-potassium alloy in diethyl ether [see eqn (4.1) in Section 4.1.3], whereas the hexamer (Me2Si)6 is oxidised by aluminium trichloride in dichlor-omethane to the corresponding cation radical. In both cases the EPR spectra of the radical ions can be interpreted in terms of a-electron delocalisation over the entire polysilane ring (see Section 10.1.4.1). In this respect, the cyclosilanes resemble aromatic hydrocarbons rather than their aliphatic analogues. 

The product consists uf various amounts of high polymer (.r is very large) and discrete cyclosilanes with n = 5-35. This is the largest homologous series of cyclic compounds now known except for the cycloalkanes. Although these compounds are formally saturated, they behave in some ways as aromatic hydrocarbons. They can be reduced to amon ladicals. and EPR spectra indicate that the unpaired electron is delocalized over the entire ring. 06... 

Cyclosilanes, cyclosilazanes, cyclosilthianes, and cyclosiloxanes are numbered in the same way as carbon compounds of similar nature. Examples ... 

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. 

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