Cyclosilane ring

Photolysis of cyclosilane rings containing heteroatoms also leads to silylenes, and sometimes this is the preferred route. An example is shown in equation 1444. Likewise, hexaneopentyl trisilaoxetane undergoes photolysis to give the silylene and a disilaoxirane... 

Reduction potentials have not been reported for cyclosilanes, but competitive experiments have established the following order of relative reducibility [( -Bu)MeSi]4>(Me2Si)5> benzene >(Me2Si)6>(Me2Si)7. The lower reduction potentials of the four- and five-membered cyclosilane rings are attributed to better orbital overlap in the SOMO in these planar (or nearly planar) rings9. 

Since only polymers derived from cyclic starting materials, which are likely to have siloxene-like structures, exhibit color and fluorescence, the polysilane ring seems to be essential for the exceptional optical properties. This is in agreement with the original idea assuming the cyclosilane ring to be the chromophore responsible for the photoluminescence of siloxenes. 

Matyjasjewski s procedure can be applied to other types of cyclosilane rings. Thus, the ROP of the five-membered cyclosilane [(PhMeSi) (Me2Si)4] can be accomplished by silylanion initiators such as Mc2PhSiK (see Eq. 7.6) [48]. 

Cyclosilane ring opening Acoxysilanes from silanes... 

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)... 

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 with hydrogen atoms directly attached to the ring do not react to give oligomers in the presence of the usual catalysts like Cp2ZrMe2 or Cp2TiMe2. 

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

From other aliphatically substituted cyclosilanes only a deca(i-butyl)-cyclopentasilane was isolated302 preparation of a ring with t-butyl group seems to be prevented by steric hindrance. 

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. 

It is perhaps relevant here that the fluorescent compound formed in the hydrolysis of iodine cyclosilanes also has rings isolated by oxygen. It is significant, moreover, that two-dimensional polysilanes become fluorescent if a slight partial oxidation takes place. 

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