Cyclopentasilanes

Conformational analysis of the novel bicyclo[3.3.0]octasilane and bicyclo[4.2.0]octasilane systems was carried out by X-ray crystallography (Figs. 19-21). The bicyclo[3.3.0]octasilane framework of trans-17 has a trans-fused structure of cyclopentasilane rings. The trans-fused structure is... 

Figure 5.2. Gel permeation chromatogram (GPC) of the liquid silicon precursor for Si film formation for (a) cyclopentasilane (CPS) and (b) UV-irradiated CPS, both of which were diluted with toluene (1 vol.%) before GPC measurements. The UV-irradiation conditions were 405 nm, 100mW/cm2, and a 10-min irradiation for 1cm3 of CPS. The broad peak around Mw = 2600 corresponds to polysilanes of various molecular weights, as a result of the photo-induced polymerization of CPS. [Reproduced with permission from Ref. 10. Copyright 2006 Nature Publishing Group.]...

Hengge, E. Bauer, G. 1973. Cyclopentasilan, das erste unsubstituierte cyclische Siliciumhydrid. Angew. Chem. 85 304-305. 

Scheme 10 Formation of sequence-ordered polysilane 19 by ROP of phenyl-substituted cyclopentasilane.

Electron diffraction data on gaseous cyclopentasilane (61) fit both the C2 and C, models. Rustad s simple MM method calculates virtually equal energies for these conformers, and 61 is likely to undergo pseudorotation, as does cyclopentane (200). 

Whereas photolysis of cyclohexasilanes usually results in silylene elimination and the successive formation of cyclopentasilanes and cyclotetrasilanes79, attachment of the Fp group to the ring as in 162 results in rearrangement to a silyl-substituted cyclopentasilane, 163, revealing that the presence of the transition metal alters the chemistry of these systems... 

Silylenes are produced by several photochemical routes. Dimethylsilylene, Me2Sit, is produced during the photolysis at 254 nm of dodecamethylcyclohexasilane, (Me2Si)6, but the cyclopentasilane and lower homologues are by-products of this approach, which... 

The first successful synthesis of a cyclosilanyl transition-metal compound was published by West and coworkers99 in 1980, who prepared methylated cyclopentasilane derivatives containing one or two [Fe(CO)2Cp] ligands from the cyclopentasilanyl chlorides and Na[Fe(CO)2Cp] (equations 28 and 29). 

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. 

Summary Mixtures of structural isomers of permethylated dichlorocyclohexasilanes and disubstituted cyclopentasilanes can easily be separated after hydrolysis. The syntheses and a crystal structure are discussed. 

Synthesis, Reactivity, and Spectroscopy of Phenylated Cyclotetrasilanes and Cyclopentasilanes... 

All cyclopentasilanes investigated so far have been prepared from deca-phenylcyclopentasilane by deaiylation with triflic acid (triflation) and subsequent reaction of the triflate compounds with lithium halides, potassiiun halides or lithium aluminum hydride. The reaction sequences are symbolized below (Scheme 1, phenyl groups are not shown). 

Mislow et al. have used empirical force fields to study structures and conformations of the noncarbon members of group 14. Initially, parameterization of MMP was extended to cover compounds containing Si—Si bonds in order to treat polysilanes.Tbe types of compound considered were disilane, trisilane, tetrasilane, hexaphenyldisilane, cyclopentasilane, and 1,3,5,7-tetramethyltetrasilaadamantane. The minimum energy conformations calculated with their parameters matched experimental data well. The bond lengths are accurate within 0.05 A and bond angles within 2°. 

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