Cyclopolysilanes

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. 

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We have attempted the polymerization of octaphenylcyclotetrasilane, which is the only commercially available strained cyclopolysilane. 

Normally, no small cyclopolysilanes are observed in these reactions. Two exceptions we have noted are the very slow reaction of phenylsilane under the influence of Cp2TiMe2 and the reaction of benzylsilane under the influence of dimethyltitanocene at very long reaction times. From both of these reactions we isolate a single isomer of the cyclohexasilane, in ca. 10 per cent yield in the case of the phenylsilane and ca. 60 per cent yield in the case of the benzylsilane. These isomers are believed to be the all-trans isomers. The phenyl derivative is identical to that... 

In the case of dichlorosilanes, oligomerization to form cyclopolysilanes occurs in high yields, with the product s ring size dependent upon the steric bulk of the starting silane (219). Boudjouk initially reported (221) the synthesis of West s novel disilene (222) upon sonication of lithium with the highly hindered bis(mesityl)dichlorosilane [Eq. (44)]. It is difficult, however, to obtain consistent results with this sonochemical synthesis of the... 

The ESR spectra of the radical anions of the ladder polysilanes show a relatively broad signal with satellites (Fig. 16). From the intensity, the satellites are attributed to the 13C nuclei at the -positions of the isopropyl groups rather than the 29Si nuclei. The number of spin couplings is equal to the number of equivalent carbon atoms at the a-positions, indicating that the spin is highly delocalized. These results resemble those of the radical anions of other cyclopolysilanes observed by ESR57 and can be explained by... 

Oxidation of 44 is completely inhibited by the addition of a small amount of 2,4,6-tri-f-butylphenol. Consequently, the oxidation of the Si—Si bond with oxygen displays the following features 1) a Si—Si bond which is either angle-strained or substituted with more than two fluorine atoms is easily oxidized 2) a radical mechanism is operative 3) the insertion of oxygen into the Si—Si bond proceeds stereospecifically . Incorporation of molecular oxygen was also observed in the direct photolysis of the cyclopolysilanes 80 and 81 (equations 88 and 89) . ... 

There have been a number of routes to silylenes that are related to the simple photolysis of the precursors. The principal photoprocesses of alkylpolysilanes in solution are (a) chain abridgement through ehmination of silylene and (b) chain scission by Si—Si bond homolysis. The photolysis of trisilanes [RR Si(SiMe3)2] or cyclopolysilanes [(RR Si) ] is a well-established method for the generation of silylene (Scheme 14.7). ... 

The permethylated cyclopolysilanes are colourless, crystalline solids. The hexamer (Me2Si)e adopts a chair conformation with negligible methyl-methyl interactions in the solid state, whereas the larger rings (Me2Si) ( = 13 or 16) are distorted in response to endocyclic interactions between methyl groups (see Figure 6.1). In solution, the permethylated cyclopolysilanes show only sharp... 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

By extension uf the reactions involved in the formation of cyclopolysilanes. West and Carbeny"7 synthesized bicydic and cugc permcthylpolysilanes such as ... 

Exceptionally long Si—Si bonds in cyclopolysilanes are found in compounds such as 75-79 which are bearing bulky substituents such as f-Bu. 

FIGURE 10. The dependence of Si—Si—Si bond angles on ring size in cyclopolysilanes... 

As mentioned above, the photolysis of cyclopolysilanes was the earliest method for the generation of silylenes in matrices. Indeed, such extrusions from oligosilanes were used... 

A third photochemical access to silylenes, beside the isomerization of silenes and the photolysis of tri- or cyclopolysilanes, is the irradiation of geminal diazidosilanes, which under nitrogen loss often gives the corresponding divalent silicon species. However, silan-imines are frequent by-products164 or in some cases even the only products (Section IV). 

The parent cyclotetrasilane Si Ix and all the perfluorinated cyclopolysilanes are so far still unknown. More recently, the efforts in cyclopolysilane chemistry are mainly directed towards polycyclic and cage-like compounds and towards partially functionalized derivatives, which may be used as synthons for larger polysilane frameworks. 

The chemistry and properties of cyclopolysilanes have been summarized in various earlier reviews. The review published in 1995 by Hengge and Janoschek provides the most up-to-date summary of the field of cyclopolysilane chemistry and covers the literature up to 19945a-d. In the current chapter, therefore, recent results gained since 1994 will be emphasized and only a short survey of older investigations will be given. The interested reader is referred to the reviews cited above for further details of previous work. Cyclopolysilanes containing endocyclic heteroatoms are also beyond the scope of this article. 

In order to quantify the dependence of the preferred ring size obtained in cyclopolysilane synthesis on the bulk of the substituent groups on the silicon Cartledge postulated a set of parameters derived from the rates of acid catalyzed hydrolysis of appropriate hydrosilanes8. The model, however, was applied only with limited success9. 

Catalytic dehydrogenative coupling of organohydrosilanes, a common method for the formation of linear polysilanes, can also be applied to cyclopolysilane synthesis. Thus, benzylsilane reacts to give all-fraws-hexabenzylcyclohexasilane in the presence of dimethyltitanocene (equation 3). However, extremely long reaction times are required and only moderate yields are obtained15. 

Cyclopolysilanes bearing several different substituents can be assembled in a step-by-step procedure from small appropriately substituted organosilane precursor molecules. An example originally presented by Uhlig16 and modified later in order to obtain the reaction product free of polymers17 is shown in Scheme 1, where... 

Compared to hydrocarbon rings, cyclopolysilanes are much more flexible due to their rather flat potential energy surfaces. Five- and six-membered cyclopolysilanes usually adopt folded conformations. The energy differences between stable conformers, however, exhibit values of only 5-9.2 kJmoP1 in SisHio and SigH. Additionally,... 

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