Cyclosilanes preparation

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)... 

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. 

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... 

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. 

Three permethylated cyclosilanes of the formula [(CH3)2Si]B with n=5-7 are prepared by allowing dimethyldichlorosilane to react with sodium-potassium alloy in tetrahydrofuran. From this reaction, an insoluble polymeric mass and a crystalline solid are produced (21, 177). The latter consists, for the most part, of dodecamethylcyclohexasilane but it also contains decamethylcyclopentasilane and tetradecamethylcycloheptasilane in small quantities. The amounts of both five- and seven-membered cyclosilanes are much increased if the reaction is worked up immediately after the addition of the dichlorosilane is completed with little or no refluxing. The three cyclosilanes can be separated by preparative gas chromatography 21). 

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. 

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. 

The triflic acid derivatives were prepared from the perphenylated cyclosilanes by dearylation with triflic acid (Eq. 3). The subsequent reaction of Si5Phg(CF3S03)2 with lithium aluminium hydride and with hydrogen chloride in a second step led to l,3-dihydrooctachlorocyclo-pentasilane(Eq. 4). 

The reaction of perphenylated cyclosilanes with hydrogen halides under pressure or with aluminum halides as catalysts allows the preparation of (SiCl2)s and (SiCl2)6 (Eq. 5) as well as the preparation of SisPhsIs (Eq. 6). 

From a synthetic point of view, obtaining difunctional organosilanes allowed the preparation of new functional cyclosilanes. Optically active oxasilacyclopentane and thiasilacyclopentane were, for example, obtained from (-)-( — )l-NpPh(MenO)SiH by use of the stereoselective reaction sequence depicted in Scheme 433,34. 

In contrast to the acyclic series, families of cyclosilanes are known for many different alkyl groups. The methyl compounds were however the first to be prepared and are the most thoroughly studied. 

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). ... 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

The preparation of polysilanes by the reaction of intermediately formed silylmetallics, derived from cyclosilanes, will be discussed in the next section. 

The usefulness of these dilithio reagents is illustrated further by the preparation of other cyclic organosilicon and cyclosilane derivatives (see also Section VI on General Trends). 

In addition, the cleavage of octaphenylcyclotetrasilane and decaphenyl-cyclopentasilane with lithium in the presence of chlorosilanes has provided access to a variety of polysilane derivatives. Other reactions describing the use of cyclosilanes for the preparation of polysilanes have been thoroughly reviewed 14),... 

The cyclohexasilane derivatives [(CO)2CpFe]—SisMen, [(CO)3PPh3Co]—SieMen, 1,3- or l,4-[(CO)2CpFe]2—SisMeio and l,4-[(CO)3PPh3Co]2-SieMem can be made quite similarly Manganese derivatives of cyclosilanes have also been prepared . An inverse salt elimination procedure, however, has to be applied in order to prevent extensive transmetallation reactions (equation 30). 

The purpose of this chapter is to discuss the structure, properties, preparation, and reactions of the cyclosilanes in the light of recent investigations of the phenyl and methyl series, and to correct some misconceptions concerning the perphenylated cyclosilanes. Because reactions of these cyclosilanes invariably lead to polysilanes, a section has been included to list these derivatives, many of them new compounds, in tabular form. 

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