Reactions of Cyclosilanes

The reaction of cyclosilanes with organometallic and organosilylmetallic reagents gives synthetically useful polysilylmetallic derivatives (see also Section VI on General Trends). 

To present a brief survey of the chemistry of the organic substituted silicon-silicon bond, as it bears on the reactions of cyclosilanes which have... 

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. 

Diene-Iron Carbonyl Complexes and Related Species R. Pettit and G. F. Emerson Reactions of Organotin Hydrides with Organic Compounds Henry C. Kuivila Organic Substituted Cyclosilanes... 

Scheme 56 Reaction of bromine with the C6o cyclosilane adduct 129a.

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. 

Reaction of 3a with an equimolar amount of (BusSiNa at -78°C with subsequent warming to room temperature leads to a mixture of various cyclosilanes. The cyclotrisilane 8 is the major component (59%), other products are the cyclotrisilane 9 (5%) and the cyclotetrasilanes 10 (8%), 11 (25%) and 12 (3%) [2] The all-c/s -cyclotetrasilane is not formed. At higher temperature (65°C) 3a reacts with U3SiNa mainly to the cyclotrisilane 9 (74 %) and not at all to the cyclotrisilane 8. The yields of the cyclotetrasilanes are not significantly temperature-dependent. 

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). 

The methyl(chloro)cyclosilanes were achieved by the reaction of the corresponding phenyl-(methyl)cyclosilanes with hydrogen chloride (Eqs 7-8). 

FIGURE 2. Yields (wt%) of cyclosilanes (EtMeSi) from the reaction of EtMeSiCl2 and potassium as a function of m left scale is for n>8. Reproduced by permission of Elsevier Sequoia from West et ai, J. Organomet. Chem., 271, 353 (1984). 

The photolysis of cyclosilanes with 254-nm UV light leads to elimination of divalent Si intermediates (silylenes), and reclosure of the ring. In addition to being a source of silylenes, this reaction is useful in the synthesis of cyclosilanesIn the permethyl series, photolysis of (Me2Si)g leads successively to (Me2Si)j and (Me2Si)4 ... 

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. 

Reaction of the appropriate diorganodihalosilane with metals gives cyclosilane derivatives such as octaphenylcyclotetrasilane (14), decaphenyl-cyclopentasilane (14), dodecaphenylcyclohexasilane (14), and dodeca-methylcyclohexasilane (14, 49, 50). The above reactions, as well as the... 

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

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 conversion of silicon-functional polysilanes (obtained from cyclosilanes) to other polysilane derivatives is also possible. Thus, reaction of... 

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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