Trisilanes cleavage

In the absence of light or a catalyst, reactions of 2 proceed by cleavage of one Si-Si bond and lead, depending on the reactants, to either acyclic trisilanes or ring-enlarged products. 

The reaction of MeLi in Et20 with alkyl- and aryl-substituted branched tetrasilanes proceeds by cleavage of one Si—Si bond, thus affording various trisilanes, which are lithiated at the central silicon atom (equation 26)62. The outcome of these reactions apparently is insensitive as to whether halide-free MeLi or the MeLi-LiBr complex is used62. The decrease in the reaction rate in the order R = Tip > Mes > Ph has been suggested by Fink and coworkers to reflect the relief of steric strain on converting the tetrasilane into the metalated silane626. 

The 1,3-silyl shift in aryl disilanes is suppressed when the aromatic ring is ortho-substituted144. An attempted silylene synthesis from 1,3-dimesitylhexamethyltrisilane 259, however, led to low yields of silylene trapping products (ca 30% generation of Me2S ). The major pathway is the homolytic cleavage of the trisilane, followed by disproportionation of the radicals 260 and 261 to the silene 262 and the disilane 263 (equation 65). 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

Photolysis at 300 nm of one of the stereoisomers of methyl(t-butyl)cyclotetrasilane (Me(t-Bu)Si)4 resulted in the formation of an equilibrium mixture of all four possible stereoisomers indicating cleavage of a ring silicon-silicon bond to a diradical, followed by reclosure. Silylenes, which could be trapped, were also formed during the process96 and low yields of linear trisilanes were also obtained. 

Disilanes and trisilanes with olefinic substituents (such as vinyl- and allyl-disilanes) react in the presence of H2PtCl6 6 H20 undergoing a Si-Si cleavage692. A second reaction sometimes occurs so that simple cleavage products are not obtained. For example, vinylpentamethyldisilane reacts with the cleavage product trimethylsilane, giving an ethane derivative ... 

Gilman found that the a, w-dihydroxy-decaphenylpentasilane formed by a normal cleavage reaction with bromine followed by hydrolysis reacts to form a trisilane when the solution is run through a column of aluminium oxide8 The terminal Si atoms are apparently cleaved by the basic OH groups of the A1203 ... 

A metal-free trimethylsilyl anion is formed from hexamethyldisilane by cleavage with TBAF in HMPA in equilibrium concentration, as revealed by H and F NMR analysis of the reaction mixture. Treatment of trisilane with TBAF provides Me3SiMe2Si /NBu4 and Me3SiF the fluoride anion attacks at the terminal silicon of trisilane [Eq. (21)] (59). Synthetic application of this species is described in Section VII. Other metal-free silyl anions are described in Section V. 

The Si-Si bond cleavage of trisilanes by MeLi in THF is applicable to the preparation of disilanyllithiums, in which a phenyl or mesityl group is attached to a-silicon [Eq. (30)] (69). A similar cleavage reaction occurs with a Na/K alloy to afford disilanylpotassium. These species couple with AdCOCl (Ad = 1-adamanthyl) to give acyldisilane in modest yields. 

Photochemically induced cleavage of hexa-t-butylcyclo-trisilane (213) affords di-t-butylsilanediyl (214) and tetra-t-... 

The reactions of halo polysilanes with organometallic reagents usually proceed with some Si—Si bond cleavage 17, 26, 29, 30, 31), and these reactions have little synthetic value for the preparation of higher polysilanes. Thus, from the reaction between ClgSi3 and phenylmagnesium bromide no trisilane product was obtained, and only tetraphenylsilane and hexaphenyl-disilane were isolated (25). On the basis of reactions of this type, it was postulated (26) that the reactivity of the Si—Si bond in halogenated polysilanes increases as the number of silicon atoms increases. 

With substituents like 9-methylfluorene and diphenylmethane, Si-C bonds can be activated for a cleavage under mild conditions. In contrast to the 9-methylfluorenyl-substituted silanes 7a and 7b, diphenylmethyl-substituted tetraorganosilanes of types 10a, 10b and roc-20 have proven to be valuable precursors for the synthesis of silyllithium reagents like 11a, 11b and rac-21 (Eq. 5). Therefore they correspond well to the silyl anion synthons B. Furthermore the bis(diphenylmethyl)-substituted silane 15 allows a sequential synthesis of unsymmetrical trisilanes and thus is a valuable silyl dianion synthon D (Eq. 6). 

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