From Monosilanes

The majority of monosilane decompositions involve the elimination of X2 from molecules of the type 81X4, or Hj from molecules of the type SiHjX and 81112X2, as shown in equations (18)-(20). The HX elimination from 8iHX3 as shown in equation (21) has also been observed. 

The photolysis of deuterium-labeled silane molecules was again studied by Ring and coworkers. For the 147.0-nm photolysis and the electric discharge decomposition of equimolar SiH4 SiD4 mixtures, they concluded from the relatively low HD, Si2HD and SiH,D yields that the main primary process in silane photolysis is the formation of SiHj via equation (22). However, in a more recent article, they have cautioned that neither reaction shown in equations (25) and (29) can be ruled out.  

Purnell and Walsh proposed earlier that if SiH4 does decompose to SiHj, a possible configuration for the transition state would involve a three-center bond in the following way  

The spin conservation rule requires that the silylene thus formed be in the singlet electronic state. 

Althoug the mercury-sensitized photolysis of the methylsilanes was consistent with the Si-H bond scission as the primary step as shown in equation (32) to give H and CHjSiHj radicals which combined to give the final products in unit quantum yields, other studies indicated the existence of silylenes as intermediates. The vacuum uv photolysis of CHjSiHj produced SiHMe as shown in equation (33). The major products from the pyrolysis of CHjSiHj are Hj 

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Entries 46 and 48 are examples of trans square-planar Pt(II) derivatives (LV X = Cl, R3 = PhMe2) the latter, with R3 = MePh(l-naphthyl), is chiral at silicon, and its structure is particularly significant because it establishes the absolute stereochemistry of the (+)-enantiomer as (S). The long Pt-Cl bond (245 pm in each case) should be noted it is attributed to the large trans influence of silicon (97, 98, 229). The simple silyl derivative in entry 50 [LV X = H, R3 = (cyclohexyPsi is prepared from monosilane and PtH2[P(cyclo-hexyl)3]2 the Si-Pt distance is appreciably greater than that in other members of this class. 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

Synthesis of disiloxanetetraols directly from monosilanes has been a difficult task, and synthesis of diphenyldisiloxanetetraol was accomplished from PhSi(OAc)3 (1), [PhSiCl2]20 (2), or [PhSi OMe)2l20 (3). In all cases, the overall yields from monosilanes were low. Quite recently, we have found a facile direct synthesis of disiloxanetetraols from trichlorosilanes. In this reaction, disiloxanetetraols were obtained as solids after work-up and washing, no separation was necessary. As an example of this synthesis, herein we describe the preparation of diphenyldisiloxanetetraol. The obtained compound was identified by comparing spectroscopic data. Chemical shift of Si NMR (-62.2 ppm in acetone-de) was identical to the reported data (-62.1 ppm (2)). 

Fig. 10. Product-time curves for the initial stages of pyrolysis of 77 torr monosilane at 430 °C. (3 min = 20 % decomposition.) (From Purnell and Walsh67.)...

Stokland74 found that SiD4 decomposed more slowly than SiH4. There seemed to be a systematic increase in the ratio of the rate coefficients, kD/kH (for the second stage of the pyrolysis), from about 0.54 at 648 °K to ca. 0.7 at 764 °K. The experimental conditions used by all investigators of monosilane pyrolysis, and their results, are summarized in Table 3. 

The possibilities of synthesis are limited by the general properties of the Si-Si bond to a small number of reaction types. Disilane derivatives were usually synthesized by a Wurtz-like reaction of monosilane halide derivatives with alkali or alkaline-earth elements. Some pyrolysis reactions that have yielded disilane derivatives are also known. Often, new disilane derivatives were prepared from common derivatives by an exchange of substituents. Some rearrangements have been observed in recent years. 

It is interesting to note that Ph2MeSiH is the major monosilane from the disproportionation of PhMeSiH2 but that BzMe iH is the major monosilane from BzMeSiH2. It therefore appears that phenyl migration is more facile than methyl migration, but methyl is more facile than benzyl migration in these disproportionations. 

Such a disproportionation might result from the preliminary dissociation of a part of the Si5Hi2, followed by hydrogenation of the remainder to form monosilane and disilane. This spontaneous disproportionation of the higher hydrides explains why long chains of silicon atoms have not been found. 

From our experience that cyclic silanes are formed more likely when using monosilanes with bulky substituents, we tried to electrolyze (undecamethylcyclohexasllanyl)dichlorosilane. However, in this... 

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