Secondary silanes

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

The dimeric complex [Pt2(/i-CO)(CO)2(/i-dppm)2] reacts with the secondary silanes SiMe2El2, SiEt2H2, SiPh2H2, and SiMePhEl2 to give complexes of the type [Pt2(/i-SiR2)(CO)2(/i-dppm)2] (8).63... 

Synthesis is typically by alkali metal-mediated coupling of dichlorosilanes or 1,2-dichlorodisilanes, although electrochemical coupling of chlorosilanes and dehydrocoupling of primary and secondary silanes also often lead to oligomeric (as opposed to high polymer) fractions. 

Catalytic alcoholysis reactions between secondary silanes and allylic alcohols give allyloxyhydrosilane products, in which Si-H and C=C double bond functionalities are present [Eq. (61)]. 

Summarizing, aminosilanes show a fast adsorption on the silica surface. An equilibrium monolayer coating is formed. Modification in aqueous solvent causes polymerization on top of the initial monolayer. For modification from organic solvent, the reactions can be better controlled. With the bifunctional AEAPTS, a secondary silane layer adsorbs on the free primary amine groups of the first monolayer. At high concentration and after long reaction times, for both aminosilane types, a further non-specific deposition occurs. 

In addition to ruthenium, Tilley and coworkers also reported that cationic iridium silylenoid complexes were efficient olefin hydrosilation catalysts [reaction (7.6)].56 This silylene complex catalyzes the hydrosilation of unhindered mono- or disubsti-tuted olefins with primary silanes to produce secondary silanes with anti Markovni-kov selectivity. Iridium catalyst 32 exhibited reactivity patterns similar to those of ruthenium 30 only primary silanes were allowed as substrates. In contrast to 30, cationic iridium 32 catalyzed the redistribution of silanes. Exposing phenylsilane to 5 mol% of 32 in the absence of olefin produced diphenylsilane, phenylsilane, and silane. 

The remaining catalyst system, a combination of the commercially available metallocenes, Cp2MCl2 (M = Ti, Zr, Hf) with BuLi was introduced by Corey and co-workers.3m 48,49 With this system there is no induction period when used for the condensation of PhSiH3, and secondary silanes can be coupled at reasonable rates at higher temperatures. 

In his first publication4 reporting the use of Cp2TiMe2 to polymerize primary silanes, Harrod also described the condensation of PhMeSiH2 in toluene and the coupling of this secondary silane occurred much more... 

As a last comment, an attempt to dehydrocouple the labeled secondary silane, PhMeSiD2, to give D(PhSiMe)xD, with Cp2ZrCl2/2 BuLi demonstrated another process that was promoted by this catalyst system.50 Analysis of the oligomer mixture produced revealed that although each oligomer had approximately two deuterium atoms (determined by mass spectroscopy), the label had scrambled between the SiH and CH (methyl group) positions as determined by H and 2H NMR spectroscopy. 

There are several reports of the condensation of bis-silanes of the type H3Si-X-SiH3 (primary silanes) and H2RSi-X-SiR H2 (secondary silanes) principally with the aim of producing polymers that would be precursors to silicon carbide ceramic material. The polymers would, in principle,... 

The carbosilane monomer, H2MeSiCH2CH2SiH3, contains both a secondary and a primary silicon center. After about 48 h with Cp2TiMe2 or Cp2TiCl2/ BuLi at r.t. the dehydrocoupling reaction occurred at the primary silane center to give a polysilane with a substituent at each silicon center that contained a tether to the secondary silane center. The initial polymer had a Mw of approximately 1000. After another 72 h at r.t., the... 

Although a transition metal was not involved, the dehydrocoupling of the secondary hydrosilane, 1,1-dihydrotetraphenylsilole, to polymers with Mw values ranging from 4000 to 6000 (polydispersities ranged between 1.1 and 1.2) in the presence of catalytic quantities of Red-Al, L(N or K)-Selectride or Super-Hydride has been reported.1353 This is the only currently published example of polymeric material produced from a secondary silane. It is interesting that this secondary silane is a heterocyclic system where steric interactions (from substituents at silicon) have been reduced, although the fact that there are phenyl groups on the a-carbon, makes this a rather extraordinary result. 

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