Reactions Involving Silicon Hydrides

Both Si-H and C—H compounds can function as hydride donors under certain circumstances. The silicon-hydrogen bond is capable of transferring a hydride to carbo-cations. Alcohols that can be ionized in trifluoroacetic acid are reduced to hydrocarbons in the presence of a silane. 

Aromatic aldehydes and ketones are reduced to alkylaromatics under similar conditions through reactions involving benzylic cations.176 

Reduction of Carbon-Carbon Multiple Bonds, Carbonyl Groups, and Other Functional Groups 

Aliphatic ketones can be reduced to hydrocarbons by triethylsilane and gaseous BF3.178 The BF3 is a sufficiently strong Lewis acid to promote formation of a carbocation from the intermediate alcohol. 

A combination of Friedel-Crafts alkylation and reduction can be achieved using InCl3 and chlorodimethylsilane. The Lewis acid presumably promotes both the Friedel-Craft reaction and the subsequent reduction.179 

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Several examples of oxidative addition reactions of compounds involving group (iv) elements, silicon and germanium, with iridium(i) complexes have been described and reviewed. In the reaction of silicon hydrides (CH3) (C2H50)3 SiH with bis(bis-l, 2-diphenylphosphinoethane)iridium(i), [Ir (diphos)2],... 

Side reactions or postcuring reactions are possible in the formation of silicone networks. In most cases, the silicon-bound hydride is in stoichiometric excess to enhance reaction rates. Disproportionation reactions involving terminal hydride groups have been reported (222). A major side reaction consumes Si—H to give redistributed siloxane in the resulting polymers and gaseous silane as a by-product. 

This review focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. In the past two decades, progress in determining the absolute kinetics of radical reactions in general has been rapid. The quantitation of kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase has been particularly noteworthy, progressing from a few absolute rate constants available before 1980 to a considerable body of data we summarize here. 

Silicones can be prepared in such a way that they contain only one percent or less of vinyl groups and silicon hydrides, which undergo a catalytic hydrosilylation reaction to give the desired cross-linking (Figure 18.3). Vinyl silanes are made by Si-H addition to acetylene, and thus two hydrosilylations are involved. 

Scheme 6.20 Reaction intermediates involved in the autoxidation of silyl substituted silicon hydrides (R = Me or MesSi)...

The reaction between olefins or acetylenes with silicon hydrides in the presence of phosphinenickel complexes has been studied for a variety of substituents on the phosphine and on the silane. In many cases two products are obtained, one of them is the expected simple adduct and the other is an adduct, the formation of which has involved an interchange of hydrogen and chlorine on silicon in the course of hydrosilylation (206) Eq. (90). When methyldichlorosilane was added to octene-1 at temperatures in excess of 120°, the two silanes obtained were CgHi7SiMeClH and CgHi7SiMeCl2. The results outlined in Table II... 

The reaction of metal carbonyl dimers with silicon hydrides also probably involves an initial oxidative addition step. Chiral silyl-cobalt and silyl-manganese carbonyl complexes have been obtained through the reaction of optically active organosilicon hydrides with metal carbonyls65 68 (equation 15 and 16). Phosphine-substituted cobalt complexes were similarly obtained by reaction of a chiral hydrosilane with Co2(CO)6L2 [L = PPh3, P(OPh)3, P(c-C6Hn)3]69. 

Hydrogen atom transfer reactions involving 17-electron hydrides have been considered in a number of cases [54, 58, 86, 99, 112] as alternatives to proton and electron transfers, on the basis of the known atom transfer processes of hydride compounds of tin, germanium, and silicon. In addition, as discussed in section 6.4.3, there is some evidence for M-H bond weakening upon oxidation, suggesting that the homolytic rupture of this bond may take place under favourable circumstances. 

Hydrosilylation.—This reaction is catalysed by the usual homogeneous catalysts. In some cases the mechanism involves insertion of the alkene into a metal-hydrogen bond, as in hydrosilylation of butadiene in the presence of PdL(PPh3)2, with L = p-benzoquinone or maleic anhydride. In other cases concerted addition of the silicon hydride to the carbon-carbon double bond is indicated, as in hydrosilylations catalysed by rhodium(i) catalysts such as RhCl(PPh3)3. In the reaction of silanes with hex-l-ene in the presence of this catalyst, rates depend on the stability of the intermediate adduct RhClH(SiR3)(PPh3)2 such an adduct was isolated in one case. Hydrosilylation of ethylene by trimethylsilicon hydride... 

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