Catalysts with silicon—hydrogen bond

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The hydrosilylation of alkenes (Equation 16.12) and alkynes (Equation 16.13), alternatively termed hydrosilation, is the addition of a silicon-hydrogen bond across the C-C TT-bond to form a new alkylsilane or vinylsilane. This reaction has been catalyzed by complexes containing many different metals, but is most commonly conducted with complexes of platinum, rhodium, and palladium. The hydrosilylation of alkenes t3q>ically forms terminal alkylsilanes as the major regioisomer, and the hydrosilylation of vinylarenes often generates the chiral branched alkylsilane. The hydrosilylation of alkynes has also been developed. As shown generally in Equation 16.13, these reactions can occur by either cis or trans addition, depending on the catalyst. In some cases, the reactions of silanes with olefins form vinylsilanes (called dehydrogenative silylation. Equation 16.14). The addition of an Si-Si bond of a disilane across an olefin has also been reported (Equation 16.15), and this reaction is called disilation of olefins. 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

EtC=CSiMe3, MeC = CSiMe2-i-Pr, and MeC = CSiMe2-/-Bu all fail to polymerize with any of the Nb and Ta catalysts62. It will be reasonable to attribute this finding to the steric effect. MeC = CSiHMe2 does not polymerize with Nb or Ta catalysts, either 64). In this case it is presumed that the active hydrogen bonded to the silicon atom in the monomer reacts with the active species of polymerization to decompose it. 

A proposed mechanism for the rhodium-catalyzed alcoholysis is represented in Scheme 49 (77). In the first step, activation of the hydrosilane occurs through oxidative addition. Formation of the alkoxysilane then takes place by nucleophilic attack of a noncoordinated alcohol molecule. The dihydro-rhodium complex 143 thus obtained liberates a hydrogen molecule upon reductive elimination. Nucleophilic cleavage of the silicon-rhodium bond, without prior coordination of the alcohol at the rhodium is supported by results obtained in asymmetric alcoholysis (cf. Sect. II-D). Optical yields were shown to be little dependent on the catalyst ligands (in marked contrast with the asymmetric hydro-silylation), indicating but weak interaction between alcohol and catalyst during the reaction. Moreover, inversion of configuration at silicon, which occurs in the particular case of methanol as solvent, is not likely to occur in a reaction between coordinated silane and alcohol. 

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... 

Templates or SDAs formed by a single molecule or ion can be used in hydro-thermal methods to shape the final products. An assembly of molecules can also act as a template to direct the formation of new materials. The well-known silica MCM-41, with a unidimensional structure of hexagonal pores, is a good example. A surfectant (e.g., NR4 cations) is used as SDA, TEOS or a similar compound as a silicon source, and water as a solvent a catalyst (acid or basic) is also added. At concentrations above the critical micellar concentration, the surfactant molecules are ordered in micelles (layers, spheres, cylinders, etc.), where the molecules are weakly bonded by van der Waals or hydrogen bonds. Supramicel-lar interactions lead to a liquid crystal (LC) structure, on whose walls the inorganic species are formed, oligomerized, and finally polymerized [44]. 

---