Silylmetallic species, reactions

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

The formation of silylmetallics by cleavage of Si—Si bonds with alkali metals is described in 5.5.42.1. Because Si—Si bonds can be formed by condensation of halosilanes with alkali metals, a direct route to silyl metallics should be possible by forming and reacting the disilane (or polysilane) with a metal without isolating the intermediate Si—Si-bonded species. However, this approach contains the same inherent restrictions as those described in 5.5.4.2.1 i.e., there must be one aryl group bonded to Si in the starting halosilane (see Table 1). Reaction of PhjSiCl does not occur with Na, but... 

To explain the predominant appearance of unusual trans addition products (Z-alkenylsilanes), Crabtree and co-workers (48) and Ojima and co-workers (74) proposed a mechanism based on silylmetallation in the migratory insertion step followed by E/Z isomerization (Scheme 9). The most significant feature of this mechanism is the exclusive insertion of alkyne into the M—Si bond, forming a (Z)-silylvinylene complex. Because of the steric repulsion between the silyl group and the substituted metal atom, the (Z)-silylvinylene intermediate isomerizes to thermodynamically favorable (E)-silylvinylene complex through either zwitterionic carbene species (74) (proposed for Rh-catalyzed processes) or metallacyclo-propene intermediate (77) (postulated for Ir-catalyzed reactions). Since the reductive elimination is the rate-determining step, (Z)-alkenylsilanes are formed as the kinetic products. 

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