Silylmetallation alkynes

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 reaction of 1-propargyltetrahydroisoquinoline 76 with PhMe2SiH, catalyzed by Rh(acac)(CO)2 at 60°C and 50 atm. CO furnished the silylcyclocarbonylation (SiCCa) product 77 in 65% yield (Eq. 22) [49]. The SiCCa reaction has been applied to the syntheses of enantiopure indolizidine skeletons, where the aminolysis of the acyl-[Rh] bond took place instead of reductive elimination after silylmetallation of the alkyne moiety [49]. 

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

An alternative route to the hydrosilylation product of monosubstituted alkynes (29) is that which involves silylmetallation of the alkynes (equation 16). A cuprate reagent (34) prepared from CuCN and 2 mol of Me2PhSiLi adds in a syn manner to the bond with the Me2PhSi group at the terminal carbon. Quenching with water affords a product of type (31). 

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. 

Silylzinc reagents are also employed for the silylmetalation reaction of alkynes and alkenes. Bis(dimethylphenylsilyl)zinc adds across these unsaturated substrates in the presence of a copper(I) catalyst (Scheme 3-34). The addition reaction proceeds in a stereospecific manner always cis adducts are produced. 

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