Silylmetallics

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... 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

Figure 13. Calculated reaction profile for the silylmetallation process required for the modified-Chalk-Harrod hydrosilylation mechanism.

According to our simulations, hydrosilylation reaction proceeds through the classic Chalk-Harrod mechanism as depicted on the right-hand-side of Figure 5. The modified or anti-Chalk-Harrod mechanism is hindered by a rather large silylmetallation barrier which is calculated to be 46 kcal/mol for the minimal QM model A. 

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). 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/hydrosilylation of enynes initiated by oxidative addition of the H-Si bond of the hydrosilane to form the Rh(iii) silyl hydride complex If (Scheme 7). Silylmetallation of the G=G bond of the enyne coupled with coordination of the pendant G=G bond could form... 

The mechanism of the intramolecular hydrosilylation catalyzed by Rh and Pt complexes was investigated by using deuterated silanes, which indicated the operation of both the traditional Chalk-Harrod hydrometallation and silylmetallation pathways accompanied by rapid P-hydride elimination [65]. This intramolecular reaction was applied to the syntheses of natural products [66],... 

Detailed mechanistic study on these intramolecular hydrosilylation of allylic O-silyl ethers 59 and allylic A -silylamincs 63 using deuterium labeling techniques shows that 5-endo cyclization giving 60 or 64 proceeds via a Chalk-Harrod type hydrometalation catalytic cycle, while 4-exo cyclization process yielding 61 or 65 includes a Seitz-Wrighton type silylmetalation mechanism89. 

A closely related synthetic method involves the addition of [C6H5(CH3)2Si]2CuLi to a-unsubsti-tuted a,/j-unsaturated carbonyl compounds to give anions 4 (R2 = h), followed by alkylation with haloalkanes18 I9. The stereochemistry of the alkylation is generally the same as that of the protonation (i.e occurs via 5 and 6) so that, in its overall effect, this method is complementary to the silylmetalation-protonation. 

Silylmetallic compounds can be prepared in synthetically useful quantities and used to generate new silicon-carbon bonds ... 

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). 

When trialkylsilanes are employed, vinylsilane is often produced as the main product. This oxidative hydrosilylation can be carried out with [Me3SiCo(CO)4], [Cp 2Rh2Cl4] (equation 29), [ Ir(OMe)(COD))2], [Cp Fe(CO)2SiMe3], or Na[HRu3(CO)i ] catalysts. This unusual reaction is understood in terms of silylmetallation rather than the ordinary hydrometallation mechanism. ... 

Solid silylmetallic reagents can be isolated. Evaporation of liq NHj siffords solid reagents, such as Et3GeK pure, solid MejSiLi is prepared by vacuum sublimation and (MejSOjSiLi S THF (tetrahydrofuran) may be recrystallized from pentane and Li(SiPh2)4Li 2 THF from cyclohexane. Crystalline 1 1 adducts, e.g., EtjSiLi-L, are formed by addition of hexamethylphosphoramide (HMPA), tetramethylethylenediamine (TMED), or 1,2-dimethoxyethane (DME) to Et3SiLi in hexane. Solid McjSiLi DME is also known . Isolation of KSiHj, free of solvent and KH is also possible . 

Two moles of silylmetallic form per mole of disilane, and the reagent obtained is salt free, but the disilane (or polysilane) must be prepared from a halosilane, which is the commercially available reagent. For a direct route from chlorosilane to silylmetallic through the disilane, which is not isolated, see 5.5.4.3. 

Organopolysilanes. Cleavage of Si—Si bonds in organopolysilanes also occurs but is not always discriminating, and mixtures of silylmetallics are formed. The best results are obtained with phenyl or silyl substituents. Polysilanes with Me substituents give the most reaetive disilanes species, but mixtures of metallic reagents are obtained. 

Reagent indicated is added and % yield corresponds to coupling product when chlorosilanes or Me PO are added, to isolated silane when H O is added and to active silylmetallic from double titration directly Biphenyl added,... 

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