Hydrosilanes, addition

Stereochemistry of Hydrosilane Addition to Alkynes Platinum and Rhodium Catalysts. Hydrosilylation of hex-1-yne with trichlorosilane in the presence of hexachloroplatinic acid gave a mixture of two vinylsilanes (eq. [39]) (190). 

Interestingly, Harrod and co-workers (259) also compared the kinetics of triphenylphosphine and hydrosilane addition to a neutral iridium complex (eq. [89]). The kinetic parameters for the phosphine and silane reaction were almost identical AHp - AHSi = 0 kcal/mol and A5p - ASsi = 13 e.u. This striking similarity is not necessarily proof that phosphine and silane addition are mechanistically analogous processes. However, this kinetic behavior is consistent with the above description of oxidative addition. 

Interestingly, Harrod and coworkers73 also compared the kinetics of triphenylphos-phine and hydrosilane addition to a neutral iridium complex (equation 18). The activation enthaplies for the phosphine and silane reaction were almost identical ... 

As to the stereochemistry of the 1-silyl isomer, it has been shown that the trans-isomer is exclusively produced through ds-addition. This is a characteristic feature of the hydrosilylation of alkynes in the presence of platinum catalysts. In contrast, trans-addition giving a ds-adduct is the predominant process on using free radical initiators37. Dichlorosilane also adds to 1-alkynes in the presence of a platinum catalyst giving a mixture of 1 1 adducts and 1 2 adduct38. Again, the mode of the hydrosilane addition is exclusively cis (see equation 8). 

On the other hand, when hydrosilane additives were introduced in the reaction mixture, a significant improvement of the cycloisomerization reaction was observed (Scheme 7.43) [64]. 

Further studies enriched the field with several palladium precatalyst as [(phen)PdMe(MeCN)]BarF [62], [(n -aUyl) Pd(MeCN)2][OTf] [63], and so on, acting as palladium-hydride species under the reaction conditions. Thus, those are providing good to excellent yields of cyclized products in a high ratio between the major regioisomers and without the need of hydrosilane additives (Scheme 7.45). 

SCHEME 7.43 Improvement of diene cycloisomerization with hydrosilane additives under palladium catalysis. 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

Structures that come to mind for assignment to die observed spectra are those that would be expected from an initial disproportionation of two primary silyl radicals to a silene and a hydrosilane, followed by an addition of another silyl radical to the silene, producing a / silylated silyl radical. Repetition of the process would eventually lead to a highly sterically encumbered and undoubtedly persistent silicon-based radical carrying only silicons in its / positions ... 

Mono- and bis(silyl)platinum(II) complexes are believed to play important catalytic roles in hydrosilylation, dehydrocoupling, and double silylation reactions with disilanes and hydrosilanes. A stable, mono(silyl)platinum(II) complex has been prepared by the oxidative addition reaction of the sterically hindered, primary arylsilane 2,6-Mes2C6H3SiH3 (Mes = 2,4,6-trimethylbenzene) to the platinum(O) species [Pt(PPr3)3] in hexane solution at room temperature.133 The colorless product m-[PLl 1(2,6-Mes2C6II3(11 )2Si)(PPr3)2] (21) was isolated as the OPPr3 adduct, and its... 

Summary Multifunctional (meth)acrylate alkoxysilanes synthesized from commercially available acrylate compounds and mercapto-substituted alkoxysilanes or hydrosilanes are used as novel precursors for inorganic-organic copolymers. The alkoxysilyl groups are available for the formation of an inorganic Si-O-Si backbone by sol-gel processing. The (meth)acrylate groups allow the additional formation of organic polymer units by thermally or photochemically induced polymerisation reactions. 

The proposed mechanism starts with a methyl group abstraction on platinum complex 416 with the borane reagent in the presence of diyne 414 (Scheme 105). The square-planar cationic diyne-platinum(n) complex 417 is converted to the octahedral platinum(rv) hydride intermediate 418 through oxidative addition of the hydrosilane. This complex decomposes rapidly with methane release to form another tetracoordinated platinum(n) species 419, followed by platinasilylation of the triple bond. The resulting vinylplatinum 420 undergoes an intramolecular carboplatination to... 

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... 

The catalytic reaction giving allenes by the addition of a hydrosilane twice to 1,3-diynes65 has been applied to the asymmetric synthesis of axially chiral allenylsilanes although the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (23, 43 )-PPM is the best catalyst for the addition of phenyldimethyl-silane to diyne 52 giving allene 53 with 22% ee (Scheme 14).66 663... 

Although it is not a catalytic asymmetric hydrosilylation, chirality transfer was reported in the palladium-catalyzed addition of an enantiomerically enriched hydrosilane to norbornene.76... 

This concerted process may operate in the case of d° early metal complexes where the oxidative addition is forbidden [194]. Nevertheless, it was postulated also in the interaction of a dihalo-ruthenium(II) intermediate and a hydrosilane... 

The nature of the M-H bond-forming step, (ii), in a given catalytic cycle depends strongly on the reducing agent used. Dihydrogen [13, 14, 17, 20, 24, 29] and hydrosilane [78, 81, 82] react mostly by oxidative addition [193, 209, 210]. For example, the product of the reaction in Eq. (23) - which is involved in an... 

Reduction of a., -unsaturated carbonyl compounds. Hydrosilanes, particularly (QH,)2SiH2, in the presence of Pd(0), and a Lewis acid, particularly ZnCl2, can effect selective conjugate reduction of unsaturated ketones, aldehydes, and carboxylic acid derivatives. Chloroform is the solvent of choice. In addition, 1 equiv. of water is required. Experiments with D,0 and (C6H,),SiD2 indicate that... 

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