Hydrosilylation of alkenes with hydrosilanes

Hydrosilylation of Alkenes with Hydrosilanes 462 Hydrosilylation with a hexachloroplatinicllV) acid catalyst 462 Hydrosilylation with a transition metal catalyst 463 Hydrosilylation with a base catalyst 464... 

The catalytic addition of organic and inorganic silicon hydrides to alkenes, ary-lalkenes, and cycloalkenes as well as their derivatives with functional groups leads to their respective alkyl derivatives of silicon and occurs according to the anti-Markovnikov rule. However, under some conditions (e.g., in the presence of Pd catalysts), this product is accompanied by a-adduct (i.e., the one containing an internal silyl group). Moreover, dehydrogenative silylation of alkenes with hydrosilanes, which proceeds particularly in the presence of iron- and cobalt-triad complexes as related to hydrosilylation (and very often its side reaction), is discussed. 

Organosilicon compounds are widely used in our daily life as oil, grease, rubbers, cosmetics, medicinal chemicals, etc. However, these compounds are not naturally occurring substances but artificially produced ones (for reviews of organosilicon chemistry, see [59-64]). Hydrosilylation reactions catalyzed by a transition-metal catalyst are one of the most powerful tools for the synthesis of organosilicon compounds. Reaction of an unsaturated C-C bond such as alkynes or alkenes with hydrosilane affords a vinyl- or alkylsilane, respectively (Scheme 16). 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Alkynes are hydrosilylated more easily than alkenes. Hexenylsilanes 579 and 580 are prepared by the Pt-catalysed hydrosilylation of hexyne with one equivalent of hydrosilane. By the reaction of two equivalents of IlSiCfr, 1,6-disilylhexane 581 is obtained as a major product rather than the 1,2-adduct 582 [223],... 

Recently, radical-initiated hydrosilylation of alkenes was studied again and it was found that trialkylsilanes, which had been poor reactants compared with HSiCl3, efficiently added to alkenes in the presence of radical initiators such as r-butyl peroxide with the use of a large excess of hydrosilanes171 The large excess of hydrosilanes suppresses the polymerization of alkenes which is a severe side-reaction under the radical-initiated process. 

Compared with hydrosilylation of alkenes, less extensive studies have been carried out on hydrosilylation of alkynes. Mono- and dihydrosilylation occur depending on conditions. Yamamoto found concomitant dimerization-hydrosilylation of 1-heptyne catalyzed by Pd-PPhs catalyst to give a mixture of products 77, 78, and 79. Their ratios depend on the hydrosilanes used. The head-tail dimer 78 was the main product when HSiCb was used. The expected monohydrosilylation... 

A proposed mechanism and stereochemical course of the 1,4-reduction are described in Fig. 2. As in the case of hydrosilylation of alkenes described in Scheme 10, the phebox-Rh(l) intermediate 16 reacts with hydrosilane to give a hydro silyl Rh(lll) species 17. The hydride attacks to the Si face on the (3-carbon atom of the a,(3-unsaturated carbonyl compound to give the R-product. On the other hand, the stereochemistry with the Re face attack involves the steric repulsion between the oxazoline substituent and the carbonyl group. In the catalytic reaction. 

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... 

Use of a hydrosilane instead of molecular hydrogen in combination with a transition metal has opened the door leading to hydrosilylation and dehydrogenative silylation of unsaturated bonds. Thus, replacement of hydrogen by a hydrosilane is a reasonable strategy to improve serious issues in hydroformylation of alkenes. Along this line, some types of silylcarbonylation were extensively studied by Murai and his co-workers. However, the silicon moiety always attaches to the oxygen atom of incorporated GO molecule (Scheme 1). 

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... 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

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