Hydrosilanes hydrosilation

In the phase-transfer processes discussed in Section 11.2 it is assumed that the anionic hydride source, i.e. borohydride or a hypervalent hydrosilicate, forms an ion-pair with the chiral cationic phase-transfer catalyst. As a consequence, hydride transfer becomes enantioselective. An alternative is that the nucleophilic activator needed to effect hydride transfer from a hydrosilane can act as the chiral inducer itself (Scheme 11.6). 

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. 

Aluminum trichloride catalyzed hydrosilation provides an interesting alternative to the free radical process. The addition of the silane was found to occur regioselectively at trisubstituted alkenes in a frans -fashion. Mechanistically it was concluded that the reaction proceeds via the addition of a silylenium cation to the multiple bond, generating the more stable (more substituted) carbocation, which abstracts a hydride from another molecule of hydrosilane (Scheme 2). ... 

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

As the hydrosilation reaction is related to hydrogenation, it also bears a certain relationship to the hydroformylation (see Hydroformylation) reaction. The Co2(CO)g catalyzed process, which formylates an alkene in the presence of a hydrosilane instead of hydrogen, gives silyl enol ethers in excellent yields (equation 27). The hydrosilane/CO system can also react with cyclic ethers or aldehydes to give co- or... 

The reaction is also utilized for cross-linking between vinyl-substituted siloxanes and oligomeric hydrosilanes in the production of room-temperature vulcanizable sihcone rubber. Reaction can be carried out without solvents, but common solvents such as benzene, toluene, dichloromethane, THF, and acetonitrile can be employed as well. A wide variety of 1-alkenes are hydrosilated regiospecifically to give... 

Isomerization does not take place on the products. No isomerization of alkenes occurs with the action of chloroplatinic acid alone, but recovered alkenes from a silane-alkene mixture often contain isomers of the alkene. D-H exchange is observed for the recovered hydrosilane from the hydrosilation mixture with DSiCb. These facts suggest the mechanism outlined in Scheme 2, which is known as the Chalk-Harrod mechanism. " The mechanism involves oxidative addition of hydrosilane to the active species of the catalyst, coordination of alkene, followed by <7 — tt conversion, and reductive elimination of the product. 

Hydrosilanes, RsSiH, can be viewed as silylated hydrogen and used as a reducing reagent. Hydrosilation (equation 20) resembles hydrogenation in this sense. Reduction of prochiral ketones through hydrosilation has been investigated extensively with platinum and rhodium complexes having... 

Hydrosilation. The second most important method for preparing or-ganosilanes on a large scale is the addition of hydrosilanes across a carbon-carbon double bond. The reaction is quite general and applies to a wide variety of substituted alkenes, dienes, and alkynes. 

Alkenes with a variety of substituents such as acetal, amine, amide, carbamate, ester, ether, isocyanate, ketone, oxirane, and sulfide can be hydrosilated, usually without affecting the functional group. However, this is not always true for example, allyl chloride gives a considerable amount of the reduction product (see equation 26). Tabled lists representative examples of hydrosilation. It is not so remarkable in the case of radical hydrosilation, but reactivity is high for hydrosilane, which has electronegative group in the case of transition metal catalyzed hydrosilation. 

Hydrosilation Agent. Another application of this difunctional hydrosilane is in the nickel-catalyzed hydrosilation of 1,7-octadiyne, which affords a 1,2-dialkylidenecyclohexane with a (Z)-vinylsilane moiety. This exocyclic silyl diene is treated with an allyl alcohol to give a silicon-tethered triene that undergoes intramolecular Diels-Alder reaction and subsequent oxidative cleavage of the silicon-carbon bond to afford a bicyclic diol (eq 2) ... 

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