Homogeneous hydrosilation reactions

The addition of Si-H bonds to organic unsaturations such as olefins, acetylenes, and ketones is known as the hydrosilation reaction. A majority of the hydrosilation reactions are catalyzed by soluble transition-metal complexes and are known as homogeneous hydrosilation reactions [43]. Catalytic hydrosilation reactions are known to be very complex reactions. However, a few generalizations have been drawn about the mechanism of such reactions (Figure 15.4). Homogeneous olefin hydrosilation are presumed to start with an obligatory oxidative addition of the Si-H bond in a cis fashion to the catalytic metal [44]. This process is followed... 

Our interest in silicon chemistry quite naturally led to a study of the hydrosilation reaction, the addition of the Si-H group across an olefin or an acetylene. This reaction is one of the most useful methods of making silicon-carbon bonds and is an important industrial process. Typically, homogeneous catalysts based on platinum, rhodium or ruthenium are used, and while very efficient, they are not recoverable(46). 

Previous work (48) with homogeneous analogues showed that Si-H oxidative additions yield cis products. A cis geometry of hydride and silyl may be allowed in catalytic hydrosilation. Because the industral homogeneous hydrosilation catalyst (62) is h PtClg we tested the activity of our surface generated reagent for the reaction of Equation 24. A suspension of the catalyst was irradiated in 1-... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Hydrosilicate formation is also in evidence in the Cu(II)-Si02 system. Via precipitation from a homogeneous solution one can obtain highly dispersed copper oxide on silica (cf. above, Fig. 9.10, where it should be noted that the Cu case is more complicated than the Mn one in that intermediate precipitation of basic salts can occur). Reaction to copper hydrosilicate is evident from temperature-programmed reduction. As shown in Fig. 9.12 the freshly dried catalyst exhibits reduction in two peaks, one due to Cu(II) (hydr)oxide and the other, at higher temperature, to Cu(II) hydrosilicate. Reoxidation of the metallic copper particles leads to Cu(II) oxide, and subsequent reduction proceeds therefore in one step. The water resulting from the reduction of the oxide does not produce significant amounts of copper hydrosilicate, in contrast to what usually happens in the case of nickel. 

In some homogeneous alkyne hydrosilations, a second product (B) is sometimes found in addition to the usual one (A). How do you think B is formed Try to write a balanced equation for the reaction, assuming an A/B ratio of 1 1 and you wilt see that A and B cannot be the only products. Suggest the most likely identity for a third organic product C, which is always formed in equimolar amounts with B. 

The author s association with industrial hydrosilation processes and research for a number of years has been particularly enlightening in connection with several commercially important reactions and their nuances, oddities and challenges. This has helped the development of certain perspectives on the reaction which will undoubtedly infiltrate parts of this review. It is hoped that these occasional views will enhanee the value of this chapter to the readership. Also, because Ojima s second review was published fairly recently there will be overlap of coverage. However, the organization of this review is not as elaborate and it will only cover hydrosilation occurring, intended to occur or perceived to occur in homogeneous media, except for brief but important comparisons with and discussion on some new developments in the area of heterogeneous catalysis. 

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