Olefins hydrosilylation, transition-metal catalyzed

Similar to olefins, acetylenes undergo facile addition reactions. Transition metal-catalyzed hydrosilylation is such a reaction that has been utilized for the synthesis of silicon-containing hyperbranched polymers. 

The first successful achievements using asymmetric homogeneous transition metal catalysis were obtained in the asymmetric hydrogenation of alkenes24 25, This method has been successfully used in many synthetic applications (Section D.2.5.1.)26-29. In addition, chirally modified versions of the transition metal catalyzed hydrosilylation of olefins and carbonyl compounds (Sections D.2.3.1. and 2.5.1.) and olefin isomerization (Section D.2.6.2.) have been developed. Transition metal catalyzed asymmetric epoxidation constitutes one of the most powerful examples of this type (Section D.4.5.2.). 

Herein, we report on a novel process for the synthesis of organomodified poiydimethyisiioxanes employing ionic liquids for the heterogenization and/or immobilization of the precious metal catalyst [13]. The advantage of this novel hydrosilylation process is that standard hydrosilylation catalysts can be used without the need for prior modification to prevent catalyst leaching. To the best of our knowledge, this is the first example of a hydrosilylation of olefinic compounds using ionic liquids (Scheme 1). However, a method for the transition metal-catalyzed hydroboration and hydrosilylation of alkynes in ionic liquids has recently been described [14]. 

In the recent years, all new mechanistic implications on the late transition metal-catalyzed hydrosilylation of olefins reported involve ruthenium complexes as model transition metal centers of molecular catalysis (86-88). 

Hydrosilylation of olefins is also a transition metal catalyzed reaction (Pt, Co, Pd, Rh, etc., derivatives being active catalysts) [86]. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

It is well documented that hydrosilylation of alkyl-substituted terminal olefins catalyzed by transition metal complexes proceeds with high regioselectivity in giving linear hydrosilylation products which do not possess a stereogenic carbon center.2 It follows that the asymmetric synthesis by use of the hydrosilylation of alkyl-substituted... 

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

Mechanism of Hydrosilylation of Olefins Catalyzed by Transition-Metal Complexes... 

The mechanism of catalytic hydrosilylation involves oxidative addition of a silicon-hydrogen bond to a metal complex as an essential step since it is here the activation of hydrosilane by the catalyst takes place. Thus, many transition metal ions and complexes, especially group VIII metals in low oxidation state containing ir-acid ligands such as CO, tertiary phosphines or olefins display catalytic activity. The sequence of unit reactions in a typical d -metal complex-catalyzed hydrosilylation is summarized as ... 

Hydrosilylation of olefins is one of the most thoroughly investigated reactions of its type. It can be catalyzed with complexes of various transition metals [1-3]. 

The mechanisms of these reactions are varied, but can still be categorized. The hydrocyanations, hydrosilylations, and many of the hydroborations, occur with late-metal catalysts. These reactions occur by oxidative addition of the H-X bond, followed by migratory insertion of the olefin into the M-H or M-X bond, and reductive elimination to form the final product. Hydrocyanation occurs by insertion of the unsaturated reagent into the M-H bond, while hydrosilylation and hydroboration have been shown to occur by insertion of the olefin into the M-H bond in some cases and into the M-X bond in others. HydrosUy-lations and hydroborations of alkenes and alkynes catalyzed by (P transition metal complexes and by lanthanides follow a different pathway because these complexes caimot undergo oxidative addition. The mechanism of the reactions catalyzed by these complexes involves u-bond metatheses. 

Although many transition metal complexes catalyze olefin isomerization, early studies have shown that the presence of silane markedly affects such reactions that occur concurrently with hydrosilylation. Double-bond migration, which is a characteristic feature of most coordination catalytic reactions, can be illustrated schematically in (Scheme 6) as a side reaction occurring during hydrosilylation (3,10,47). 

Some of these redistributions may be initiated either by homolysis or catalytic activation or even by conventional acidic or basic catalysts, but a number of them proceed more readily in the presence of olefins. These reactions are fast and yield a large number of products that also contain adducts formed during the preliminary redistribution of the silane substrate(s) prior to their hydrosilylation. Redistributions of silicon compovmds catalyzed by transition metals occur when at least one Si—H bond is present in the molecule, since the Si—H bond is the most labile of those undergoing oxidative addition to yield a silyl metal hydride. 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

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