Transition-metal catalysts, hydrosilylation using

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). 

To date, the reductive cyclization of allenic alkenes remains undeveloped. However, the reductive cyclization of activated alkene partners in the form of 1,3-dienes and conjugated enones has been achieved using late transition metal catalysts. Indeed, the hydrosilylative dimerization of 1,3-dienes reported in 1969 appears to be the first reductive... 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

The author hopes that this chapter has convinced the readers of the value of homogeneous catalysis for the synthesis of organophosphorus compounds and for organo-heteroatom compounds in a broader sense. Hydrosilylation and hydroboration are indispensable modern synthetic reactions in this category. The H-P addition reactions herein described joins them as a third member. Although this chapter does not cover, the addition reactions of the S-P and Se-P bonds in thiophosphates [39] and selenophosphates [40] to alkynes also proceed in the presence of transition metal catalysts. In view of the wide use of phosphorus compounds, the new procedures will find practical applications. 

The reactivity of a transition metal catalyst can be modified by redox modification of its oxidation level. In a remarkable example the complex 2, which is formed by the hydrogenation of a precursor complex, may be switched between the +1 and +2 from by the use of an electrode. The +2 form is highly effective at the hydrogenation of alkenes, whilst the +1 complex is more effective for other applications such as the hydrosilylation of ketones526. 

These hydroxy-1,1 -binaphthyl functionalised NHC ligands can be used in asynunetric catalysis. Catalytic reagents performed with transition metal catalysts carrying these ligands include olefin metathesis [19,80,86], allylic alkylation [17,18,88] and hydrosilylation of ketones [85]. 

Stereoselective introduction of a silyl moiety may also be realized by intramolecular hydrosilylation, for example, using a silyl group anchored to a neighboring hydroxy group in the presence of a transition metal catalyst such as platinium or rhodium. This reaction has been used for the stereoselective synthesis of 1,3-diols 9—11Sc. 

The hydrosilylation reaction can also be conventionally conducted by reaction of an olefin and an SiH-f mctional polydimethylsiloxane in the presence of a standard transition metal catalyst, and after the reaction the catalyst can be extracted with an ionic liquid. In some cases, the use of an ionic liquid in the hydrosilylation process even improved the quality of the polyethersiloxanes with respect to color compared to the standard process. An explanation might be the avoidance of catalyst reduction leading to the formation of colloidal metal particles, which tend to color the product slightly brownish. In other words, the ionic liquid seems to have a stabilizing effect on the catalyst. 

A novel transition metal-catalyzed hydrosilylation process is described. The use of an ionic liquid in this process allows for the immobilization, heterogenization, and recovery of the expensive precious metal catalyst as well as its direct reuse in a subsequent hydrosilylation reaction. From an economic and ecological point of view, this process perfectly fits in the concept of "Sustainable Chemistry". Future research activities will aim at the prolongation of the catalyst life-time. For this, it is necessary to gain a deeper understanding of the catalytically active species in the catalyst/ionic liquid solution. 

Hydrosilylation. Traditional methods of hydrosilylation involve the use of transition metal catalysts. However, Lewis acids such as AICI3 also show such reactivity. 

Silanes bearing Si—H bonds are of basic interest in catalytic reactions. Transition metals or transition metal complexes, respectively, are able to catalyze hydrosilylation which is a useful approach to polysilanes. The major drawback of this method is the relatively low molecular weight of the polysilanes generated. The reaction pathway, as well as the reaction products, depend on the nature of the educts and particularly on the catalyst which is used. Numerous transition metals can be effective catalysts either as metal or in compounds. For a general overview some key reactions of transition metal catalysts are given in Scheme 5. The hydrosilylmetal species (a), which is... 

Hydrosilylation requires a transition-metal catalyst, such as [HiPtCle], and results in very good yields of the product alkenyl silanes, but often as mixtures of regio- and stereoisomers (2.66). More recently, it has been found that hydrosilylation of internal alkynes using the catalyst [Cp Ru(MeCN)3]Pp6 occurs by trans addition, and this can be followed by protodesilylation to provide a route to E-alkenes. " For example, ruthenium-catalysed hydrosilylation of methyl 2-octynoate with triethoxysilane, followed by fluoride-promoted desilylation of the intermediate regioisomeric alkenyl silanes, gave ( )-methyl 2-octenoate (2.67). 

As mentioned earlier, hydrosilylation can be used as an alternative route of ketone reduction. By performing the addition of hydrosilanes in the presence of suitable chiral transition metal catalysts, prochiral ketones are transformed in the subsequent hydrolysis step into optically active alcohols (5) ... 

Transition-metal-catalyzed hydrosilylation was first reported in the late 1950s with catalysts based on platinum, ruthenium, and iridium chlorides. For industrial applications, chloroplatinic acid (H PtCl nHjO) has been used extensively and is highly active for this process. This catalyst has become known as Speier s catalyst. This catalyst is spectacularly reactive, as indicated by the low catalyst loading for the reaction in Equation 16.17. A Pt(0) complex containing vinylsiloxane ligands (platinum divinyltetramethyl-disiloxane) shown in Figure 16.1 has also been used frequently in industrial settings as a catalyst for hydrosilylation. Tliis catalyst has become known as Karstedt s catalyst. ... 

Organosilicon compounds are largely produced by the hydrosilylation of unsaturated organic substrates [47]. Various transition metal catalysts have been used to obtain alkyl-SiR products from the reaction of H-SiRj with an alkene. Alkene insertion into an M-Si bond is recognized as a fairly common process which plays a key role in catalytic hydrosilylation processes. The reaction of 1,3-butadiene (3-6) with triethylsilane in the presence of [Cr(CO)g] under photochemical condition yields exclusively the ds-1,4-adduct, ds-l-(triethylsilyl)-2-butene (7) (Scheme 10.7) [48]. In all cases, 1-4 addition products form in major, however, in some cases 1-2 addition product (9) also forms in minor yield. Formation of product 12 can be rationalized in terms of double bond migration subsequent to the initial hydrosilylation (Scheme 10.7). 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

Hydrosilylations with transition metal catalyst tend to be a cis-addition [14]. Recently, by Doyle and Shanklin, hydrosilylation to acetylene is carried out in the presence of carbon monoxide to give Z-alkenealdehyde in high yield as shown in eq. (8.24). Rhodium is used as the catalyst and since this hydrosilylation is hydroformylation, it is called silylcarbonylation [18]. 

Platinum and rhodium catalysts have been the most frequently used catalysts among all the metal-based catalysts for the hydrosilylation of alkenes to date. In particular, a variety of rhodium catalysts have been extensively studied , while the development of other Group Vni transition metal catalysts such as those of palladium and ruthenium continues. In general, the hydrosilylation of an alkene gives the corresponding silylalkanes with varying regioselectivity (equation 1). 

HSi(OEt)3 is an efficient reagent for the hydrosilylation of alkenes, alk3tnes, and conjugated dienes (eq 1). The reaction is promoted by a variety of transition metal catalysts, including platinum, ruthenium, and rhodium. Chloroplatinic acid hexahydrate, H2PtCl6-6H20, is by far the most widely used catalyst and has been shown to be very efficient, particularly in the hydrosilylation of alkenic substrates. ... 

---