Hydrosilylation platinum catalysts for

Oxidation State. Homogeneous catalysis normally involves the metal in changes of oxidation state. It can therefore speed up reactions if the catalyst already contains the metal in an oxidation state involved in the catalytic cycle. However, these oxidation states are often lower than those of readily available compounds. Thus a reduction step is generally involved in the preparation of homogeneous catalysts. An example of this is the use of platinum catalysts for hydrosilylation. 

The choice of an appropriate support is of no less importance than that of active phase of a catalyst. We have focused our attention on the application of hydrophobic supports to prepare effective platinum catalysts for hydrosilylation since our preliminary experiments have shown that in a number of hydrosilylation reactions hydrophobic material-supported catalysts appeared to be superior to those based on hydrophilic supports such as alumina and silica. We have also aimed at selecting such supports which, in addition to their hydrophobicity, do not have acid centers on their surfaces, and due to this, they do not catalyze undesirable side reactions of isomerization. The supports selected for our study were styrene-divinylbenzene copolymer (SDB) and fluorinated carbon (FC), because nonfunctionalized SDB is free of acid sites and surface acidity of FC is extremely weak (H 9). The performance of SDB- and FC-supported platinum catalysts was studied in several reactions of hydrosilylation. 

FIGURE 15.1 Activity of polymer- and carbon-supported platinum catalysts for hydrosilylation of allyl glycidyl ether with heptamethyltrisiloxane at 100°C. 

Platinum compounds are the most active catalysts for hydrosilylation. Compounds such as and PtCl2(CH2CHCOCH2)2 are effective. In the... 

Platinum compounds and complexes are the most important and commonly used catalysts for hydrosilylation processes [7 - 9]. Platinum catalysts tolerate a variety of functional groups, but some impurities may interact with them leading to catalyst poisoning [10]. This has stimulated much research aimed at employing other transition metal compounds as potential catalysts. For example, Rh(I) complexes are selective and active hydrosilylation catalysts [11] and more resistant to poisoning than the platinum ones [12]. 

Scheme 1. New homogeneous platinum catalysts for the hydrosilylation reaction.

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. ... 

Hydrophobicity of catalyst support, although very important to many hydrosilylation reactions carried out in heterogeneous systems, is not the only factor deciding of the performance of supported platiniun catalysts for hydrosilylation. Results obtained by using different platinum com-poimds for the preparation of catalysts for hydrosilylation show that the... 

Tests for multiple uses of catalysts for hydrosilylation of hexadecene and octene with triethoxysilane have shown that yields of desirable products are maintained on a high constant level for three runs and then they decline due to platinum leaching from the catalysts. 

Finally, poly-NHC complexes of other metals have occasionally been employed as catalysts for hydrosilylations as well. The group of Hollis reported in 2012 on dinuclear rhodium(I) complexes with a bridging dicarbene ligand (structure 78, Fig. 26) as catalysts for the hydrosilylation of phenylacetylene with dimethylphenylsilane. Results were comparable to those previously obtained with platinum dicarbene complexes, which is in contrast to previous reports on Rh-catalyzed hydrosilylations, in which the Z-beta product is predominantly formed without production of the alpha isomer. 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Denmark pursued intramolecular alkyne hydrosilylation in the context of generating stereodefined vinylsilanes for cross-coupling chemistry (Scheme 21). Cyclic siloxanes from platinum-catalyzed hydrosilylation were used in a coupling reaction, affording good yields with a variety of aryl iodides.84 The three steps are mutually compatible and can be carried out as a one-pot hydro-arylation of propargylic alcohols. The isomeric trans-exo-dig addition was also achieved. Despite the fact that many catalysts for terminal alkyne hydrosilylation react poorly with internal alkynes, the group found that ruthenium(n) chloride arene complexes—which provide complete selectivity for trans-... 

The ratio of the three products depends on the reacting silane and alkyne, the catalyst, and the reaction conditions. Platinum catalysts afford the anti-Markovnikov adduct as the main product formed via syn addition.442- 146 Rhodium usually is a nonselective catalyst404 and generally forms products of anti addition.447 151 Minor amounts of the Markovnikov adduct may be detected. Complete reversal of stereoselectivity has been observed.452 [Rh(COD)Cl]2-catalyzed hydrosilylation with Et3SiH of 1-hexyne is highly selective for the formation of the Z-vinylsilane in EtOH or DMF (94-97%). In contrast, the E-vinylsilane is formed with similar selectivity in the presence of [Rh(COD)Cl]2-PPh3 in nitrile solvents. 

Although homogeneously catalyzed reactions of platinum complexes are mostly concerned with hydrogenation, hydroformylation, isomerization and hydrosilylation reactions, the complexes trans-PtHX(PPh3)2 (X = C1, Br, I) have been used used as catalysts for the oxidative chlorination of n-pentane. H2PtCl6 and K2PtCl are used as oxidants.201... 

Intramolecular hydrosilylations of functionalized alkenes followed by hydrogen peroxide oxidation provide powerful methods for organic syntheses86-88. The reactions of allylic O-dimethylsilyl ethers 59 promoted by platinum catalysts, e.g. Karstedt s catalyst and Pt(PPh3)2(CH2=CH2), or rhodium catalysts, e.g. Rh(acac)(COD) and [RhCl(CH2=CH2)2]2> proceed via 5-endo cyclization to give oxasilacyclopentanes 60 with a couple of exceptions in which siloxatanes 61 are formed (Scheme ll)87,89. 

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