Platinum hydrosilation catalysts

Newer silicone adhesives having solids levels up to 97% are also commercially available [109]. Instead of using silanol condensation reactions, they rely on addition chemistry between vinyl functional silicone oligomers and silicon hydride terminated silicones. This addition reaction is typically facilitated with platinum derived catalysts. This hydrosilation process can be run at reduced oven temperatures, but the finished products typically do not yield the same balance of properties as seen for condensation cure materials. 

CYCLOHEXEN-l-ONE. Importantly, only 1.6 equivalents of Ag20 are required for efficient coupling. The final preparation in this series illustrates the hydrosilation of racemic 3-butyn-2-ol catalyzed by a phosphine based platinum(O) catalyst. The resultant racemic (E)-vinylsilane is then resolved with a commercially available lipase and subjected to a Johnson ortho ester Claisen rearrangement to afford [3R- AND 3S-]-(4E)-METHYL... 

Recently, a new class of platinum silyl complexes was reported (89-91). The complexes of type 49, formed from LPtfQH (L = bulky phosphine) and trialkylsilanes, are extremely efficient hydrosilation catalysts. Upon heating, 49 is converted into 50 via an ill-defined a-elimination process. In all examples of redistributions, the presence of at least one hydro-... 

Although the platinum-catalyzed reactions are not as practical as palladium used for the same purpose, several types of unique discoveries still are noteworthy. Besides electrophihc C-H activation, the platinum complexes are efficient catalysts for hydrosilation see Hydrosilation Catalysis) that is important to silicon polymer industry. Platinum carbene catalysts are prepared from Karstedt catalyst (9) and imidazolium salts. The trials of model hydrosilylation reactions show very high yields of regioselective products (<1% impurity) with remarkable TON (see Turnover) (<30ppm catalyst load) (Scheme 81). ... 

The platinum catalyst is effective in very small amounts, and can be introduced as H2PtCl6 or as elemental platinum on an inert support. A particularly active catalyst is the soluble platinum complex of divinyltetramethyldisilox-ane, CH2=CHSiMe2-0-SiMe2CH=CH2. The hydrosilyla-tion reaction operates through the Chalk-Harrod mechanism or one of its variants. bz jn these mechanisms, the first step involves the conversion of a metal alkene complex to a metal alkene silyl hydride complex. In addition to platinum, recently ruthenium, rhodium, palladium, copper, and zinc complexes are being studied as hydrosilation catalysts. " ... 

The hydrosilation of simple olefinic bonds is mature technology, and platinum-based catalysts (such as 1 [13] and 2 [14]) are now available of such a level of efficiency that the catalyst cost is... 

Catalysis. Platinum-catalyzed hydrosilation is used for cross-linking siUcone polymers and for the preparation of functionally substituted silane monomers (250). The most widely used catalyst is chloroplatinic acid (Spier s catalyst), H2PtCl3. Other compounds that catalyze the reaction include Pt(Il)... 

Silicon—Ca.rbon Thermoset. The Sycar resins of Hercules are sihcon—carbon thermosets cured through the hydrosilation of sihcon hydride and sihcon vinyl groups with a trace amount of platinum catalyst. The material is a fast-cure system (<15 min at 180°C) and shows low moisture absorption that outperforms conventional thermosets such as polyimides and epoxies. Furthermore, the Sycar material provides excellent mechanical and physical properties used in printed wiring board (PWB) laminates and encapsulants such as flow coatable or glob-top coating of chip-on-board type apphcations. 

Platinum compounds Hydrosilation cross-linking of silicone polymers Hydrogenation, isomerization and hydroformylation of alkenes Automobile exhaust catalyst Sensitization dermatitis... 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

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). 

The original patent uses platinum as the catalyst and calls for temperatures of 100-300° and pressures of 45-115 psi(47). We found that such rigorous conditions are not required for the hydrosilation reaction with most commercial sources of platinum on carbon. Usually vigorous stirring at slightly elevated temperatures, 40-80°, at 15 psi will give moderate yields of the product. The rates and yields are usually highly dependent on the method of preparation of the catalyst. However, ultrasound permits the reaction to occur at a useful rate at 30° at atmospheric pressure(48) ... 

Stone and co-workers (16) recently announced that they had isolated and characterized a novel class of crystalline platinum(II) complexes prepared at room temperature by oxidative addition of hydrogen or of R3SiH (R3 = Me2Et, Me2CH2Ph, Cl3, or Et3)to (cyclo-C6H1, P Pt (C2H4. All these complexes were found to be excellent catalysts for hydrosilation... 

Isomerization was faster than hydrosilation and more facile than with platinum or rhodium catalysts. Isomerization occurred with (MeO)3SiH, which does not cause isomerization with platinum. These hydrosilations were most successful below 60°C and proceeded smoothly even at 0°C after an induction period. Above 60°C, the catalyst decomposed and became ineffective. 

The hydrosilation of butadiene, isoprene and other simple conjugated dienes with platinum catalysts has not been intensively studied. It usually occurs by 1,4 addition and only at elevated temperatures (10). Although palladium catalysts are practically ineffective to hydrosilate an alkene, they are very active under mild conditions with conjugated dienes to give two principal products, sometimes in 100% yields. With triethoxy silane, butadiene, and PdCl2(PhC=N)2 (5.5 x 103 moles/mole) at 22°C for 24... 

These studies were extended to hydrosilation of cyclopentadiene with trichlorosilane (52). This is most difficult with platinum catalysts. Palladium complexes favored production of 1 1 adducts as a mixture of 3- and 4-trichlorosilylcyclopentene. Nickel complexes produced substantial amounts of 1 2 adducts as trichlorosilyl-substituted 4,7-methylene-4,7,-8,9-tetrahydroindanes, with the exception of nickel tetracarbonyl, which was very active and selectively formed almost exclusively 3-trichlorosi-lylcyclopentene with no 1 2 adduct. 

In the Pt(0)-catalyzed hydrosilation, the addition of a catalytic amount of Na(0) appears to be necessary to achieve high levels of regioselectivity (cf., 1,1- vs 1,2-disubstituted vinylsilane). Without the inclusion of Na(0), the reaction yields a minor regioisomer, ( )-2-(dimethylphenylsilyl)-1 -buten-3-ol, 2b, in a ratio of 10-15 1. The checkers found that the reaction was considerably faster and higher-yielding with 60 mg of the catalyst, bis(T(-divinyltetramethyldisiloxane)tri-tert-butyIphosphine-platinum(O). 

Si-H bonds add across olefins over platinum catalysts. This reaction (hydrosilation) is used in silicone polymer manufacture, when the silane is a hydrogen bearing oli-go(alkylsiloxane). With some types of olefin there have been reports of runaways to explosion because of unexpectedly fast reaction. Dangerous substrates recorded are 2-allylphenols and ethenylsiloxanes. Very low levels of catalyst (ppm) and good cooling are recommended. 

Although the mechanism of the platinum-catalyzed hydrosilation is poorly understood, it seems probable that an intermediate with a platinum-silicon bond is formed, with which the olefin and hydrogen may also be complexed. The cleavage of pentamethyldisilane may be rationalized by considering nucleophilic attack of isopropyl alcohol, which is used for preparing the catalyst solution, on the silicon atom that has become attached to platinum and thus vulnerable to the attack by any Lewis base (see also Section IV, A, 2,f B, 2,b). 

The vinyldisilane (XXVIII) undergoes chloroplatinic acid-catalyzed hydrosilation with trimethylsilane with great ease at 40° C to give (/J-tri-methylsilylethyl)pentamethyldisilane in 93% yield, which can also be obtained in 80% yield by the addition of pentamethyldisilane to vinyl-trimethylsilane in the presence of tra r-dichloro(ethylene)(pyridine)-platinum(II) as catalyst in benzene at 45° C (128). 

Difficulties (41, 42) associated with the use of pentamethyldisilane in hydrosilations catalyzed by usual metal complex catalysts led Yamamoto et al. to study the interaction of penta- and tetramethyldisilanes with transition metal complexes (43). Bis(phosphine)platinum chlorides effectively catalyzed the redistribution in Eq. (66) (R = Me, H). Yamamoto et al. 

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