Ethylene platinum-catalyzed hydrosilylation

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

A recent detailed theoretical study of the platinum-catalyzed hydrosilylation of ethylene [15] led to a conclusion that this process proceeds through the Chalk-Harrod mechanism. The rate-determining step in this mechanism is the isomerization of the Pt(silyl)(alkyl) complex formed by ethylene insertion into the Pt-H bond, and the activation barrier of this step is 23 kcal moP for R = Me and -26 kcal mol for R = Cl). In the modified Chalk-Harrod mechanism, however, the rate-determining step is ethylene insertion into the Pt-SiRa bond and its barrier is 44 kcal moP for R = Me and 60 kcal moP for R = Cl. 

There are numerous applications where cure reaction by-products are not acceptable as they may contaminate other sensitive areas of the devices, and where fast deep section cure is required. These constraints have directed the choice to the platinum-catalyzed hydrosilylation addition cure system. The reaction involves the addition of a hydrido silane (SiH) to an alkenyl organic group (-CH=CH2), typically the vinyl or hexenyl groups. The product of the reaction is the ethylenic bridge (-CH2-CH2-). The alkenyl groups are usually placed at the end of the polymer base chains in structures such as The SiH groups are usually placed in a combed structure within the polymeric or copolymeric chains called cross-linker M-D -M or M-D -Dy-M. (The M, D, etc. nomenclature is explained in Silicones structures.) Typical values of n cover a wide range from 100 to 1000, while the values of x and y vary from 3 to 100. 

Metal and Supported Metal Catalysts. Another group of catalysts active in the addition of the Si—H bond to unsaturated compounds are metals supported on inorganic materials or carbon. Initially, only a platinum catalyst supported on carbon, silicates, and silica appeared to be effective in the reactions of trichlorosi-lane with ethylene, acetylene, butadiene, allyl chloride, and vinylidene fluoride. However, it was soon established that other metals could also be used to catalyze hydrosilylation reactions (viz. Rh, Ru, Pd, Ni, and Ir). These metals are usually supported on active carbon, y-Al203, Si02, or CaCOs. Platinum supported on carbon (usually in 5 wt% concentration) is the most common and most efficient metal catalyst for the polyaddition and hydrosilylation of carbon—carbon multiple bonds (3) (for recent review, see (125)). 

Sasaki S, Mizoe N, Sugimoto M (1998) Theoretical study of platinum(0)-catalyzed hydrosilylation of ethylene. ChaUc-Harrod mechanism or modified Chalk-Harrod mechanism. Organometallics 17 2510-2523... 

The migratory insertion (silyl migration) of alkene into the M—Si bond is a key step for the dehydrogenative silylation catalyzed by late transition metal complexes. The first convincing results for mechanistic pathways involving this step were presented by Seitz and Wrighton (45) and obtained in a photochemical study of the reaction with [(Me3Si)Co(CO)4] complex. The insertion of ethylene into the Co—SiMes bond was spectroscopically confirmed. In addition, as already mentioned, a theoretical study of the hydrosilylation of ethylene explained the preference of rhodium over platinum complexes as catalysts in these reactions (41,43). 

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