Platinum catalyzed hydrosilation

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)... 

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 platinum-catalyzed or thermal ring opening polymerization of the substituted silacyclobutane monomers yields linear polycarbosilanes. The platinum-catalyzed hydrosilations of the AB (allyl) and AB2 (diallyl) monomers yield linear and hyperbranched polycarbosilanes, respectively. A wide range of random copolymers are readily available from polymerization of mixtures of the silacyclobutane or AB and AB2 monomers. The preparation of random copolymers is often of interest for achieving desired physical properties. 

We also explored an alternate synthesis of Mono(F). Figure 3 outlines the four step synthetic procedure used to prepare Mono(F). The first step consisted of the rhodium catalyzed hydrosilation reaction of tetramethyldisiloxane with one equivalent of allyloxytrimethylsilane (77). The next step consisted of the platinum catalyzed hydrosilation of the disiloxane silicone hydride intermediate with allyloxyoctafluoropentane. Both hydrosilation reactions were monitored for extent of reaction (loss of Si-H) by NMR spectroscopy. The third step in the reaction consisted of an acetic acid catalyzed deprotection of the trimethylsilyl group using a 10% solution of acetic acid in methanol. The deprotection was quantitative with no apparent degradation of the siloxane linkage. The final step consists of the reaction of the deprotected disiloxane (used as is) with methacryloyl chloride. The final purified product, as expected, is fi ee of the methacrylate reduction by-products. 

Various methods can be applied for the synthesis of POSS molecules with functionalizations at multiple silicon vertices. In the majority of cases eightfold-functionalized molecules are formed. The probably most studied octafunctional POSS molecules are octa(hydrido)silsesquioxane, octa(vinyl)silsesquioxane, and octa(phenyl)silsesquioxane. The first one is prepared via hydrolytic condensation of trichlorosilane in the presence of FeCls [18]. The vinyl derivative can be prepared treating trichlorovinylsilane with hydrochloric acid and ethanol [19]. Both POSS molecules can be derivatized applying typical functionalization reactions for Si-H or Si-CH=CH2 in the comers, such as platinum-catalyzed hydrosilation reactions (Fig. 5) [20, 21],... 

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... 

A patent (FR 2474891 see CA 96 124298c) assigned to B.A. Ashby, GE Company, claims a platinum-siloxane complex which catalyzes hydrosilation of vinylsiloxanes or SiOH groups in the preparation of silicone resins and rubbers. 

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). 

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). 

Until relatively recently, less attention has been given to catalysis of redistribution reactions by transition metal complexes. Redistributions have been observed during the course of platinum-catalyzed hydrosila-tion hence, the scrambling reaction can be a nuisance by decreasing the yield of desired hydrosilation products. A noteworthy example is the H/Cl exchange that occurs during the hydrosilation of allyl chloride [Eq. (10)]. 

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). ... 

Other substituent groups are usually introduced via Hydrosilation reactions. In this important reaction, Si-H bonds add across C=C or C=C bonds in 1,2 fashion to give products. Many transition metals and their complexes will catalyze hydrosilation, but the usual catalyst is platinum see Hydrosilation Catalysis). Thus CF3CH2CH2Si(Me)Cl2 is made by hydrosilation of 3,3,3-trifluoropropene with methyldichlorosilane (equation 12). n-Alkylmethyldichlorosilanes can be made similarly, by hydrosilation of l-aUcenes. Vinylmethyldichlorosilane may be obtained in analogous fashion by hydrosilation of acetylene, as shown in equation (13). An alternate route to the same compound is hydrosilation of vinyl chloride, followed by dehydrochlorination (equations 14 and 15). 

A vinyl-terminated siloxane polymer is mixed with a compound containing several Si H groups, such as dJ, (MeSiHO)4 in the presence of a platinum catalyst, hydrosilation takes place to give Si H2 H2-Si crosslinks (equation 27). A related platinum-catalyzed system employs two siloxane prepolymers, one containing some Si H bonds and the other Si vinyl groups. The platinum catalyst is active in extremely small amounts, so that its use is practical despite the high cost of platinum. 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

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... 

Intramolecular Hydrosilation of Allyl Amines. The amino group of an allyl amine is silylated with chlorodimethylsilane, and then the resulting silazane is subjected to standard platinum-catalyzed cyclization and oxidation steps to form a syn-2-amino alcohol (eq 1). Exclusive formation of the four-membered cyclic product and the high stereoselectivity should be noted. ... 

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... 

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. 

The main product in hydrosilation of a,P-unsaturated ketones and aldehydes catalyzed by chloro-platinic acid, platinum on alumina, or metallic nickel is the corresponding silyl enol ether. With nickel catalyst, product distribution is highly dependent on the enone structure. Hydridosilanes add to a, -unsaturated esters, producing the corresponding silyl enolate as well as carbon silylated products. The course of addition depends on substrate structure and the hydridosilane utilized. Thus, triethylsilane undergoes 1,4-addition to methyl acrylate in the presence of chloroplatinic acid, while trichlorosilane with either chloroplatinic acid or Pt/C gives the -silyl ester (Scheme 65). ... 

---