Chloroplatinic acid, as catalyst

At 165°C and in the presence of chloroplatinic acid as catalyst, isoprene reacts with trichlorosilane, metbyldicblorosilane, ethyldichlorosilane, ben2yldichlorosilane, and diben2ylchlorosilane (72). The addition is 1,4- with the substituted silane group attaching to the first carbon atom. 

With chloroplatinic acid as catalyst and under similar conditions, not only cleavage of the silicon-silicon bond, but also addition of trimethylsilane, formed from cleavage, to the starting olefin occurs to some extent. 

Even though the addition of silicon hydrides to vinyl compounds often takes the form of /3-addition, exceptions do exist. For example, the reaction of methyldichlorosilane and styrene catalyzed by chloroplatinic acid or Pt/C yielded 53% /3-addition and 33% a-addition product (Ryan and Speier, 1959). For polysiloxanes, Huang and Wu (1992) studied the synthesis of polymer (3.40) using NMR analysis. The result showed that at 50 °C using chloroplatinic acid as catalyst, the product was actually a copolymer with isomeric structures, 58% of which were from //-addition ((3.40)), the other 42% were from the a-addition (3.41). 

The addition of dichlorosilane to 1,4-cyclohexadiene (352) in the presence of hexa-chloroplatinic acid as catalyst unexpectedly affords 3-dichlorosilylcyclohexene (353) in 96% yield184. This, on heating in the presence of the catalyst, rearranges into 7,7-... 

The hydrosilylation of conjugated dienes such as butadiene and isoprene using chloroplatinic acid as catalyst proceeds only at elevated temperatures giving a mixture of products. The hydrosilylation of butadiene with dichloromethylsilane affords a 1 1 mixture of monoadduct and diadduct (combined yield 50%), the former including 2-butenylsilane as major product (85-90%) and 3-butenylsilane as minor product (15—10% )79 (equation 29). 

The addition of Si—H bonds and the reactions of silyls with platinum complexes is of significance because of the early discovery of chloroplatinic acid as a hydrosilylation catalyst.53 This section focuses on the formation of hydrides from silanes. 

Siloxane Polymers. The synthesis of the ferrocene-modified siloxane polymers (A - E) has been described previously (25,27,32). Briefly, the methyl(2-ferrocenylethyl)-siloxane polymers were prepared by the hydrosilylation of vinylferrocene with the methylhydrosiloxane homopolymer or the methylhydrosiloxane-dimethylsiloxane copolymers (m n ratios of 1 1, 1 2, and 1 7.5 see Figure 1) in the presence of chloroplatinic acid as a catalyst. The methyl(9-ferrocenylnonyl)siloxane-dimethylsiloxane (1 2) copolymer was prepared via hydrosilylation of 9-ferrocenyl-l-nonene with the methylhydrosiloxane-dimethylsiloxane (1 2) copolymer. The molecular weight range of these ferrocene-modified siloxane polymers is approximately 5,000-10,000. Purification of the polymers was achieved by reprecipitation from chloroform solution, via dropwise... 

Methyl-( -ferrocenylethyl)- and methyl-[ -(r,3 -dimethylferrocenyl)ethyl]siloxane polymers 53 and 54, respectively were prepared by the hydrosilylation of vinyl-ferrocene and l,T-dimethylferrocene-3-vinylferrocene with methyl hydrosiloxane (molecular weight was originally reported to be 2270) or methylhydrosiloxane-dimethylsiloxane copolymer (molecular weight was originally reported to be 2000 — 2100) in the presence of chloroplatinic acid as a catalyst. The synthetic route is given in Seheme 10-25 [62], The reaction was monitored by IR spectroscopy until the complete disappearance of the Si-H absorption at 2161 cm". ... 

This mechanism (Scheme 2), originally derived from studies of chloroplatinic acid as a precursor (Pt catalyst), presents an conventional oxidative addition-reductive elimination steps to explain the hydrosilylation. The oxidative addition of trisubstituted silanes, HSiR3 to a metal alkene complex configuration (usually... 

In the presence of chloroplatinic acid as the catalyst, silane adds to the double bonds of the tetrafunctional crosshnking agent with the formation of stable Si-C bonds ... 

Mechanism and Side Reactions. Many reviews and articles have been published on the mechanism of hydrosilylation catalysis by late transition metal complexes proposed in 1965 by Chalk and Harrod. This mechanism, originally derived from the studies of chloroplatinic acid as a precursor (Pt catalyst), provided a qualitative rational generalization of the role of catalyst of other transition metal complexes (37). 

Miscellaneous. Chloroplatinic acid is used in the production of automobile catalysts. Platino-type prints based on reduction of Pt(II) to Pt(0) by a photosensitive reducing agent such as iron(III) oxalate are used in art photography (261,262). Infrared imaging devices based on a platinum siLicide detector have been developed (263). 

There are two ways to produce acetaldehyde from ethanol oxidation and dehydrogenation. Oxidation of ethanol to acetaldehyde is carried out ia the vapor phase over a silver or copper catalyst (305). Conversion is slightly over 80% per pass at reaction temperatures of 450—500°C with air as an oxidant. Chloroplatinic acid selectively cataly2es the Uquid-phase oxidation of ethanol to acetaldehyde giving yields exceeding 95%. The reaction takes place ia the absence of free oxygen at 80°C and at atmospheric pressure (306). The kinetics of the vapor and Uquid-phase oxidation of ethanol have been described ia the Uterature (307,308). 

When a carrier is impregnated with a solution, where the catalyst deposits will depend on the rate of diffusion and the rate of adsorption on the carrier. Many studies have been made of Pt deposition from chloroplatinic acid (HgPtClg) with a variety of acids and salts as coim-pregnants. HCl results in uniform deposition of Pt. Citric or oxalic acid drive the Pt to the interior. HF coimpregnant produces an egg white profile. Photographs show such varied distributions in a single pellet. 

Commercial C.p. chloroplatinic acid varies somewhat in its purity. In this work that from the Mallinckrodt Chemical Works, St. Louis, was used and gave very satisfactory results. Since small amounts of impurities in the catalyst are important factors in the rate of reduction of certain types of compounds, this question of impurities in the chloroplatinic acid must be taken into account (Note 13). In a large proportion of the reductions studied, platinum oxide prepared from the chloroplatinic acid mentioned gave as good results as that from spectroscopically pure chloroplatinic acid made according to the directions of Wichers.1... 

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... 

The most common catalyst used to date is chloroplatinic acid (also known, after its discoverer, as Speier s catalyst) it is now clear that, contrary to earlier views (23), hydrosilylation is a homogeneous process (25, 208). A major problem is that of reproducibility, and efforts are being made to utilize soluble transition metal complexes. Information about such systems has been used in the interpretation of some related catalytic heterogeneous reactions (232). 

Figure 4.11 reveals that Pt is present on the surface of the catalyst as an oxide, in combination with hydrocarbon species (a contaminant during sample preparation) and as a chloride (derived from the Pt precursor, chloroplatinic acid). The results show the composition of the washcoat to be Pt and Rh on alumina and ceria. 

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