Silicon—platinum bonds

Contrary to previous reports suggesting colloidal metal as the active species in Pt-catalyzed hydrosilylations, the catalyst was found to be a monomeric platinum compound with silicon and carbon in the first coordination sphere.615 The platinum end product at excess olefin concentration contains only platinum-carbon bonds, whereas at high hydrosilane concentration, it is multinuclear and also contains platinum-silicon bonds. An explanation of the oxygen effect in hydrosilylation was also given to show that oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. 

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 reactivity of platinum silylenoid 27 was explored with traditional silylene trapping reagents. While the silylenoid did not react with triethylsilane or 2,3-dimethyl-1,3-butadiene, phenylacetylene was a viable substrate, providing the me-tallocyclohexadiene 29 (Scheme 7.4).54 The formation of platinum complex 29 was hypothesized to occur via platinum cyclobutene intermediate 28, which formed on insertion of the acetylene into the platinum-silicon bond. A second molecule of phenylacetylene was then inserted into the remaining platinum-silicon bond to provide the observed product. 

Platinum-silicon bonds may be made by the reaction of trans-[PtH(ClXPMe2Ph)2] with R3SiH in the presence of solvent, provided that R is an electron-withdrawing group such as Cl or aryl. For Cl3SiH reaction occurs at RT but for triarylsilanes at 80-90°C ... 

A reasonable mechanism for the formation of 5 involves the initial insertion of the cyano group into one of the platinum-silicon bonds, leading to intermediate 6, followed by cyclization to the imine 7. 

Thus, several complexes with platinum-silicon or germanium bonds have been obtained ... 

With platinum-silicon compounds described in this chapter, evidence supporting two-electron, three-center Pt-H-Si bonding seems to be strong, and the phenomenon is perhaps expected in view of the existence of numerous... 

A carbon-silicon bond can be cleaved by a process of (3-methyl elimination. Thermolytic rearrangement of platinum(II) complex 76 yields methyl-plati-num(II) complex 78 [93]. Generation of (ri2-silene)platinum intermediate 77by (3-methyl elimination is followed by migration of the other silylmethyl ligand to the silicon terminus of the r)2-silene ligand. 

Several mechanisms for curing gels are possible however, most have limitations involving either processing or the final gel properties. Condensation type cures form water or alcohol by-products which cause outgassing and voids. Free radical peroxide-activated addition cures make it difficult to control gel consistency from batch to batch. These problems are not evident in the most prevalent cure mechanism used today, the addition of silicon-bonded hydrogen atoms to silicon-bonded olefinic radicals, usually vinyl, in the presence of a few parts per million of a platinum catalyst (I). This system creates no by-products and is easily controlled. 

After considerable experimentation, a similar hydrosilylation protocol was used as a key step for the syntheses of jatrophatrione and citlalitrione by Paquette and co-workers.32 Following the stereoselective reduction of a tricyclic ketone with lithium aluminum hydride to provide alochol 28, silylation and platinum catalyzed hydrosilylation were effected to produce 29. Finally, the carbon-silicon bond was successfully cleaved to generate diol 30 in an impressive 93% yield. 

Figure 3 shows the assembly of the test element-aluminum heat sink and the silicone-bonded fiberglass holder in which it is mounted so as to confine heat exchange with the liquid to the top surface of the foil. The foil element was 0.00508-cm-thick chemically pure platinum, cut in a ring of mean radius of 3.325 cm and radial width of 0.200 cm. The effective length of the element was such as to give 3.81 cm of heat transfer surface. The heat... 

In the last few years the design and use of various disilane compounds has gained importance because of the reactivity of the Si-Si bond and the large potential for organic synthesis involved with it. Many publications offer us numerous examples of possible reactions at the silicon-silicon bond such as addition reactions with C-C double bonds or C-C triple bonds [1, 2], addition reactions with C-element multiple bonds (e.g. aldehydes, quinones, isocyanides) [3-5] or metathesis [6, 7] and cross-metathesis [8]. In the most cases the existence of a catalyst (palladium, platinum or nickel complexes) for activation of the silicon-silicon a bond is indispensable for a successful transformation [9-11]. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex. ... 

The order of stabilities of the adducts was the same as that observed previously for additions of the hydrosilanes to complexes [14, 32c, 18], i.e., negative substituents such as alkoxy groups or chlorine atoms on silicon stabilize the adducts. Furthermore, rate measurements have indicated that the structure of hydrosilanes does not affect markedly the rate ( i) of the forward reaction (oxidative addition), but affects strongly the rate ( i) of the reverse reaction (reductive elimination). This latter fact, in addition to a possible dependence of the stability of metal-silicon bonds on metal species (c.g., rhodium vs, platinum) will be reflected in the catalysis by particular metal complexes, which is clearly shown in the following sections. Another approach has been to study the stereochemistry of an optically active... 

Si>Si bond exists in the platinum-silicon 4HR and is consistent with our bonding description. Electron impact mass spectral data also provides supporting evidence for the bonding descrip-... 

X-ray crystallography. The observation that our platinum-silicon 4MR can be formed from reagents with Si-Si single bonds is also consistent with the P2 h bonding description. West has prepared monoplatinum complexes of disilenes from disilenes or disilanes by three routes. One route, which is similar to the reaction depicted in Eg.4, is shown in Eg.5. The platinum disilene complexes are described by the two resonance forms drawn in Eg.5. (It is interesting to note that oligomeric silanes are obtained as by-products of Eg.5.)... 

The temperatures quoted are those at which decomposition becomes appreciably rapid. Presumably metal fluorides are also formed in these decompositions but this has not been proven. A reaction which is apparently the reverse of these pyrolyses was mentioned above, namely the addition of platinum-fluorine bonds to tetrafluoroethylene 118). The thermal decomposition of perfluoroalkyl and polyfluoroalkyl derivatives of main group elements such as boron, silicon, or tin was mentioned in earlier sections of this chapter. Transfer of fluorine atoms from the side chains on heating was also a characteristic property. However, it is interesting to compare the reaction (97),... 

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

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