Transition metal catalysts with platinum

Both silylmagnesium and silylaluminium species add to terminal alkynes in the presence of a transition-metal catalyst with high regio- and stereoselectivity. For example, platinum-catalysed silamagnesiation (3) followed by aqueous quenching provides exclusively ( )-l-silylalk-l-enes. 

Most alkenes react quantitatively with molecular hydrogen, Hg in the presence of a transition metal catalyst to give alkanes. Commonly used transition metal catalysts include platinum, palladium, ruthenium, and nickel. Idelds are usually quantitative or nearly so. Because the conversion of an alkene to an alkane involves reduction by hydrogen in the presence of a catalyst, the process is called catalytic reduction or, alternatively, catalytic hydrogenation. 

Of associated relevance are investigations that show that the imidazolium cation of the ionic liquid may be reactive towards dissolved transition metal catalysts, with the isolation and characterisation of a carbene-platinum complex [642] (CSD refcode TETVOC), mpt. 193°C, from one such system This provides a link to the extensive literature [643-645] on imidazolylidene carbenes and related metal imidazolylidene complexes. More recendy, the crystal structures of [Ag(l-ethyl-3-methylimidazol-2-ylidene)2][B(CN)4] (mpt. 100-104°C) [464] and the lower-melting [Ag(l-ethyl-3-methylimidazol-2-ylidene)2][N(S02CF3)2]... 

Some gold catalyst species proved to be better than platinum in the intramolecular reactions of unactivated alkenes, as studied by Widenhoefer et al. [61-63]. Gold was allowed to work under mild conditions and the scope of the reaction was also broader than with other late-transition-metal catalyst systems, leading to the formation of five-and six-membered rings. 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

There has been considerable interest in binuclear and polynuclear metal complexes as models for intermediates proposed to be formed during reactions which are heterogeneously catalysed by transition metals (1). Since platinum is one of the most versatile catalysts, we have begun an investigation into the synthesis, and chemical and catalytic properties of some binuclear organo-platinum complexes. In this article some hydrido and methyl complexes will be described, and a preliminary account of catalysis with binuclear complexes given. In addition, structural studies indicate that Pt-Pt bonding interactions may take several different forms in these complexes and so the nature of the Pt-Pt bond will also be discussed. 

Because oxidations with oxygen are free-radical reactions, free radicals should be good initiators. Indeed, in the presence of hydrogen bromide at high enough temperatures, lower molecular weight alkanes are oxidized to alcohols, ketones, or acids [5 7]. Much more practical are oxidations catalyzed by transition metals, such as platinum [5, 6, 55, 56], or, more often, metal oxides and salts, especially salts soluble in organic solvents (acetates, acetylacetonates, etc.). The favored catalysts are vanadium pent-oxide [3] and chlorides or acetates of copper [2, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66], iron [67], cobalt [68, 69], palladium [60, 70], rhodium [10], iridium [10], and platinum [5, 6, 56, 57]. 

Since Pt dissolution is favored by high electrode potential, relative humidity, and temperature, the possibility to limit the risk of electrocatalyst aging is based on the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which is characterized by higher potential with respect to anode, and by adoption of operative conditions not too severe in terms of humidity and temperature. While this last point requires interventions on the membrane structure, the study of catalyst materials has evidenced that a minor tendency to sintering can be obtained by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode catalyst [59, 60], suggesting a possible pathway for future work. On the other hand also the potential application of non-platinum catalysts is under study, in particular transition metal complexes with structures based on porphyrines and related derivatives have been proposed to substitute noble metals [61], but their activity performance is still far from those of Pt-based catalysts. 

Hydrocarboxylation is the formal addition of hydrogen and a carboxylic group to double or triple bonds to form carboxylic acids or their derivatives. It is achieved by transition metal catalyzed conversion of unsaturated substrates with carbon monoxide in the presence of water, alcohols, or other acidic reagents. Ester formation is also called hydroesterification or hydrocarb(o)alkoxylation . The transition metal catalyst precursors are nickel, iron or cobalt carbonyls or salts of nickel, iron, cobalt, rhodium, palladium, platinum, or other metals4 5. 

The first variable in catalytic hydrogenation is the catalyst. The most commonly used heterogeneous catalysts are platinum, palladium, nickel, rhodium, nickel, and ruthenium. Rylander gave references for the preparation of the most common catalysts, shown in Table 4.14.31 In some cases, salts of transition metals are used rather than the metal itself although the pure metal adsorbed on a support (see above) is also commonly used. Hudlicky presented an order of relative reactivity with propene for groups 8-10 (VIII) transition metal catalysts,341 based on the work of Mann and Lien.342 xhe order given is ... 

Sila- and 1,3-disila cyclobutanes possess an increased reactivity because of their ring strain [106] and can be transformed into polycarbosilanes by ring opening polymerization using either thermal activation [107], anionic polymerization with organolithium compounds [108], or mostly platinum [109] or other late transition metal catalysts [110]. 

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