Platinum-catalysed reactions

Mechanism (C) was proposed for the platinum-catalysed reaction to explain the observed order of 1.5 in hydrogen. The mechanism comprises steps (1), (3)—(6), (9) and the reaction... 

In the platinum-catalysed reaction, it has been observed [155] that the effect of increasing the hydrogen/propadiene reactant ratio was to increase the yield of propane without affecting the yield of propene. This has been interpreted as showing that propane may be formed directly from adsorbed propadiene by a mechanism which does not involve adsorbed propene as an intermediate. However, no conclusions were drawn regarding the nature of the surface intermediates involved in such an interconversion. 

In the rhodium- and platinum-catalysed reactions [167], it is of significant interest that the 7V-profiles calculated from the distributions of deuterium in the n-butane (Table 30) appear to bear no clear relationship to the 7V-profiles of the n-butenes formed simultaneously. This observation has been interpreted as indicating that either the butene which undergoes further hydrogenation never desorbs as butene, or that the sites responsi-... 

The work-up of the reaction mixture and the analysis were conducted in the same way as in the platinum-catalysed reaction. 

Platinum-catalysed reaction of olefin epoxides to SiH compounds, or hydrosilation, is an elegant and convenient means of sythesizing epoxy-functional silicones and siloxanes (1). The basic reaction is presented as equation 1 ... 

If an addition alkene is present, then the cyclopropyl carbene intermediate may be intercepted to effect cyclopropanation of the second alkene. Cyclopropanation can become the dominant pathway, as in the platinum-catalysed reaction of enyne 6.266 (Scheme 6.108). ° Whether this occurs, however, once... 

SN displacement reactions, 27-9 a-Selenocyclohexanones, 77 Senecioyl chloride, 33 Silmagnesiation, platinum-catalysed, 8 Silametallation of terminal alkynes, 7-9 Silver acetylide, 49 Silver trifluoroacctate, 42,127 Silyl cuprates, 7... 

Hydroformylation represents the most industrially important homogeneous catalysed reaction by volume [2, 3]. The petrochemical, agrochemical and pharmaceutical industries are particularly interested in this transformation. The reaction uses syngas (COiH mix) and a catalyst, commonly rhodium or platinum, to transform an olefin into an aldehyde (Scheme 9.1) [4]. 

Before showing how direct peroxidation of active group works, it should be mentioned that platinum catalyses oxidation of alcohols at ambient temperature in aldehydes (provided they are primary). The exothermicity of the reaction is sufficient to cause the alcohol to combust by heating a thread of platinum at a high temperature. Note that 2-butanol (but not isopropanol) is classified in list B, producing peroxides that become expiosive at a certain concentration in the lists of peroxidable compounds set up by Du Pont de Nemours company (see p.261). 

The synthesis of precursors A and B (see Figure 12.10) has been described by Michalczyk et al. [102]. These compounds can be synthesised from platinum-catalysed hydrosilylation reactions, that is addition reactions between Si—H and C=C groups in the presence of a catalyst. Once the pure precursors are obtained, BSG can be synthesised by incorporation of calcium alkoxide during polycondensation of the precursors. 

Metallacyclobutanes have been proposed as intermediates in a number of catalytic reactions, and model studies with isolated transition metallacyclobutanes have played a large part in demonstrating the plausibility of the proposed mechanisms. Since the mechanisms of heterogeneously catalysed reactions are especially difficult to determine by direct study, model studies are particularly valuable. This article describes results which may be relevant to the mechanisms of isomerization of alkanes over metallic platinum by the bond shift process and of the oligomerization or polymerization of alkenes. 

Trifluoroacetic acid is the best solvent for several platinum-catalysed hydrogenation processes. For example the rate of reduction of ketones to secondary alcohols is about three times faster than in acetic acid. The ketones are taken in concentrated solutions to keep the reactions faster. 

Kinetics is the study of the factors which influence reaction rates. Enzyme-catalysed reactions are subject to the same principles of rate regulation as any other type of chemical reaction. For example, the pH, temperature, pressure (if gases are involved) and concentration of reactants all impact on the velocity reactions. Unlike inorganic catalysts, like platinum for example, there is a requirement for the substrate (reactant) to engage a particular region of the enzyme known as the active site. This binding is reversible and is simply represented thus ... 

In conclusion, in the instance of the platinum-catalysed hydrosilylation reaction the molecular insight is behind the industrial practice for an industrial... 

Platinum, palladium, and rhodium all catalyse reactions that transform some of the noxious gases in the exhausts of cars into less... 

Over all the metals studied, except cobalt, nickel and copper, the selectivity and stereoselectivity decreased slightly as the reaction proceeded. In addition to the products shown in Table 20, in the rhodium- and iridium-catalysed reactions small yields (2—3%) of buta-1 2-diene were also observed. For all the catalysts, except rhodium, iridium and platinum, which were not investigated, the initial rate kinetic orders were unity in hydrogen and zero or slightly negative (Ni) in but-2-yne. 

From these results it can be seen that in the hydrogenation of 1,2-dialkylcyclohexenes the expected cis-1,2-dialkylcyclohexane is not the sole product. Similarly, in the hydrogenation of the 1,4-dialky lcyclohexenes where both the cis- and trans-cyclohexanes are expected, the trans-isomer being the thermodynamically more stable, the stereoselectivity varies from metal to metal. Thus with palladium, the cis/trans ratio approaches the equilibrium composition, whereas with platinum and rhodium, the equilibrium composition is never approached. It is also instructive to note that in the palladium-catalysed reactions, hydrogenation is accompanied by extensive alkene isomerisation [220—223], whereas with rhodium and platinum, little or no isomerisation is observed [220,... 

The platinum-catalysed intramolecular domino annulation reaction of o-alkynylben-zaldehydes has been described as a versatile approach to naphthalenes with annulated carbocycles or heterocycles of various sizes (Scheme 32).94 A plausible mechanism for the platinum(II)-catalysed annulation reaction shows that the double annulation process most probably proceeds through the benzopyrylium cation (117), which results from the nucleophilic attack of the carbonyl oxygen at the alkyne, activated by the Lewis-acidic platinum salt. A subsequent intramolecular Huisgen-type 3 + 2-cycloaddition of the second alkyne is assumed to generate intermediate (118). Rearrangement to (119) and the formal 4 + 2-cycloaddition product (118) leads to the aromatized final (116), liberating the active catalyst. In the case of FeCl3 as the Lewis acid, we assume that intermediate (118) is oxidatively transformed to (121). 

Platinum-catalysed intramolecular hydroamination of unactivated alkenes with secondary alkylamines has been reported. Thus, a number of y- and 5-aminoalkenes reacted in the presence of a catalytic 1 2 mixture of [PtCl2(H2C=CH2)]2 (2.5 mol%) and PPh3 in dioxane at 120 °C for 16 h to form the corresponding pyrrolidine derivatives in moderate to good yields. The reaction displayed excellent functional group compatibility and low moisture sensitivity.92... 

Mainly the hydrogenations are mediated through a platinum-group-metal-catalysed reaction with chiral ligands bound to the metal. Koten and coworkers210 proposed the use of Ru- and Rh-containing catalysts immobilised on silica in asymmetric hydrogenation. As a chiral compound, they used BINAP (2,2 -bis(diphenylphosphino)-1,1 -binaphthyl) (Scheme 4). 

Chlorostannate liquids such as [C4Ciim][Sn2Cl5] have also been shown to be a potential alternative in i.e. platinum-catalysed hydroformylation, where they activate the catalyst precursor in a Lc vis-acid ase reaction, as depicted in Scheme 4.5.[39]... 

Gold- and platinum catalysed polycyclizations have also been performed under solvent free conditions (Figure 2.12). These isomerization reactions, atom-efficient by their very nature, offer an expedient and stereoselective route to various polycyclic products. Although studies are ongoing, the catalyst loading can be reduced to 0.5% and in some cases the product can be isolated by distillation. In other cases, petroleum ether was used to purify the crude reaction mixture by column chromatography. 

If a catalyst has a different physical state from the reaction mixture that it is catalysing, it is said to be a heterogeneous catalyst. An example of this would be a metal catalyst, such as platinum, which catalyses the hydrogenation of gaseous alkenes. If, however, the catalyst has the same physical state, it is said to be a homogeneous catalyst, for example, the solvated proton in an acid catalysed reaction that is occurring in solution. 

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