Olefins, complexes with platinum

Other mechanisms of insertion than the type shown in Eqs. (c) and (d) may exist, ESR evidence, for example, supports a radical mechanism for the reaction of alkyl-platinum complexes with olefins . 

The Duphos ligand (157) has been used to prepare organometallic complexes of palladium.385 Rhodium complexes of ligands (156), (157), or (159) are efficient catalysts for enantioselective hydrogenations337-341,343,345,386-388 and furthermore, in the case of platinum complexes with (157), as catalysts for the asymmetric hydrophosphination of activated olefins.71... 

Chloroplatinic acid, platinum on carbon or alumina, platinum(II) complexes with olefins, and platinum(O) complexes with phosphines have been used . Among them,... 

Chloroplatinic acid, platinum on carbon or alumina, platinum(II) complexes with olefins, and platinum(O) complexes with phosphines have been used for hydrosilylation1,2. Chloroplatinic acid, H2PtCl6-6H20, is by far the most commonly used and efficient catalyst particularly for the hydrosilylation of a variety of olefinic substrates1 4. Chloroplatinic acid is frequently used as a solution in isopropanol and this system is referred to as Speier s catalyst , in recognition of the work of J. L. Speier in the discovery of its usefulness. 

Complexes of Platinum(0) with Olefins and Related Molecules. In the period of this Report the geometries of ten complexes of bis(triphenylphosphine)-platinum(O) with unsaturated molecules, such as olefins, acetylenes, and dioxygen, have been described. In these complexes the metal co-ordination is trigonal planar, one site heing occupied by a more or less symmetrically co-ordinated multiple bond, as in (71). A selection of distances and angles... 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

A closely related dicationic platinum complex has been shown to transform efficiently /3-citronellene into cis-thujane in a highly diastereoselective manner, which mimics terpene biosynthesis.362 Also, using platinum(n) catalysis, Widenhoefer has reported an intramolecular alkylation of indoles with unactivated olefins, which can be carried out in an enantioselective fashion (Scheme 99).363... 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

The mechanistic study on the hydrophosphination of activated olefins, in conjunction with rapid inversion of the configuration at the phosphorus center, was elaborated to develop asymmetric hydrophosphination catalyzed by a chiral phosphine platinum complex although the % ee is not excitingly high yet (Scheme 9) [15]. 

Nucleophihc attack of methoxide ion on olefinic and acetylenic groups of suitable unsaturated phosphine complexes of platinum(II) takes place to give cyclic, a-bonded complexes. This reaction appears to occur whether or not the unsaturated group is coordinated to the metal (22). But-3-enyl diphenylphosphine forms a chelate complex with platinum(II) chloride which on treatment with sodium hydrogen car-... 

The sulphide ligands containing one butenyl group form chelate complexes with platinum(II) and palladium(II) halides. The chelated butyl pentenyl sulphide complexes could only be obtained for platinum-(II). The compounds containing two group VI atoms functioned as bidentate ligands by donation from the two sulphim atoms only. All these chelated olefinic sulphide complexes react with simple monodentate... 

Platinum(II) and ruthenium(II) complexes with chiral modified diphosphines like 47 or tetradentate P2N2 ligands like 48 have been used for the asymmetric epoxidation of olefins with hydrogen peroxide with ee values of 18-23%, which increased up to 41% when cationic solvato derivatives such as P2Pt(CF3)(CH2Cl2)(BF4) are used . Similar chiral inductions were reported for Ru derivatives, although the nature of the active intermediate was still in question. ... 

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

The metal hydride mechanism was first described for the cobalt-carbonyl-catalyzed ester formation by analogy with hydroformylation.152 It was later adapted to carboxylation processes catalyzed by palladium136 153 154 and platinum complexes.137 As in the hydroformylation mechanism, the olefin inserts itself into the... 

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