Oxidative addition active catalysts

T-Olefin platinum(O) complexes are important starting materials for oxidative addition see Oxidative Addition) or catalysts. Karstedt s catalysts, which are the most active ones for hydrosilylation, have been structurally characterized and found to show the structure of Pt2(M y M y )3 (9), wherein = divinyltetramethyldisiloxane." A styrene analogue Pt°(styrene)3 provides a convenient route to get an r-alkyne platinum complex by displacement (Scheme 27). DFT calculations indicate that aUcyne in the... 

Keywords 8-Acylquinoline 8-Quinolinyl ketone All-carbon quaternary centers Bond insertion Carboacylation Carbon-carbon bond activation Chelation Cyclometalation Directed metalation Directing group Metallacycle Oxidative addition Rhodium catalyst... 

The best procedures for 3-vinylation or 3-arylation of the indole ring involve palladium intermediates. Vinylations can be done by Heck reactions starting with 3-halo or 3-sulfonyloxyindoles. Under the standard conditions the active catalyst is a Pd(0) species which reacts with the indole by oxidative addition. A major con.sideration is the stability of the 3-halo or 3-sulfonyloxyindoles and usually an EW substituent is required on nitrogen. The range of alkenes which have been used successfully is quite broad and includes examples with both ER and EW substituents. Examples are given in Table 11.3. An alkene which has received special attention is methyl a-acetamidoacrylate which is useful for introduction of the tryptophan side-chain. This reaction will be discussed further in Chapter 13. 

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

Such a complex, cw-Rh(CO)2I2, is the active species in the Monsanto process for low-pressure carbonylation of methanol to ethanoic acid. The reaction is first order in iodomethane and in the rhodium catalyst the rate-determining step is oxidative addition between these followed by... 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

The catalytic activity of doped nickel oxide on the solid state decomposition of CsN3 decreased [714] in the sequence NiO(l% Li) > NiO > NiO(l% Cr) > uncatalyzed reaction. While these results are in qualitative accordance with the assumption that the additive provided electron traps, further observations, showing that ZnO (an rc-type semi-conductor) inhibited the reaction and that CdO (also an rc-type semi-conductor) catalyzed the reaction, were not consistent with this explanation. It was noted, however, that both NiO and CdO could be reduced by the product caesium metal, whereas ZnO is not, and that the reaction with NiO yielded caesium oxide, which is identified as the active catalyst. Detailed kinetic data for these rate processes are not available but the pattern of behaviour described clearly demonstrates that the interface reactions were more complicated than had been anticipated. 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

When NOj levels are measured electrochemicaUy, NO and NO2 can lead to opposing signals because NO is oxidized and NO2 tends to be reduced. Moreover, it is preferred to obtain a total NO, measurement instead of only one of the constituents. The latter can be achieved by catalytically equilibrating the feed with oxygen before contact with the sensor by coating an active zeolite layer on top or placing a active catalyst bed in front of the sensor. Both approaches have been demonstrated successfully with a Pt-Y zeohte as active catalyst [74, 75]. The additional advantage of the filter bed is a reduction in the cross-sensitivity with CO due to CO oxidation above 673 K. 

V (2 ), Cr ( ), Zr (1 ), or Ta (1 ). The role of these promoters in the air cathode is unclear, and some have suggested that the active catalysts are alloys of the Ft with the transition metal (1,4) which form during heat-treatment of the oxide impregnated precursor. In the first section of this paper, we review the work from the Lawrence Berkeley Laboratory on the study of the mechanism of promotion of air cathode performance by these transition metal additives. 

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