Catalytic cycle involving coordination

These catalysts are activated by hydrogenation of the cyclooctadiene ligand, which releases cyclooctane and opens two coordination sites at iridium. The mechanism has been probed by computational studies.40 It is suggested that the catalytic cycle involves... 

This behavior, as well as complementary observations, can be explained on the basis of the reaction mechanism depicted in Scheme 5.3. The main catalytic cycle involves three successive forms of the enzyme in which the iron porphyrin prosthetic group undergoes changes in the iron oxidation state and the coordination sphere. E is a simple iron(III) complex. Upon reaction with hydrogen peroxide, it is converted into a cation radical oxo complex in which iron has a formal oxidation number of 5. This is then reduced by the reduced form of the cosubstrate, here an osmium(II) complex, to give an oxo complex in which iron has a formal oxidation number of 4. 

On the basis of DFT calculations, a catalytic cycle involving a copper vinylidene intermediate has been proposed (Scheme 9.22) [44]. The reaction is initiated by copper acetylide (138) formation. Sharpless and coworkers next invoke an unusual [3 + 3]-cycloaddition that would be forbidden by orbital symmetry, were it not stepwise. Coordination of an azide to complex 138 generates a zwitterionic complex (139). Internal nucleophilic attack of the acetylide moiety of 139 on the electrophilic... 

The catalytic cycle involves the oxidative addition of RX to Pd(0), coordination and migratory insertion of CO leading to cr-acylpalladium complexes, transmetallation of the latter by organometallic compounds, followed by reductive elimination (Scheme 1). 

Kinetics. Since the hydrogenation of 1-hexene is accompanied by isomerization to the internal olefin, the catalytic cycle involves an alkyl intermediate which must be formed by inserting the coordinated 1-alkene. Reaction 7 proposes a mechanism for the hydrogenation ... 

In the first step of the catalytic cycle a coordinatively unsaturated Pd(0) species - which is formed in situ from Pd(OAc)2 and PPh3 -inserts into the alkenyl-I bond of 8 to give 42 (syn addition). Next an insertion of the terminal olefin into the cr-alkenyl-C-Pd bond forms the six-membered ring in 43. The stereochemistry can be explained by 41 reaction of the si-face of the exomethylene group involves a nearly coplanar orientation of the Pd-C bond and the C-C-zrbond. The siloxy substituent is placed pseudo-equatorially. 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

The chemical transformation of Ru-complexes in faujasite-type zeolites in the presence of water and of carbon monoxide-water mixtures is reviewed and further investigated by IR, UV-VIS spectroscopic and volumetric techniques. The catalytic activity of these materials in the watergasshift reaction was followed in a parallel way. The major observations could be rationalized in terms of a catalytic cycle involving Ru(I)bis and triscarbonyl intermediates stabilized in the supercages of the faujasite-type zeolite. The turnover frequency of this cycle is found to be determined by the nature, number and position of the charge compensating cations, as well as by the nature of the ligands present in the Ru-coordination sphere. 

Since such catalytic cycles involve transformations of organic species at a metal centre, a first step must be the formation of an organometallic. These can be formed by a direct reaction in which an organic reactant replaces a solvent or ligand molecule in the coordination sphere of a metal complex. For example, the ethylene-palladium(II) complex Pd(ri -C2H4)Cl2 2 can be formed by the direct reaction of PdCl2 with ethylene (Equation 1), and the butadiene-iron complex is also formed directly by replacement of two CO s on Fe (Equation 2). 

The catalytic cycle involving [RuCbCPPhsjs], one of the more active transfer-hydrogenation catalysts, is shown in Scheme 10. The catalyst first forms an alkoxido complex, with elimination of HCl, when allowed to react with the secondary alcohol. This pentacoordinate complex forms an 18-electron species by coordinating a molecule of alkene. The alkoxido ligand transfers its Q -deuterium atom to the metal, after which the ketone oxidation product of the secondary alcohol is eliminated. The steps are believed to occur in... 

The reaction of some acyclic and cyclic nitrones with methacrolein in the presence of catalytic amounts of Rh or Ir complex 92 was studied. Some intermediates involved in the process were isolated and characterized and, accordingly, a catalytic cycle involving [M]-aldehyde, [M]-nitrone and [M]-adduct species was proposed. All reactions occurred with complete diastereoselectivity and good enantioselectivity. Cyclic nitrones such as 91 were added slowly to the mixture of aldehyde and 92 to avoid nitrone coordination to the metal <05JA13386>. 

However, little is known as to why selective generation of CO takes place in the electrochemical reduction by these Ni macrocycle species adsorbed on the surface of an Hg electrode in H2O. Sakaki et al. carried out SCF ah initio calculations for [NiF(NH3)4] as the model of [Ni(cyclam)] adsorbed on Hg (Fig. 5), and a NiX7ji-C02 adduct with an OCO angle of 135.3° is suggested as the active species during CO2 reduction (66, 67). The increase in electron density of an 0-atom of the terminal CO2 from -0.33e (free CO2) to -0.58e upon coordination to Ni supports the proposed mechanism for the catalytic cycle involving an hydroxycarbonyl intermediate. 

As Scheme 1 illustrates, part of the proposed catalytic cycle involves intermediate A which can either trap H20 (route I) to release the amide and regenerate the catalyst, or become inactive by product coordination (route II). A positive note involves the regioselectivity of the catalyst as only the C=N position is hydrated when bifunctional substrates, such as acrylonitrile, are used. At 75 °C, CH2=CHC(0)NH2 was produced exclusively with no detectable amounts of HOCH2CH2C=N and (N=CCH2CH2)20, which are drawbacks of other processes [66]. 

In contrast with the initial report employing boronic esters that follow a Pd(II)/ (0) pathway [52], the current methodology is proposed to occur via a Pd(II)/ (IV) mechanism. No reaction was observed in the presence of a Pd(0) source or in the absence of the silver salt. A plausible catalytic cycle involves an initial C-H activation step of palladium complex A to provide palladacycle B, followed by oxidative addition of the aryl iodide to give a highly reactive Pd(IV) complex C, which undergoes rapid reductive elimination to provide the desired product and a Pd(II) complex D (Scheme 30). The last step is loss of iodide, mediated by the silver salt. The use of the weakly coordinating -NHTf group by the Yu lab has allowed for the development of the first intermolecular and enantioselective C(sp )-H activation via a Pd(n)/(IV) catalytic cycle. 

Interesting information about the elementary steps involved in the formation of the styryl derivatives Os (iJ)-CH=CHPh Cl(CO)(PR3)2 has been obtained by using the alkyne dicarbo lic methyl ester as a model. This alkyne initially coordinates to OsHCl(CO)(PPr3)2 in a position trans to the hydride, followed by a rearrangement to the cis-isomer, and subsequent insertion to yield the corresponding alkenyl species. The slow step of this catalytic cycle involves the... 

The plausible mechanism of the catalytic cycle involves oxidative addition of the cyclic dichalcogenide to the ML species, alkyne coordination and insertion into the M—Z bond, and reductive elimination (Scheme 3.73) [123]. Although only... 

The mechanism for the catalytic oxidation of methane by Pt(II) was first put forth by Shilov. The first step of the catalytic cycle involves the formation of a methylplatinum(II) intermediate 16 via the C-H activation of hydrocarbon methane with Pt(II) 15. A methylplatinum(IV) species 17 is obtained by the oxidation of methylplatinum(II) intermediate 16 (second step). The formation of the product takes place through reductive elimination from the methylplatinum(IV) 17 either via coordination to water or by the nucleophilic attack at the carbon by an external nucleophile such as water or chloride ion. Considerable amount of experimental data supports has been offered for supporting the given general mechanism (Fig. 6). 

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