Oxidative additions, viii

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

The viscosity of the oxidized polymer (VIII) was determined using DMF as a solvent. Chloroform was not a good solvent because it was too volatile and resulted in poor reproducibility. The reduced viscosities are plotted against polymer concentration (Figure 6). Polymer VIII behaved like a polyelectrolyte, the reduced viscosities increased sharply on dilution in a salt free solution. The addition of 0.01 M KBr did not completely suppress the loss of mobile ions however, at 0.03 M KBr addition a linear relationship between the reduced viscosities and concentration was established. 

The mechanism proposed for this allenic Alder-ene reaction is shown in Scheme 8.3. The rhodium] I) catalyst coordinates with the allenyne V forming intermediate VI, which undergoes an oxidative addition to form the metaUocycle VII. The metaUocycle then undergoes a /9-hydride elimination producing triene intermediate VIII, which... 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

Scheme 23 Reagents and conditions (i) CH2Cl2 (ii) CH2X2 (X = Br, I), —80 °C (iii)reductive coupling (iv) oxidative addition (v) SN2 (vi) halide abstraction (vii) migratory insertion (viii) CD2Cl2, r.t.

Besides these determinations of static structures, further studies of the kinetics of oxidative addition to Ir(I) complexes have been made. Beginning with Ir(CO)H(PPh3)3 (VIII), the first step is seen as dissociation to the four-coordinate planar intermediate (IX). This then deforms and undergoes concerted cis addition to yield the adduct (V) (179,224). In a comparison of group IV hydrides, PhsMH, the rates increased in the order Sibond energy but to the effects of differing polarizability and solvation of M-Ir products on the activation energy for addition. 

We are here concerned with various organometallic reactions for which there is evidence that organic free radicals are implicated in the reaction pathway. Many of these arc formally two-electron oxidative additions or their retrogressions, the reductive eliminations (Section V,A). We shall focus attention on systems in which transition metal Group VIII complexes are involved. 

The reaction between a low-valent Group VIII metal complex and an alkyl halide belongs to the class known as oxidative addition and has attracted much study and controversy as to the mechanism. Recent evidence suggests free radicals as intermediates in many cases. The oxidative-addition reaction is of widespread occurrence and importance in transition metal chemistry, due in part to its use in synthesis and to its implication in many catalytic systems. In one of its forms it is described by... 

Apart from the well-known oxidative additions to Ir1, hydrogen can also be added to Ir111. In the case of (21-VIII) the reaction proceeds at room temperature the resulting Irvtetrahydride reductively eliminates H2 only on heating above 130°C and is a highly active catalyst for the transfer dehydrogenation of cyclooctane.28... 

Oxidative addition of organic halides to low-valent metal complexes generates reactive metal alkyls that can then be used in insertion, coupling, carbonylation-decar-bonylation and cyclization reactions for organic synthesis. These transformations can be made catalytic after development of the stoichiometric chemistry using the more stable metal alkyls. This section surveys the reactions of alkyl, aryl and acyl halides with transition metal complexes of the groups IIIA (lanthanides and actinides), IVA-VIII and IB. 

The reactivity of transition-metal complexes toward oxidative addition increases on going from right to left across a period and on going down a given triade.g., for the d -4-coordinated complexes. In general, one-electron oxidations are more common among the lanthanides, actinides and early transition metals, whereas two-electron oxidations are more common in the later transition metals. The d -Pt(0) and -Pd(0) complexes, e.g., are more reactive toward RX oxidative addition than toward oxidative addition, whereas the opposite is true for the other group VIII metal complexes. 

Group VIII Fe, Ru, Os. Five-coordinated complexes of the iron triad metals M(0) and M(I) undergo oxidative addition with alkyl, aryl and acyl halides to give 6-... 

Group VIII—Co, Rh, Ir. The lr(0) 18-electron complex, Ir(NO)[P(C6Hj)3]3, undergoes oxidative addition of CH3I with loss of one phosphine ligand to give a 5-coordinated complex ... 

Group VIII—Ni, Pd, Pt. The Ni triad provides the widest reactivity toward oxidative addition of alkyl halides. It is in this triad that the extreme sensitivity of the oxidative addition reaction to changes in ligands, the organic halide and reaction conditions (solvent and T) are most felt. 

The reactivity of Pt(0) complexes toward aryl halides closely parallels that of the Ni(0) and Pd(0) analogs. The trans-oxidative addition product is obtained, and second-order kinetics are observed. The greater stability of the metal-aryl compared with the metal-alkyl bond in group VIII d °-M(0) systems is attributed to the greater electronegativity of the sp carbons in the aryl ligands . 

The basic mechanism of the Heck reaction (as shown below) of aryl or alkenyl halides or triflates involves initial oxidative addition of a pal-ladium(O) species to afford a a-arylpalladium(II) complex III. The order of reactivity for the oxidative addition step is I > OTf > Br > Cl. Coordination of an alkene IV and subsequent carbon-carbon bond formation by syn addition provide a a-alkylpalladium(II) intermediate VI, which readily undergoes 3-hydride elimination to release the product VIII. A base is required for conversion of the hydridopalla-dium(II) complex IX to the active palladium(O) catalyst I to complete the catalytic cycle. 

Sections 10.3.6.1-10.3.6.7 contain a critical summary of reactions involving oxidative addition and reductive elimination processes occurring at group VIII metal centers. 

Oxidative addition of halogen-containing compounds to organo-group VIII complexes dates from the 1950s ... 

Oxidative addition to ruthenium and osmium four-coordinate complexes occurs readily. These complexes are excellent starting materials for group VIII complexes. Addition of formaldehyde to complexes M(CO)L(PPh3)2 (L = CO or PPhs selection of L is metal dependent) leads to oxidative addition products, a reaction of relevance to Fischer-Tropsch processes. The ruthenium complex is proposed as an intermediate only the osmium complex has been isolated ... 

Oxidative Addition and Reductive Elimination Reactions of Group VIII Cobalt, Rhodium, and Iridium... 

The mechanism of catalytic hydrosilylation involves oxidative addition of a silicon-hydrogen bond to a metal complex as an essential step since it is here the activation of hydrosilane by the catalyst takes place. Thus, many transition metal ions and complexes, especially group VIII metals in low oxidation state containing ir-acid ligands such as CO, tertiary phosphines or olefins display catalytic activity. The sequence of unit reactions in a typical d -metal complex-catalyzed hydrosilylation is summarized as ... 

The exchange reaction of methane with a lutetium complex apparently proceeds via the transition state VIII-1 rather than via oxidative addition involving intermediate VIII-2. Quantum-chemical calculations have been carried out [23] for the mechanism including inter alia a four-electron four-center transition... 

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