Ligand transfer oxidation

Diaryl tellurides undergo facile ligand-transfer oxidations with [bis(acyl-oxy)iodo]arenes in chloroform to give stable diaryltellurium dicarboxylates 12 (Scheme 7) [23]. Similar ligand-transfer oxidations of triarylbismuthanes and triarylstibanes with BAIB in dichloromethane leading to Bi(V) and Sb(V) diacetates 13 and 14 have also been reported [24,25]. The triarylbismuth diacetates were employed for high yield Cu(II)-catalyzed arylations of a series of aryl-amines [24]. 

Since these reactions are influenced by changes in the redox potential of the metal complex, it is possible to change from one process to the microscopic reverse process by changing the ligands attached to the metal. For example, with acetate ligands cobalt(II) is stable with respect to cobalt(III), and, in the presence of bromide ions, cobalt(III) is reduced by alkyl radicals in a ligand transfer oxidation ... 

A further modification106 is achieved by intercepting the allylic radicals with Cu(II) shown in Eq. (82). The 5-carboxypentyl radical may also be directly intercepted by Cu(II) to afford co-hexenoic acid [Eq. (83)]. Also, the ligand transfer oxidation of alkyl radicals by cupric halides may be employed to produce the corresponding co-halo acids, e.g.,... 

Facile ligand-transfer oxidation of alkyl radicals is accomplished by copper(II) halides or pseudohalides 143a). Two processes occur simultaneously (1) oxidative substitution via cationic intermediates and an alkylcopper species, as in electron-transfer oxidation processes and (2) homolytic atom transfer. The former is akin to the oxidative displacement... 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

In the presence of manganese triacetate and the azide anion in refluxing acetic acid alkenes are converted to 1,2-diazides in satisfactory yield, most probably through a ligand transfer-oxidation mechanism74. 

The Cu(II)X2-CuR reaction results in formation of oxidative coupling and ligand-transfer oxidation products, R—R and R—X, respectively, e.g. ... 

In the process of Scheme 9 the attack to the protonated base successfully competes with a ligand-transfer oxidation of the alkyl radical 13 [Eq. (8)]... 

A chain mechanism is proposed for this reaction. The first step is oxidation of a carboxylate ion coordinated to Pb(IV), with formation of alkyl radical, carbon dioxide, and Pb(III). The alkyl radical then abstracts halogen from a Pb(IV) complex, generating a Pb(IIl) species that decomposes to Pb(II) and an alkyl radical. This alkyl radical can continue the chain process. The step involving abstraction of halide from a complex with a change in metal-ion oxidation state is a ligand-transfer type reaction. 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

The oxidation of n-butanal by CUCI2 in dimethylformamide showed simple second-order kinetics in the presence of lithium chloride . At 83 °C, 2 s 2x10 1.mole". sec". a-Monohalogenation occurs in 97% yield. Cu(Il)-catalysed enolisation followed by ligand-transfer is proposed. a-Halogenation of acetone is accomplished by CUCI2, viz. 

The methanolic cupric bromide oxidation of propargyl alcohol to trans-BrCH-CBrCH20H (30%) and Br2C=CBrCH20H (18%) and, under other reaction conditions, Br2C-CBr-CH20H (93 %) follows simple second-order kinetics with a rate coefficient of 1.5 x 10 l.mole . sec at 64 °C. A mechanism of ligand-transfer in a 7t-complex is proposed. ... 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

Meerwein Arylation Reactions. Aryl diazonium ions can also be used to form certain types of carbon-carbon bonds. The copper-catalyzed reaction of diazonium ions with conjugated alkenes results in arylation of the alkene, known as the Meerwein arylation reaction.114 The reaction sequence is initiated by reduction of the diazonium ion by Cu(I). The aryl radical adds to the alkene to give a new (3-aryl radical. The final step is a ligand transfer that takes place in the copper coordination sphere. An alternative course is oxidation-deprotonation, which gives a styrene derivative. 

A general mechanistic description of the copper-promoted nucleophilic substitution involves an oxidative addition of the aryl halide to Cu(I) followed by collapse of the arylcopper intermediate with a ligand transfer (reductive elimination).140... 

Organoiron complexes (7) are converted in high yield into ammonium salts (8) these in turn undergo oxidatively induced ligand transfer and cyclization to give azetidinones (9) in moderate yields (Scheme 9). Formation of the trans product (9b) indicates a stereochemical sequence of trans addition to the olefin complex followed by carboxamidation with retention of configuration at the C—Fe bond. 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

The radical >C(Ar)-C < is oxidized by ligand transfer as Jenkins and Kochi (1972) indicated. If the cation-radical [>C=C<]+ obtained as a result of the initial electron transfer is not fully consumed in the reaction, it is reduced by Cu(I) and returns in the form of its geometrical isomer. In the olefin cation-radical state, cis—>trans conversion has to take place, and it indeed takes place in the systems considered (Obushak et al. 2002). 

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