Thallium ions, oxidation reactions

The equation for a net chemical reaction represents the overall transformation of reactants into products. Thus, thallium Ill) ions oxidize iron(II) ions according to Eq. (1-1), and a secondary amine reacts with an aryl chloride as in Eq. (1-2). 

A mechanism involving an intermediate enediolate ion with the rate of reaction being equal to the rate of enolizatiori was proposed. The kinetics and mechanism of thallium(III) oxidation of cellobiose in acid medium have been determined the acid-catalyzed reaction was first order in each reactant, and the active oxidant was thallium(III) diacetate. The sugar products were identified as D-gluconic acid and glucose. ... 

Solvatomercuration (40) is a polar addition which follows the Markovnikov rule. Because acetoxymercuric ion is a soft acid, the reaction proceeds facilely. Thallium ions are also soft, hence they attack multiple bonds efficiently. As Tl(III) is also an oxidant, the initial solvatothallation products often undergo rearrangements (41,42) unobserved during solvatomercuration. 

The oxidative bromination of Cs-Cg olefins can be carried out using a system composed of catalytic thallium(iii) oxide (0.2 M of Tl(iii)) and a vanadium-based heteropolyacid cocatalyst (HPA-n = H3+ PMOi2 V 04o n = 2-8) in the presence of hydrogen bromide solution (1.0 M of Br ) and oxygen as co-oxidant. The HPA-n cocatalyst can promote oxidation of thal-lium(i) to thallium(iii) in the presence of bromide ions. For instance, the reaction of 1-hexene with the thallium(iii)/HPA-6/bromide system at 100 °C and 8 bar of oxygen led to a mixture of 1,2-dibromohexane and hex-enebromohydrin along with a small amount of 2-hexanone in different ratios. In the absence of thallium(m) species, the oxidation process proceeds slowly, presumably due to the HPA-6 anion-mediated oxidation of bromide to bromine. 

Several other useful reviews of reactions involving metal ions have also been published. Redox reactions of chromium(m)-amine species have been described and a survey has been made of the solution chemistry together with reaction paths involved in the redox reactions of various plutonium species. Oxidation reactions of thallium(m) have also been described. Developments in the redox chemistry of peroxides have been reviewed, the nature of the reactions which involve iron(iii) in various complexed forms providing a fascinating example of the manner in which geometry and co-ordination to the metal centre greatly affect the reactivity of the system. Redox properties of cobalt chelates, with delocalized... 

A similar situation exists in the hydroxymercuration and thallium(m) oxidation of cycloalkenes and cycloalkanes, the rate laws showing strict second-order behaviour for both metal ions. In the thallium(ni) reactions two products have been observed, a 1,2-diol and a ketone or aldehyde. Two-electron coupling with trifluoroacetate has been shown to promote oxidative phenol coupling. The mechanism of oxidation of olefins by palladium(n) has been investigated. In the presence of PdClg in aqueous media, the data conform to the rate expression... 

Consider the oxidation of mercurous ion by thallium(3+) ion in aqueous solution.13 The reaction and rate law are... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

The utility of thallium(III) salts as oxidants for nonaromatic unsaturated systems is a consequence of the thermal and solvolytic instability of mono-alkylthallium(III) compounds, which in turn is apparently dependent on two major factors, namely, the nature of the associated anion and the structure of the alkyl group. Compounds in which the anion is a good bidentate ligand are moderately stable, for example, alkylthallium dicar-boxylates 74, 75) or bis dithiocarbamates (76). Alkylthallium dihalides, on the other hand, are extremely unstable and generally decompose instantly. Methylthallium diacetate, for example, can readily be prepared by the exchange reaction shown in Eq. (11) it is reasonably stable in the solid state, but decomposes slowly in solution and rapidly on being heated [Eq. (23)]. Treatment with chloride ion results in the immediate formation of methyl chloride and thallium(I) chloride [Eq. (24)] (55). These facts can be accommodated on the basis that the dicarboxylates are dimeric while the... 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

During oxidation of tin(II) ions by hydrogen peroxide, iodine, bromine, mercury(ir) and thallium(III) the induced reduction of cobalt(in) complexes cannot be observed. Therefore, it was concluded that these reactions proceed by 2-equivalent changes in the oxidation states of the reactants. 

Aromatic radical-cations are generated by pulse-radiolysis of benzene derivatives in aqueous solution. Radiolysis generates solvated electrons, protons and hydroxyl radicals. The electrons are converted by reaction with peroxydisulpbate ion to form sulphate radical-anion, which is an oxidising species, and sulphate. In another proceedure, electrons and protons react with dissolved nitrous oxide to form hydroxyl radicals and water, Hydroxyl radicals are then made to react with either thallium(i) or silver(i) to generate thallium(ii) or silver(ll) which are powerfully... 

The reaction of 3-(3,4-dimethoxyphenyl)propanoic acid with thallium(III) trifluoroace-tate in the presence of boron trifluoride etherate leads to a mixture of the dihydrocoumarin (574) and the spirolactone (572) (78JOC3632). It is suggested that these products arise through an initial one-electron oxidation to the radical cation, the fate of which may vary. Thus, intramolecular reaction with the carboxyl group gives the radical (571) and eventually the spirolactone. Alternatively, capture of the radical ion by solvent and further oxidation affords the radical (573), whereupon an intramolecular Michael addition to the carboxyl group and aromatization lead to the dihydrocoumarin (Scheme 218) (81JA6856). 

Silver salts are also common activators in numerous Suzuki coupling reactions. The earliest example could probably be found in the Kishi s palytoxin synthesis. Silver oxide as well as thallium hydroxide provided dramatic rate enhancements in the couplings of vinylboronic acids (Scheme 10.41).69 Both thallium and silver ions are most probably abstracting halide in palladium intermediates, but silver is clearly the most efficient. Moreover, with the right counterion, the silver salt also acts as a mild base and activator. 

We suggest that electron transfer and electrophilic substitutions are, in general, competing processes in arene oxidations. Whether the product is formed from the radical cation (electron transfer) or from the aryl-metal species (electrophilic substitution) is dependent on the nature of both the metal oxidant and the aromatic substrate. With hard metal ions, such as Co(III), Mn(III), and Ce(IV),289 reaction via electron transfer is preferred because of the low stability of the arylmetal bond. With soft metal ions, such as Pb(IV) and Tl(III), and Pd(II) (see later), reaction via an arylmetal intermediate is predominant (more stable arylmetal bond). For the latter group of oxidants, electron transfer becomes important only with electron-rich arenes that form radical cations more readily. In accordance with this postulate, the oxidation of several electron-rich arenes by lead(IV)281 289 and thallium(III)287 in TFA involve radical cation formation via electron transfer. Indeed, electrophilic aromatic substitutions, in general, may involve initial charge transfer, and the role of radical cations as discrete intermediates may depend on how fast any subsequent steps involving bond formation takes place. 

More involved studies of the oxidation of plant phenols [27], as well as the introduction of thallium and hypervalent iodine complexes and the use of electrochemical methods, have emphasized the importance of another intermediate involved in oxidative coupling reactions, namely the phenoxonium ion 8 [28-30]. Due to its ionic nature, reaction through an oxo-nium ion can improve the regioselectivity of bond formation and lead to fewer unwanted products (for example, no coupling via the oxygen atom). The coupling reaction can then be viewed as an electrophilic aromatic substitution between 17 and a nucleophilic aromatic unit 15 (Scheme 5). 

Coordination OrganometalUc Chemistry Principles Electron Transfer Reactions Theory Macrocyclic Ligands Metal Ion Toxicity Oxides Solid-state Chemistry Polynuclear Organometallic Cluster Complexes Sulfur Organic Polysulfanes Thallium Organometallic Chemistry. 

Thallium Dorfman and Gryder, investigating the kinetics of the Ce(IV)-Tl(I) reaction, concluded that the mechanism involves two 1-equivalent oxidation steps with a Ce(rV) dimer, with T1(II) as an intermediate. They pointed out that the reaction may be complicated by the formation of OH or NOs free radicals that oxidize TI(I). Sinha and Mathur studied the Ce(IV)-Tl(I) reaction in the presence of the catalysts Ag(I) and Os(VIII). The rate was found to be first order with respect to both Ce(IV) and T1(I) but unaffected by Ce(III) or T1(III). Bisulfate and hydrogen ions acted as inhibitors. The slow step in sulfuric acid medium was considered to be... 

Calculating The concentration of thallium(I) ions in solution may be determined by oxidizing to thallium(III) ions with an aqueous solution of potassium permanganate (KMn04) under acidic conditions. Suppose that a 100.00 mL sample of a solution of unknown T1+ concentration is titrated to the endpoint with 28.23 mL of a 0.0560M solution of potassium permanganate. What is the concentration of T1+ ions in the sample You must first balance the redox equation for the reaction to determine its stoichiometry. 

A homogeneous catalyst exists in the same phase as the reactants. Ceric ion, Ce4+, at one time was an important laboratory oxidizing agent that was used in many redox titrations (Section 11-8). For example, Ce + oxidizes thallium(I) ions in solution this reaction is catalyzed by the addition of a very small amount of a soluble salt containing manganese(II) ions, Mn +. The Mn + acts as a homogeneous catalyst. 

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