Redox substitution

Redox substitution reactions can be photoinitiated. Taube first proposed that the photo-catalyzed substitution of PtCll- occurs by an electron-transfer process (equation 560) to give a kinetically labile platinum(III) intermediate.2040 Further work on this system has shown that the exchange occurs with quantum yields up to 1000,2041-2043 and the intermediate has beer assigned a lifetime in the fis range.2044 Recently the binuclear platinum(III) complexes Pt2(P2OsH2)4Xr (X = Cl, Br, I) have been found to show similar behavior and both photoreduction and complementary redox reactions are again proposed to explain the substitution behavior.1500... 

Because of the slow substitution chemistry of iridium, sequential reaction steps are well separated in terms of reaction time, so that the initial product of the redox-substitution reaction undergoes further simple substitution only with prolonged heating. 

G. Zotti, G. Schiavon, S. Zecchin, A. Berlin, G. Pagani and A. Canavesi, Mixed-valence conduction in redox-substituted polythiophenes. Enhancement of redox electron-exchange rate by polyconjugated chains, Synth. Met, 76, 255-258 (1996). 

Ligand substitution at platinum(iv) generally involves a redox substitution mechanism, as in the recent example of halide substitution reactions of rranj-[Pt(en)2Cl2] + and of rra j-[Pt(en)(LL)X2] +, where LL = NNN N -tetramethylethylenediamine. Conversely, reduction of PtCl4 by phosphine in the presence of iodide ion involves preliminary substitution... 

In recent years interest has developed in the mechanisms of electrochemically-induced catalytic substitution reactions of metal carbonyls. Darchen, Mahe, and Patin i have shown a general mechanism for redox substitution, depicted as a so-called "square-scheme". Scheme 7.6 ... 

Types of Chelometric Titrations. Chelometric titrations may be classified according to their manner of performance direct titrations, back titrations, substitution titrations, redox titrations, or indirect methods. 

The standard-state electrochemical potential, E°, provides an alternative way of expressing the equilibrium constant for a redox reaction. Since a reaction at equilibrium has a AG of zero, the electrochemical potential, E, also must be zero. Substituting into equation 6.24 and rearranging shows that... 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

Emulsion Polymerization. In this method, polymerization is initiated by a water-soluble catalyst, eg, a persulfate or a redox system, within the micelles formed by an emulsifying agent (11). The choice of the emulsifier is important because acrylates are readily hydrolyzed under basic conditions (11). As a consequence, the commonly used salts of fatty acids (soaps) are preferably substituted by salts of long-chain sulfonic acids, since they operate well under neutral and acid conditions (12). After polymerization is complete the excess monomer is steam-stripped, and the polymer is coagulated with a salt solution the cmmbs are washed, dried, and finally baled. 

A useful approach to the substitution of ring C—H positions lies in the activation of the heteroaromatic system by an A-oxide group, initiating a formal intramolecular redox reaction. 1-Methyllumazine 5-oxide reacts with acetic anhydride in a Katada rearrangement... 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

Imidazole with [Re(CO)3(phen)Cl] or [Re(CO)3(phen)(CF3S03)] in the presence of sulfuric acid gives [Re(CO)3(phen)(im)]2SO (95ICA(240)169). Imidazole with [Mri2(CO) g] gives [Mn2(CO)g(imH)] (84P707). This path involves the nucleophilic substitution of the carbonyl ligand. However, it is complicated by some redox... 

From Table 3, it can be seen that the reactivity of acyl acetanilide, such as BAA or AAA, is higher than that of the other reductant reported from our laboratory, i.e., acetanilide (AA), N-acetyl-p-methylaniline (p-APT), acetylacetone (AcAc), and ethyl acetoacetate (EAcAc). Moreover, the promoting activities of derivatives of acetoacetanilide were affected by the ortho substituent in benzene ring, and the relative rate of polymerization Rr) decreased with the increase of the bulky ortho substituent to the redox reaction between Ce(IV) ion and substituted acetoacetanilide. 

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