Mechanisms internal redox reactions

Illustrated in Scheme 7.8 are the mechanisms that give rise to the products shown in Scheme 7.7. These mechanisms involve either electrophilic attack or an internal redox reaction. The internal redox reaction shown in Scheme 7.8 involves proton trapping from the solvent or from the hydroquinone hydroxyl group as shown. This process has been documented for the mitomycin system50 and also occurs in many quinone methide systems.25,30,31... 

These possibilities arise because of the presence of species, such as water and surfactant, with which the primary radical can Interact. Three possibilities are Illustrated in Figure 18. In each case, the radical activity is associated in the first Instance with a species in which cobalt(III) has been reduced to cobaltCII) and the acetylacetonate ligand has rearranged to give a free radical on the methylenic carbon atom. In the first possibility, the monomer reacts directly with this species, and propagation then proceeds in the normal way. The consequence of such a mechanism would be that the polymer produced would contain both cobalt (albeit perhaps more loosely bound than in an acetylacetonate) and a moiety derived from acetylacetone. In the second possibility, the species which results from the Internal redox reaction interacts with another molecule in the reaction system (such as water) in such a way that the radical-bearing entity is displaced from the metal complex. 

Polyaniline provides the prototypical example of a chemically distinct doping mechanism [33,34], Protonation by acid-base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base) to metal (the emeraldine salt). The doping mechanism is shown schematically in Fig. II-2. The chemical structure of the semiconducting emeraldine base form of polyaniline is that of an alternating copolymer, denoted as [(1A)(2A)] , with... 

An important biological requirement of iron-enterobactin systems is the release of iron from the highly stable [Fe(ent)] complex once inside the cell. The redox potential for [Fe(ent)j] (estimated as ca. — 750 mV at pH 7) is probably too negative to allow reduction to Fe" by known biological reductants at physiological pH. At lower pH the redox potential may shift towards positive values, however. In one mechanism for the release of iron from enterobactin at low pH it is proposed that an internal redox reaction occurs between Fe " and catechol to generate a blue Fc"-semiquin-... 

An internal redox reaction of 2-picoline-l-oxide (X-90, X = H) and its alkyl derivatives with nitrite ion and an acylating agent yields 2-cyanopyridines OC-91, X = H). The following mechanism is proposed ... 

From the initiative of the director of Polarographic Institute, professor A. A. Vlcek (1927-1999) (Fig. 3.1.9), a regular international electrochemical forum called Heyrovsky Discussion was fotmded in 1967, to which foreign scientists both from East and West were being invited. Every year a different topic was selected the first discussion in 1967 was about adsorption on electrodes and its effect on electrode processes, the next one in 1968 was about adsorption and processes on catalytic electrodes, and in 1969 it was about the mechanism of redox reaction proper (cf. Sect. 3.2). 

Mechanism of Action. Under photolysis, acidic, and electron bombardment conditions, the transformation of o-nitrobenzyl alcohol or its derivatives involves an internal redox reaction sequence followed by liberation of the deprotected alcohol or amine (eq 1). Analogously, the photorearrangement of esters of o-NBA, obtained through its reaction with acid chlorides or anhydrides, also induces an internal redox reaction (eq 2). 

The PR studies on XO and XDH provide qualitatively similar sequences of internal ET reactions and illustrate the usefulness of this method in delineating enzymatic redox reaction mechanisms (167-169, 172). No one to date has attempted to obtain more quantitative information regarding the ET reactivity of the individual sites. Better characterization of the individual ET steps in XO will surely be required for this purpose. 

Reactions of amminecobalt(m) complexes as oxidants are being further investigated. Under pseudo-first-order conditions, the two reactions observed in the reduction of [Co(NH3)4(OH)2] + by cysteine have been ascribed to the faster redox involving the traizj-isomer and a slower cis reaction. The order with respect to complex is unity and that for cysteine is zero. The mechanism proposed involves the rapid pre-equilibrium to form [(RS)Co(NH3)4(OH2)] + complexes which then undergo a rate-determining internal redox process. More data are required for these systems which appear to involve a pre-equilibrium rate much greater than expected for a low-spin [Pg.88]

The substrate requires a hydride acceptor proximal to a C—H bond serving as hydride donor, and the reaction is initiated by a hydride shift (or related H-atom-transfer step), which formally oxidizes the carbon donor and reduces the hydride acceptor. The new C-X bond will be formed at the hydride-donor atom, after the hydride shift takes place. The defining characteristic for these reactions is the functionalization of a C-H bond concurrent with a hydride shift. The names proposed by Sames ( HT-cyclization ) and Akiyama ( Internal Redox Cascade ) would seem more appropriate if focusing more on the unique hydride-shift mechanism that draws together a diverse group of substrates, at least for intramolecular examples. 

---