Redox cycles, reversible

Keywords C - C bond formation Co-reductant One-electron reduction Radical Reversible redox cycle... 

The redox interaction with a co-reductant permits the formation of a reversible redox cycle for one-electron reduction as shown in Scheme 2. Furthermore, the function of transition metals is potentially and sterically controlled by ligands. A more efficient interaction between the orbitals of metals and substrates leads to facile electron transfer. Another interaction with an additive as a Lewis acid towards a substrate also contributes to such electron transfer. 

A vanadium catalyst is essential although the combination of Zn and MejSiCl is capable of reductive dimerization of aldehydes [20]. A reversible redox cycle for the in situ generated low-valent vanadium species mediating the electron transfer is achieved in the presence of Zn as the stoichiometric co-reductant (Scheme 4). 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

Figure 27.11 illustrates a third dual-electrode arrangement that permits enhancement of the response by reversible redox cycling. Many more electrons are therefore transferred than would be the case with a single electrode, and the current is amplified dramatically. This concept does not work well with conventional LC columns because the volume flow rate is too large to permit a significant number of redox cycles. Nevertheless, the concept is certainly interesting, and, as reversed-phase capillary columns are developed, it may well have some practical value. A detailed treatment of multiple-electrode LCEC has been published [24]. 

There are two reversible redox cycles, I II and I III. Upon reoxidation, water is evolved in I - II, but not in III - I. In the case of HjPMoi204o, the redox cycle is nearly reversible when the average extent of reduction is less than 3e /anion at 573 K. The second redox cycle (I - III) tends to dominate at high temperatures and for extensive reductions. 

The blue copper sites seem generally to be involved in electron transfer, with the copper cycling between the +1 and +2 states. The mechanistic details of the reversible redox cycle can, however, display unexpected complexity.34... 

Not only are Li+ intercalation and H+ intercalation in these solids closely similar processes, but topotactic Li+/H+ exchange has been observed on complex oxides [614] this is further evidence that soft chemistry is related to properties of the radial Schrodinger equation, the occupied angular wavefunctions being the same throughout the reversible redox cycle, so that bonds are not broken and bond directions, in particular, are hardly altered. In this respect, soft chemistry resembles physisorption, as opposed to chemisorption (see section 11.3). 

The Cu(II) complex with polyaniline (emeraldine base) exhibits a higher catalytic efficiency for the dehydrogenative oxidation of cinnamyl alcohol into cin-namaldehyde. Iron(III) chloride is similarly used instead of copper(II) chloride. The catalytic system is applicable to the decarboxylative dehydrogenation of man-delic acid to give benzaldehyde. The cooperative catalysis of polyaniline and cop-per(II) chloride operates to form a reversible redox cycle under oxygen atmosphere as shown in Scheme 3.4. The copper salt contributes to not only oxidation process but also metallic doping. The reduced phenylenediamine anionic species appear to be stabilized by the metallic dopants. 

The redox interaction between transition metals and redox-active ligands is likely to permit a smooth reversible redox cycle in the transition metal-catalyzed oxidation reactions. Actually, the Wacker oxidation reaction of a terminal olefin proceeds catalytically only in the presence of a catalytic amount of polyaniline or polypyrrole derivative as a cocatalyst in acetonitrile-water under oxygen atmosphere to give 2-alkanone (Scheme Copper-free catalytic systems are... 

The cyclic voltammograms of all the monomers show an irreversibile oxidation peak, followed by a fast chemical process, which is attributable to the polymerization the electrodes covered by 1 /u-m film, upon transfer into blank solution, show the reversible redox cycles attributable to the electroactivity of the polymers. 

The larger A cations, usually alkaline or rare earth elements, with inert d" or f electronic structure, act as structural stabilizers and do not offer much to the redox catalytic activity. The smaller B cations can be 3d, 4d, or 5d transition metal elements and are the main catalytic sites/centers in such solids due to their ability to undergo reversed redox cycles without destruction of the structure. In the vast majority of catalytic studies, 3d cations are employed due to obvious economic reasons. Nevertheless, almost 95% of the elements of the periodic table can participate in perovskites and these many possible combinations result in a plethora of diverse, and sometimes imexpected, properties of such solids, which have been nick-named chemical chameleons. 

The conjugated polymer complexes composed of polyanilines or polypyrroles are performed to afford the redox systems depending on their structnres and redox properties. As mentioned in the Sect. 3.2, the complexation with copper salts can affect the redox properties of polyanilines to form the reversible redox cycle [17]. The conjugated polymer complex can serve as an oxidation catalyst [18,20,23,24], wherein the coordination of the QD moiety might play an important role in a reversible redox processes of the complexes. Polyanilines or polypyrroles are effectively employed as a redox-active ligand in the Wacker reaction as mentioned in Sect. 3.2. 

In the case of samples II and III, where an excess of Bronsted acid OH groups was present, after reoxidation a secondary dehydroxylation was found by IR measurements, which most likely resulted in the formation of [In-0-In] + complexes [cf. Eq. (41)]. However, upon reduction of these reoxidized and dehy-droxylated samples, IR evidenced a complete restoration of the excess Bronsted acid OH groups [271]. Thus, in this case, the reversible redox cycle is described by the following scheme ... 

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