Cation-radicals confined

Cations, radicals and anions can be formally considered as being derived from a parent compound by loss of H , H- or respectively. Cations are also commonly considered as being formed by addition of to a site bearing an unshared electron pair. The result of such processes can be indicated by adding suffixes, modifying them, or modifying a prefix. This discussion is confined almost entirely to examples in which the affected site is part of a heterocyclic ring. 

Although phase-transfer catalysis has been most often used for nucleophilic substitutions, it is not confined to these reactions. Any reaction that needs an insoluble anion dissolved in an organic solvent can be accelerated by an appropriate phase transfer catalyst. We shall see some examples in later chapters. In fact, in principle, the method is not even limited to anions, and a small amount of work has been done in transferring cations, radicals, and molecules. The reverse type of phase-transfer catalysis has also been reported transport into the aqueous phase of a reactant that is soluble in organic solvents. ... 

An enormous amount of work has been done in this wide field and a number of excellent reviews on different aspects of sulfur electrochemistry has been published [1-7], so here we confine our attention to some principal reactions and interesting apphcations of both anodic and cathodic activation of sulfur-containing molecules. Compared to other chalco-genides, sulfur has frontier orbitals that have volume, symmetry, and energy more suitable for efficient interaction with adjacent carbon atoms. The ionization of molecular sulfur requires about 10 eV. Conjugation of the pz orbitals of sulfur with a 7T-system lowers the ionization potential by ca. 2 eV. For this reason, compounds of divalent sulfur undergo oxidation rather easily often giving rise to cation radicals or dications. The stability of this species is in line with the... 

Organic molecules spontaneously form corresponding cation-radicals on inclusion within activated zeolites (Yoon and Kochi 1988, Yoon 1993, Pitchumani et al. 1997). Zeolites are crystalline alu-mosilicate minerals that are widely used as sorbents, ion exchangers, catalysts, and catalyst supports. As zeolites act as electron acceptors due to the presence of Lewis- or Broensted-acid sites, confined organic compounds occur to be electron donors. Frequently, the interaction of electron donor with electron acceptor centers spontaneously generates cation-radicals and traps the ejected electrons. 

Cation-radicals, stabilized in zeolites, are excellent one-electron oxidizers for alkenes. In this bimolecular reaction, only those oxidizable alkenes can give rise to cation-radicals, which are able to penetrate into the zeolite channels. From two dienes, 2,4-hexadiene and cyclooctadiene, only the linear one (with the cylindrical width of 0.44 nm) can reach the biphenyl cation-radical or encounter it in the channel (if the cation-radical migrates from its site toward the donor). The eight-membered ring is too large to penetrate into the Na-ZSM-5 channels. The cyclooctadiene can be confined if the cylindrical width is 0.61 nm, however the width of the channels in Na-ZSM-5 is only 0.55 nm. No cyclooctadiene reaction with the confined biphenyl cation-radical was detected despite the fact that, in solution, one-electron exchange between cyclooctadiene and (biphenyl) proceeds readily (Morkin et al. 2003). 

The restriction for a nucleophile to penetrate and react with the confined cation-radical sometimes leads to unexpected results. Comparing the reactions of thianthrene cation-radicals, Ran-gappa and Shine (2006) refer to the zeolite situation. When thianthrene is absorbed by zeolites, either by thermal evaporation or from solution, thianthrene cation-radical is formed. The adsorbed cation-radical is stable in zeolite for a very long time. If isooctane (2,2,4-trimethylpentane) was used as a solvent, tert-butylthianthrene was formed in high yield. The authors noted it is apparent that the solvent underwent rupture, but the mechanism of the reaction remains unsolved. ... 

Confinement of ion-radicals considerably changes their reactivity. What is more important for practical applications is that the confinement increases the ion-radical stability. For instance, the cation-radicals of polyanilines (emeraldines) sharply enhance their thermodynamic and kinetic stabilities when they are formed encapsulated in cucurbituril (Eelkema et al. 2007). Emeraldines have electric condnctivity as high as 1 X 10 cm (Lee et al. 2006). Encapsulation of emer-... 

Since overlap of the spectra of the TNB anion radical and the anthracene cation radical is virtually confined to the central feature of the anion spectrum, observation of the intensity of one of the outer features permits separate assessment of the anion-radical concentration (Figure 2c). As in a previous investigation (2) a quantitative study of the enhancement of the ion-radical spectrum in the presence of coadsorbate was therefore possible by using a calibration curve in which the intensity of the outer line of the TNB spectrum was plotted against the doubly integrated area of the whole of the TNB spectrum in a separate series of experiments. Figure 3 shows the effect of added anthracene and perylene on the surface concentration of TNB anion radicals. A tenfold increase in the TNB radical concentration was observed in the presence of either hydrocarbon. Addition of naphthalene, on the other hand, produced no enhancement of the TNB anion-radical concentration. 

Cation radicals are formed by the removal of one electron from a neutral molecule. The result is the formation of a species which is at the same time a cation (the positive charge caused by the loss of an electron) and a radical (the remaining unpaired electron). One-electron oxidations may be achieved with a variety of chemical oxidants, e.g. concentrated sulfuric acid, by physical means, e.g. photoionization, pulse radiolysis, and electron impact (mass spectrometry), and by anodic oxidation. In this section we shall confine ourselves mainly to chemical oxidations, to oxidations on acidic surfaces, and to photoionizations. Anodic oxidation is dealt with more fully in section 2 and in Volume 12 of this series (Eberson and Nyberg, 1976). Formation by electron impact ionization will not be discussed here. 

B The monomer is absorbed onto the electrode surface and chain lengthening occurs as a heterogeneous reaction between the surface-confined oligomer and monomer cation radical. 

When generated in zeolites, alkene or arene radical cations react with the parent molecules to form ti-dimer radical cations. For example, 2,3-dimethyl-l-butene and benzene formed 91 + and 92 +, respectively. The confinement and limited diffusion of the radical cation in the zeolite favor an interaction between a radical cation and a neutral parent in the same channel. 

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