Of ion-radical

Peak Potentials and Estimated Lifetimes of Ion Radicals in Electbochemical Oxidation-Reduction of Isoindolbs ... 

Bond cleavage in ion radicals often involves the expulsion of an ionic moiety from the parent molecules. Rearrangements of ion radicals involving an intramolecular bond cleavage, as for example the opening of a small ring,78-81 belong to the same class of reactions. Their dynamics should therefore lend themselves to the same type of analysis. 

The cleavage of ion radicals may be a homolytic process rather than an... 

Follow-up reactions of ion radicals as critical (reactive) intermediates 228... 

The examples in Schemes 14-17, which clearly demonstrate the critical role of ion radicals in D/A reactivity, suggest a thorough review of the diverse reaction pathways that are available to cation radicals and anion radicals as follows. 

The explanation for the dichotomy between aromatic nitration versus de-alkylative oxidation in equation (84) lies in the dynamics of ion-radical triad (which is predictably modulated by solvent polarity and added inert salt). For example, the nonpolar dichloromethane favors aromatic nitration via a radical-pair collapse of ArH+, NO, 239 i.e.,... 

A route to the resolution of this conundrum is provided by the photoactivation of the donor-acceptor complex to the ion-radical pair, as described in equation (98). In this case, the close interrelationship between the photochemi-cally-produced or vertical (nonadiabatic) ion pair formed in equation (98) and the thermally accessed or contact (adiabatic) ion pair in equation (99) is illustrated in Scheme 28, where the asterisk identifies the contact ion-pair in the gas phase and s represents the solvation of ion-radical pair. According to... 

A somewhat similar type of ion radical is made by oxidizing amines ... 

When the reaction is well-established as a radical one it is still possible to find explanations of polar substituent effects, usually in terms of ion-radical intermediates, dipole-dipole repulsion, solvation, or charge-transfer structures like those postulated for -complexes.488 Reac-... 

CLEAVAGE OF ION RADICALS. REACTION OF RADICALS WITH NUCLEOPHILES... 

FIGURE 3.22. Endergonic homolytic dissociation of ion radicals, a Potential energy curves, b Bronsted plots combining the effect of diffusion and activation. kBT/h= 1013s-1, 4/= 1010 s-1, kdif = 1010M- s-1, Drx-w + 2o = 0.3eV, r = 298K. Dotted line log[Pg.225]

In the stepwise case, the intermediate ion radical cleaves in a second step. Adaptation of the Morse curve model to the dynamics of ion radical cleavages, viewed as intramolecular dissociative electron transfers. Besides the prediction of the cleavage rate constants, this adaptation opens the possibility of predicting the rate constants for the reverse reaction (i.e., the reaction of radicals with nucleophiles). The latter is the key step of SrnI chemistry, in which electrons (e.g., electrons from an electrode) may be used as catalysts of a chemical reaction. A final section of the chapter deals... 

Recently the data concerning to interaction of propanthiole with chlorine dioxide in 8 solvents have been published [1], In this work it was shown, that the dependence of process rate from solvents properties is satisfactory described for seven solvents, after the exclusion of data for ethyl acetate, by the Koppel-Palm four parameters equation (coefficient of multiple correlation R 0,96) at determining role of medium polarity (coefficient of pair correlation between lg(k) and (s - l)/(2e + 1) - r 0.90). Chemical mechanism of the reaction including the formation of ion-radical RS H and radical RS has been proposed by authors [ ] ... 

In order to establish the generality of ion-radical pair formation by the charge-transfer activation of EDA complexes, let us focus on a few diverse... 

Ions or radicals formed from a substrate further react with other ions or radicals. There are many reactions that include one-electron transfer before the formation of ions or radicals. Sometimes, electron transfer and bond cleavage can take place in a concerted manner. The initial results of one-electron transfer involve the formation of ion-radicals. 

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

Equal or nonuniform distribution of spin density can occur among individual atoms of the molecular carcass. This kind of distribution defines the activity of one or another position in an ion-radical. From the point of view of organic synthesis, properties of ion-radicals such as stability, resistance to active medium components, capacity to disintegrate in the required direction, and... 

This section shows that substituent effects in organic ion-radicals are quite different from those of their parent neutral molecules. Amino and nitro compounds are good examples to show that conventional ideas may not be applicable to the chemistry of ion-radicals. 

This means that 4-nitrostilbene is a more effective electron acceptor than nitrobenzene. This theoretical conclusion is verified by experiments. The charge-transfer complexes formed by nitrobenzene or 4-nitrostilbene with Af,Af-dimethylaniline have stability constants of 0.085 L mol or 0.296 L mol respectively. Moreover, the formation of the charge-transfer complex between cis-4-nitrostilbene and A/,Af-dimethylaniline indeed results in cis-to-trans conversion (Dyusengaliev et al. 1995). This conversion proceeds slowly in the charge-transfer complex, but runs rapidly after one-electron transfer leading to the nitrostilbene anion-radical (Todres 1992). The cis trans conversion of ion-radicals will be considered in detail later, (see sections 3.2.5.1, 6.4, and 8.2.1). 

The sulfate anion-radical is not a very strong hydrogen acceptor. It acquires the atomic hydrogen from organic substrates at significantly smaller rates as compared with the rates of one-electron oxidations. For instance, dehydration rate constants are 10, 10 , and 10 L mol s for methanol, tert-bntanol, and acetic acid, respectively (Goldstein and McNelis 1984, Zapol skikh et al. 2001). Snch a peculiarity is very important for the selectivity of ion-radical syntheses with the participation of S04. ... 

Therefore, this chapter provides a summarized data on the preparation of organic ion-radicals as independent particles that can be free or bound with counterions in ion pairs. The chapter considers liquid-phase equilibria in electron transfer reactions and compares electrode and liquid-phase processes for the same organic compounds. Isotope-containing molecules have specific features as ion-radical precursors, therefore, the generation of the corresponding ion-radicals is considered in Section 2.6 of this chapter. This chapter also pays some attention to the peculiarities of ion-radical formation in living organisms. 

This class of ion-radicals is characterized by the localization of an unpaired electron at the atom bearing a free (valence) electron pair. Although their applicability in organic synthesis remains an open question, the preparative methods and electron structure of carbene ion-radicals attract some attention of the researchers. Probably, it is an initial step to a new chapter in organic ion-radical chemistry. 

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