Radicals, reaction with nucleophiles

As in reduction reactions, two possible mechanisms exist for substitution reactions (1) electron-radical, involving the intermediate formation of radicals and their reaction with nucleophiles X ... 

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

The overall conclusion from the reaction of BP and 6-substituted BP radical cations with nucleophiles of various strengths is that weak nucleophiles display higher selectivity toward the position of highest charge localization. Thus another important factor in the chemical reactivity of radical cations is represented by the strength of the nucleophile. 

Owing to their cationic and radical character, organic cation radicals also participate in a variety of bimolecular reactions with nucleophiles, bases, radicals, etc., as illustrated by the following examples. 

To favor the coupling reaction, the competing side reaction of the radical cation with nucleophiles must be suppressed by the use of a medium of low nucleophilicity. The solvent of choice is dichloromethane. Especially in elec-troanalytic studies, neutral alumina is frequently added to suppress hydroxy-lation of the radical cation [162]. The reversible cyclic voltammetric behavior of radical cations is also enhanced in mixtures of methylene dichloride, triflu-oroacetic acid, and trifluoroacetic anhydride (TFAn) with TBABF4 as supporting electrolyte. With acetonitrile as solvent... 

Pandey and co-workers have generated arene radical cations by PET from electron-rich aromatic rings [119]. The photoreaction is apparently initiated by single-electron transfer from the excited state of the arene to ground state 1,4-dicyanonaphthalene (DCN) in an aerated aqueous solution of acetonitrile. Intramolecular reaction with nucleophiles leads to anellated products regio-specifically. The author explains the regiospecifidty of the cyclization step from... 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-... 

Thus, almost all the reactions of lignin substructure model dimers by the enzyme are explained on the basis of cation radical intermediates and their subsequent reactions with nucleophiles such as H2O and intramolecular hydroxyl groups, and with radicals such as di oxygen (for non-phenolic substrates), or on the basis of phenoxy radical intermediates (for phenolic substrates). 

A laser flash technique can be used to generate radical-cations and follow the kinetics of their reaction with nucleophiles [68, 69]. The experimental technique... 

As mentioned above, it is difficult to find organic compounds which are suitable as redox catalysts for oxidations. This is the case because organic cation radicals, which are mostly the active forms in indirect electrochemical oxidations, are usually easily attacked by nucleophiles, thus eliminating them from the regenerative cycle. Therefore, the cation radicals must be stabilized towards the reaction with nucleophiles. Nelson et al. demonstrated that the cation radicals of triaryl amines and related compounds are very stable if the para positions of the aryl... 

X = O, S Bonds in heavy print are of highest order N, R, E show sites of reaction with nucleophiles, radicals... 

The 2,3-double bond in benzo[6]thiophene-1,1-dioxides undergoes addition reactions with nucleophiles in a manner comparable to that of other a,/9-unsaturated sulfones no aromatic properties are detectable in this way for the thiophene ring.718-723 For example, thiophenol and p-thiocresol give the adducts (340 R = H or Me) in the presence of base.723 However, if the aryl mercaptan and the sulfone are heated together, a radical reaction occurs to give the corresponding 2-substituted compound.723 In contrast to the behavior of aryl mercap-... 

In the context of mechanistic studies, the electrochemical behavior and reactions with nucleophiles of 4-chloro-2,6-diphenylpyrylium and 4-chloro(bromo)flavylium have been studied <1999CHE653>. The proposed mechanism for nucleophilic substitution in halogen-substituted pyrylium and flavylium salts passes through formation of a charge-transfer complex that is converted into an ion-radical pair by simple electron transfer. Heterocyclic cleavage of the C-halogen bond occurs at the stage of the radical or the adduct from the reaction of the pyrylium salt and the nucleophile. In this study, an amine nucleophile was used however, the data are likely relevant for other types of nucleophiles as well (Scheme 5). 

In the S l reactions with nucleophiles of the types RS , PhS, PhSe", PhTe , Ph2As and Ph2Sb , fragmentation reactions of the radical anions of the substitution products are often observed. A review on the phenomenon of radical anion fragmentation in the course of aromatic S l reactions has appeared in 1982692. The mechanism of fragmentation was also very clearly described in a report on photostimulated reactions of haloarenes with benzeneselenate ions693. In the photostimulated reaction of />ara-iodoanisole (247) with benzeneselenate ions (248) not only the straightforward substitution product 249 was obtained, but also the symmetrical diphenyl selenide (250) and the di-(/ -anisyl) selenide (251) (equation 182). 

Oxidation of carboxylic acids can be classified into two major categories, formation of radical intermediates followed by dimerization and generation of cation intermediates followed by reaction with nucleophiles (equation 54). The reaction is controlled by a variety of factors including anode material, ano potential, current density, solvent, supporting electrolyte, structure of R and temperature. 

Another general problem, perhaps with somewhat more impact on homogeneous solution chemistry, is whether the reactions of radical ions with nucleophiles or electrophiles, depending upon the charge, go directly via the radical ions or if reactive doubly charged ions must first be formed by disproportionation. The reaction of thianthrene radical cation with water (Murata and Shine, 1969 Shine and Murata, 1969) is a typical example which was proposed to involve the disproportionation mechanism (71 and 72). The... 

Parker, 1980). The essential feature of this mechanism is that the initial interaction between the radical cation and pyridine involves the formation of a 7t complex. Under conditions where the reaction is first order, (88) was supposed to be rate-determining. When the concentrations of DPA- and the complex becomes very low the steady state concentration of DPA /pyr could become so low that (88) and (89) can be treated as equilibria and the rate is determined by (90). This mechanism is very similar to that proposed for the reactions of a number of radical cations with nucleophiles on the basis of homogeneous kinetic studies (Svanholm et al., 1975 Svanholm and Parker, 1976a,b) and reflects the complexities of the type that have been observed during the protonation of DPA. Since the ambiguity remains between the data from different laboratories, the mechanism of the pyridina-tion of DPAJ cannot be considered to be established. 

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