Reactions of cation radicals with nucleophiles

Mechanisms of the reactions of cation radicals with nucleophiles. 68... 

THIS CHAPTER IS CONCERNED WITH A REACTION of aromatic and hetero-cyclic cation radicals about which only little is so far known their ability to react with neutral radicals. The reaction is expressed simply for the coupling of an aromatic cation radical (ArH +) with a radical (R-) in equation 1. This simple equation, presently only poorly documented, is nevertheless part of current thinking in two reactions of wide scope electrophilic aromatic substitution and reactions of cation radicals with nucleophiles. The product of equation 1 is a a complex, (ArHR)+, which is structurally the same as that... 

Disproportionation is an important concept in cation radical reactions. Many reactions of cation radicals with nucleophiles have the stoichiometry of eqn (99) (or a variant of it). Without the aid of... 

The main emphasis of this chapter will be on our attempts to find evidence for radical coupling in reactions of the thianthrene cation radical (Th +) with nucleophiles, that is, for the occurrence of steps such as equations 7 and 8. However, because discussions of reactions of cation radicals with radicals are relatively sparse, we will refer first to other works that principally deal with aromatic nitration. 

Many electrochemical studies involve reactions of the electrogenerated cation radicals with added nucleophile or in coupling reactions a number of these are discussed in the sections on reactions of cation radicals. Added nucleophile causes the disappearance of the CV cathodic (reversal) peak current and usually an increase in the forward (anodic) peak current because the RNu species formed in the reaction of cation radical and nucleophile is more easily oxidized than the parent [see reactions (73)—(75)]. Moreover, the oxidation peak is shifted towards negative potentials [see... 

To illustrate such a feature, let us compare the reactions of A, A-dimethylaniline cation-radicals with nucleophiles in their dependence on the manner of the one-electron oxidation of the parent molecule. Works by Kirchgessner et al. (2006), Weinberg and Reddy (1968), and Andreades and Zahnow (1969)... 

Eberson and co-workers have recently discussed the probability that the interaction of ion-radicals with nucleophiles and electrophiles is subject to orbital symmetry constraints.31,32 This follows the observation that with perylene the cation-radical (18) the preferred course of reaction with halide ions is electron transfer rather than nucleophilic addition, whereas with the phenothiazine cation-radical (19) nucleophilic attack by Cl" and Br occurs. 

The question of dimerization mechanism has also surfaced in the study of reactions of cation radicals. The anodic oxidation of 4,4 -dimethoxystilbene is accompanied by the formation of dimeric products, the nature of which depends upon the nucleophile involved in the reaction as indicated in Scheme 5 (Parker and Eberson, 1969 Eberson and Parker, 1970 Steckhan, 1978 Burgbacher and Schafer, 1979). The reaction was studied by spectroelectro-chemistry (Steckhan, 1978) in acetonitrile containing methanol. Competing mechanisms were proposed involving the reactions of the radical cation (D7) with methanol (134) and by dimerization (135) giving rise to rate law (136). Theoretical working curves were used to find the best fit of the data... 

Shine and co-workers have carried out product studies of the reactions of fV-substituted phenothiazine cation-radicals with nucleophilic re-agents. As with the cation-radicals of thianthrene (178) and phenoxathiin (198), addition of certain nucleophiles at S may occur to give, ultimately, such adducts as 270-272 (see Sections III,C,4,b V,B,1). Attack by other nucleophiles may result in reduction by electron transfer or substitution at position 3. Shine has reviewed much of this work. The mechanisms of such reactions have been controversial (see Section... 

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... 

Scheme 1 Intramolecular reaction of a radical cation with a nucleophile.

Alkylimidazolinm tetraflnoroborates are, for example, ionic liquids at room-temperature that can provide an anion to stabilize an intermediate cation-radical with no possibility of nucleophilic attack on it. Ionic liquids have a huge memory effect, and their total friction is greater than that of conventional polar solvents. Thus, the total friction of l-ethyl-3-methylimidazolium hexafluoro-phosphate is about 50 times greater than that of AN (Shim et al. 2007). The solvent effects of ionic liquids on ion-radical ring closures deserve a special investigation. The ring closure reactions can be, in principal, controlled by solvent effects. 

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). 

Rate constants for quenching of 1-7 by methanol and acetic acid in hexane solution from fluorescence quenching and quantum yield data are 10 M l-s-l-. Limiting quantum yields for adduct formation are 0.1. The observation of reactions of protic solvent with 1-7 but not 1-t may reflect the longer lifetime and/or enhanced reactivity of the cyclic molecule. While photo-induced nucleophilic addition is a common reaction of aryl olefins, it is normally observed to occur only under conditions of electron-transfer sensitization (139). Under these conditions, it is the aryl olefin cation radical which undergoes nucleophilic attack. The reaction of 1-7 with protic solvents appears to be the only reported example of nucleophilic trapping of an aryl olefin it,it singlet state (140). 

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5k2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

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