Reactions of cation radicals with

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

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

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... 

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. 

In regard to the reactions of cation radicals with stable radicals, such as l,l-diphenyl-2-picrylhydrazyl, and with well-known spin traps, virtually nothing can be found in the literature. The pyridine cation radical has been reported to add to the spin trap phenyl-N-tert-butyl nitrone (37). 

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 question of disproportionation is quite controversial in reactions of cation radicals with water. Murata and Shine (1969) found that the reaction of thianthrene cation radical (Th +) with water gave quantitatively equal amounts of thianthrene (Th) and thianthrene... 

Reaction of cation radicals with chloride and bromide ions is not too well defined in the literature. Phenothiazine cation radical perchlorate in acetonitrile solution containing halide ion gave 70-80% yields of 3-chloro- and 3-bromophenothiazine according to (130). Small amounts (4%) of 3,7-dihalogenophenothiazine were also... 

Little is to be found in the literature on the reaction of cation radicals with alkylamines. Shang and Blount (1974) found that DPA + reacts with triethylamine in an ECE sequence to give the... 

As far as we are aware, reactions of cation radicals with carbonyl compounds were until recently very rare. Cyclization of the 6-/3-ketopropionic ester during oxidation of the metalloporphyrin [52] by iodine is thought by Cox et al., (1969) to occur in the... 

Some vinyl compounds can function as donor molecules because they possess a low ionization potential. The acceptors can be neutral molecules, like quinones, anhydrides, nitrile compounds, etc. They can also be ionic intermediates, such as metal ions, ionized acids, and carbon cations. An interaction of an acceptor with a donor is followed by a subsequent collapse of the charge transfer complex. This can result in formation of cation radicals capable of initiating cationic polymerizations. The exact mechanism of the reaction of cation radicals with olefins is still not completely determined. 

It was postulated that organosilver radicals were formed by reaction of CH2OH radicals with Ag+ cations. In order to solve a problem of organosilver radicals structure we carried out the experiments with 109Agi-NaA zeolite exposed to 13CH3OH. 

The chemical reaction mechanism of electropolymerization can be described as follows. The first step in course of the oxidative electropolymerization is the formation of cation radicals. The further fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable case the next step is a dimerization reaction, and then stepwise chain growth proceeds via association of radical ions (RR-route) or that of cation radical with a neutral monomer (RS-route). There might even be parallel dimerization reactions leading to different products or to the polymer of a disordered structure. The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction... 

Table 7.2 Relative rates of reaction of alkyl radicals with the pyridinium cations 7.33...

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... 

The spectral nature for the transients formed in the OH reaction with -hydroxybenzaldehyde (Fig. 8) was found to be different from those recorded with its ortho- and meta-isomcrs. In addition to a single peak around 370-410 nm observed with the latter, a more intense peak at 325 nm by four folds was seen. Furthermore, this peak decayed faster with a first-order rate constant k = 5.5 x 10 s"h This decay was found to be acid-catalyzed. In the reaction of OH radical with hydroxybenzaldehydes, the time resolved spectral changes are interpreted in terms of the formation of phenoxyl radical via intermediate radical cation in the case of ortho- and /rm-isomers whereas phenoxyl radical formation by dehydration seems to be the predominant reaction pathway for the w/Jt/r-isomer. 

Given that single-electron oxidation is mechanistically important in these reactions, the thermodynamic criteria discussed earlier can be used, together with the known reactions of cation radicals, to predict new organic oxidation products. For example, there is a wide variety of oxidations that proceed by photoelectrochemical methods. Alcohols, for example, can be oxidized to the corresponding carbonyl compounds in high yield [106-118] (Eq. 13). 

The transient absorption spectrum of the OH adduct of dimethylaniline has a broad band centered at ca 380 nm. Decay of this species results in a growth of absorption in the 450 nm region where the radical cation has a strong absorption. Assignment of the radical cation spectrum was supported by eomparison with the spectra obtained in the eleetron-transfer reaction of sulfate radicals with DMA, and with DMA+ spectra generated by flash photolysis and eleetroehemical methods [101, 102]. The spectral characteristics of the observed transient species are summarized in Table 2. 

The mechanism of hydroxyl-radical-mediated oxidation of aromatie amines has recently been reinvestigated by Tripathi and Sun, who used time-resolved Raman spectroscopy as a diagnostic tool. Their studies suggest that the initial transient formed in the reaction of hydroxyl radicals with the neutral / -phenylene diamine is the amine radical cation and not the OH-adduet of the amine as originally believed. 

Fig. 5-8 Interactions of frontier orbitals for the reaction of alkyl radicals with substituted pyridinium cations...

Ionizing radiation (y- and X-rays, high energy electrons and other particles) is absorbed by molecules rather indiscriminately, so that most of the energy is absorbed by the solvent and not by the solutes that are present at low concentrations. Thus radiation chemistry involves in most cases the reactions of solvent radicals with the solutes. Deposition of ionizing radiation leads, as the name implies, to ionization of the solvent, i.e. formation of electrons and radical cations. These undergo subsequent processes to form a complex mixture of species. In many solvents, however, the primary events are followed by solvent-specific reactions, which result in the formation of one or two main radicals that can undergo simple reactions with the solute. Thus, despite the complexity of the early events, radiation chemistry may provide a means to study reactions of simple radicals or reduction... 

We believe, then, that the reactions with 5-hexenylMgCl, CPMMgCl, and BuMgCl are the first coupling reactions of alkyl radicals with a cation radical to have been reported (43). 

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... 

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