Organic radical ions reactive intermediates

Microwave or radio frequencies above 1 MHz that are appHed to a gas under low pressure produce high energy electrons, which can interact with organic substrates in the vapor and soHd state to produce a wide variety of reactive intermediate species cations, anions, excited states, radicals, and ion radicals. These intermediates can combine or react with other substrates to form cross-linked polymer surfaces and cross-linked coatings or films (22,23,29). 

Reactive radical ions, cations and anions are frequent intermediates in organic electrode reactions and they can serve as polymerization initiators, e.g. for vinylic polymerization. The idea of electrochemically induced polymerization of monomers has been occasionally pursued and the principle has in fact been demonstrated for a number of polymers But it appears that apart from special cases with anionic initiation the heterogeneous initiation is unfavorable and thus not competitive for the production of bulk polymers A further adverse effect is the coating of electrodes... 

Such radicals or ion pairs are formed transiently as reactive intermediates in a very wide variety of organic reactions, as will be shown below. Reactions involving radicals tend to occur in the gas phase and in solution in non-polar solvents, and to be catalysed by light and by the addition of other radicals (p. 300). Reactions involving ionic intermediates take place more readily in solution in polar... 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

Electron Transfer (ET) is a basic chemical process and is the fundamental step in oxidation-reduction reactions. Therefore it can be found in organic chemistry, inorganic chemistry, material science and biochemistry. The study of the structure and reactivity of radical ions, the primary species formed upon electron transfer of parent closed-shell systems, has been a topic of interest for several years. Initially, the research in this field focused on the experimental exploration of molecular structures in terms of their hyperfine structure. However, the interest has shifted during the past decade to the mechanistic studies of these reactive intermediates. 

Many anodic and cathodic conversions are not mediated by soluble redox couples or by redox coatings but are essentially initiated by a one-electron charge transfer between the electrode and the organic molecule-generating radical ions or radicals as first, reactive intermediates ... 

Schmittel M, Ghorai MK (2001) Reactivity patterns of radical ions - a unifying picture of radical-anion and radical-cation transformations. In Balzani V (ed) Electron transfer in chemistry, vol 2. Organic molecules. Wiley-VCH, Weinheim, pp 5-54 Schoneich C, Bonifacic M, Dillinger U, Asmus K-D (1990) Hydrogen abstraction by thiyl radicals from activated C-H-bond of alcohols, ethers and polyunsaturated fatty acids. In Chatgilialoglu C, Asmus K-D (eds) Sulfur-centered reactive intermediates in chemistry and biology. Plenum, New York, pp 367-376... 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

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