Coupled radical reactions that

In that earlier study an examination of the available sulfur chemical kinetics lead to the identification of the following 8 fast coupled radical reactions that could account for the measured concentration profiles. 

The studies of the biosynthesis of erythrina alkaloids got off to a false start. In a beautiful experiment, A. I. Scott and colleagues showed49,50 that the whole erythrina skeleton could be constructed in one simple phenolate coupling radical reaction. 

Finally, some termination step occurs, two of which are shown in the scheme. The most common is coupling, in which two radicals combine, leading to one larger macromolecule. Polystyrene radicals typically undergo termination by coupling. Another reaction that is common with some monomers (e.g., methyl methacrylate) is called disproportionation in which on the reaction of two radicals, a hydrogen atom transfers from one species to the other. 

An intriguing copper-catalyzed radical reaction that involves a formal C-H bond activation has been reported by Nakamura. The reaction efficiently couples an arylacetylene or enyne to a penta(aryl)[60]fullerene bromide in a formal [4 -I- 2] fashion to form a dihydronaphthalene ring fused to a fullerene sphere [35]. 

In general, for polymers, it is the free radical generation that dominates. This leads to free radical reactions that depend on the chemistry of the polymer and may be coupled with the environment. Such reactions can result in main chain scission, recombination, or disproportionation as exampled below in polyethyl... 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

It is considered that the stannyl or silyl radical and the alkyl radical are reactive intermediates in these reactions. In contrast to the selective formation of the arylchalcogenosilanes in the above radical reactions, the cross-coupling reaction of a hydrosilane with alkyl(aryl)sulfides catalyzed by palladium nanoparticles results in the selective formation of the corresponding alkylthiosilanes.42... 

A first turning point in the dichotomy between radical and ionic chemistry is located at the level of the primary radical, usually an ion radical, formed upon single electron transfer to the substrate. If, for a reduction, the reaction medium is not too acidic (or electrophilic), and for an oxidation, not too basic (or nucleophilic), radical reactions involving the primary radical, such as self-coupling, have a first opportunity to compete successfully with acid-base reactions. In this competition, the acidity (for a reduction) or basicity (for an oxidation) of the substrate should also be taken into account insofar as they may lead to father-son acid-base reactions. It should also be taken into consideration that the primary radical may undergo spontaneous acid-base reactions such as expelling a base (or a nucleophile) after a reduction, and an acid (or an electrophile) after an oxidation. 

An evaluation of numerous clays, including kaolins and bentonites of comparable particle size and distribution, revealed a wide variation in the properties of LDEE-clay composites prepared under identical conditions with similar loadings. This may be attributed to the interference with the radical reactions involved in the coupling sequence. This is confirmed, in part, by the finding that the most effective clays were those which were reported to have been treated with sodium polyphosphate to improve their dispersibility in water during papermaking processes. Solomon reported (1) that treatment of the clays which inhibited radical reactions with sodium polyphosphate reduced the inhibition. 

Second-order irreversible chemical reaction following a reversible electron transfer dimerization. It is quite common in chemical reactions that newly formed radicals couple to each other. This also often happens in the electrochemical generation of radicals according to a dimerization process that can be written as ... 

There are a number of other reactions that can be used cycloaddition of an azide group allowed binding of polystyrene [43], while a radical coupling was exploited to graft polymers prepared by a nitroxide-mediated radical polymerization [121]. Other end groups could be used it was shown above how amide bonds were utilized to attach water-soluble polymers amino or hydroxyl moieties are other conventional groups. 

Electron transfer from the alkene leads to a radical cation that can undergo coupling (Scheme la). The radical cation can also react with the nucleophilic heteroatom of a reagent to afford addition or substitution products (Scheme lb). Adducts can be likewise obtained by oxidation of the nucleophile to a radical that undergoes radical addition. Reactions between alkenes and nucleophiles can be realized too with chemical oxidants that are regenerated at the anode (mediators) (see Chapter 15). Finally, cycloadditions between alkenes can be initiated by a catalytic anodic electron transfer. These principal reaction modes are subsequently illustrated by selected conversions. 

Perhaps the best-known and most widely appreciated electrochemical transformation is the Kolbe oxidation (see also Chapter 6) [1, 2, 31]. The process involves the one electron oxidation of the salt of a carboxylic acid, and the loss of carbon dioxide to afford a radical, R, that subsequently engages in coupling reactions. Both symmetrical (R + R ) and nonsym-metrical (R + R ) radical couplings are known and are illustrated in the following discussion. The nonsymmetrical variety (often referred to as a mixed or hetero coupling) is remarkable given that it requires the cogeneration and reaction of more than one reactive intermediate. 

For aminophenols, one-electron oxidation and the proton elimination can run together in one stage. This leads to a cation-radical containing O and +NH3 fragments within one and the same molecular carcass (Rhile et al. 2006). Such concerted reactions are classified as proton-coupled electron transfer (Mayer 2004). Proton-coupled electron transfer differs from conventional one-electron redox reaction in the sense that proton motion affects electron transfer. Because the transfers of a proton and an electron proceed in a single step, we can say about the hydrogen-atom transference, (H+ -I- e)=H. It is the fundamental feature of proton-coupled electron-transfer reactions that the proton and electron are transferred simultaneously, but from different places (see Tanko 2006). 

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