Radicals, anions conjugated

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

The persistent radical anion II was obtained by chemical or electrochemical reduction of the parent neutral compound. The EPR spectrum of II is composed of a triplet of triplet (ap(2P)=3.50 mT and ap(2P)=0.89 mT) characteristic of a planar conjugated structure (Fig. 9) [87]. Amazingly, the dianion III was found to be paramagnetic exhibiting an EPR spectrum composed of a distorted dou-... 

The ladder polysilanes are highly a-conjugated systems, and they are easily oxidized and reduced to give unique oxidation products and persistent radical anions. 

The radical anions of polysilanes have been studied as unique a-conjugated radical ion species.2,4,56,57 Although many radical anions of polysilanes have been reported, most of them are unstable species that can only be observed at low temperatures. Quite recently, we found that the radical anions of longer ladder polysilanes are persistent at room temperature.58... 

The conductance of the perylenebisimide (PBI) 15 was measured by the STM-BJ technique as 1 nS [73]. Note that the thiophenol handles are not conjugated to the central core, contributing to the small value. Electron transport was temperature-independent, indicating a tunneling mechanism. However, when a gate electrode reduced the core to its radical anion, the conductance became thermally activated, indicating that electron transport then follows a hopping mechanism into and out of the core. 

The CVA method shows that electrochemical reduction of 3,3 -bis(2-R-5,5-dimethyl -4-oxopyrrolidinylidene)-1,1 -dioxides (237) (Fig. 2.20), which per se are dinitrones conjugated with a C=C double bond, is an EE process that produces stable radical anions and dianions. Oxidation is an EEC- or EE-process that gives stable RC and dications (431, 432). 

In this analysis, the activation barrier for both C1-C6 and C1-C5 cyclizations of enediyne radical-anions can be described as the avoided crossing between the out-of-plane and in-plane MOs (configurations). One-electron reduction populates the out-of-plane LUMO of the enediyne moiety. At the TS (the crossing), the electron is transferred between the orthogonal re-systems to the new (in-plane) LUMO. This effect leads to the accelerated cyclization of radical-anions of benzannelated enediynes, a large sensitivity of this reaction to re-conjugative effects of remote substituents and the fact that this selectivity is inverse compared to that of the Bergman cyclization. Similar electronic effects should apply to the other reductive cyclization reactions that were mentioned in the introduction. 

A new class of conjugated hydrocarbons is that of the fullerenes [11], which represent an allotropic modification of graphite. Their electrochemistry has been studied in great detail during the last decade [126]. The basic entity within this series is the Ceo molecule (23). Because of its high electron affinity, it can be reduced up to its hexaanion (Fig. 4) [14,127]. Solid-state measurements indicate that the radical anion of Ceo reversibly dimerizes. NMR measurements confirm a u-bond formation between two radical anion moieties [128,129]. 

Radical anions resulting from cathodic reductions of molecules react with electrophilic centers. As an example (Scheme 8), the reduction of compounds in which a double bond is not conjugated with a carbonyl group, involves an intramolecular coupling reaction of radical anion with alkene [12]. 

An alternative route to phenolate-like EGBs is through the cathodic reduction of quinonemethides, (36), [82, 83]. The advantage of these PBs is that they are reduced at modest potentials, which allow EGB formation to take place in situ, and they are ultimately converted into phenols that are easily reoxidized to (36) either by air or by anodic oxidation (60-70% yield) [82]. The radical anion (36a) is expected to have basicity similar to that of (35) , whereas the pK of the conjugate acid of the dianion formed by further reduction can be assumed close to that of triphenylmethane, 30.6. 

Reduction of the aromatic nucleus in AjjV-dimethylbenza-mide occurs by an initial single electron transfer to give a radical anion. Protonation of the radical anion generates a radical and a second electron transfer gives the amide enolate 1. Protonation of the cross-conjugated trienolate moiety in 1 occurs carbonyl group to give the cyclohexa-1,4-diene 2. ... 

In a moderately alkaline medium, the ter Meer reaction proceeds through a considerable induction period the kinetic curves are S-shaped. Peroxide compounds and UV irradiation accelerate the process (Bazanov et al. 1978). Radical traps inhibit the reaction (as discussed earlier). This indicates the radical nature of the process. The rate of formation of active radical centers obeys the second-order equation in the total concentration of chloronitroethane introduced into the reaction. In nonionized substrate and anion conjugated with it, the reaction is a first-order one. The rate of the whole reaction is independent of the nitrite concentration. 

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

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