Dimerization of radicals

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

This reaction resembles decarboxylation of carboxylates during electrode one-electron oxidation (Kolbe reaction). Kolbe reaction also consists of one-electron oxidation, decarboxylation, and culminates in dimerization of alkyl radicals just after their formation at the electrode surface. When the sulfate radical acts as a one-electron oxidant, the caboradical dimerization is hampered. The radicals can be used in preparative procedures. One typical example is alkylation of heterocyclic nitrogen bases (Minisci et al. 1983). This difference between Kolbe reaction and the reaction with the help of a dissolved electrode (the sulfate radical) deserves some explanation. The concentration of the one-electron oxidation products in the electrode vicinity is significantly higher than that in the bulk of the solution. Therefore, in the case of anode-impelled reactions, the dimerization of radicals produced from carboxylates proceeds easily. Noticeably, 864 secures the single electron nature of oxidation more strictly than an anode. In electrode reactions, radical intermediates can... 

Isomerization, Oxidation, and Dimerization of Radical Cations of Stiibene Derivatives... 

An updated book on organic radical ions3 comprises an extensive (presumably almost complete) set of hyperfine data and summarizes methods for their generation. Another book concentrates on chemical properties of radical ions4. Valuable information on timescales and the energetics leading to the dimerization of radical anions and cations (and neutral radicals) based on tetracyanoethylene, tetracyanoquinodimethane, substituted benzoquinones (DDQ and chloranil) as acceptors and octamethylbiphenylene is reported by Kochi and coworkers5. 

Anodic oxidation of enamine ketones or esters in CH30H-NaC104 at a graphite anode gives substituted pyrroles in 15-45% yield.101 Formation of the symmetrically substituted pyrroles 47 indicated radical dimerization of radical-cations formed as primary products from 46. This process leads to dications from which the pyrroles can be formed by cyclization and elimination of an amine [Eq. (44)]. 

Indoxyl and thioindoxyl are easily oxidized, e.g. by K3Fe(QsT respectively, via dimerization of radical intermediates (263). 6... 

The use of alkali melals for anionic polymerization of diene monomers is primarily of historical interest. The electron-transfer mechanism of the anionic polymerization of styrenes and 1,3-diencs initiated by alkali metals has been described in detail the dimerization of radical anion intermediates is the important step. 

The crystals of radical 29, which are diamagnetic by electron spin resonance (ESR), consist of cofacial dimers with four dimers per unit cell linked by two long, albeit unequal (3.053 and 3.309 A), interannular S-S bond <2005IC1837>. This mode of association has not hitherto been observed for 1,2,3-dithiazolyls. Dimers of radical 29 do not form 7i-stacked arrays instead, they adopt the closed packed herringbone arrangement which allows for a clustering of the radical heads so as to maximize S- -S and S- -N contacts. 

O. Hammerich, M. F. Nielsen, The Competition Between the Dimerization of Radical Anions and Their Reactions with Electrophiles, Acta Chem. Scand. 1998, 52, 831-857. 

The mode of asymmetric induction can be rationalized from the mechanism of the photopinacolization in the presence of aliphatic amines. The electron transfer from the amine to the excited triplet ketone furnishes charge transfer complex 5, from which a radical pair is formed by protoirtransfer. The weakly coordinated chiral amine seems to favor the dimerization of radical 6 from the si face leading to the (/ , ft)-enantiomer 3. The much lower selectivities observed with methanol as the cosolvent (3% ee at 27°C) indicate dipolar or hydrogen bonding interactions between the chiral diamine and the prochiral radical (Scheme 4). 

AH°q -10.7 Real mol ) indicate a higher stability of the sulfur containing radical (86NJC345). A similar study concerning the dimerization of radicals 53 and 54 was carried out by Kawata and Niizuma (89BCJ2279). Owing to steric hindrance of the phenyl groups in the dimers, the dimerization equilibria were found to be endothermic. 

Fluorine is usually so reactive that its use results in numerous side reactions, such as halogen-fluorine exchange, replacement of hydrogen, rearrangements or dimerization of radical intermediates. The addition of fluorine may be controlled, if fluorine is diluted with an inert gas and the temperature is lowered, or by using the Jet Fluorination technique, which was developed by Bigelow and co-workers (see also Vol. ElOa, pl59ff). 

The dimerization of radical anions derived from 9-X-substituted anthracenes (Scheme 4), where X is an electron-withdrawing substituent, is related to electrohydrodimerization and might be expected to be less complex since proton donors are not involved in the formation of the products (Hammerich and Parker, 1981b). The reactions, where X is NOj, CHO, or CN, were studied by LSV and DCV. Primarily on the basis of the near independence of the reaction rates on temperature, the simple dimerization mechanism was excluded. It was proposed that the overall reaction consists of two reversible steps (i) formation of a radical anion dimer complex in which the two anthracene moieties are not bonded at the 10 positions and (//) the rearrangement of the complex to the stable dimeric dianions. The rate of the reaction was found to be independent of the water concentration in DMF. The radical... 

The trap is to form the product by dimerizing these allylic radicals. Dimerizing of radicals is known (in the acyloin reaction, for example, p. 1032), and these radicals will sometimes dimerize ut it is a rare process. 

Typical examples of the application of DCV for kinetic and mechanistic studies include the cleavage of the carbon-halogen bond in the radical anions of aromatic halides [89], the protonation of radical anions [87,90-92], and the dimerization of radical ions [84]. 

Logarithmic analyses can be carried out on the neopolarogram in much the same way as with classical polarography (see Sec. V). Examples that illustrate the application of the convolution technique include reversible dimerization of radical cations [114], the study of dissociative electron transfer reactions [175,176], and investigations of the possible potential dependence of the transfer coefficient a [174,177]. 

Cholesterone, progesterone, and other eneones in the cholestane series have a highly sterically hindered tail position and afford only head-to-head pinacols at a mercury cathode in ethanolic buffers at both pH5 and pH 12.5 [142-144]. The stereochemistry of the pinacols formed from some cyclohexenones similar of structure to the steroids has been studied in some detail [145,146]. Dimerization of radicals derived from XXXVII is highly enantioselective. It occurs by reaction between radicals from the same enantiomeric configuration to give only one of the possible racemic pinacols XXXVIII. Where an angular methyl substituent is present as in XXXIX, the dimerization is less enantioselective. It is... 

Table 21 Selected Rate and Equilibrium Constants for Dimerization of Radical Anions Derived From Substituted Aromatics... 

Alkenes without - -M substituents can be coupled if they are dialkylated at one carbon atom of the double bond and are unsubstituted at the other (Table 6, number 11). This is a further indication that the dimerization of radical cations responds sensitively to steric hindrance. 

The dependence on drop-time and/or concentration of the electroactive substance was verified for the reduction of carbonyl compounds (Mairanovskil, 1961) and of the tropylium ion (Zuman et al., 1961). In both cases dimerization of radicals is considered as the second-order reaction. 

Starting with C(HgCl), finally the dimeric products hexalithioethane (CjLig) and tetralithioethylene (C Li ) (ratio 4 3) have been obtained predominantly, without any doubt attributable to dimerization of radical intermediates The direct mercury-lithium exchange reaction therefore is not very suitable for the synthesis of CLi. 5 >. 

As mentioned before, a radical cation of HTM 2 is more stable than that of HTM 1 by two factors. As shown in structure A and B, the naphthyl amine radical cation B is preferred because it has two more resonance forms than cation A. It is well known that molecules having more resonace form are more stable. In addition, cation B can be stabilized further by steric effect. A bulky naphthyl moiety which is bigger than phenyl moiety can retard dimerization of radical cations. 

These radicals are the basis for a complex sequence of reactions involving radicalization of ions and dimerization of radical ions, resulting in polysulfide anions (Sx ), e.g. ... 

This initiation process is thus similar to alkali metal initiation in (a). That this reaction occurs is shown by electron spin resonance measurements, which indicate the complete disappearance of radicals in the system immediately after the addition of monomer. The monomer in these systems often has a lower electron affinity than the polycyclic hydrocarbon, but dimerization of the monomeric radical anion [Eq. (8.15)] drives the equilibrium of reaction (8.14) to the right. Dimerization of radical centers is highly favored by their high concentrations, typically 10 -10 M and the large rate con-stants (10 -10 L/mol-s) for radical coupling. (Note that the dimerization occurs to form the styryl dicarbanion instead of CH2CH0CH0CH2 , since the former is much more stable.) The styryl dianions are colored red (the same as styryl monocarbanions formed via initiators such as n-butyllithium). Anionic propagation occurs at both carbanion ends of the styryl dianion ... 

The initiation process is thus similar to alkali metal initiation described earlier [cf. Eq. (8.10)]. The dimerization of radical anions is highly probable because of their high concentrations, typically jlO -10" M, and the large rate constants (10 -10 L/mol-s) for radical coupling (Odian, 1991). Anionic propagation takes place by monomer addition at both carbanion ends of the styryl dianion ... 

Dimerization of radical ions depends not only upon the radicars stability but also upon the 7r-eneigy changes that accompany the reaction. ... 

Dimerization of radical cation, loss of proton and formation of neutral dimer. 

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