Dimerization between radicals

This favorable situation may not be encountered in every case. With radical reductions endowed with high intrinsic barriers, the half-wave potential reflects a combination between radical dimerization and forward electron transfer kinetics, from which the half-wave potential cannot be extracted. One may, however, have recourse to the same strategy as with the direct electrochemical approach (Section 2.6.1), deriving the standard potential from the half-wave potential location and the value of the transfer coefficient (itself obtained from the shape of the polarogram) under the assumption that Marcus-Hush quadratic law is applicable. 

Fig. 39). The ESR data for dimethylselenide dimer cation radical 107c are consistent with two-center three-electron bonding, in which two electrons are in a cr bond between the two chalcogens, while the third electron is in a o orbital. 

Scheme 29 describes a plausible mechanism for the formation of the products which fit the observed coulometric (n 0.45 F/mol) and preparative results. The intramolecular cyclization process involves a dimerization between a radical cation 52a and the ketene imine 52 to form the intermediate radical cation 52b which then cyclizes to the radical 52c which can abstract a hydrogen atom leading to 54 or can be further oxidized and transformed through a cyclization and deprotonation reaction to 53 which involves 1 F/mol. However, it seems that the [2 -1- 3]-cycloaddition between the parent compound 52 and the cation 52d giving rise to 55 is the fastest reaction as compared with the intramolecular cyclization of 52d to 53. This can also explain the low consumption of electricity. 

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... 

Hence, in toluene, no dimer anion-radicals are observed. Encounters between anion-radicals and neutral molecules result in electron transfer instead. Charge transfer from the solute anion-radicals to toluene is unlikely, since toluene is a poor electron acceptor. Consistent with this fact, no differences were observed in the anion-radical ESR-coupling constants in toluene when compared to those in -hexane (Werst 1993). Charge (not electron) transfer from the toluene molecules to the... 

The equilibrium between radical-anion and dimer for pyridine and quinoline has been examined in a number of aprotic solvents. Radical-anions of pyridine dimer-ise rapidly in liquid ammonia in tire presence of alkali metal ions [15] In hex-amethylphosphoramide with alkali metal counter ions, the monomer is detectable in an equlibrium concentration [16], The monomeric species can be stabilised by substituents and 2- or 4-cyanopyridines give radical-anions which persist in liquid ammonia while 3-cyanopyridine radical-anion dimerises with a rate constant of 2 x 10 [17], Quinoline radical-anion is stable in hexamelhylphosphoramide [16] but in liquid ammonia it dimerises irreversibly [18]. 

Under ordinary conditions, reduction of these imines in dimethylformamide is a two-electron process involving saturation of the carbon-nitrogen double bond [181] because the radical from protonation of the radical-anion is more easily reduced than the starting imine. Immonium salts with two or more phenyl substituents are reduced reversibly in acetonitrile to the radical-zwitterion such as 42. Other immo-niura salts, e.g. 43, are reduced irreversibly to the dimer [182]. Radical-zwitterion intermediates generated from immonium salts exhibit nucleophilic character on carbon. Intramolecular interaction between the reduced immonium function and a... 

Remarkable enhancements of the unimolecular c-t isomerization of c-S with p-MeO and oxidation of S with -MeO are explained by charge-spin separation in such S Unimolecular c-t isomerization of such c-S proceeds with a chain mechanism, while regioselective oxidation occurs in such S because of the spin localization. Cycloreversion of t,c,t-TPCB occurs to give a a-St 2, while the photochemical cycloreversion of TPCB and t,t,t-TPCB gives Tr-St 2 and t-St /t-St pair, respectively. Radical cations of phosphorus compounds (9 and 10 form intramolecular rr-dimer between two Nps from which Np 2 forms. Formation of intermolecular a-dimer of aromatic acetylene (11 - and 12 -) and intramolecular dimer of 13 and diarylmethanoT was observed, and the n = 3 rule is not effective for intramolecular dimer -. 

In early studies, flash vacuum pyrolysis, a method that has proven very valuable in preparative studies of closed-shell compounds,was regarded as the method of choice for the production of radicals for matrix isolation studies. " The disadvantage of this method, which is very well suited for preparative studies of closed-shell compounds, is that the reaction occurs on the walls of a hot tube whose surface may trap radicals (this problem may be alleviated by coating the inside of the tube with gold ). Also, unless a very low vacuum can be maintained in the pyrolysis mbe, collisions between radicals may lead to gas-phase dimerization. 

The photoreduction of polymer pendant viologen by 2-propanol was reported to proceed by the successive two-electron transfer processes between the adjacent viologen units and the propanol which is a two-electron reducing agent44). Preferential formation of a dimeric cation radical of viologen observed was ascribed to the polymeric structure and the two-electron process. These fundamental studies on polymeric electron mediators contribute to the construction of solar energy conversion systems. 

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). 

Once formed, the dihydrofuryl radicals may undergo a variety of reactions as known from homogeneous chemistry including disproportionation, addition to double bonds, electron transfer and dimerization. Surprisingly, the latter pathway is followed to about 90 % as indicated by the complete material balance [145]. It is therefore likely that C-C coupling occurs between radicals within the water-2,5-DHF surface layer. This is corroborated by the quadratic dependence of the initial... 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

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... 

Pyryliiim [181-183] and isobenzopyrylium [184] salts have been shown to be polaro-graphically reducible in a one-electron reduction. In cyclic voltammetry in aprotic solvents, 2,4,6-trisubstituted pyrylium salts show two peaks the shape of the peak depends on the rate of dimerization. This process occurs more rapidly at C-4 than at C-2 (C-6) [182, 183], and the dimerization takes place spontaneously for 4-unsubstituted pyrylium salts. The equilibrium between radicals and dimer is displaced in favor of the radicals on introduction of electron-withdrawing substituents such groups enhance the aromatic character of the radical [185]. If the reduction of pyrylium salts is made in the presence of an alkyl iodide a fair yield of the 4-alkylated 4i/-pyrane is isolated with the dimer [182]. 

In dry MeCN, n = 0.6 was found for 51, but the value increased to 0.94 on addition of water up to 10% [13]. For 52a in very dry MeCN, the mechanism of the reductive dimerization was examined and the experimental results were interpreted as an RS mechanism under these conditions [125]. However, addition of water increased the rate of reaction considerably, and in the presence of water the kinetic measurements were in accord with the RR mechanism [126]. In DMF, addition of water also accelerates the dimerization process for 52 [7,10], similar to what is observed for many monoactivated alkenes (Sec. II.A.7). The accelerating effect of water in DMF on the dimerization of 52a has been studied in greater detail [15]. On the basis of a reaction order in water close to 1, it was suggested that the dimerization reaction takes place between a free radical anion and a hydrogen-bonded radical anion [15]. The involvement of hydrogen bonding between radical anions and water may also account for the low activation energies found for the reductive dimerization of 52a in MeCN [126] and in DMF [10] (see Table 11). 

In most cases of cross-dimerization between carbonyl and unsaturated groups in the previous section, there has been an ambiguous point on whether the carbonyl or unsaturated group is first reduced to give a reactive intermediate. Hence, the stereochemical course of the reaction was not sufficiently discussed. In this section, the stereochemical course of cross-dimerization, in which it has mechanistically been clarified that the carbonyl groups are reduced to give the corresponding radical or anion intermediate species (XIV-XVI) that react with the unsaturated bonds such as C=C, C=N and C=N is discussed in detail. 

Dimerization between 2 transient radicals Dimerization between transient and a persistent radical... 

Scheme 9.1 Dimerization between two transient radicals (left) and a transient and a persistant radical (right)...

The fluorescence quenching dynamics of excited state electron donors by various pyrimidine and 5,6-dihydropyrimidine substrates have been examined and found to obey the Rehm-Weller relationship." In addition, an unexpected difference was observed between the reduction potentials for the trans-syn and cis-syn diastereoisomers of dimethylthymine cyclobutane dimers, and this has been ascribed to a stereoelectronic effect in the cis-syn dimer anion radical resulting from an unfavourable charge-dipole interaction between the added electron and the O carbonyl group of the pyrimidine ring... 

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