Dimerization reactions radical

Preferential C-N coupling is also observed for oligomeric radicals (Scheme 5.9).117 A ketenimine (21) is the major product from the reaction of the "dimeric" MAN radical 18 with cyanoisopropyl radicals (15). Only one of the two possible ketcnimincs was observed a result which is attributed to the thermal lability of ketenimine 19. If this explanation is correct then, although C-N coupling may... 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

A more complete coverage of the literature on electronic spectra of radicals is presented in our paper submitted for publication in Fortschr. Chem. Forsch. (Topics in Current Chemistry), where theafi initio studies are also reviewed and the existing open-shell computational procedures discussed. Recently we performed semiempirical all-valence-electron calculations on ground-state properties and electronic spectra of small radicals (Zahradnik, R., and P. Carsky, Theoret, Chim. Acta, 27, 121 (1972) and Carsky, P., M. Machacek, and R. Zahradnik, Coll. Czech. Chem. Commun., in press) and on equilibrium constants of dimerization reactions of small radicals (Zahradnik, R., Z. Slanina, and P. (5arsky, to be published). 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

The intermediately formed alkyl radical could be either further oxidized in presence of IPT and water molecules yielding corresponding alcohols or recombine with other radical (dimerization reaction) ... 

The sulfonium dimer can in turn be oxidized at a higher potential, which is the diffusion-limited oxidation current of Ph2S. For this reason, the concentration of the latter in the reaction layer closer to the electrode is practically zero, so a dimeric dication radical formed from [(Ph)2S-C6H4SPhj can only react with this cation. Thus, no trimers but only tetramers are obtained under such conditions (Scheme 12) [52, 54, 55]. 

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... 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

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... 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

Reduction of phenyl vinyl sulphones in dimethylformamide containing phenol as proton donor causes loss of phenylsulphinate ion. The reaction probably involves a series of electron and proton addition steps [74]. In absence of a proton source, phenyl vinyl sulphone radical-anion undergoes a dimerization reaction discussed on p. 57. Reactions of alkyl substituted vinyl sulphones are complicated by alkene migration in the presence of electrogenerated bases. Dimers are formed and further reduction leads to loss of phenylsulphinate ion [81] (Scheme 5.3). 

A third important reaction of aromatic radical-cations is carbon-carbon bond formation with a further aromatic substrate. This reaction is limited to the oxidation in acetonitrile of substrates with electrondonating substituents. Radical-cations from benzene, naphthalene and anthracene form a-complexes but do not form a a-bonded reaction intermediate. Tlie dimerization reaction has been investigated both by pulse-radiolysis [22] in water and by electrochemical methods [27] in acetoni-... 

The radical site of the intermediates in the dimerization reaction is stabilised by resonance and examples have been noted where dimerization occurs by substitution onto the aromatic ring. This is the major reaction course for the reduction of 1-acetylnaphthalene [3 ] which yields 1 in alkaline solution by a radical-ion radical coupling. In the reduction of acetophenone, small amounts of related reaction products, 2 and two diastereomers of 3, can be detected. The yields of these compounds are increased by the reduction of acetophenone encapsulated in a cyclodextrin [32],... 

Bromopyrogallol Red, hydrogen peroxide determination, 628-9 Bronchial epithehal cells, IR spectrophotometry, 683 Br0nsted acids, olefin epoxidation, 471 BSA see Bovine semm albumin BTSP see Bis(trimethylsilyl) peroxide 2-Butanone peroxide, hydroperoxide determination, 686, 688, 689 -2-Butene, final ozonide, 721 t-Butoxy free radical, a-methylstyrene dimer reaction, 697... 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

The radical anions have been characterized by their ESR spectra which have been assigned using deuterium labeling 45). Above -90°, the radical anions undergo a dimerization reaction 42, 45). 

Using this method, the dimerization reactions of short lived methyl-diphenylamine and diphenylamine radical cations are successfully investigated61. In this study tris(4-bromophenyl)amine cation radical (TBPA,+) was used as 1 . Oyama et al.,94 used tris(2,4-dibromophenyl)amine (TDBPA) as the reaction initiator (M,+) and spectroscopically detected anthracene derivative cation radicals in acetonitrile using the ESTF method. This approach holds good potential for evaluation of the reactivities of short lived cation radicals. 

Reversible dimerization reactions have been described for a number of aromatic radical ions. In aromatic oligomers and polymers with a more extensive delocalization of the charge and the spin such dimerizations are disfavoured. This was shown in a study of several aromatic molecules with extended it systems at different redox stages179. An extensive overview on fluorinated aromatic radical anions has been presented180. 

Eq. (25) 5 . Various products have been reported in the photocatalyzed oxidation of dimethyl sulfide depending on the initial concentration of thioether. On TiO2, CdS, or ZnSe, a Stevens rearrangement occurs, Eq. (26) The key intermediate appears to be a dimethylsulfide dimer cation radical, and the reaction is only efficient in protic solvents. 

The formation of propene oxide as a side product of the acrolein formation or dimerization reactions is reported by many authors. Daniel et al. [95,96] demonstrated that propene oxide is formed by surface-initiated homogeneous reactions which may involve peroxy radical intermediates. The epoxidation is increased by a large void fraction in the catalyst bed or a large postcatalytic volume. In view of these results, the findings of Centola et al. [84] are understandable, as the wall of the empty reactor may have been sufficiently active to initiate the reaction. 

Although there is little doubt that the electron transfer reaction (Reaction 2) is involved in the over-all reaction (21), the suggestion that quantitative yields of disulfide (13) arise from the dimerization of thiyl radicals is inconsistent with the observed behavior of other free radicals (24). It seems preferable to suggest that some kind of coordination occurs as a prerequisite to the transfer of electrons (12,15). In this case, metal-thiol complexes should be formed as intermediates in the oxidation, in which the metal acts not only as an electron acceptor but also to locate the resultant thiyl entities in close proximity, thereby favoring dimerization reactions and producing disulfide. The electrons gained by the metal may then be passed on to an oxygen molecule. The over-all reaction may be represented as... 

The formation of relatively stable semiquinone radicals by electrolytic reduction of quinones has been established by a variety of methods. Some semiquinone radicals undergo reversible dimerization reactions to form peroxides. 

These findings show that by dimerization to diphenol, structures are being formed which are less stable towards oxidation (radical one-electron transfer reactions), explaining dimerization by radical coupling during oxidative degradation from the energetic point of view. 

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