Conclusions on the radical cation mechanism

It was already mentioned [reactions (8) and (9) and the associated text, p. 94] that the first situation in which a radical ion of a spin trap was suggested to be involved (Crozet et al., 1975) was the reaction between an alkyl iodide and a thiolate ion in the presence of TBN [2], This compound is reduced reversibly at -1.25 V, and with °(RS /RS ) around 0.2 V reaction (8) is endergonic by 1.4 eV, not a favourable precondition for an ET reaction. Therefore, it is likely that some other mechanism is responsible for the observations made. 

The quite negative reduction potentials of spin traps (Table 2) make them less amenable to participation in the radical anion mechanism, as first established in the cathodic reduction of benzenediazonium salts at a controlled potential in the presence of PBN (Bard et al., 1974). In fact, the lower cathodic limit of the spin trapping method is set not by the nitrone but by the spin adduct formed. 

A different electrochemical approach was applied to the cathodic reduction of sulfones in W,JV-dimethylformamide (Djeghidjegh et al., 1988), for example t-butyl phenyl sulfone, which is reduced at a more negative potential ( pc = -2.5 V) than is PBN (-2.4 V). Thus, the electrolysis of a mixture of PBN and the sulfone would possibly proceed via both true and inverted spin trapping. If a mediator of lower redox potential, such as anthracene (-2.0 V), was added and the electrolysis carried out at this potential, it was claimed that only the sulfone was reduced by anthracene - with formation of t-butyl radical and thus true spin trapping was observed. It is difficult to see how this can be reconciled with the Marcus theory, which predicts that anthracene - should react preferentially with PBN. The ratio of ET to PBN over sulfone is calculated to be 20 from equations (20) and (21), if both reactions are assumed to have the same A of 20 kcal mol-1. 

The ready protonation of radical anions under conditions of proton availability causes other problems to appear, as for example shown by the stepwise cathodic reduction of PBN to the corresponding imine and amine [reactions (59) and (60)] during which the intermediate radicals [21] and [22] appear and become trapped by PBN (Simonet et al., 1990). 

As already pointed out, determining the mechanism of formation of hydroxyl spin adducts in aqueous media under oxidizing conditions is a particularly urgent problem in view of its implications in biochemistry and biology. In 

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The conclusion that results from these observations is that the barrier for enolization is apparently much smaller in the radical cation of 7-methyl-l-indanone than it is in that of 4,7-dimethyl-l-indanone, which raises the question why this is so (the width of the barrier is probably not affected by this change). As the 4-methyl group is obviously not directly involved in the enolization process, it must exert a remote, possibly electronic, influence on the process. The answer to this question came from considering the mechanism of photochemical enolizations of enones, which proceed through - Jt states, as illustrated in Figure 4.9a, because in the 3t Jt states a proton transfer would need to take place that leads to a high-energy zwitterionic intermediate [18]. 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

Diverse lines of evidence support the conclusion that a chemical mechanism is operative in certain reactions of the type under consideration. In certain systems this evidence is quite direct.160 Thus, when Cr2+ reacts with pentaammine-0-(pyrazinecarbonylato)cobalt(III), the first very rapid reaction phase (k > 106 M-1 s-1) leads to a green intermediate which in a slower first-order process (k = 4.5 x 102 s-1) produces Co2+(aq) and a Cr111 chelate of the new ligand. ESR measurements on the reaction mixture in a rapid flow apparatus show that the green intermediate is a radical cation.161... 

The present observations could well be explained on the same basis since hexamethylcyclotrisiloxane polymerizes via both anionic and cationic mechanisms (5). (No evidence exists to date for free radical polymerizations.) The greater activity of H2O as an inhibitor, as compared with NH3, is explained on the basis of a more rapid ion transfer. Since this would be a hydrogen ion rather than a hydride ion, it further suggests an anion as the primary polymerization species in the present polymerization. The authors feel, however, that further work is required before definite conclusions can be drawn regarding the polymerization mechanism or mechanisms. 

Photocyclization of iV-chloroacetyl amines has been used previously in the synthesis of nitrogen heterocycles, and the reaction of the substituted amine (166) leads to a benzazepinone that can be elaborated to give pseudoprotopine alkaloids. Y-Chloroacetyl derivatives of the seven isomeric indolylethylamines give azepinoindoles and azocinoindoles by photocyclization. Quantum yields for the reaction are correlated with calculated (CNDO/2 and INDO) electron densities, and on this basis mechanisms are suggested the conclusion is that both indole radical cations and indolyl radicals (for the 1-substituted compounds) are... 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

In the presence of 1-5% of iron trichloride or TBPA under conventional conditions, 60-70% yields of the adduct are formed even at -50 C, in times as short as 15 min.43 Therefore, the conclusion is that, imder appropriate conditions, the anthracene radical cation is most probably on the reaction pathway, in contrast with the case of maleic anhydride cycloaddition. A coincidence seems then to exist between the possibility of a mechanism involving the diene radical cation and the presence of a sonochemical effect. 

The mechanism for the fluorination of benzene and its derivatives with xenon difluoride in the presence of hydrogen fluoride has been discussed independently in more detail, but similar conclusions about the participation of radical-cations in the early stages of the reaction have been reached. However, the effects of added chlorine and of hydrogen chloride on fluori-nations by this method indicate that free radical-cations are involved only in the formation of biaiyls, the halogenobenzenes themselves being formed directly from a complex of the substrate and fluorinating agent. 

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