Spin trapping inverted

Oxidative Alkoxylation of Nitrones to a-Alkoxy Nitrones and a-Alkoxy Substituted Nitroxyl Radicals The first direct experimental evidence of the possibility to carry out radical cation nucleophilic addition to nitrones with the formation of nitroxyl radicals has been cited in Section 2.4. Further, such a reaction route was referred to as inverted spin trapping this route is an alternative to a conventional spin trapping (508-512). Realization of either mechanism depends on the reaction conditions namely, on the strength of both nucleophile and oxidant. The use of strong oxidants in weak nucleophilic media tends to favour the radical cation mechanism... 

For the first time, the possibility of carrying out preparative inverted spin trapping was demonstrated by the oxidative methoxylation of heterocyclic nitrones derived from AH -imidazole-1,3-dioxide (219) (Scheme 2.79) (513, 514). 

Extended studies later showed that the radical cation mechanism of equations (6) and (7) is prevalent in strongly oxidizing systems (Eberson, 1992 Eberson and Nilsson, 1993), especially under photolytic conditions where excited states are formed (Eberson, 1994). The latter normally can act as strong oxidants or reductants, as in equation (11), and thus can create radical ions under seemingly mild conditions. Since this type of mechanism was judged to be much more common than thought, the name inverted spin trapping was coined for it in view of the inverted situation of electron demand which appears when an electron is formally transferred between the spin trap and the nucleophile or electrophile see equations (13)-(16). 

As already mentioned in Section 1, the radical ion-mediated mechanism, often to be denoted as inverted spin trapping in the following, was discussed and experimentally supported in several isolated cases in the period between 1975 and 1990, but was not subjected to systematic study. In addition, numerous studies were performed on systems which, in the light of later developments, must have involved inverted spin trapping, but which were interpreted differently. Two particularly interesting cases involve the formation of fluoro and acetoxy spin adducts. 

Table 5 Standard potentials of redox couples Nu /Nu of interest in inverted spin trapping under oxidizing conditions."...

Tetramethyl-l-pyrroline-l-oxide ([11] TMPO) underwent inverted spin trapping but only with one nucleophile, triethyl phosphite. This is expected in view of the even lower redox reactivity of TMPO, E° = 1.8 V. 

From the above, it is evident that every photochemical system must be carefully analysed in order to establish the nature of the process of spin adduct formation. Not all systems have the inbuilt diagnostic features of the fluoride or carboxylate nucleophiles, and it must therefore be accepted that mechanistic certainty will be difficult to attain. It also must be remembered that many studies in the past were designed without regard to the inverted spin trapping mechanism and are difficult to judge owing to lack of critical experiments to test this particular aspect. 

The electrochemical behaviour of PBN +-cyanide ion is identical to that found in the two cases of inverted spin trapping described above, namely that attack at PBN + occurs via the softer carbon atom of CN". This contrasts with an observation of the cyano adduct to PBN formed by irradiation of Mo(CN) -... 

The formation of the trinitromethyl adduct of PBN by photolysis of PBN and tetranitromethane (Okhlobystina et al., 1975) is an unequivocal case of inverted spin trapping. These components give an orange-red CT complex in, for example, dichloromethane when this solution is irradiated by light which only can excite the CT complex (A > 430 nm) the spin adduct (N02)3C-PBN is formed via reaction (46) (Eberson et al., 1994b). This adduct is highly persistent. When the solution is acidified by —2% trifluoroacetic acid, irradiation does not lead to spin adduct formation owing to protonation of trinitromethanide ion. 

Di-t-butylethylene [5] has one unique property in comparison with other spin traps, in that it cannot be excited under normal photolysis conditions (Amax = 185 nm). Imidyl-[5] are formed by UV photolysis of ImX-[5] solutions, and it is obvious that the mechanism must involve excitation of ImX to ImX around 205 nm, followed by either homolytic cleavage of the excited state or oxidation of [5] (Table 3) [E°(ImX /ImX ) is estimated to be very high, 6 V] to give [5] + and create conditions for inverted spin trapping. 

In photochemical reactions, the role of DBPO will undoubtedly be that of an electron acceptor from an excited state species, as shown in reaction (51). Thus, inverted spin trapping will be feasible and an unambiguous interpretation of the appearance of PhCOO—ST will be difficult. In HFP the very strongly attenuated reactivity of benzoate ion should, however, make the homolysis mechanism predominate, as indicated by the appearance of both PhCOO—ST and Ph-ST (ST = PBN or DMPO) in the photolysis of DBPO and ST (Eberson et al., 1996a). 

Much work conducted in low-temperature matrices has shown that the primary chemical process induced by y-irradiation is formation of electrons (e ) and positive holes (h+), the latter eventually leading to the formation of radical cations of the component(s) with the lowest ionization potential (Symons, 1997). This means that an added spin trap may be transformed into its radical cation by y-irradiation and thus create conditions for inverted spin trapping, as already described for PBN and DMPO above in experiments designed to study this aspect. 

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. 

Thus there is little doubt that the hydroxyl radical, if generated by an unambiguous method such as pulse radiolysis, can be trapped by PBN or DMPO, even if the former has several deficiencies, among them low trapping efficiency and short half-life of HO-PBN. The problem in hydroxyl radical trapping thus rests with the possible competition from the nucleophilic addition-oxidation mechanism, as exemplified in reaction (69) for DMPO and Ox-Red as a general one-electron redox system, or the inverted spin trapping mechanism (70). The treatment to follow will mostly be limited to DMPO. 

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