Radical cation mechanism

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... 

Under similar reaction, the action of XeF2 on PBN and DMPO led to similar spin adducts, proving the radical cation mechanism (524). Unlike this reaction, the formation of fluoro-containing spin adducts when using a weaker oxidant such as /V-lluorodibenzsulfonamide [(PhSC>2)2N-F] is believed to involve the Forrester-Hepburn mechanism (524). 

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). 

The electrochemistry of RH-Nu systems is well established (Eberson and Nyberg, 1976 Eberson et al., 1991 Childs et al., 1991). The radical cation mechanism has been shown to prevail for most situations where Nu = F , Cl-, RCOCT, OCN", CN", NO-r, Py and triethyl phosphite, all of them nucleophiles that are difficult to oxidize (Table 5). The initial formation of Nu" is indicated for the redox-reactive SCN", NJ, I- and N02, with Br and (N02)3C occupying a somewhat indeterminate position. 

All of the direct fluorinations reported appear to be addition-elimination processes with solvent involvement (Scheme 42). A study of the mechanism and stereochemistry of uracil and cytosine fluorination using fluorine and acetyl hypofluorite has implicated a radical-cation mechanism (86JOC1466). The effect of acetate ion on the products proved to be important. In its absence both m-isomers (49) and trans-isomers (50) were observed in the reaction mixture, but only 50 [and 5-fluorouracil (51)] in its presence. The process has been rationalized in terms of the reaction diagram shown in Scheme 43. NMR studies have revealed that the acetate from the solution containing acetate ion, rather than the residue from acetyl hypofluorite, binds to the 6-position of uracil to form the intermediates (49 and 50). Acetate is a sufficiently strong base to induce trans-elimination of acetic acid from the cis-isomer (49). 5-Fluorouracil (51) was obtained in 45% yield from these reaction sequences (86CJC424). 

The same electron transfer mechanism was proposed by Heiba et a/.242 247 and was supported by the observation by ESR of the radical cations of several arenes when they were treated with Co(III) acetate in trifluoracetic acid.248 Cobalt(III) is a stronger oxidant in trifluoracetic acid than in acetic acid217,249 (see later). In some cases (with electron-rich aromatics), radical cations were observed in acetic acid.242 Further evidence for the radical cation mechanism was obtained in the oxidation of p-methoxybenzyl phenyl sulfide.242 The pro-... 

A radical cation mechanism is also supposed to be operative in the photosubstitution of chloride by cyanide in A-methyl-3-chlorocarbazole637. 

Because of the low reduction potential of TCB, the energy content of the radical ion pair resulting from the reaction of triplet excited sensitizer 3TCB and many olefins is insufficient lead to the triplet state of the olefins. In this case the isomerization takes place via a chain radical-cation mechanism [194, 195], The same electron acceptor also induces the electron transfer-sensitized valence isomerization of quadricyclane to norbomadiene [196, 197],... 

It is assumed that, in the reaction of arenes with aryllead triacetates, the arylation takes place via the corresponding cationic TC-complexes. In the reaction with lead tetraacetate, an electron transfer-radical cation mechanism was postulated. 1 3 jn agreement with this assumption, 4,6,8-trimethyl-... 

The fluorination of uracil and cytosine using fluorine and acetyl hypofluorite involves a radical-cation mechanism <86J0C1466>. 

Support for a nitrogen radical cation mechanism in P450-catalyzed A-dealkylation reactions... 

---