Unstable radical pair

As for the salt formation and single-electron transfer, thermodynamics for simple redox processes may be applied to predict their selectivity. As a first approximation, a cation with red lower and higher than 0.2 V would give a salt and a radical pair, respectively, when combined with [2 ]. In practice, the cations which were found to give salts with [2 ] have red values more negative than —0.8 V. On the other hand, quantitative single-electron transfer has been observed from [2 ] to the heptaphenyltropylium ion which is relatively unstable p/fR+ —0.54 in methanol (Battiste and Barton, 1968) and E ed —0.30 V vs. Ag/Ag in acetonitrile (Kitagawa et al., 1992). 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

From the viewpoint of the reaction mechanism, the emphasis in Scheme 5 is focused on products with the general formula R3MCH2CHB1CH2CCI3 (M = Ge, Sn) with a halogen atom in a /3-position to the element M (the so-called normal addition product). These compounds are believed to be unstable and to decompose with the elimination of an R3M radical. The phenomenon is referred to as /S-decomposition or /3-cleavage (Scheme 5). The mechanism presented in this scheme lacks the radical pair stages, while the experimental results47,48 demonstrate CIDNP effects observed for the initial compounds and the main reaction products of the interaction of R3MCH2CH=CH2 with CChBr (Table 6). Thus Scheme 5, which is based on the analysis of the reaction products, needs to be refined. 

Incorporation of triethylamine into the reaction medium produced more reduction product presumably due to electron transfer from the triethylamine to the excited alkyl halide. This results in a weakly-bound amine-alkyl halide pair [57]. The alkyl halide radical anion releases X- (Scheme 17). In a related example, it is known that solutions of aliphatic amines in CC14 are unstable to light quickly forming white crystalline precipitates [60]. The initial reaction is formation of a singlet radical pair via excitation of a ground state charge-transfer complex. 

The fate cS the contact ion pair [RH A 1 is critical to electrm-transfer oxidation. Oxidative efficimey is the highest with those organic donors diat yield unstable radical cations, such as hexamethyl(Dewar benzene), which und goes spontaneous rearrangement (equation 7). > ... 

Another possibility is that an unstable dimer is form rapidly, followed by slow rearrangement, presumably by dissociation and recombination within a radical pair complex, to the more stable dimer (Eq. 17). Absorption spectra may help to distinguish these two mechanisms in spite of the similarity of dimer qj tra to those of radicals. 

It is beheved that in biological systems NO reacts with superoxide O2 to form the cytotoxic ONOO. In biological media, low concentration levels of HCO3 prevent the toxic effects of ONOO, due to the formation of an unstable adduct with the composition ONO2CO2, which homolyzes to give the NO2/CO3 radical pair. ... 

If the lifetime of the alkyl radical is extremely short, in other words, if a certain given radical is very unstable (i.e., very reactive), the lifetime of a radical pair, consisting of an alkyl and a ketyl radical, after electron transfer and homolysis, would be extremely short. Subsequently, electron transfer and radical combination follow each other, with almost no separation no differentiation can be made between a two-step electron-transfer mechanism and a one-step polar mechanism. For steric reasons, one would expect the following sequence of reactivities in a concerted mechanism tertiary < secondary < primary, whereas the reverse is to be expected for an electron-transfer mechanism, in view of the stability of radicals. 

N-CIDNP was used to investigate photoinduced nitrations of aromatic compoimds ArH with tetranitromethane. The polarizations invariably arise from radical pairs ArH NO, but the experiments reveal different pathways of formation of these pairs. With a substrate such as 1,2-dimethoxybenzene, the precursor multiplicity is triplet and the pairs are produced by a dissociative electron transfer from the aromatic compound to tetranitromethane, which then cleaves into an NO2 radical and C(N02)3. In contrast, the very similar substrate anisole (methoxybenzene) exhibits polarizations indicating a singlet precursor, and the radical pair is thought to be formed by decomposition of a preceding unstable diamagnetic intermediate, most probably a nitro-trinitromethyl adduct. ... 

The alkyl halide radical anion is very unstable and short-lived, decomposing to an alkyl radical and it halide ion. In fact, the electron transfer very-likely occurs dissocialively. forming the radical directly with the transfer of the electron (eq. 2.15). T his is supported theoretically 43. and is consistent with the fact (see below) that the ease of reduction increases with the stability oflhc radical formed. Low temperature LRR studies indicate only weak polarization interaction between the radical and the halide ion 44. Nevertheless, it has been suggested that even this weak interaction might affect the partition of radical species during the lifetime of a radical pair 45. lit contrast. 

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