Nitrogen-centered radical anions

Among the early examples were the reduction of organic nitro and nitroso compounds which yielded relatively long-lived nitrogen-centered radical anions. One of the questions raised in those days, especially by electrochemists, was whether the reduced nitro moiety existed exclusively in an anionic form or could establish an acid/base equilibrium as formulated in eq. 1 for the nitrobenzene case. 

Free radical rearrangements 6 Nitrogen-centered radicals 7 Nitroxyl radicals 7 Aryloxy radicals 8 Sulfur-centered radicals 8 Ketyl radicals 9 Radical cations and anions 10... 

Other methods, among which thermolysis or photolysis of tetrazene [59], photolysis of nitrosoamines in acidic solution [60], photolysis of nitrosoamides in neutral medium [61], anodic oxidation of lithium amides [62], tributylstannane-mediated homolysis of O-benzoyl hydroxamic derivatives [63, 64], and spontaneous homolysis of a transient hydroxamic acid sulfinate ester [65] could have specific advantages. The redox reaction of hydroxylamine with titanium trichloride in aqueous acidic solution results in the formation of the simplest protonated aminyl radical [66] similarly, oxaziridines react with various metals, notably iron and copper, to generate a nitrogen-centered radical/oxygen-centered anion pair [67, 68]. The development of thiocarbazone derivatives by Zard [5, 69] has provided complementary useful method able to sustain, under favorable conditions, a chain reaction where stannyl radicals act simply as initiators and allow transfer of a sulfur-containing... 

Oxidation of N1 and N2 cannot be established. However, one-electron reduction is feasible in a straightforward way. Exposure with ahcali-metal mirrors in THE or dimethoxyethane under super dry condidons aUows the detection of EPR spectra attributable to the radical anions Nl and N2. The hfc values of the pairwise equivalent nitrogen nuclei are 0.420/0.394 and 0.430/0.340 mT, respectively. This is approximately half the size of the corresponding values of (mono) diazenes and reveals that the spin is (almost) evenly distributed between the virtually equivalent nitrogen centers. 

The OH-adduct radicals can become deprotonated at nitrogen. As a consequence of this, water is eliminated from the adduct. This reaction results in the formation of a heteroatom-centered, oxidizing radical e.g. reaction (11)] this may also be formed directly from the deprotonated nucleobase at higher pH [reaction (10)]. At high pH, this radical can undergo a second deprotonation [reaction (12)]. However, this radical anion is not the thermodynamically favoured species under these conditions, and is subsequently reprotonated at carbon (i.e. the heteroatom-protonated acid has a lower p a value than its carbon-protonated isomer) [reaction (14)]. The hydroxyl group can also be eliminated by acid catalysis, which gives rise to a radical cation. These reactions will be discussed below in a separate section. 

Two distinct primary transients have been observed by optical spectroscopy in its pulse radiolysis [54]. One of these is not affected by O2 and has been attributed to an A-centered radical cation, A/ N-EDTA , directly bridged to the second EDTA nitrogen. Using strong reductants as probes, e.%. N,N,N N -tetramethylphenylenediamine, G(A/ A/ -EDTA) = 1.6 x 10 mol J has been obtained. Besides generating iV iV-EDTA, the OH radicals produce C-centered radicals by H-abstraction. These have reducing properties and are rapidly oxidized by tetranitromethane, giving rise to nitroform anion, G(NF ) = 4.2 x 10 mol J . The C-centered radicals react rapidly k = 7.6 x 10 dm mol s ) with O2, and subsequent fast 02 elimination. The Schiff bases thus formed hydrolyze to the final products. 

Product studies have demonstrated that I-phenyl and 1,1-diarylalkene radical cations react with nitrogen-centered nucleophiles such as amines and pyridines by both addition and deprotonation. The addition reactions occur by a mechanism analogous to that shown in Scheme I for methanol addition. Deprotonation by an amine or pyridine base is an alternate possibility for radical cations derived from 2-alkyl-substituted alke-nes and leads to an allylic radical (Eq. 19). Reduction of this radical by the sensitizer radical anion generates an anion that is protonaled at either the original position to regenerate starting... 

Scheme 3-5). Ohya-Nishiguchi et al. (1980) noted that such a large localized spin density is very rare in a ir-electron system of purine s size and should have important application to its chemical reactivity. Reactions such as protonation should take place preferentially at position 6. This was deduced from the result of molecular orbital calculations (Nakajima Pullman 1959). According to Fukui s frontier electron theory (Fukui et al. 1952), such areaction should take place at the position where the frontier electron density is the largest. The calculations clearly indicate that the large electron density is at position 6. Scheme 3-5 describes the protonation of the purine anion radical (Yao Musha 1974). Protonation indeed takes place at position 6. After that, the radical center appears at the cyclic nitrogen in the vicinal 1 position.

In the nitrogen and boron analogs depicted in Scheme 3-52, two methyl groups provide a sufficient shielding at the NR2 centers (R = Me), while two mesityl groups are needed for protection of the BR2 centers (R = 2,4,6-trimethyl phenyl). Electrochemical studies of l,4-bis(dimesitylboryl)benzene have shown two well-separated one-electron reduction processes, with the formation of the corresponding anion radicals and dianions, respectively (Fiedler et al. 1996). According to UV/vis/near-IR and ESR spectroscopic data... 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

The same Romanian group has measured ESR spectra for the cation- and anion-radicals 82 and 83, respectively, of 2-nitrodibenzo[l,4]dioxin. As expected, the cation-radical is heterocycle-centered, showing a very small substituent-nitrogen hyperfine splitting, whereas the opposite is true for the... 

The symbol N indicates an excited azide radical, but the authors apparently did not distinguish between this entity and the azide ion, N3, nor explain its presence in the crystal. The three nitrogen molecules were assumed to diffuse out of the lattice, leaving the electron occupying two adjacent anion vacancies, i.e., an Fj center. 

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