Pyridyl radical cation

Very little has been reported regarding 2,4 -bipyridine. As expected, it is quaternized preferentially on the y-pyridyl ring " and with excess methyl iodide the diquaternary salt l,r-dimethyl-2,4 -bipyridinium diiodide is ob-tained. ° The latter salt, like diquat, is reduced to a radical cation by one-electron reducing agents. The potential E of the one-electron transfer in aqueous solution is —0.64 lV-alkyl-2,4 -bipyridinones have been... 

Crude 2,8-phenanthroline, prepared from irradiating trans-1-(3-pyridyl)-2-(4-pyridyl)ethylene, has been directly converted into the 2,8-dimethyl diquaternary salt with trimethyloxonium fluoroborate. This salt too is reduced by a one-electron transfer to the corresponding radical cation. The reduction potential (E0) is —0.47 volt in aqueous solution.15,307... 

Pyridinium cations are reduced electrochemically or by metals to neutral radicals of considerable stability, especially when merostabilization by an a- or y-substituent occurs thus (381) has been isolated. Bispyridinium compounds are particularly readily reduced to radical cations, such as (383). Radical (383 R = Me) is the active species of the herbicide paraquat. Pyridyl radicals without such stabilization dimerize and form bispyridinium compounds by oxidation. 

Radical reactions are facilitated by the fact that pyrroles and indoles can form reasonably stable radical cations in some cases. For instance, photoarylation of indole by 2-iodopyridine is controlled by a photochemical electron-transfer reaction leading to the combination of the indole radical cation and the 2-pyridyl radical. The direction of attack is controlled by the relative spin densities of the possible radical cations. In polar solvents, substitution is favored at positions 3,6, and 4, whilst in nonpolar solvents, there is a preference for substitution at positions 2 and 7 (Scheme 35) (88CPB940). 

The intermediacy of a cation, formed by electron transfer within a photochemically created radical pair, was also invoked to explain the results obtained upon photolysis of 2-bromo-, 2-chloro- and 2-iodopyridine in methanol, ethanol and acetonitrile-water376. The major products are 2-methoxypyridine, 2-ethoxypyridine and 2-acetamidopyridine + 2-hydroxypyridine. In all cases pyridine was the minor reaction product, in contrast with the 3- and 4-halopyridines which produce pyridine exclusively, via a radical process. It is proposed that the unshared electron pair on the nitrogen atom assists in the formation of the 2-pyridyl cation. The presence of cupric salts increases the relative amounts of products formed via ionic reactions because Cu2+ can accept an electron from the 2-pyridyl radical. 

D. W. Clack, J. C. Evans, A. Y. Obaid, and C. C. Rowlands, The assignment of the hyperfine coupling constants for the pyridyl protons in aryl and alkyl A-substituted bipyridylium radical cations, Tetrahedron 39, 3615-3620 (1983). 

Zhang et al. [32] have reported the formation of a series of radical cations via the sonolysis of aqueous A-tetraalkyl-p-phenylenediamines. It is suggested that on sonolysis the aqueous solution forms hydroxyl radicals and via a single electron transfer the corresponding alkyl radical cation is produced. In this study both hydroxyl and the jV-tetraalkyl-p-phenylenediamine radicals were identified by ESR techniques. Christman et al. [33] obtained evidence for the production of free radicals in aqueous solutions due to microsecond pulsed ultrasound. Employing spin traps such as 5,5-dimethyl- 1-pyrrolidine-jV-oxide (DMPO) and 4-pyridyl-l-oxide-A-rert-butylnitrone (4-POBN) the ESR spectra obtained provided evidence for the formation of the free radicals OH- and H-. 

Impure 3,7-phenanthroline, obtained by the irradiation of trans- - l-pyridyl)-2-(3-pyridyl)ethylene, was converted into the 3,7-dimethyl diquaternary salt with trimethyloxonium fluoroborate. This salt is reduced to the corresponding radical cation at a potential (E of —0.44 volt in aqueous solution. - ... 

A 30 G splitting triplet superimposed on a large singlet is also observed after irradiation of the polymer at 77°K [344]. The contribution of the triplet was greatly enhanced by the presence of electron acceptors [344]. It thus seems clear that the additives markedly affect the mechanism of radiolysis by interaction with the cation radical. Additives with a hi i electron affinity inhibit cation and electron recombination and thus increase the yield of a-pyridyl radicals which are involved in crosslink formation. Additives that have a lower ionization potential than pyridine (9.28 eV) would interact with the polymer radical-cation according to... 

Good electron donors such as sulfides, phosphines, or arsines can react with N-fluoropyridinium cation by a single-electron transfer (SET) pathway. This conclusion was reached after finding products known to be derived from free-radical processes. For example, it is believed that the SET process is operative in the reaction of sulfides (74) to give pyridyl-substituted sulfides (78) through the intermediary of a radical 75 and a radical cation 76 (Scheme 7). In addition to 78 this reaction produces a dimer of a radical 77 derived from the radical cation (76) and a number of other products known to be formed from 76 or 77 (93JHC329). The... 

Formation of neutral bipyridyls requires overall abstraction of a hydrogen atom from each pyridine molecule and is much more difficult to explain. Possibilities include the deprotonation of pyridine cation radical to give, for example, the 2-pyridyl radical, or... 

M.C. Richoux, P. Neta, P.A. Christensen, and A. Harriman, Formation and Decay of Zinc Tetrakis(N-methyl-3-pyridyl)-porphine x-Radical Cation in Water, J. Chem. Soc. Faraday Trans. 2, 82 (1986) 235. 

Functionalization of Ceo with a porphyrin on one side and with a pyridyl ligand on the other side leads to the supramoleciflar triad 39 formed by a covalent-CO ordination approach (Fig. 31a) [18,20,24]. The photophysical properties of this system were studied in detail and show that energy-transfer occurs from the singlet excited state of the free-base porphyrin to the zinc(ll) porphyrin. A charge separated state with formation of the Ceo radical anion and the zinc(II) porphyrin radical cation is produced at an increased efficiency upon complex formation of the array. 

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect >99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

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