Pyridine radical cations

At the same time the pyridine is oxidized to a pyridine radical cation which by dimerization and proton loss forms 4,4 -dipyridyl. However, if 2.4.6-triphenylpyridine 31 is used instead of pyridine,it forms the stable radical cation 33, which can be observed in the ESR-spectrum in addition to the pyryl radical 22... 

The gas-phase reactivity of pyridine radical cation was studied and it was shown to combine with molecules of acetylene to provide aza-polycyclic aromatic structures, such as the quinolizium cation. Similar processes were demonstrated for the radical cation of pyrimidine. It is suggested that similar chemical processes are involved in formation of aza-polycyclic aromatic compounds in interstellar clouds and solar nebula. 

Tire self-chemical ionization reaction of CS2 under chemical ionization conditions (approx. 1 Torr) generated 83, which sulfurized pyridine (97MI1) and nitriles (97JPC6970) to give the corresponding cation radicals 61 and 62, respectively. Ab initio calculations on 83 at the G2 (MP2, 8VP) level revealed that the ylide radical cation form 63 is more stable than the dithiiranethione radical cation form (64) by 42 kJ/mol (97JPC6970). 

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

When 10-phenylphenothiazine (104) (and 10-phenylphenoxazine) was brominated in acetic acid a number of products were isolated. Pyridine perbromide, though, only brominated the phenyl substituent (Scheme 47). The suggestion that acetic acid bromination might involve the radical cation of the substrate (104) was confirmed by generating the radical cation of the substrate (104) with perchloric acid prior to bromination. Again a 43% yield of the 3-bromo product and multiple bromination products were observed (Scheme 47). The reaction of 10-phenylphenoxazine with pyridine perbromide appeared to be at least partially electrophilic the products... 

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

For unsubstituted PAH, such as benzo[a]pyrene (BP), pyridinium or acetoxy derivatives are formed by direct attack of pyridine or acetate ion, respectively, on the radical cation at C-6, the position of maximum charge density (Scheme 1). This is followed by a second one-electron oxidation of the resulting radical and loss of a proton to yield the 6-substituted derivative. For methyl-substituted PAH in which the maximum charge density of the radical cation adjacent to the methyl group is appreciable, as in 6-methylbenzo[a]-pyrene (6-methylBP) (Scheme 2), loss of a methyl proton yields a benzylic radical. This reactive species is rapidly oxidized by iodine or MnJ to a benzylic carbonium ion with subsequent trapping by pyridine or acetate ion, respectively. 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

In conclusion, structures containing polyiodide anions, with cationic aromatic ligands as counter parts of formulae [(L)(HL+)] (I ) are known to be synthesized by the treatment of the appropriate amide with HI [26-28], In contrast, the complexes with PYOH, in the present case, were formed by the direct reaction of 2-hydroxypyridine with di-iodine in a molar ratio of 2 1 and 1 2. This is a redox reaction, where 2-hydroxy-pyridine firstly is oxidized to pyridinone-2 radical cation. In the case of 2-hydroxy-pyridine however, peroxide structures are not formed like disulphides in the case of PYSH. Polyiodide anions are simultaneously produced in this case This should be a consequence of redox differences between -SH and OH groups and may be proven a useful pathway for the synthesis of polyiodide materials. 

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

Oxidation of acridine in anhydrous acetonitrile leads to a dimer 65 formed by reaction of the nitrogen in one molecule of the substrate with the point of highest positive charge density in a radical-cation [208]. Anodic oxidation of neat pyridine... 

A quite different mode of reaction was observed for the reactions between thianthrene radical ion(l+) and the heterocyclic bases pyridine (72JOC2691) and 2,3-diazabicyclo[2,2,2]oct-2-ene (88JA7880) thianthren-2-yl-N salts were obtained in each case. It was shown that 2 mol equivalents of the radical cation are required, the byproduct being thianthrene. 

The one-electron reduction of diquaternary salts of 2,2 -bipyridine has attracted much attention. When an aqueous solution of diquat dibromide (75) is treated with a one-electron reducing agent, such as zinc dust, the solution acquires an intense green color. This is due to the formation of the stable radical cation 87. The one-electron transfer is completely reversed by air. In theory the radical cation 87 can take up another electron to form the neutral species 88. The stability of the radical cation 87 is due to the ability of the odd electron to be located at any of the nuclear positions because of the near coplanarity of the two pyridine rings. The potential 0 of the first one-electron transfer occurs at — 0.35 V in aqueous solution... 

For pyridine N- oxides, direct oxygen loss may occur, but for quinoline and isoquinoline N- oxides (118) a 1,2-shift is an alternative, giving the corresponding -one radical cation (119) (68T3139), while -one derivatives themselves readily lose CO, as shown for N-methylquinolin-2-one (120) and acridinone (121) molecular ions (67AJC1179, B-71MS364). 

The elusive radical cation of pyridine (140) has been obtained by irradiation of pyridine in CFCb at 4 K (79MI20403) and g values and hyperfine coupling constants have been measured for the parent molecule and deuterated derivatives. This species is of cr-type, the odd electron spending most of its time in the N sp2 lone pair orbital. Radical cations and anions of pyridinium bis(alkoxycarbonyl)methylides have been produced in the former case (78CC817) as a cyclopropenone complex, and in the latter by reduction of pyridinium bis(methoxycarbonyl)methylide with sodium (79JMR(35)l7l). The coupling constants in the ESR spectrum of both the radical cation and the anion agree to some extent with simple Huckel MO calculations. 

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

The position of phosphorus with respect to nitrogen in the periodic table led to the expectation that it should be much easier to remove one electron from X -phos-phorins than from pyridines (see also p. 37). Indeed, soon after the synthesis of 2.4.6-triphenyl-X -phosphorin 20 by Markl, we discovered that addition of 2.4.6-triphenoxyl 57 in benzene induces oxidation to the very stable radical cation 55... 

X -phosphorins have physical properties which are rather similar to those of pyridines. But the chemistry of X -phosphorins is very different, due mainly to the phosphorus atom which can easily lose one electron to produce a stable radical cation, or accept one or more electrons to yield a radical anion, dianion or radical trianion. Nucleophiles add to stable X -phosphorin anions. In contrast to pyridine chemistry, no stable X -phosphorinium compound (corresponding to a N-alkyl-pyridinium salt) could be isolated. Instead the electron shell of phosphorus is enlarged by addition of an electrophile yielding a X -phosphorine derivative. 

The reaction was carried out in CH3CN-Et4NC104 with addition of pyridine as base, using controlled potential electrolysis and a divided cell. The yield of (33) varied greatly, depending on the method of electrolysis. Oxidation of 32 in the presence of pyridine gave 33 in 60-85% yield, whereas the electrolysis without pyridine lowered the yield to 10-20%, and products of hydrolysis, because of accumulation of the acid in the anodic compartment, were identified. The mechanism of the reaction proposed on the basis of electroanalytical results involves the cyclization of the radical-cation or its deprotonation as the rate-determining step.78... 

The substituted 1,4-dihydropyridines 156 in dry acetonitrile, containing Bu4NC104, are oxidized in a one-electron step leading presumably to a radical-cation (157).235 The final product obtained in dry acetonitrile is a substituted pyridine (158) or pyridinium derivative (159), a two-electron product. This indicates a disproportionation of the initially formed radical-cation. By adding water to act as a base, the wave doubled in height indicating a deprotonation of the radical cation 157 to a radical that is oxidizable... 

The primarily formed radical-cation dimerizes at C-3 to bis(l-phenyl-A2-pyrazolin-3-yl) if C-3 is unsubstituted.281 By-products include biphenyl derivatives formed by an alternative dimerization of the parent radical-cation. The major product is further oxidized under the reaction conditions to stable cations. By blocking the para position of the phenyl ring in the 1-position, a persistent radical would be expected, and in the presence of a base, e.g., pyridine, the corresponding pyrazole was obtained.284,285 The anodic oxidation of 1,5-diphenyl-3-(4-hydroxycoumarinyl)-A2-pyrazoline in CH3CN-Et4NC104 solution, containing pyridine, resulted in the isolation of l,5-diphenyl-3-(4-hydroxycoumarinyl)pyrazole in 95% yield when the para position was not blocked.290... 

Azacyclohexatriene-2-ylidene (3), the 2-isomer of pyridine, has been generated by one-electron oxidation of the corresponding radical cation in neutralization-reionization mass spectrometry.17 It was determined by ab initio H-F calculations that the charge polarization of the radical fonned by H-abstraction from pyrazine can be... 

Heterocyclic substrates in SET processes have been widely studied, including the reactions of diliydroiiicotiiiamide,116 pyridine, and quinoline117 and also phenoxazine and phenotiiiazines.118 Phenothiazine has also been shown by ESR analysis to undergo an electron-transfer reaction witii its radical cation with an appreciable 15N/14N isotope effect.119 The reaction of phenazine di-A -oxidc radical cations with hydrocarbons shows evidence of non-radical processes.120... 

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