1- Methyl pyridine reaction with radicals

The bipyridyl herbicide Paraquat is made by reduction of pyridine to radical ions, which couple at the para positions. Oxidation and reaction with methyl bromide gives paraquat. Diquat is formed by dehydrogenation of pyridine and quaternization with ethylene dibromide. 

Oxidation of methyl ethyl ketone in H20 in the presence of catalysts (Fe3+ and the complexes Cu2+-pyridine, Fe3+-phenanthraline, Mn2+-phenanthraline) proceeds by a peculiar mechanism [173,174]. No chain reaction with propagation by the step R02 + RH takes place under these conditions. Hydrogen bonding, R02. .. HOH, hinders this reaction. Oxidation starts with attacks on the enol form of the ketone by metal ions to form a radical R followed by... 

A detailed laser flash photolysis study of the reactions of the 4-methoxystyrene radical cation and its P-methyl and p.p-di-methyl analogs with amines and pyridines in acetonitrile and aqueous acetonitrile has been carried out. " Representative kinetic data are summarized in Table 6 and cover approximately 4 orders of magnitude in time scale. A combination of transient spectra, product studies, and redox potentials has been used to establish which of the three possible reactions contributes to the measured rate constant for any given radical cation/amine pair. For example, transient spectra obtained after quenching of the radical cations with either DABCO or aniline clearly show the formation of the amine radical cation, consistent with the fact that both of these amines have substantially lower oxidation potentials than any of the three styrenes. Reaction with primary amines occurs by addition, as evidenced by the formation of a transient in the 300-nm region that is assigned to the substituted benzyl radical. These results are consistent with the high oxi-... 

Rate constants for the reaction of substituted pyridines with the 4-methoxystyrene radical cations have also been measured (Table 6). The bulky 2,6-di-tert-butyl pyridine reacts with the two methyl-substituted radical cations with rate constants of approximately 10 M s , but is substantially less reactive towards 4-methoxystyrene. This reaction has been attributed to deprotonation since electron transfer would be endergonic by -0.7 V and since the effects of methyl substitution at the P-carbon are opposite to those observed for other nucleophilic additions. 2,6-Dimethylpyridine also reacts with the two methyl-substituted radical cations with rate constants of 10 M s, but is approximately I order of magnitude more reactive towards the 4-methoxystyrene radical cation. The latter reaction must be nucleophilic addition since this radical cation cannot undergo deprotonalion. Product studies have confirmed that the reaction of 2,6-dimethylpyridine with the p-methyl-4-methoxystyrene radical cation is deprotonation. The major product of irradiation of a mixture of 1,4-dicyanobenzene, 4-methoxystyrene, and 2,6-dimethylpyridine is the rearranged tautomer, 3-(4-methoxyphenyljpropene, formed by a deprotonation, reduction, protonation sequence as shown in Eq. 19. By contrast to these... 

Carbon dioxide radical anions, C02 , are commonly used in aqueous chemistry as a reducing agent for metalloporphyrins or as intermediate in the formation of superoxide anion. COf has been reported to undergo efficient electron transfer reactions with methyl violo-gen, quinones, alkyl halides, fumarates, nitro and nitrosobenzenes and chlorinated benzaldehydes. With nitrobenzenes and chlorinated benzaldehydes, electron attachment occurs on the nitro and aldehyde groups, respectively. CO2 radicals have also been reported to add to some unsaturated compounds such as acrylamide and pyridin-3-ol. Efficient hydrogen abstraction from mercaptobenzenes have also been reported. 

Metal-complexed ligand radicals are formed in the oxidation of iron(II) diimine complexes with cerium(IV) in sulfate media. The reaction with /r/5-[2-pyridinal-a-methyl(methylimine)]iron(n) ([Fe(pmm)3], studied at lower acid concentrations (< 4 M H2SO4) is a complex process involving formation of Fe(II)- and Fe(III)-ligand oxidized species, a total of 10-11 mol of oxidant being required for reaction with each iron(II) complex. The [Fe(pmm)3] species undergoes intramolecular reductions to Fe(II) and a radical which may be further... 

Minisci reactions have also been applied to these compounds. formation by exposure to w-CPBA and O-methylation with Meerwein s reagent converted 54 into 55. Nucleophilic attack of the hydroxymethyl radical, generated with ammonium sulfate, provides an alternate route to 2-hydroxymethyl pyridines 56. 

In a search for new spasmolytics, Heilbron and Hey and their co-workers synthesized a number of pyridylquinolines by coupling diazotized arainoquinolines with pyridine. Pyridine has also been substituted by the 3-pyridyl radical in the Gomberg reaction, the 2-and 3-substituted products being obtained in 55 and 5% yield, respectively, together with a third product obtained in 20% yield w hich w as tentatively formulated as the 4-isomer. The same radical w as coupled with 4-methyl- and 4-ethyl-pyridine and gave in each case mixtures of the twm possible substitution products in which the radical had coupled predominantly with the carbon atom of pyridine adjacent to the alkyl substituent, as showm in Eq. (19). ... 

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. 

Observation of the list of borazine derivatives produced photochemically (Table 3) reveals that all of the reagents yielding photochemical products have an electronegative element (either 0,N,F,C1, or Br) which substitutes at the boron site. No B-C bonds have been formed photochemically, even when, as in the CH3Br and the HFA reactions, methyl or perfluoromethyl radicals are present Also, no B-C bonded compounds were formed when borazine was photolyzed with benzene, pyridine, cyclopentadiene, allene, ketene, acetylene, or acetone. 

The fact that such selectivity was not found with homolytic alkylation of nonprotonated heteroaromatics (Table I) or with homocyclic aromatics indicates that polar factors play a major role in the reactivity of alkyl radicals with protonated bases. These effects were determined by the study of the relative reaction rates in the alkylation of 4-substituted pyridines in acidic medium. The results obtained with methyl, n-propyl, w-butyl, sec-butyl, i-butyl, and benzyl radicals are summarized in Table III. 

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