Pyridyl radicals

Pyridyl radicals, 4-methoxycarbonyl-l-methyldihydro-ES 2, 145 3-Pyridylmethanol as metabolite... 

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). ... 

Tricyclic derivative 186 was obtained by treating solution of 184 with tris(trimethylsilyl)silane (TTMSS) and azobisisobutyronitrile (AIBN). The pyridyl radical 185 is the suggested intermediate product (Scheme 4)... 

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 effects of substituents upon the orientation of the entering phenyl radical and upon the reactivity of the pyridine nucleus have not received much attention. 4-Picoline is substituted by the 3-pyridyl radical (from the Gomberg-Hey reaction) to give a mixture containing 15% of the product of attack at the a-position (163) and 55% of product of attack at the /3-position (164). A very similar result was... 

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. 

Selenophene /3-diketones can be used as extractants for the separation and isolation of metals. The advantages of selenophene /3-dike-tones were revealed by comparison of their dissociation and distribution constants with those of acetylacetone, benzoylacetone, thenoyl-trifluoromethylacetone, etc. Selenophene /3-diketones containing a trifluoromethyl group and a pyridyl radical were of particular interest. 2-Acetoacetylselenophene127 is better for the extraction of thorium from water than acetylacetone, previously extensively used. 

Selenophene /3-diketones containing a-, /3-, or y-pyridyl radicals can be used for extraction of neodymium the extraction percentage is approximately 90%. The most practical is w-picolinoyl-2-aceto-selenophene which gives a high percentage of metal extraction at a... 

The absorption of the pyridyl radical is considerably higher than that of unsubstituted phenyl. There is a clear band at about 261 nm with smaller bands (shoulders) at 269 and 265 nm (see figure 8). 

Reactions with Radicals Reactions of Pyridyl Radicals... 

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