1 -Ethyl-4- pyridinyl radical

A nice example is the disproportionation reaction of the l-ethyl-4-(meth-oxycarbonyl)pyridinyl radical (43), which leads to the ion pair (43a)l(43b) [215, 216]. 

Halogen abstraction by the stable free radical 1 -ethyl-4-(methoxycarbonyl)pyridinyl (Py ) proceeds by the mechanism shown in Eq. (5-66) [214, 570]. The first step, which is rate-determining, is a transfer of the halogen atom to the pyridinyl radical. 

Table 5-11. Rate constant solvent effects for the reaction of haloalkanes with l-ethyl-4-(methoxy-carbonyl)pyridinyl radicals at 25 °C [215, 570],...

Ethyl-, 1-propyl-, 1-bntyl- and l-benzyl-pyridinyl radicals ... 

In order to establish the mechanism of the reaction, the solvent effect on the reaction of pyridinyl radical with dibromomethane was investigated. As the results listed in Table 1 show, there is no solvent effect on the rate of the reaction. How could one reconcile the formation of a salt with the lack of solvent polarity effect on the rate Since the initial state (Py + RX) is not very polar (pyridinyl radical with a 1-ethyl group is soluble in n-hex ine, BrCH2Br has a dipole moment of ca. 1 Debye), the lack of... 

TABLE I. Rate Constants for the Reaction of l-Ethyl-4-carbomethoxy-pyridinyl Radical with Halides and Polyhalides at 25 C... 

In the course of an attempted titration of l,l -trimethylene-bis-(4-carbomethoxypyridinium) ion (++) with l-ethyl-4-carbomethoxy-pyridinyl radical ( )> we discovered that these species formed a strong complex, quite stable in acetonitrile at room tenq)erature. (Eq. 13). Epr spectra exhibited exchange narrowing and possibly two overlapping spectra, but the most significant property of the complex was a near infrared band 1360 nm (e g 1500)) which... 

Simple pyridinyl radicals, Py, like those derived from 1-ethyl-4-carbomethoxy-pyridinium ion through l reduction, have proven useful for the investigation of properties of radicals, including complexes, and the study of the mechanisms of their reactions. Discoveries include (a) distinction between atom-transfer and electron-transfer reactions by means of solvent effect on rate constants, (b) intramolecular radical complexes (ir-mers) within pyridinyl diradicals and pyridinyl diradical metal halide complexes, (c) occurrence of either electron-transfer or dimerization between pyridinyl radicals in water according to structure and (d) con5)lexation between pyridinium ions and pyridinyl radicals. Consideration of actual and potential biological roles for pyridinyl radicals is aided by these discoveries. 

The pure l-ethyl-4-carbomethoxypyridinyl radical 4") was isolated by distillation from the mixture produced by sodimn amalgam reduction of the corresponding pyridinium iodide Flash photolysis of 4 l also yielded the pyridinyl... 

Stable radicals can be isolated in pure form, e.g., l-ethyl-4-carbomethoxy-pyridinyl ( ) Griller and Ingold have proposed the term penristent to refer to long-surviving radicals. (Neither stable nor persistent can be defined precisely since exposure of the radicals to different reactants will l wi to different... 

Indole carboxylic acid 187 was converted via a Barton ester to fused indole 194 (11 examples, 5-79% yield). Barton ester 189 was formed by treatment of indole 187 with S-(l-oxido-2-pyridinyl)-l,l,3,3-tetramethyl thiouronium hexafluorophosphate (188, Garner s HOTT reagent) in the absence of light. Upon refluxing in MeCN, the Barton ester 189 decomposes to give nucleophilic ethyl radical 190, which adds to the unsubstituted carbon of alkyne 191 furnishing vinyl radical 192.This species cyclizes onto the C2 position of the indole to provide 193, which aromatizes to dehver the final product 194. The sequence proceeds without the need for an initiator or metal catalyst (14JOC5903). 

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