SELECTIVE SUBSTITUTION OF UNPROTONATED PYRIDINES BY ALKYL RADICALS
The reactions of dioxanyl and cyclohexyl radicals with 2- and 3-X-pyridines (X=CN, COMe, CO2Me) give a single substitution product deriving by addition at the 5- and 6-position respectively; with 4-X-pyridines substitution occurs preferentially at the 3-position.If the reactions are carried out with protonated pyridines other positional isomers are obtained.From the synthetic point of view the two procedures are therefore complementary.The change in positional selectivity on passing from unprotonated to protonated aromatic substrates is discussed and interpreted on the basis of the different nature of the transition state of the addition step.
Chianelli, D.,Testaferri, L.,Tiecco, M.,Tingoli, M.
p. 657 - 663
(2007/10/02)
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