Polar Effects in Free-Radical Reactions. Rate Constants in Phenylation and New Methods of Selective Alkylation of Heteroaromatic Bases

The rate constants for the addition of the phenyl radical to protonated and unprotonated 4-substituted pyridines have been determined by competition with chlorine abstraction from CC14. The constants range from 2 X 105 to 6 X 106M-1 s-1depending on the substituent and on the degree of protonation. The phenyl radical shows a clear-cut nucleophilic character. On the basis of these rate constants, the use of phenyl radical from diazonium salt or benzoyl peroxide to generate alkyl radicals by iodine or hydrogen abstraction has been developed as a general procedure for the alkylation of heteroaromatic bases. This reaction is characterized by high yields and selectivities. © 1986, American Chemical Society. All rights reserved.