Electrochemical Determination of Absolute and Relative Reactivities of Phenyl Chalcogenide Anions toward Aryl Radicals

Article abstract of DOI:10.1021/jo00305a020

Absolute and/or relative reactivities of PhE- anions (E = Se, Te) toward the radicals resulting from the reductive cleavage of 9-bromoanthracene and 2- and 4-bromobenzophenone were determined by electrochemically stimulated S_RN1 substitution in MeCN and Me2SO.Cyclic voltammetry and preparative-scale electrolyses were carried out under experimental conditions where the nucleophilic attack (rate constant k₂) was competing only with H atom abstraction (rate constant k_H) and so k₂/k_H ratios could be measured.Taking into account results previously published by Saveant et al. concerning k_H and the reactivity of PhS- nucleophile toward aryl radicals, the following conclusions were drawn out.The absolute reactivity of PhSeNBu4 toward 9-anthryl radical is 12 times higher in MeCN (k₂ = 8.1 x 10⁹ M^-1 s^-1) than in Me2SO (k₂ = 6.8 x 10⁸ M^-1 s^-1).In Me2SO, the relative reactivities of PhSLi, PhSeLi, and PhTeLi are 1, 1.4, and 3.2.In MeCN the rate k₂ is close to the diffusion limit in the nucleophilic attack of PhSeNBu4 and PhSNBu4 upon radicals corresponding to reduction of bromobenzophenone.