ELECTROCHEMICAL DETERMINATION OF ABSOLUTE AND RELATIVE REACTIVITIES OF PHENYL CHALCOGENIDE ANIONS TOWARD ARYL RADICALS

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 SRN1 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 kH) and so k2/kH ratios could be measured. Taking into account results previously published by Saveant et al. concerning kH 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-1s-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 k2is close to the diffusion limit in the nucleophilic attack of PhSeNBu4 and PhSNBu4 upon radicals corresuponding to reduction of bromobenzophenone. © 1990, American Chemical Society. All rights reserved.