Reactions of Cation Radicals of EE Systems. 7. Mechanistic Considerations and Relative Reactivities of Nucleophiles in Reaction with the Cation Radical of 9,10-Diphenylanthracene

The reactions of the cation radical of 9,10-diphenylanthracene (DPA+1·) with neutral protic, neutral aprotic, and anionic aprotic nucleophiles in acetonitrile have been examined by spectroelectrochemical and stopped-flow kinetic methods. In these reactions, both reduction of and addition to the cation radical are observed. It is found that reaction type (electron transfer vs. addition) can be predicted from a consideration of the oxidation potential of the nucleophile relative to that of DPA. For those cases in which the nucleophile behaves as a reductant (H2S, Br-, I-, SCN-, and CN-), the experimental rate law is found to be first order with respect to both nucleophile and cation radical concentrations and independent of the concentration of the precursor (DPA), indicating rate-determining encounter between nucelophile and cation radical. Furthermore, the dynamics of the addition reactions of H2O, 4-cyanopyridine, pyridine, and piperidine to DPA+· are described by a rate law of the same form. In concert with the observed reaction stoichiometries, these kinetic data indicate that addition occurs via the half-regeneration mechanism. The relative reactivities of these nucleophiles toward DPA+· in acetonitrile (both addition and electron transfer cases) are found to parallel those reported for these same nucleophiles in the SN2 displacements of iodide from methyl iodide in methanol. In addition to affording a means of predictability of the dynamics of reactions of nucleophiles with this carbon-centered cation radical, the linearity of this correlation which includes both electron transfer and addition reactions suggests a common transition state for both reaction types.