Electron-Transfer Oxidation of 9-Substituted 10-Methyl-9,10-dihydroacridines. Cleavage of the C-H vs C-C Bond of the Radical Cations

Electron-transfer oxidation of various 9-substituted 10-methyl-9, 10-dihydroacridines (AcrHR) by Fe(ClO₄)₃ and [Fe(phen)₃](PF₆)₃ (phen = 1, 10-phenanthroline) results in cleavage of the C(9)-H or C(9)-C bond of AcrHR^•+ depending on the substituent R. Transient electronic absorption spectra as well as electron spin resonance (ESR) spectra of AcrHR^•+ have been detected by using a stopped-flow spectrophotometer and a rapid mixing flow ESR technique, respectively. The hyperfine splitting constants (hfs) are determined by comparing the observed ESR spectra with those from the computer simulation. Comparison of the hfs values with those expected from the molecular orbital calculations indicates the structural change of AcrHR^•+ with the substituent R, which is reflected in the selectivity of the C-H vs C-C bond cleavage of AcrHR^•+ depending on the substituent R. The decay rates of AcrHR^•+ obey the mixture of first-order and second-order kinetics due to the deprotonation (or the C-C bond cleavage) and disproportionation reactions, respectively. Both the first-order and bimolecular second-order decay rate constants of AcrHR^†+ are reported. The first-order decay rate constant for the deprotonation of AcrHR^•+ by the C-H bond cleavage decreases with the substitution in order R = primary > secondary > tertiary alkyl groups, while the first-order decay due to the C-C bond cleavage becomes dominant with tertiary alkyl groups. The one-electron oxidation potentials of various AcrHR have been determined directly by applying fast cyclic voltammetry. The pK_a values of AcrHR^•+ (R = H and Me) have also been evaluated by analyzing the dependence of the first-order deprotonation rate constants on the concentrations of HCIO₄.