Dehydrogenation versus oxygenation in two-electron and four-electron reduction of dioxygen by 9-alky-10-methyl-9,10-dihydroacridines catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins in the presence of perchloric acid

Dehydrogenation of 10-methyl-9,10-dihydroacridine (AcrH₂) by dioxygen (O₂) proceeds efficiently, accompanied by the two-electron and four-electron reduction of O₂ to produce H₂O ₂ and H₂O, which are effectively catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins in the presence of perchloric acid (HClO₄) in acetonitrile (MeCN) and benzonitrile (PhCN), respectively. The cobalt porphyrin catalyzed two-electron reduction of O ₂ also occurs efficiently by 9-alkyl-10-methyl-9,10-dihydroacridines (AcrHR; R = Me, Et, and CH₂COOEt) to yield 9-alkyl-10- methylacridinium ion (AcrR⁺) and H₂O₂. In the case of R = Bu^t and CMe₂COOMe, however, the catalytic two-electron and four-electron reduction of O₂ by AcrHR results in oxygenation of the alkyl group of AcrHR rather than dehydrogenation to yield 10-methylacridinium ion (AcrH⁺) and the oxygenated products of the alkyl groups, i.e., the corresponding hydroperoxides (ROOH) and the alcohol (ROH), respectively. The catalytic mechanisms of the dehydrogenation vs the oxygenation of AcrHR in the two-electron and four-electron reduction of O ₂, catalyzed by monomeric cobalt porphyrins and cofacial dicobalt porphyrins, respectively, are discussed in relation to the C(9)-H or C(9)-C bond cleavage of AcrHR radical cations produced in the electron-transfer oxidation of AcrHR.