A Mechanistic Dichotomy in Two‐Electron Reduction of Dioxygen Catalyzed by N , N ’‐Dimethylated Porphyrin Isomers

Selective two‐electron reduction of dioxygen (O 2 ) to hydrogen peroxide (H 2 O 2 ) has been achieved by two saddle‐distorted N , N ’‐dimethylated porphyrin isomers, an N 21, N ’22‐dimethylated porphyrin ( anti ‐Me 2 P ) and an N 21, N ’23‐dimethylated porphyrin ( syn ‐Me 2 P ) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti ‐Me 2 P with higher turnover number (TON=250 for 30 min) than that by syn ‐Me 2 P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins ( anti ‐Me 2 Iph or syn ‐Me 2 Iph ) by spectroscopic measurements. The rate‐determining step in the catalytic ORRs was concluded to be proton‐coupled electron‐transfer reduction of O 2 with isophlorins based on kinetic analysis. The ORR rate by anti ‐Me 2 Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn ‐Me 2 Iph by external protons was observed. The different mechanisms in the O 2 reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O 2 with inner N H protons of the isophlorins.