Rate-Limiting Step of the Sulfoxidation Reaction of the Oxoiron(IV) Porphyrin π-Cation Radical Complex: Electron Transfer-Coupled Bond Formation Mechanism

Sulfoxidation reactions mediated by oxoiron(IV) porphyrin π-cation radical species (Compound I) are key reactions in chemical and biological reactions. A single electron transfer (ET) process from a sulfide to Compound I has been proposed in the rate-limiting steps of many sulfoxidation reactions, although it is endergonic. To clarify whether the ET takes place in the rate-limiting step of the sulfoxidation reaction, we comprehensively performed the Marcus plot analysis with Compound I and sulfides having various redox potentials and steric effects. The slopes of the Marcus plots for the sulfoxidation reactions do not match the slope expected from the ET reaction. However, the change of the slope for the sulfoxidation reaction is identical with the slope expected from Marcus's theory, providing strong evidence for the participation of the ET process in the rate-limiting step. The slope of the Marcus plot for the change in the redox potential of the sulfide is smaller than that of Compound I. As the sulfide is bulkier, the reaction becomes slower, and the slope of the Marcus plot becomes smaller. These results suggest that the O-S bond formation process is also involved in the rate-limiting step. All results in this study are reasonably explained by the ET coupled bond formation mechanism, in which the endergonic ET process from the sulfide to Compound I is coupled with the exergonic O-S bond formation process in the rate-limiting step of the sulfoxidation reaction.