Kinetic studies of oxygen atom transfer reactions from trans-dioxoruthenium(VI) porphyrins to sulfides

Abstract

The kinetics of the reactions of three trans-dioxoruthenium(VI) porphyrin derivatives with organic sulfides were measured. The dioxo systems studied were 5,10,15,20-tetramesityl porphyrin-dioxoruthenium(VI) ( 2a), 5,10,15,20-tetraphenylporphyrin-dioxoruthenium(VI) ( 2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-dioxoruthenium(VI) ( 2c). Species 2 were competent oxidants and reacted rapidly with thioanisoles to generate the corresponding sulfoxides. Typical second-order rate constants determined from pseudo-first-order kinetic studies for sulfoxidation reactions are 8–60 M − 1 s − 1 , which are 3 orders of magnitude larger in comparison with those of well studied alkene epoxidations and activated C–H bond oxidations by the same dioxo species. For a given sulfide substrate, the reactivity order for the dioxoruthenium(VI) species was 2a < 2b < 2c, which is in agreement with expectation on the basis of the electron-withdrawing and steric effects of the porphyrin macrocycles. Various para-substituted thioanisoles react in a narrow kinetic range with the same dioxo species. The kinetic results obtained in this study indicate a concerted oxygen atom transfer and/or electron transfer followed by oxygen transfer mechanism from oxidant to sulfide. Competition kinetic reactions with a catalytic amount of porphyrin ruthenium(II) species and a terminal oxidant give relative rate constants for sulfoxidations of competing substrates that are somewhat smaller than the ratios of absolute rate constants, implying a multiple oxidant model for sulfoxidation reactions.