Effect of redox potential on manganese-mediated benzylalcohol and sulfide oxidation

Tridentate 1,3-bis($2^{\prime}$-Ar-imino)isoindolinato manganese(II) complexes were found to efficiently catalyze the mild oxidation of organic sulfides to sulfoxides and benzyl alcohols to benzaldehydes with mCPBA and PhIO, but they proved almost ineffective by the use of $t$BuOOH and H2O2 at room temperature. The effect of electron-withdrawing and electron-donating substituents on the substrates (thioanisole and benzylalcohol), and the redox properties of the metal center by varying the aryl groups on the bis-iminoisoindoline were investigated, and showed a significant impact on the catalytic C–H oxidation and sulfoxidation reactions. Based on these results, including the linear correlations between the oxidation reactivity of the catalysts and $\mathrm{Mn}^{\mathrm{III}}/\mathrm{Mn}^{\mathrm{II}}$ redox potentials, the Hammett correlation with $\rho = -0.27$ for 4R-PhSMe and $\rho = -0.27$ for 4R-PhCH2OH, electrophilic oxomanganese(IV) intermediate has been suggested as key oxidant. Furthermore, the small negative slope $({-}0.5)$ from the $\log k_{\mathrm{rel}}$ versus $E^{\mathrm{o}}_{\mathrm{ox}}$ for 4R-PhSMe gives clear evidence for the direct oxygen atom transfer (OAT) mechanism instead of electron transfer (ET) mechanism between the $\mathrm{Mn}^{\mathrm{IV}}\mathrm{O}$ and PhSMe.