Reversible Deactivation of Manganese Catalysts in Alkene Oxidation and H₂O₂ Disproportionation

Mononuclear MnII oxidation catalysts with aminopyridine-based ligands achieve high turnover-number (TON) enantioselective epoxidation of alkenes H2O2. Structure reactivity relations indicate a dependence enantioselectivity and maximum TON on the electronic effect peripheral ligand substituents. Competing H2O2 disproportionation is reduced by carrying out reactions at low temperatures slow addition H2O2, which improve TONs for alkene but mask substituents turnover frequency (TOF). Here, in situ Raman spectroscopy provides time resolution needed to establish that minimum TOFs are greater than 10 s–1 complexes [Mn(OTf)2(RPDP)] [where R = H (HPDP-Mn) OMe (MeOPDP-Mn) RPDP N,N′-bis(2″-(4″-R-pyridylmethyl)-2,2′-bipyrrolidine)]. Simultaneous headspace monitoring reveals proceeds concomitant substrate ratio toward ligand-dependent. Notably, rates both decrease over under continuous due progressive catalyst deactivation, indicates same responsible reactions. Electrochemistry, UV/vis absorption, resonance spectroelectrochemistry undergo an increase state within seconds form dynamic mixture MnIII MnIV species, composition depending temperature presence alkene. However, it formation these (resting states), rather degradation, especially temperatures, hence, intrinsic observed TOFs. These data show interpretation effects reaction efficiency (and conversion) respect oxidant needs consider reversible deactivation relative importance various pathways.