Reversible Deactivation of Manganese Catalysts in Alkene Oxidation and H₂O₂ Disproportionation

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