Lewis versus Brønsted Acid Activation of a Mn(IV) Catalyst for Alkene Oxidation

Lewis acid (LA) activation by coordination to metal oxido species has emerged as a new strategy in catalytic oxidations. Despite the many reports of enhancement of performance in oxidation catalysis, direct evidence for LA-catalyst interactions under catalytically relevant conditions is lacking. Here, we show, using the oxidation of alkenes with H₂O₂ and the catalyst [Mn₂(μ-O)₃(tmtacn)₂](PF₆)₂ (1), that Lewis acids commonly used to enhance catalytic activity, e.g., Sc(OTf)₃, in fact undergo hydrolysis with adventitious water to release a strong Brønsted acid. The formation of Brønsted acids in situ is demonstrated using a combination of resonance Raman, UV/vis absorption spectroscopy, cyclic voltammetry, isotope labeling, and DFT calculations. The involvement of Brønsted acids in LA enhanced systems shown here holds implications for the conclusions reached in regard to the relevance of direct LA-metal oxido interactions under catalytic conditions.