Hydrogen atom abstraction by a high-valent manganese(V)-oxo corrolazine

Abstract

High-valent metal-oxo complexes are postulated as key intermediates for a wide range of enzymatic and synthetic processes. To gain an understanding of these processes, the reactivity of an isolated, well-characterized Mn(V)-oxo complex, (TBP8Cz)MnVO (1), (TBP8Cz = octakis(para-tert-butylphenyl)corrolazinato(3-)) has been examined. This complex has been shown to oxidize a series of substituted phenols (4-X-2,6-t-Bu2C6H2OH, X = C(CH3)3 (3), H, Me, OMe, CN), resulting in the production of phenoxyl radicals and the MnIII complex [(TBP8Cz)MnIII] (2). Kinetic studies have led to the determination of second-order rate constants for the phenol substrates, which give a Hammett correlation ((log k''x/k''H) vs sigmap+) with rho = -1.26. A plot of log k versus BDE(O-H) also reveals a linear correlation. These data, combined with a KIE of 5.9 for 3-OD, provide strong evidence for a concerted hydrogen-atom-abstraction mechanism. Substrates with C-H bonds (1,4-cyclohexadiene and 9,10-dihydroanthracene) are also oxidized via H-atom abstraction by 1, although at a much slower rate. Given the stability of 1, and in particular its low redox potential, (-0.05 V vs SCE), the observed H atom abstraction ability is surprising. These findings support a hypothesis regarding how certain heme enzymes can perform difficult H-atom abstractions while avoiding the generation of high-valent metal-oxo intermediates with oxidation potentials that would lead to the destruction of the surrounding protein environment.