Oxidation of m-chlorobenzoic acid by Mn(V)O complexes

Manganyl [Mn(V)O] complexes have garnered considerable attention over the past 15 years as the reactive species in asymmetric olefin epoxidation, as catalysts for green chemistry and as possible intermediates in photosynthetic water oxidation chemistry. One method of preparing Mn(V)O complexes is by the reaction of an oxo-transfer oxidant with a precursor Mn(III) compound. Different oxidants have been employed to serve this function including sodium hypochlorite (NaOCl), iodosylbenzene (PhIO) and meta-chloroperbenzoic acid (m-CPBA). In this report we present the reactivity of high valent manganese complexes with meta-chlorobenzoate (m-CBA) and show that these manganese compounds are capable of carrying out single electron oxidations of the oxidant to form m-CBA radicals. This organic radical is unstable and decomposes to form carbon dioxide and, ultimately, m-chlorophenol. Two manganyl compounds are shown to carry out this chemistry including one containing the oxidation resistant ligand [Mn(V)O(HMPAB)]− (where H4HMPAB=1,2-bis(2-hydroxy-2-methylpropanamido)benzene) and [Mn(V)O(salen*)]+ (where H2salen*=[(R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine]) which has been a catalyst in Katsuki–Jacobsen chiral epoxidation reaction. We demonstrate that both Mn(V) complexes are unstable in the presence of m-CBA in favor of lower valent Mn compounds.