Homogeneous oxidation of C-H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

Oxidations of C-H bonds with m-chloroperoxybenzoic acid (m-CPBA) catalyzed by transition metal complexes are known to proceed through a number of routes, from the non-selective free radical to selective concerted and metal-mediated ones. However, there is a lack of understanding of the m-CPBA oxidative behavior, reaction mechanisms and factors that trigger its activity. An experimental and theoretical investigation of sp(3) C-H bond oxidation with m-CPBA in the presence of the heterometallic pre-catalyst ((Co4Fe2O)-Fe-III-O-III(Sae)(8)]center dot 4DMF center dot H2O (1) (H(2)Sae = salicylidene-2-ethanolamine) and HNO3 promoter has been performed herein. The catalytic system 1/HNO3/m-CPBA allows mild hydroxylation of tertiary C-H bonds with 99% retention of stereoconfiguration of model alkane substrates, supported by high TOFs up to 2 s(-1) (for cis-1,2-dimethylcyclohexane) and TONs up to 1.4 x 10(4) (at 50 degrees C). The catalytic effect of 1 is seen at the ppm level, while 1000 ppm (0.1 mol%) loading allows 1000-fold increase of the initial reaction rate up to 9 x 10(-5) M s(-1). The reaction mechanism was investigated by means of combined kinetic studies (including isotope effects), isotopic labeling (O-18(2), (H2O)-O-18, D2O), ESI-MS spectroscopy and OFT theoretical studies. The results suggest that the main oxidation pathway proceeds through a concerted mechanism involving a cobalt-peroxo C-H attacking species or via a cobalt-oxyl species (rebound process), rather than a free-radical pathway. Remarkably, the Co(III) catalyst does not change its oxidation state during the most energetically favored pathway, consistent with a metal-ligand cooperativity. The chlorobenzene radical is responsible for H abstraction in the non-selective side route, which is efficiently suppressed by the acidic promoter. Finally, signs for slow direct oxygen exchange between m-CPBA and water in the presence of a proton or a metal complex are found, suggesting that the results of O-18-tests should be treated cautiously when m-CPBA is used as the oxidant.