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 sp3 C–H bond oxidation with m-CPBA in the presence of the heterometallic pre-catalyst [CoIII4FeIII2O(Sae)8]·4DMF·H2O (1) (H2Sae = 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 × 104 (at 50 °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 × 10−5 M s−1. The reaction mechanism was investigated by means of combined kinetic studies (including isotope effects), isotopic labeling (18O2, H218O, D2O), ESI-MS spectroscopy and DFT 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 18O-tests should be treated cautiously when m-CPBA is used as the oxidant. This journal is © The Royal Society of Chemistry

Nesterova O.V.1 , Kuznetsov M.L.1 , Pombeiro A.J.L. 1, 2 , Shul'pin G.B. 3, 4 , Nesterov D.S. 1
Royal Society of Chemistry
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  • 1 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, 1049-001, Portugal
  • 2 Peoples' Friendship University of Russia (RUDN University), Research Institute of Chemistry, Moscow, 117198, Russian Federation
  • 3 Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, Moscow, 119991, Russian Federation
  • 4 Chair of Chemistry and Physics, Plekhanov Russian University of Economics, Moscow, 117997, Russian Federation
Ключевые слова
Catalysis; Catalysts; Cobalt compounds; Isotopes; Metal complexes; Oxidation; Transition metals; Behaviors reaction; C-H bond; Experimental investigations; Fe-systems; Homogeneous oxidation; M-chloroperoxybenzoic acids; Mechanistics; Reaction mechanism; Theoretical investigations; Transition-metal complex; Free radicals
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