Novel Mn(III)chlorins as versatile catalysts for oxyfunctionalisation of hydrocarbons under homogeneous conditions

The novel fluorinated metallochlorin 5,10,15,20-tetrakis(pentafluorophenyl)-tetrahydro-IH-N-methyl-pyrrolo[3,4-b]-porphyrinato manganese(III), Mn(chlor)-1, and its methylated derivative, Mn(chlor)-2, have been studied as catalysts in the oxyfunctionalisation of cyclohexane with two oxidants, namely iodosylbenzene (PhIO) and H2O2. For comparison reasons two metalloporphyrins have been also used: the neutral 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato manganese(III), Mn(porph)-1, and the cationic 5,10,15,20-tetrakis(2,3,5,6-tetrafluoro-4-trimethylammoniumphenyl)porphyrinato manganese(III), Mn(porph)-2. It has been possible to verify a different reactivity in the H2O2-oxyfunctionalisation of alkanes from studies using the radical inhibitor bromotrichloromethane and competitive cyclooctane/ciscyclooctene oxidations. The results suggest that the formation of alcohols and ketones from the oxidation of cyclohexane occurs mainly from a free alkyl radical mechanism, which requires a Mn-v = O species to abstract a hydrogen atom from the alkane to generate the alkyl radical. On the other hand, the Mn(chlor)/H2O2 system possibly yields, in a first step, the Mn-III-OOH species, but this is not reactive enough to generate Mn-v = O. The Mn(chlor) hydroperoxy complex shows low efficiency in the production of the alkyl radical, leading to small amounts of alcohol and ketone. However, it seems to be an efficient species for selective epoxidation, as can be observed in the competitive cyclooctane/cis-cyclooctene oxidation reaction. (c) 2005 Elsevier B.V. All rights reserved.