Hydrocarbon Functionalization by the (Iodosylbenzene)manganese(IV) Porphyrin Complexes from the (Tetraphenylporphinato)manganese(III)-Iodosylbenzene Catalytic Hydrocarbon Oxidation System. Mechanism and Reaction Chemistry

Article abstract of DOI:10.1021/ja00349a025

The two types of complexes isolated from the reaction of (tetraphenylporphinato)manganese(III) derivatives, XMn^IIITPP, with iodosylbenzene - IVTPP(OIPh)>2O, 1, X = Cl^- or Br^-, and IVTPP>2O, 2, X = N3^- - are capable of oxidizing alkane substrates in good yields at room temperature.Several lines of evidence establish the intermediacy of free alkyl radicals in the reactions of 1 and 2 with alkanes.Oxygen exchange with water in both the iodosyl (Mn-O-I) and μ-oxo (Mn-O-Mn) moieties of 1 suggests the formation of oxo manganese porphyrin complexes from these moieties.Hydrogen abstraction from the alkane substrate by an oxo manganese porphyrin intermediate is postulated to be mechanism for reaction of 1 and 2 with alkanes.Observation of a monomeric manganese(IV) porphyrin intermediate by EPR spectroscopy during the reactions of 1 with alkanes is consistent with the formation of a hydroxymanganese(IV) porphyrin complex resulting from substrate hydrogen abstraction by an oxo intermediate.The formation of RX product from oxidation of RH by 1 has been determined to result from ligand-transfer oxidation of free alkyl radicals by the porphyrin complexes in solution.Through competition reactions and time-dependent product formation studies, ligand-transfer oxidation by XMn^IIITPP was found to be the major pathway for RX production.Observation of Mn^IITPP by EPR spectroscopy during the reactions of 1 with alkanes supports this conclusion.Formation of ROH product may result from ligand-transfer oxidation of free radicals or from the collapse of an intermediate caged radical pair.The mechanism of ROH product formation in the caged radical pair is postulated to be an outer-sphere electron-transfer process due to the expected slow rate of inner-sphere ligand transfer for the high-spin d³ hydroxymanganese(IV) porphyrin complex.Thus the ability of the substrate radical to undergo electron-transfer oxidation determines the ratio of radicals that undergo cage escape to give free radicals to radicals that undergo oxidation and subsequent formation of alcohol product in the caged species.Studies with tertiary substrates support these conclusions.