Kinetics and Mechanism of Oxidation Reactions of Porphyrin−Iron(IV)−Oxo Intermediates

Abstract

The kinetics of the reactions of three porphyrin-iron(IV)-oxo derivatives with alkenes and benzylic alcohols were measured. The iron-oxo systems studied were 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (2a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(IV)-oxo (2c). Species 2 were stable for hours at room temperature as dilute solutions in acetonitrile and reacted hundreds to thousands of times faster in the presence of high concentrations of substrates. Typical second-order rate constants determined from pseudo-first-order kinetic studies are 1-2 x 10(-2) M(-1) s(-1) for reactions with styrene and 3 x 10(-2) M(-1) s(-1) for reactions with benzyl alcohol. The reactivity order for the iron-oxo species was 2a > 2b > 2c, which is inverted from that expected on the basis of the electron demand of the porphyrin macrocycles, and the oxidation reaction was suppressed when excess porphyrin-iron(III) complex was added to reaction mixtures. These observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be an iron(IV)-oxo porphyrin radical cation, that is the true oxidant in the reactions. Analyses of the kinetics of oxidations of a series of para-substituted benzylic alcohols with Hammett sigma+ -substituent constants and with a dual-parameter method developed by Jiang (Jiang, X. K. Acc. Chem. Res. 1997, 30, 283) indicated that considerable positive charge developed on the benzylic carbons in the oxidation reactions, as expected for electrophilic oxidants, and also that substantial radical character developed on the benzyl carbon in the transition states.

Selective photocatalytic epoxidation of cyclooctene by molecular oxygen in the presence of porphyrin sensitizers

“…However, high yields of ClMnTDCPP, ClMnTPFPP and ClFeTPFPP are explained based on the existence of a parallel route, type III mechanism in addition to type II for metalloporphyrins. The trend in the stability of metalloporphyrins and the yield of the cyclooctene oxide is similar to the oxidation product obtained via the formation of a p-cation radical oxo species formed in the cytochrome P-450 model system [27]. The reactivity order of the metals is also in accord with ease of activity of formation of high valent metal oxo species [28].…”

Transition Metal Complexes and the Activation of Dioxygen

“…It was proposed that these results together seemed to indicate a disproportionation of the formed Fe(IV)-oxo species to give the corresponding Fe(IV)-oxo π* cation-radical intermediate and the Fe(III) precursor, and that in fact the Fe(IV)-oxo π* cation-radical species was the active oxidizing agent. However, this hypothesis is speculative, as this species was not directly observed [91].…”

Production of a Putative Iron(V)−Oxocorrole Species by Photo-Disproportionation of a Bis-Corrole−Diiron(IV)−μ-Oxo Dimer: Implication for a Green Oxidation Catalyst

“…True metal(V)-oxo complexes, which are generally rare and elusive, are known to be more reactive than the metal(IV)-oxo ligand radical cation valence tautomers. For example, porphyrinmanganese(V)-oxo complexes display higher reactivity than the analogous iron(IV)-oxo porphyrin radical cation,5,16-18 and a recently reported oxoiron(V) complex supported by a tetraanionic ligand showed unprecedented reactivity 19. We reported photo-induced ligand cleavage reactions that generated highly reactive corrole- and porphyrin-iron-oxo intermediates that are best described as iron(V)-oxo species 20-22.…”

“…Most commonly, a macrocycle-iron(III) species is oxidized to an iron-oxo species with a formal oxidation state on iron of +5, and the intermediate high-valent iron-oxo complex then oxidizes an organic substrate. Catalytic oxidations accomplished with iron in a formal +4 oxidation state, which might disproportionate to give a higher valent iron-oxo species,5 are possible. In this regard, cofacial bis -porphyrin-diiron(III) μ-oxo complexes6,7 have drawn considerable attention because these systems can effect organic oxidations in catalytic cycles using molecular oxygen as a terminal oxidant and photo-disproportionation of the μ-oxo dimer to give transient porphyrin-iron(IV)-oxo species 8-11…”