Rapid, Reversible Oxygen Atom Transfer between an Oxomanganese(V) Porphyrin and Bromide: A Haloperoxidase Mimic with Enzymatic Rates

Abstract

Synthetic manganese porphyrins and related systems have been used extensively in chemical modeling of biological monooxygenation reactions catalyzed by heme proteins. [1] They are also versatile catalysts for the oxygenation of alkanes, alkenes, and nitrogen-and sulfur-containing compounds using oxygen donors such as iodosylbenzene, sodium hypochlorite, alkyl, aryl, and hydrogen peroxide, amine Noxides, and molecular oxygen. [2] Only recently has the key oxomanganese(v) intermediate been well characterized. [3] Here we report that the oxomanganese(v)-5,10,15,20-tetrakis(N-methyl-2-pyridyl)porphyrin (1) can efficiently transfer its oxo ligand to bromide ion, and that this oxo transfer is rapid and reversible. [Eq.(1)]The forward reaction mimics the halide oxidation reaction catalyzed by haloperoxidases, [4] while the reverse reaction is the catalyst activation step in substrate oxygenation by manganese porphyrins. This well-behaved equilibrium allows the assignment of a free energy change for the reaction depicted in Equation (1).OxoMn V TM-2-PyP (1) has unusual stability in aqueous solution compared to other oxoMn V porphyrin intermediates. [3b] It can be generated by the stoichiometric reaction of Mn III TM-2-PyP [5] (2) with oxidants such as HSO 5 À , m-CPBA (chloroperoxybenzoic acid), and OCl À . We have found that hypobromite, a weaker oxidant, [6] is also able to generate 1 smoothly. Figure 1 a shows the reaction between 5 mm 2 and 50 mm HOBr/OBr À[7] at pH 8.5 monitored by stopped-flow spectrophotometry. [3a,b] Clear isosbestic points were observed at 392, 444, and 558 nm. Remarkably, the identical isosbestic behavior was also found for the reverse reaction, oxoMn V Br À at higher bromide concentration. Typical spectral changes observed for the oxo-transfer reaction from Figure 1. Time-resolved UV/Vis sepctra for the reaction of a) 5 mm Mn III TM-2-PyP (2) and 50 mm HOBr/OBr À ; b) 5mm Mn V TM-2-PyP (1) and 50 mm Br À at pH 8.5 (10 mm Na 2 B 4 O 7 /H 2 SO 4 buffer). For both reactions there were 60 scans in 120 ms. Every fourth scan is shown.1 to Br À are shown in Figure 1 b. The generation of hypobromite, which is favored by excess bromide and lower pH, was confirmed by observing the diagnostic bromination reaction of phenol red. [8] The pH dependence of the rate of oxo transfer to bromide was examined between pH 5.2 and 9.0 (I 0.25 m NaClO 4 ). The reaction was found to be first-order in both oxoMn V and Br À , and independent of the buffer concentration. Kinetic profiles were obtained by monitoring oxoMn V (1) at 433 nm. Pseudo-first-order fitting of the kinetic data to a single exponential was carried out with at least six concentrations of Br À at each pH value. The apparent second-order rate constant k app was calculated from the slope of the linear plot of k obs versus C Br À . Our results show that 1 was nearly as effective an oxygen donor to Br À (3.8 Â 10 5 m À1 s À1 at pH 7.0) as myeloperoxidase compound I (1.1 Â 10 6 m À1 s À1 at pH 7.0), [9] and much more effective than vanadium bromopero...

Hydrogen bond controlled formation of trans-dihydroxo porphyrinato platinum(IV) complexes: Synthesis, characterization and catalytic activity in olefin epoxidation

“…Investigations in this area have been concentrated on the mechanism of the function of heme-containing enzymes such as cytochrome P-450 by designing metalloporphyrins as models [22][23][24]. Considerable attention has been paid to the oxidation of hydrocarbons catalyzed by Fe-, Mn-, and Ru-porphyrins [1,15,[25][26][27][28][29]. Although platinum porphyrins have been studied as oxygen sensing probes [30][31][32], photosensitizers [33][34][35], potential antitumor agents [36], and molecular conductors [37,38], there aren't any studies on platinum porphyrins as oxidation catalysts.…”

Oxygen Activation Chemistry of Pacman and Hangman Porphyrin Architectures Based on Xanthene and Dibenzofuran Spacers

“…Lastly, we also examined the stereochemistry of the epoxidation of cis-stilbene by 33 (202)(203)(204)(205). The high cis-epoxide/trans-epoxide ratio of 6 AE 1 obtained is consistent with a predominant oxomanganese(V) porphyrin oxidizing agent (206)(207)(208)(209).…”

Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

“…Meunier et al have described the chlorination of dimedone by the water-soluble porphyrin Mn(III)TMPS supported on a polyvinylpyridinium polymer [32]. The direct oxidation of bromide by oxygen atom transfer from oxo-Mn(V) complexes has been observed in our laboratory as well as the reverse reaction, i.e., oxidation of the Mn(III) porphyrin with HOBr/ OBr À [33,34]. Recently, Naruta et al [35] reported the formation and characterization of a dinuclear oxo-Mn(V) porphyrin complex.…”