Detection and Characterization of an Oxomanganese(V) Porphyrin Complex by Rapid-Mixing Stopped-Flow Spectrophotometry

Abstract

The first detection and characterization of oxomanganese(V) porphyrin complexes under ambient catalytic conditions is described. The reaction of (tetra-(N-methylpyridyl)porphyrinato)manganese(III) [Mn(III)TMPyP] with a variety of oxidants such as m-chloroperoxybenzoic acid (m-CPBA), HSO5 -, and ClO- has been shown to produce the same, short-lived intermediate (1) by stopped-flow spectrophotometry. The Soret maximum of 1 was found at 443 nm, intermediate between that of oxomanganese(IV) (428 nm) and Mn(III)TMPyP (462 nm), thus facilitating its detection. The rate of formation of 1 from Mn(III)TMPyP followed second-order kinetics, first order in Mn(III) porphyrin and first order in oxidant. The rate constants have the following order: m-CPBA (2.7 × 107 M-1 s-1) > HSO5 - (6.9 × 105 M-1 s-1) ≈ ClO- (6.3 × 105 M-1 s-1). Once formed, the intermediate species 1 was rapidly converted to oxoMn(IV) (2) by one-electron reduction with a first-order rate constant of 5.7 s-1. The oxoMn(IV) species 2 was relatively stable under the reaction conditions, decaying slowly to Mn(III)TMPyP with a first-order rate constant of 0.027 s-1. The identity of 1 as an oxomanganese(V) complex was indicated by its reactivity. The one-electron reduction of 1 to oxoMn(IV) was greatly accelerated by nitrite ion (k = 1.5 × 107 M-1 s-1). However, the reaction between nitrite and oxoMn(IV) is much slower (k = 1.4 × 102 M-1 s-1). The oxoMn(V) intermediate 1 was shown to be highly reactive toward olefins, affording epoxide products. By contrast, oxoMn(IV) (2) was not capable of effecting the same reaction under these conditions. In the presence of carbamazepine (3) efficient oxygen transfer from the highly reactive oxoMn(V) (1) to the olefin (second-order rate constant of 6.5 × 105 M-1 s-1) resulted in the conversion of 1 directly back to Mn(III)TMPyP without the appearance of the stable oxoMn(IV) intermediate 2. With m-CPBA as the oxidant in the presence of H2 18O, the product epoxide was shown to contain 35% 18O, consistent with an O-exchange-labile oxoMn(V) intermediate. Nitrite ion inhibited the epoxidation reaction competitively by one electron reduction of the oxoMn(V) intermediate to the unreactive oxoMn(IV).

Epoxidation of unfunctionalized olefins by Mn(salen) catalyst using organic peracids as oxygen source: A theoretical study

“…Unfortunately, recent experimental mechanistic studies of the Mn III (salen)-catalyzed epoxidation by peracids lack solid kinetics data due to very fast (within 1 min) epoxidation rates even at Ϫ78°C (14,(16)(17)(18)(19)35). Nevertheless, for closely related Mn III (porphyrin) catalysts the rates of the Mn V oxo species formation and͞or subsequent epoxidation have been estimated in water and acetonitrile at room temperature and agree with our predictions (51,52).…”

Spectroscopic observation of iodosylarene metalloporphyrin adducts and manganese(V)-oxo porphyrin species in a cytochrome P450 analogue

“…It exhibits promising catalytic behaviour for a host of oxidative reactions, and so a primary goal of cytochrome P450 research is in the application of these enzymes, or their derivatives, in catalytic oxidation of unactivated C-H bonds, such as the hydroxylation of cyclohexane. Many synthetic metalloporphyrin model compounds have been synthesized to probe the mechanisms of the behaviours of cytochrome P450 oxygenases [3][4][5][6][7][8][9][10][11][12][13] , but the detailed pathways are still elusive, especially for the steps involving highly reactive intermediates. For a long time, it has been believed that oxoiron (IV) porphyrin p-cation radical (named as Compound I) is the unique reactive intermediate in cytochrome P450 catalysed oxidative reactions (Fig.…”

Tetra-crowned porphyrin as P450 biomimetic model for carbamazepine oxidation

“…The latter compound is then hydrolyzed to carbamazepine trans-diol (CBZ-DiOH) by the enzyme epoxide hydrolase (Figure 3 Carbamazepine oxidation catalyzed by metalloporphyrins has already been investigated in homogeneous medium 21,22 and supported systems. 23 However, to the best of our knowledge, studies using crowned metaloporphyrins as catalysts for the oxidation of this drug have not yet been reported.…”