Catalytic activities of Schiff base transition metal complexes

Ranging from the oxidative conversion of water to O(2) to the elegant hydroxylation of olefins and to oxidative dehydrogenation of alcohols Ru-mediated oxidations are finding increasing application due to the unique properties of this extremely versatile transition metal, whose oxidation state can vary from -II to +VIII. Covering recent developments in both homogeneously and heterogeneously catalysed oxidations (in liquid-phase as well as in novel reaction media), this tutorial review aims to provide investigators with a general picture of the chemical and structural origins of the excellent performance of many ruthenium catalysts and to promote further advancement that, it is envisaged, will soon benefit society at large.

Ru-based oxidation catalysis.

The reactions catalyzed by the cytochrome P450 family of enzymes have challenged and intrigued chemists for more than two decades. Alkane hydroxylation and olefin epoxidation, particularly, have attracted a sustained worldwide effort, the allure deriving from both a desire to understand the details of biological oxygen activation and transfer and, as well, the sense that catalysts for the practical application of these principles to organic synthesis and to large-scale process chemistry could be of considerable economic value. Cytochrome P450 is able to incorporate one of the two oxygen atoms of an O2 molecule into a broad variety of substrates with concomitant reduction of the other oxygen atom by two electrons to H2O.1 Cytochrome P450 enzymes have been isolated from numerous mammalian tissues (e.g., liver, kidney, lung, intestine, adrenal cortex), insects, plants, yeasts, and bacteria.2 Cytochrome P450 is known to catalyze hydroxylations, epoxidations, N-, S-, and O-dealkylations, N-oxidations, sulfoxidations, dehalogenations, and other reactions.1 The reactive site of all of these enzymes is extraordinarily simple, containing only an iron protoporphyrin IX (1) (Fig. 1) with cysteinate as the fifth ligand, leaving the sixth coordination site to bind and activate molecular oxygen. The local environment of oxygen binding and activation is also very simple, with mostly hydrophobic protein residues and a single threonine hydroxyl which is essential for catalysis for some but not all P450s. The principal catalytic cycle of cytochrome P450 has been much discussed and often reviewed, but the essential features have been agreed upon now for some time. The essential steps involve (Scheme I): (1) binding of the substrate, (2) reduction of the ferric, resting cytochrome P450 to the ferrous state, (3) binding of molecular oxygen to give a ferrous cytochrome P450-dioxygen complex, (4) transfer of the second electron to this complex to give a peroxoiron(III) complex, (5) protonation and cleavage of the O-O bond with the concurrent incorporation of the distal oxygen atom into a molecule of water and the formation of a reactive iron-oxo species, (6) oxygen atom transfer from this oxo complex to the bound substrate, and (7) dissociation of the product. While the first three steps of the enzymatic process have been monitored spectroscopically, the transfer of the second electron, the O-O bond cleavage, and the oxidation of the substrate occur too rapidly and have yet to be observed.3

Models and Mechanisms of Cytochrome P450 Action

Manganese(III) 5-(pentafluorophenyl)-10,15,20-tri(2,6-dichlorophenyl)porphyrin, Mn(PFTDCPP), and manganese(II) 2,3,7,8,12,13,17,18-octachloro-5-(pentafluorophenyl)-10,15-20-tri(2,6-dichlorophenyl)porphyrin, Mn(PFTDCCl 8 PP), have been synthesised and used as catalysts in hydrocarbon oxidations by iodosylbenzene and hydrogen peroxide both in solution and covalently bound to aminopropylated silica. The former shows higher efficiency in the epoxidation of alkenes by iodosylbenzene, whereas the perchlorinated manganese porphyrin is more efficient in the hydroxylation of alkanes by this oxidant. The supported manganese(III) porphyrin show the same activity as its homogeneous analogue. With hydrogen peroxide as oxygen donor, Mn(PFTDCPP) is a stable and effective catalyst in the presence of imidazole. The perchlorinated analogue is a poor catalyst with this oxidant. The eight additional chlorine atoms on the porphyrin ring stabilise Mn(II) and unfavour the formation of the active species, Mn V O.

Oxidation of alkanes and alkenes by iodosylbenzene and hydrogen peroxide catalysed by halogenated manganese porphyrins in homogeneous solution and covalently bound to silica

Effective systems for the catalytic oxygenation of hydrocarbons have been developed based on metalloporphyrins containing iron, manganese and ruthenium. These metalloporphyrin catalysts were inspired by studies of the mechanism of the heme oxygenase cytochrome P450 and related enzymes. Oxometalloporphyrins, which have now been spectroscopically characterized, have very high reactivity as oxygen transfer agents. A combination of experimental and computational methods has begun to provide an understanding of the structure and reactivity of these oxometalloporphyrin complexes and the mechanisms of oxygen transfer reactions they catalyze.

Reactivity and mechanisms of metalloporphyrin‐ catalyzed oxidations

The halogenated porphyrin, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyriato-iron(III) chloride, [Fe(TFPPBr8)Cl], catalyzes the oxidation of cyclohexene in the presence of molecular oxygen or iodosobenzene. With PhIO, 77% epoxide is observed, consistent with a mechanism involving a high-valent metal-oxo species. With dioxygen, however, allylic alcohol and ketone are observed, suggesting a different mechanism. The relatively high activity of the [Fe(TFPPBr 8 )Cl] O 2 system suggests that the reaction involves the formation and decomposition of alkyl peroxides.

On the mechanism of catalytic alkene oxidation by molecular oxygen and halogenated iron porphyrins

Modeling studies support a radical-chain autoxidation mechanism for the aerobic oxidation of isobutane catalyzed by halogenated porphyrin iron complexes. A key role of the electronegative halogen substituents is to increase the FeIII / IIredox potential and thereby accelerate oxidation of the intermediate t-butyl hydroperoxide by (porph)FeIII. The electronic structures of electronegatively substituted salen iron complexes have been characterized by several techniques, and related to changes in catalytic activity for oxidation of cyclohexene. The crystal and molecular structure of [Fe((NO2)4salen)(H2O)Cl] is reported.

Eva R. Birnbaum

Papers

On the mechanism of catalytic alkene oxidation by molecular oxygen and halogenated iron porphyrins

How do electronegative substituents make metal complexes better catalysts for the oxidation of hydrocarbons by dioxygen

Catalysis of aerobic olefin oxidation by a ruthenium perhaloporphyrin complex