Competitive Reaction of Axial Ligands during Biomimetic Oxygenations

Abstract

Nitrogenous bases have commonly been employed as axial ligands for metalloporphyrins in biomimetic model compounds and catalytic oxygenation chemistry. The addition of bases such as pyridines or imidazoles to metalloporphyrin-catalyzed hydrocarbon oxidation reactions is known to affect catalyst selectivity and turnover rate; this effect has been correlated with the electron-donor ability of the ligand. We have found that the role of pyridine in these reactions is far more involved than that of a simple axial ligand: pyridine is a competitive substrate and is converted in high yield to the N-oxide. Subsequently, both of these species act as ligands to the metal center. Thus, catalytic systems containing oxidizable pyridines involve complex equilibria with multiple forms of ligated catalyst, and kinetic results should be interpreted with caution. Alternatives to free pyridine were tested, including a pyridine “tail” which is covalently attached to the porphyrin.

Ionic Manganese Porphyrins with S-containing Counter Anions: Mimicking Cytochrome P450 Activity for Alkene Epoxidation

“…Expectantly, the reactions catalyzed by 3 or 4 gave low styrene conversions (73% for 3, 60% for 4) under the identical conditions (Entries 9, 13 of Table 1), indicating that sulfur in the counter anion indeed contributed to the improved activity of metalloporphyrins. These results further confirmed that S-containing anion may function as the axial ligand with advantage of stoichiometric usage, avoiding the retarded activity when much excess ligands surrounding the manganese porphyrin center [13]. It is well known that the axial ligands like nitrogenous bases, originated from the applications as the substituents for the axial cysteine thiolate (P-450) or histidine imidazole in the natural metalloproteins, were required basically in metallporphyrins catalyzed epoxidations to enhance the strength of the metal-O bond, consequently with benefits of improvements in the reactivities and selectivities of the catalysts, as well as protection against the formation of unreactive l-oxo Mn IV -porphyrin dimmers [13,27].…”

“…These results further confirmed that S-containing anion may function as the axial ligand with advantage of stoichiometric usage, avoiding the retarded activity when much excess ligands surrounding the manganese porphyrin center [13]. It is well known that the axial ligands like nitrogenous bases, originated from the applications as the substituents for the axial cysteine thiolate (P-450) or histidine imidazole in the natural metalloproteins, were required basically in metallporphyrins catalyzed epoxidations to enhance the strength of the metal-O bond, consequently with benefits of improvements in the reactivities and selectivities of the catalysts, as well as protection against the formation of unreactive l-oxo Mn IV -porphyrin dimmers [13,27]. The sulfur atom as an electron-donor also exhibited a favorable ligation to Mn ion owing to the better mixing of its orbital with manganese 3d orbital [20].…”

“…The presence of N-containing bases greatly enhance the product distribution, the rate of the overall reaction, and the stability of the catalyst toward degradation by facilitating the formation of the proposed active Mn(V)=O porphyrin intermediate, and preventing its dimerzation into an unreactive and autodestructive l-oxo Mn IV -porphyrin dimmer [12]. Nevertheless, the problems coming from this strategy are to form inactive 6-coordinate bis-ligated complex in the presence of excess axial ligands, which may result in retardation in reaction rates, and the sacrifice of axial ligands (pyridine/imidazole or its derivatives) by the oxidants, which is the reason for excess axial ligands required [13]. The way to circumvent such drawback is to attach the oxidation-tolerant axial ligands to metalloporphyrins stoichiometrically [14][15][16][17].…”

Molecular Iron-Based Oxidants and Their Stoichiometric Reactions

“…Most Fe(IV)-oxo complexes have been prepared using PhIO, or a derivative, as oxidant. This strategy has also commonly been employed for the preparation of the "high-valent" complexes of other first row transition metal ions, e.g., Cr(V)oxo [96], Co(IV)oxo [97], Mn(IV)oxo [98], and Mn(V)oxo [99]. All these species are formed in accordance to the general reaction Scheme 15.…”

Interfaces e dispositivos baseados em porfirinas supramoleculares

“…Mais recentemente, Collman et alli têm voltado os esforços na síntese e exploração de análogos estruturais e funcionais com estruturas bastante próximas ao do centro da citocromo c oxidase [316,[333][334][335][336][337][338][339][340][341][342] . Nesse caso, o papel do centro bimetálico e dos efeitos cooperativos da estrutura orgânica no ambiente ao redor do sítio de coordenação do O 2 vêm sendo investigados.…”