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.