Competitive reaction of axial ligands during biomimetic oxygenations

Nitrogenous bases have commonly been employed as axial ligands for metallop orphyrins in biomimetic model compounds and catalytic oxygenation chemistry . The addition of bases' such as pyridines or imidazoles to metalloporphyri n-catalyzed hydrocarbon oxidation reactions is known to affect catalyst sel ectivity and turnover rate; this effect has been correlated with the electr on-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: py ridine is a competitive substrate and is converted in high yield to the N-o xide. Subsequently, both of these species act as Ligands to the metal cente r. Thus, catalytic systems containing oxidizable pyridines involve complex equilibria with multiple forms of ligated catalyst, and kinetic results sho uld be interpreted with caution. Alternatives to free pyridine were tested, including a pyridine "tail" which is covalently attached to the porphyrin.