| Abstract
| - We have found pyridine to play a dual role as both ligand and competitive substrate in metalloporphyrin-catalyzed hydrocarbon oxygenations. The oxidation product, pyridine N-oxide, also acts as a ligand to the metal center. These phenomena greatly complicate the kinetic profiles of these catalytic systems, and results should be interpreted with caution. Alternatives to free pyridine were tested, including a pyridine “tail” which is covalently attached to the porphyrin.
- Nitrogenous bases have commonly been employed as axial ligands for metalloporphyrins in biomimetic modelcompounds and catalytic oxygenation chemistry. The addition of bases such as pyridines or imidazoles tometalloporphyrin-catalyzed hydrocarbon oxidation reactions is known to affect catalyst selectivity and turnoverrate; this effect has been correlated with the electron-donor ability of the ligand. We have found that the role ofpyridine in these reactions is far more involved than that of a simple axial ligand: pyridine is a competitivesubstrate and is converted in high yield to the N-oxide. Subsequently, both of these species act as ligands to themetal center. Thus, catalytic systems containing oxidizable pyridines involve complex equilibria with multipleforms of ligated catalyst, and kinetic results should be interpreted with caution. Alternatives to free pyridine weretested, including a pyridine “tail” which is covalently attached to the porphyrin.
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