OXIDATION OF OXYGEN-LABELED PHENYLACETIC ANHYDRIDE BY PYRIDINE N-OXIDE . RELATIVE NUCLEOPHILICITIES OF PYRIDINE AND PYRIDINE N-OXIDE

The reaction of pyridine N-oxide with phenylacetic anhydride, uniformly labeled with 18O, produces unlabeled benzaldehyde, indicating that the oxygen atom of the product is derived from the amine oxide. When this reaction is conducted in a large excess of pyridine, the ratio of benzaldehyde to N-benzylpyridinium ion produced is 0.12-0.23, indicating that pyridine N-oxide is substantially more nucleophilic than pyridine toward the intermediate. By the use of competition experiments, it was determined that the ratios of nucleophilic attack of pyridine to that of the N-oxide on methyl iodide, benzhydryl bromide, and p-methoxybenzhydryl bromide are 8-15, 5.7, and 3.8, respectively. It is concluded that attack by pyridine N-oxide becomes relatively more favorable as the positive charge on the electrophile increases. This implies that the carbon atom of the intermediate which is to become bonded to the N-oxide group in the oxidative decarboxylation bears a high positive charge. It is suggested that the α-carboxybenzyl cation (3) or its conjugate base is the intermediate which is directly attacked by pyridine N-oxide. © 1969, American Chemical Society. All rights reserved.