Rearrangement of tertiary amine oxides—X : The mechanism of the reaction of 4-picoline N-oxide with n-butyric anhdyride

Abstract

The reaction of 4-picoline N-oxide with n-butyric anhydride has been investigated using 18O as tracer. The main products, a mixture of 3-butyroxy-4-methylpyridine and 4-butyroxy-methylpyridine, obtained by the reaction of 4-picoline N-oxides and n-butyric anhydride uniformly labeled by 18O have been subjected to 18O-analysis. When no solvent is used, the 18O concentration of the ester mixture is nearly identical with that expected when the reaction proceeds via an intermolecular process. Whereas, when xylene is used as solvent, the concentration of 18O of the ester mixture becomes quite small and approaches the value expected from the radical cage mechanism. However, n-butyroxy radicals undergo a radical transfer reaction with n-butyric acid, while the 18O concentration of the ester mixture decreases substantially when DPPH is added to the reaction mixture of 4-picoline N-oxide and n-butyric anhydride. These observations and others, support the mechanism of the reaction of 4-picoline N-oxide and n-butyric anhydride as a combination of an intermolecular heterolytic process, an intermolecular homolytic reaction and a radical cage path, and the ratio of these processes changes with a change of solvent.