On the Role of β Hydrogen Atoms in the Hydrodenitrogenation of 2-Methylpyridine and 2-Methylpiperidine

The hydrodenitrogenation (HDN) of 2-methylpyridine and its intermediate products 2-methylpiperidine, 1-aminohexane, and 2-aminohexane was studied. The presence of most intermediates could be explained by a combination of pyridine ring hydrogenation, piperidine ring opening by elimination, and nitrogen removal by elimination, as well as by nucleophilic substitution of the amino group by a sulfhydryl group, followed by elimination of H2S or hydrogenolysis of the C–S bond. Aminoalkenes, which are expected to be the primary products of the ring opening of alkylpiperidine, were not observed, probably because of fast hydrogenation to the corresponding amines. The ring opening of 2-methylpiperidine occurred preferentially between the nitrogen atom and the methylene group, rather than between the nitrogen atom and the carbon atom bearing the methyl group. This was confirmed by comparative HDN experiments of piperidine, 2-methylpiperidine, and 2,6-dimethylpiperidine. Although the methyl groups offer extra β hydrogen atoms, these primary hydrogen atoms are not used for elimination. Instead, the methyl groups hinder the adsorption leading to the elimination of the β hydrogen atoms on the side of the molecule bearing the methyl group.