Stereochemistry of Hydrodenitrogenation: The Mechanism of Elimination of the Amino Group from Cyclohexylamines over Sulfided Ni–Mo/γ-Al2O3 Catalysts

The hydrodenitrogenation of cyclohexylamine and of the diastereomers of 2-methylcyclohexylamine and 2,6-dimethylcyclo-hexylamine was studied at 200 to 350°C and 50 bar pressure over a sulfided Ni–Mo/γ-Al2O3 catalyst. The removal of the amino group from cyclohexylamines occurs primarily by means of a β elimination mechanism. Elimination of a β hydrogen atom attached to a tertiary carbon atom is faster than that of a β hydrogen atom attached to a secondary carbon atom. The rate of elimination also depended on the stereochemical configuration of the amino group with respect to the β hydrogen atoms. Elimination was more rapid when the configuration of the amino group was anti periplanar to the β hydrogen atom (in a chair conformation) than when it was syn periplanar (in a boat conformation). This is consistent with an E2 elimination mechanism. The relative elimination rates of all the diastereomers of the substituted cyclohexylamines were rationalized in terms of the stereochemical relation between the amino group and a β hydrogen atom, the number and type of β hydrogen atoms, and the energy required to attain the required chair/boat conformation(s). In contrast to previous proposals for anti elimination of H2O from alcohols, it was shown that an anti elimination mechanism is possible on the surface of heterogeneous catalysts if SH or OH groups protrude from the catalyst surface.