Investigation of the steric course of the CN bond breaking in the hydrodenitrogenation of alkylamines

The steric course of the hydrodenitrogenation reactions of 2-(R)- and 2-(S)-butylamine to 2-butanethiol and di-sec-butylamine was studied at 3 MPa and 300 °C over sulfided NiMo/γ-Al2O3. After separation of the thiol from the dialkylamine and unreacted alkylamine, the chirality of the thiol and unreacted alkylamine was determined by analysis of the diastereomers formed by reaction with Moscher's acid chloride, (S)-(+)-α-methoxy-α-(trifluoromethyl)phenylacetylchloride. The dialkylamine was analyzed by direct chiral chromatography without derivatization, using another type of chiral column. When 2-(S)-butylamine was used as reactant, the 2-butanethiol product was completely racemic, whereas the di-sec-butylamine consisted of 48% (R,S), 32% (S,S), and 20% (R,R) isomers and the 2-butylamine of 70% (S) and 30% (R) isomers. These results cannot be explained by a classic SN2 substitution mechanism. Two similar mechanisms can explain all products and their configurations, as well as the racemization of the amine reactant. In one mechanism the amine first reacts by dehydrogenation to an imine, whereas in the other the amine first reacts by electron and proton transfer to an imine cation. Thereafter, the addition of H2S or an amine molecule to the imine or to the imine cation occurs, and, after ammonia or amine elimination and hydrogenation, a thiol or a dialkylamine is formed.