Asymmetric synthesis of ( S)-alkylamines via reductive transamination of ketones over carbon-supported palladium catalysts

Abstract

Methylethylketone (MEK) and methoxyacetone (MEOAC) were transaminated with benzylamine (BzNH 2) or l-(−)-α-methylbenzylamine [ l-(−)-α-MeBzNH 2] over Pd/C catalysts in the presence of chiral modifiers ( l-alanine, l-alaninol, l-phenylalaninol, l-lysine and methyl-, t-butyl- and benzyl-esters of l-alanine) introduced either by incipient wetness impregnation or by equilibrated impregnation onto the catalyst surface. In the first step of the transamination the Schiff base was formed from the ketone and aralkylamine which was then hydrogenated to secondary amine in the second step. In the third step of the transamination the hydrogenolysis of the secondary amine resulting in primary alkylamine and a hydrocarbon was carried out. In the transamination of both MEK and MEOAC on Pd/C catalysts the highest enantiomeric excess was observed in cyclohexane (ee=−20%–21%) using l-alanine alkyl esters or l-alaninol as modifier. In different solvents the ee of the corresponding primary amine increased in the order: MeOH<Dioxan<H 2O<CH. Upon using ( l)-(−)-α-MeBzNH 2, as a chiral transaminating agent, ( S)-methoxyisopropylamine was obtained, with ee value around 70%. The conformational analysis of N-(methoxy-2-propylidene)-methylbenzylamine strongly supports the validity of Prelog's rule in the enantio- or diastereoselective hydrogenation of Schiff base over heterogeneous catalysts.