Synthesis of primary amines from the renewable compound citronellal via biphasic reductive amination

Abstract

The reductive amination of the natural product citronellal with ammonia is presented as a new and atom economic way to its primary amine derivatives. The aqueous ammonia phase contains the homogeneous catalytic system [Rh(cod)Cl]2/TPPTS in a biphasic solvent system, whereas the starting material and the products remain in the apolar solvent phase. This concept supresses side reactions effectively, achieving a high yield of primary amines of up to 87%. Systematic investigations demonstrate that the cleavage of the secondary imine as an undesired by-product is necessary in achieving high selectivites, which can be controlled by the reaction conditions. Surfactants, ionic liquids or native cyclodextrins and their derivates prove to be useful phase transfer agents for optimising the interaction between the organic and the aqueous phase. The use of the ionic liquid [DecMIM]Br and the cyclodextrin derivative methyl-β-cyclodextrin provided especially fast and accurate phase separation.