CO2-Protected Amine Formation from Nitrile and Imine Hydrogenation in Gas-Expanded Liquids

Abstract

Gas-expanded liquids are tunable media for reaction and separation. We report that gas-expanded liquids under CO2 pressure are unique media for amine formation and separation. In the heterogeneous hydrogenation of benzonitrile and phenylacetonitrile with NiCl2/NaBH4 in CO2-expanded ethanol, the primary amines are protected by CO2 so that the yield of the primary amines is greatly increased and the production of the secondary amines is effectively suppressed. In the homogeneous hydrogenation of benzonitrile and phenylacetonitrile with RhH(P-i-Pr3)3 and benzophenone imine with Rh(1,5-C8H12){P(C6H5)3}2]PF6 in CO2-expanded tetrahydrofuran, the primary amines are separated in situ in the form of solid carbamic acids and/or ammonium carbamates with increased yield while the catalyst remains in the solution. These results demonstrate the potential for using modest pressures of CO2 to facilitate reactions as well as to separate products.