DFT Study on Mechanism of Acetylene Hydroamination Catalyzed by Metal Chloride

Hydroamination of alkynes and ammonia is a promising route to synthesize enamines. In order to explore the mechanism of the hydroamination between the carbon-carbon unsaturated bond and ammonia, the pathway for hydroamination of acetylene with ammonia in gas phase and solvent media are investigated using DFT method. Without catalysts, the reaction is prohibited due to the extremely high energy barrier of ammonia nucleophilic addition. With metal chloride catalysts (i.e., ZnCl2, AlCl3, CuCl2), the whole catalytic cycle is facile to proceed via formation of the more stable co-adorption complex C2H2-ZnCl2-NH3, C2H2-AlCl3-NH3 or C2H2-CuCl2-NH3, ammonia nucleophilic addition, the first proton transfer, the particular metal chloride migration and the second proton transfer step. The excessive ammonia molecules not only act as the nucleophile, but also an efficient proton transfer agent, substantially facilitating two proton transfer steps. In gas phase, ZnCl2 and AlCl3 finally leads to imine product, whereas CuCl2 produces enamine product due to high energy barrier of the second proton transfer step. In acetone and toluene solvents, the second proton transfer is drastically enhanced and the final product for all catalysts is imine. Thus, the metal chloride catalytic hydroamination of acetylene with ammonia is kinetically favorable.