Toward a Low-Barrier Transition-Metal-Free Catalysis of Hydrogenation Reactions:  A Theoretical Mechanistic Study of HAlX₄-Catalyzed Hydrogenations of Ethene (X = F, Cl, and Br)

Ab initio molecular orbital theory at the MP2/6-311+G(3df,2p)//B3-LYP/6-31G(d) level has been used to study the transition-metal-free catalysis of the hydrogenation of ethene. Catalysis by HX, (HX)₂ and HAlX₄ (X = F, Cl, and Br) has been examined. Both concerted pathways and stepwise pathways involving CH₃CH₂X-type intermediates have been characterized. The former are energetically preferred in the case of the HX- and (HX)₂-catalyzed reactions. However, for the HAlX₄-catalyzed hydrogenations, concerted and stepwise mechanisms are found to have similar barriers. The HAlX₄ species are found to be very effective hydrogenation catalysts, reducing the barrier for the hydrogenation of ethene from the value of 367 kJ mol^-1 in the uncatalyzed process to less than 100 kJ mol^-1 for all the halogens (X).