Toward a Low-Barrier Transition-Metal-Free Catalysis of Hydrogenation Reactions: A Theoretical Mechanistic Study of HAlX_4-Catalyzed Hydrogenations of Ethene (X = F,Cl,and Br)

摘要

Ab initio molecular orbital theory at the MP2/311+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 HAlX_4 (X = F,Cl,and Br) has been examined.Both concerted pathways and stepwise pathways involving CH_3-CH_2X-rype intermediates have been characterized.The former are energetically preferred in the case of the HX- and (HX)_2-catalyzed reactions.However,for the HAlX_4-catalyzed hydrogenations,concerted and stepwise mechanisms are found to have similar barriers.The HAlX_4 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).