Toward a Low-Barrier Transition-Metal-Free Catalysis of Hydrogenation Reactions:  A Theoretical Mechanistic Study of HAlX<sub>4</sub>-Catalyzed Hydrogenations of Ethene (X = F, Cl, and Br)

2000-07-18T00:00:00Z (GMT) by Stefan Senger Leo Radom
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)<sub>2</sub> and HAlX<sub>4</sub> (X = F, Cl, and Br) has been examined. Both concerted pathways and stepwise pathways involving CH<sub>3</sub>CH<sub>2</sub>X-type intermediates have been characterized. The former are energetically preferred in the case of the HX- and (HX)<sub>2</sub>-catalyzed reactions. However, for the HAlX<sub>4</sub>-catalyzed hydrogenations, concerted and stepwise mechanisms are found to have similar barriers. The HAlX<sub>4</sub> species are found to be very effective hydrogenation catalysts, reducing the barrier for the hydrogenation of ethene from the value of 367 kJ mol<sup>-1</sup> in the uncatalyzed process to less than 100 kJ mol<sup>-1</sup> for all the halogens (X).