Non-catalytic hydroamination of alkenes: a computational study

The detailed reaction profiles of the neutral–neutral as well as the cation–neutral direct hydroamination reactions between ethylene and ammonia are analyzed using MP2 (Full)/6-31++G(2df,2p) and B3LYP/6-31++G(2df,2p) methodologies. Analysis shows that both neutral–neutral, as well as the cation–neutral reactions are exothermic and the latter is >100 kJ/mol more exothermic than the former. Calculations show that a very large barrier height (>200 kJ/mol), and very large negative reaction entropy prevent the neutral–neutral reaction from proceeding in the forward direction. Analysis of the cation–neutral reaction, which is barrierless (the transition state is more stable than the reactants) and highly exothermic, indicates that the direct hydroamination reaction is thermodynamically attainable via a cation–neutral reaction pathway without a catalyst. Our calculations also suggest that although the cation–neutral direct hydroamination reaction is very fast, the cation of either ethylene or ammonia goes through a structural relaxation process before reacting with the other neutral reactant.