The mechanism of a carbonyl-ene addition reaction catalyzed by a bis(oxazoline) copper (II) complex has been studied using DFT methods. We find that the reaction proceeds by a stepwise mechanism with very low barriers and have identified the role of the metal catalyst. We find that the more computationally economic ONIOM method gives accurate geometries for the stationary structures on the potential energy surface but that accurate energetics must be calculated at the full DFT level.