Transition Metal Mediated Epoxidation as Test Case for the Performance of Different Density Functionals: A Computational Study

Energy profiles for olefin epoxidation with the cationic [Mn(acacen‘)] + catalyst (acacen‘ = - O(CH) 3 N−C 2 H 4 −N(CH) 3 O - ) have been calculated using pure (BP86) and hybrid (B3LYP) density functional methodologies. For the reaction, triplet and quintet energy hyper surfaces have been considered. The BP86 calculations allow for a rationalization of a reaction occurring under spin conservation. On the other hand, the B3LYP calculations suggest a reaction profile involving an early spin-crossing step, strongly supporting two-state reactivity. Further, the BP86 calculations suggest the existence of a metallacycle as possible reaction intermediate, a proposition not supported by the B3LYP approach. The two different computational approaches result not only in a quantitatively, but also in a qualitatively different description of the epoxidation reaction. This in turn implies different models for chirality transfer associated with related reactions employing chiral catalytic systems.