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)₃N-C₂H₄-N(CH)₃O ^-) 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.