Elucidating steric effects on enantioselective epoxidation catalyzed by (salen)Mn in metal-organic frameworks

The steric effects of a metal-organic framework (MOP) on the enantioselectivity of a (salen)Mn were studied using classical atomistic modeling. Rotational energy profiles for the approach of 2,2-dimethyl-2H-chromene to the active site of (salen)Mn were mapped for the homogeneous catalyst and the catalyst immobilized as a linker in a MOF. The model corroborated that the Re enantioface is favored by the homogeneous catalyst. It was shown that the predicted enantioselectivity when chromene approaches the more accessible (salen)Mn in the MOF is highly sensitive to the distance between the (salen)Mn linkers of the interpenetrated frameworks. Calculated rotational energy profiles for a hypothetical non-interpenetrated MOF revealed that this MOF catalyst should exhibit enantioselectivity intermediate to the interpenetrated MOF and the homogeneous catalyst. Finally, the continuous chirality measure of the catalyst was found to correlate well with the energy of the homogeneous catalyst-reactant complex, suggesting that high chirality content is related to high enantioselectivity. This correlation, however, did not apply for some of the MOF catalysts. For heterogeneous catalysts, the mechanism of asymmetric induction depends on the steric environment in addition to the chirality of the catalyst. (C) 2010 Elsevier B.V. All rights reserved.