Donor-ligand effect on the product distribution in the manganese-catalyzed epoxidation of olefins: A computational assessment

Based on the assumption that radical species are key intermediates in the manganese-salen-catalyzed epoxidation of olefins, the final step in this oxygen transfer reaction has been investigated in theoretical calculations based on density functional (DF) theory. Pure DF methodology (BP86) as well as hybrid approaches (B3LYP*) have been considered. The main body of the work employs a Mn(acacen') model system, whereas selected reaction steps have been calculated for Mn(salen) complexes as well. Catalysts considered are of the type [Mn]-Cl, [Mn](+), [Mn](+)-ON-Py, and [Mn](+)-OPR3. Regardless of the DF method chosen, the calculations suggest that formation of aldehyde byproduct originates from a five-coordinated manganese-salen complex [Mn]'. Whereas no significant donor-ligand effect was found for six-coordinated compounds [Mn](+)-ON-Py and [Mn](+)-OPR3, the energetics of the ligand dissociation step that furnishes the [Mn]+ species are donor-ligand dependent. The resulting mechanistic proposal is in qualitative agreement with recently published experimental data on epoxide to aldehyde product ratios (Collman, J. P.; Zeng, L.; Brauman, J. I. Inorg. Chem. 2004, 43, 2672-2679). A product formation pathway incorporating a manganoxetane intermediate does not present a viable alternative.