Asymmetric Epoxidation vs syn-Hydroxy-Acyloxylation of Olefins in the Presence of Sterically Demanding Nonheme Manganese Complexes

Herewith, we present another facet of the versatile catalytic reactivity of bulky bis-amino-bis-pyridylmethyl Mn complexes: besides highly enantioselective (up to 99% ee) epoxidation, benzhydryl-substituted Mn catalysts have been found to convert olefinic substrates into the products of syn-addition to the C═C bond, hydroxy-carboxylates, that can prevail under certain conditions. The mechanism of syn-hydroxy-acyloxylation is discussed, with the data obtained providing evidence in favor of the so far lacking direct enantioselective OH and OC(O)R transfer from the high-valent Mn active species to the olefinic C═C group. Such a mechanism is conceptually reminiscent of Rieske dioxygenases that catalyze enantioselective 1,2-dihydroxylation of C═C bonds, simultaneously incorporating two cis-ligands from the reactive metal center into the resulting diol in a syn-selective fashion.