Asymmetric Epoxidation vs <i>syn</i>-Hydroxy-Acyloxylation of Olefins in the Presence of Sterically Demanding Nonheme Manganese Complexes
Varvara A. Sherstyuk, Roman V. Ottenbacher, Evgenii P. Talsi, Konstantin P. Bryliakov
Abstract
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.