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Dioxidomolybdenum(VI) Complexes Containing Ligands with the Bipyrrolidine Backbone as Efficient Catalysts for Olefin Epoxidation



Air-stable chiral dioxidomolybdenum(VI) complexes of the type [MoO2(L)] (L = L12–; 1, L = L22–, 2) were prepared by the reaction of [MoO2Cl2] with the bis(phenol) ligands 1,4-bis(2-hydroxy-benzyl)-(S,S)-2,2′-bipyrrolidine [(S,S)-H2L1] and 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-(S,S)-2,2′-bipyrrolidine [(S,S)-H2L2). They were characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis, which revealed a distorted octahedral coordination geometry around the molybdenum(VI) center with a cis-α configuration. The Mo–Ooxo bond lengths are almost equal and vary only between 1.701 and 1.7091 Å. Complexes 1 and 2 were found to be efficient and selective catalysts for various olefin epoxidation reactions. Catalyst loadings of 0.5 mol-%, the use of 2 equiv. of oxidant (tert-butyl hydroperoxide), as well as unchanged catalyst performance over three consecutive catalytic runs demonstrate the excellent stability of the catalysts. Various challenging substrates including 1-octene or limonene were converted selectively to their corresponding epoxides. However, no asymmetric induction was observed.