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

Abstract

AbstractAir‐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.