Diastereoselective Synthesis and Catalytic Activity of Two Chiral cis-Dioxidomolybdenum(VI) Complexes

Two enantiomerically pure chiral dioxidomolybdenum(VI) complexes (1 and 2) of the type [MoO2L] (L = dianionic, tetradentate ONNO-ligand) were synthesized and investigated in enantioselective oxidation reactions. The solid-state structures of complex 1 and 2, determined via single-crystal X-ray diffraction analysis revealed two fundamentally different coordination geometries: a C1-symmetric cis- isomer (-1), and a C2-symmetric cis- isomer (-2). In both cases, only one of the two possible helical enantiomers (- or -helix) was formed. The complexes were examined as precatalysts in the epoxidation of the challenging prochiral substrate trans-stilbene, using either tert-butyl hydroperoxide (TBHP) or cumyl hydroperoxide (CHP) as oxidants. The asymmetric cis- complex 1 was found to be significantly more active in the epoxidation than its cis- counterpart 2, asymmetric induction was, however, negligible for both complexes. The complexes were also tested in catalytic enantioselective sulfoxidation reactions where chiral induction could be achieved, albeit small. The observed putative molybdenum oxido-peroxido intermediate 1-O-2 could be identified as an important precomplex before formation of the active catalyst in sulfoxidation.