Mechanistic Insight into Olefin Epoxidation Catalyzed by Rhenium(V) Oxo Complexes That Contain Pyridazine-Based Ligands

Three novel tridentate pyridazine phenolate ligands were prepared in high yields by Schiff-base condensation of salicylic aldehyde with various pyridazine hydrazines (substituent R in the 6 position: R = Cl (HL(Cl)), (t)Bu (HL((t)Bu)), or tol (HL(tol))). They react with [ReOCl(3)(OPPh(3))(SMe(2))] to form rare mononuclear trans-dichloro oxo complexes of general formula [ReOCl(2)(L(R))] with R = tol (1), (t)Bu (2), or Cl (3) as confirmed by single-crystal X-ray diffraction analyses of 1 and 2. They were found to be catalysts for oxidation of cyclooctene to the corresponding epoxide by tert-butyl hydroperoxide (TBHP). Extensive UV-vis and NMR spectroscopic investigations followed by evaluation using the powerful Mauser method revealed mechanistic details. This showed the catalyst precursor [ReOCl(2)(L)] (2) to be transformed into the rhenium(VII) compound [ReO(3)L] (4) in a two-step reaction via intermediate INT which is tentatively assigned to [ReO(2)L]. Confirmation gave the isolation of 4 by reaction of 2 with excess of TBHP. Monitoring the catalytic oxidation reaction by UV-vis spectroscopy clearly excludes the two rhenium(V) compounds 2 and INT from being the catalytically active species as their formation is several orders of magnitude faster than the observed catalytic epoxidation reaction.