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

Abstract

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.

Oxorhenium(V) Complexes with Phenolate–Pyrazole Ligands for Olefin Epoxidation Using Hydrogen Peroxide

“…Using H 2 O 2 , where the media is biphasic (H 2 O and 1,2-C 2 H 2 Cl 2 ), the polar substituents might allow for reaction of the catalytic species with H 2 O 2 across the phase boundaries, permitting the use of H 2 O 2 as oxidant. In a previous investigation on the mechanism of TBHP catalyzed epoxidation with a Re(V) complex, we proposed a cationic trans-dioxorhenium species as a likely intermediate . If a similar mechanism applies here, the polar substituents might facilitate the formation of such a charged species, resulting in the overall higher catalytic activity.…”

“…Rhenium(V) complexes, initially investigated for their role in oxygen-atom transfer reactions, have found use as moisture-stable catalysts in some epoxidation reactions. − In early investigations, such compounds coordinated by bidentate or tetradentate Schiff base ligands were found to catalyze the epoxidation of cyclooctene with tert -butylhydroperoxide (TBHP) and gave yields of up to 66%. Also, oxorhenium(V) complexes with pyridylalkoxide ligands showed catalytic activity in olefin oxidation but only up to 30% conversion .…”

Novel rhenium(iii, iv, and v) tetradentate N2O2 Schiff base mononuclear and dinuclear complexes

“…5–9 Methyltrioxorhenium(VII) Schiff base complexes have been evaluated as potential catalysts for epoxidation reactions 6, 7 while Re(V) salen complexes are being probed as potential oxidation and epoxidation catalysts. 5, 8, 9 Various rhenium Schiff base complexes have also been explored for potential applications to diagnostic and therapeutic nuclear medicine. 10–25 …”

Dioxomolybdenum(VI) Complexes with Pyrazole Based Aryloxide Ligands: Synthesis, Characterization and Application in Epoxidation of Olefins

“…A possible approach is the preparation of rhenium complexes in lower oxidation states, such as oxorhenium(V) compounds, or oxomolybdenum(VI) compounds as they tend to be more stable toward protic conditions. In this endeavor, we have developed and investigated as epoxidation catalysts several oxorhenium(V) complexes. − With one of them we were able to get an insight into the mechanism and found that the rhenium(V) precursor is stepwise oxidized to rhenium(VII) prior to the catalytic reaction which is again sensitive toward decomposition . However, we have recently prepared oxorhenium(V) complexes equipped with aryloxide-pyrazole ligands and investigated their epoxidation chemistry .…”