Transition Metal Manganese and Main Group Gallium Complexes as Catalysts for Alkene Epoxidation

Abstract

Epoxides are important organic synthesis precursors and widely used in industry. Alkene epoxidation has been under extensive investigation over decades. Synthetic models have been established for catalytic epoxidations, employing transition and main group metals. Epoxidation proceeds either through high valent metal-oxo species and/or a Lewis acid mechanism for oxygen transfer to substrate. Specifically, by high valent metal-oxo species, Jacobsen’s catalyst (MnIII-salen) and non-porphyrin MnII complexes can catalyze enantioselective epoxidation and electron-rich terminal alkene epoxidation respectively. Sharpless catalyst (TiIV-tartrate) and Al3+ centered complexes catalyze epoxidation through Lewis acid mechanisms. Manganese complexes with phenanthroline derivatives ([Mn(R-phen)2Cl2]Cl, R = NH2, Me, H, Cl, NO2) as catalysts are reported. The MnII complexes catalyze epoxidation of a variety of substrates with electron-rich alkenes being favored. The installation of functional groups on the 5-position of phenanthroline ligand plays important roles in influencing catalyst reactivity. The more electron-donating the functional group is, the more reactive it is. The reactivities with different substituent groups on the ligand have been correlated with Hammett constant, which showed a good linear relationship. EPR and spectroscopic studies indicated possible involvement of high valent MnIV-oxo species which decays quickly under experimental conditions. Gallium complexes are effective catalysts as well. [Ga(phen)2Cl2]Cl is reported to be the first homogeneous gallium catalyst for alkene epoxidation with peracetic acid. Compared with other group 13 metals, the activity of [Ga(phen)2Cl2]Cl is high with low loading of 1%. The epoxide is the only product, indicating good selectivity. Furthermore, six gallium(III) complexes with N-donor ligands were synthesized to study the mechanism of Ga(III)-catalyzed olefin epoxidation by peracetic acid. Although the complexes with relatively electron-poor phenanthroline derivatives display faster initial reactivity, the Ga(III) complexes with the polydentate pyridylamine ligands appear to be more robust, with less noticeable decreases in their catalytic activity over time. The more highly chelating trispicen and tpen are associated with markedly decreased activity.