One of the primary challenges in catalyst investigation for olefin epoxidation is the development of sustainable systems. Under ecological aspects, using an aqueous solution of hydrogen peroxide as oxygen source is advantageous, but requires a phase transfer catalyst, due to the biphasic composition of the system. Recently, it was found that perrhenate acts as catalyst by activation of H2O2 in an outer sphere fashion under micellar conditions. The micelles incorporate the organic substrate and are formed by surface-active imidazolium cations, the counterpart of the catalytically active anion. Since the rhenium center is not actively involved in the activation of hydrogen peroxide, we have investigated whether other anions with similar structural and electronic properties can activate H2O2 via H-bonds to epoxidize unfunctionalized olefins. Among the low-priced oxo-anions, nitrate was found to enable a similar mechanism, even though in a somewhat decreased activity. This is the first time reported that nitrate acts as a catalyst and serves as a model for other metal-free oxo-anions such as dialkyl phosphates, which can be tuned by their organic substituents. To extend the performance of the bifunctional catalyst concept with metal-based anions, tungstate was chosen as a more active alternative to perrhenate since its reactivity towards olefins with H2O2 is well-known for phase-transfer catalysis systems. Cryo-TEM imaging was used to show the micelle swelling from 10 to 20 nm upon uptake of organic substrate, proving its solubilization in the aqueous phase. The active species, a highly potent tetraperoxo tungstate was confirmed by 183W-NMR spectroscopy and ESI-MS by addition of H2O2. Our research shows that epoxidation under mild and micellar conditions with H2O2 is an alternative to existing biphasic approaches and enables activation of simple oxo-anions by H-bonding for their catalytic use.

Micellar approaches towards mild biphasic catalytic epoxidation of olefins with hydrogen peroxide

2019 Fall National Meeting

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