Abstract
Hydrophobic imidazolium-based ionic liquids (IL) act as catalysts for the epoxidation of unfunctionalized olefins in water using hydrogen peroxide as oxidant. Although the catalysts are insoluble in both the substrate and in water, surprisingly, they are very well soluble in aqueous H2 O2 solution, owing to perrhenate-H2 O2 interactions. Even more remarkably, the presence of the catalyst also boosts the solubility of substrate in water. This effect is crucially dependent on the cation design. Hence, the imidazolium perrhenates enable both the transfer of hydrophobic substrate into the aqueous phase, and serve as actual catalysts, which is unprecedented. At the end of the reaction and in absence of H2 O2 the IL catalyst forms a third phase next to the lipophilic product and water and can easily be recycled.
Highlights
For more than two decades, research on ionic liquids (IL) has been enjoying ever-growing academic and industrial interest.[1]
We recently reported on the extraction of [ReO4]- by organic receptors from the aqueous phase into toluene, which leads to the ‘switching on’ of the catalytic activity of perrhenate for epoxidation and allows for a simple catalyst extraction and reusability.[ 12 ]. This effect was originally ascribed to the hydrophobic character of ILs rendering hydrogen bonding between [ReO4]- and H2O2 favorable, since compounds such as potassium or ammonium perrhenate do not show any activity
In this work we have investigated the interplay between the degree of ion pairing, selectivity and solubility effects in the presence of IL catalysts and the consequences on their catalytic activity
Summary
For more than two decades, research on ionic liquids (IL) has been enjoying ever-growing academic and industrial interest.[1]. We show that the substituents at the imidazolium ring have a dramatic effect on the solubility of the IL catalysts in aqueous H2O2, affecting the catalytic activity.
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