Solvent-free selective epoxidation of cyclooctene using supported gold catalysts: an investigation of catalyst re-use
Selective oxidation is of immense importance in the synthesis of chemical intermediates and the epoxidation of alkenes by the electrophilic addition of oxygen to a carbon-carbon double bond remains one of the most significant challenges in oxidation catalysis. Although molecular oxygen is the most environmentally benign oxidant in many cases, far more reactive forms of oxygen are required to achieve reaction, and this can lead to by-products with a heavy environmental burden with respect to their disposal. We show that gold supported on graphite is a very effective catalyst for the epoxidation of cis-cyclooctene as long as catalytic amounts of a hydroperoxy species are present at the start of the reaction. Using mild solvent-free conditions the hydroperoxy initiator persists in solution for only a few minutes, being initially adsorbed on the catalyst surface. Subsequently, it decomposes to establish a reactive species that can propagate the selective oxidation process we observe. The observation of an induction period may in part be due to the adsorption of the radical initiator blocking surface sites as well as the establishment of the reactive species. We confirm that graphite is the best support and that tert-butyl hydroperoxide is the preferred initiator. We report extensive studies concerning the reusability of the gold/graphite catalyst as catalyst reusability is a key feature of green chemistry. The catalyst is found to be inhibited by the epoxide product but we demonstrate the effect of this is negligible for reused catalysts over a long reaction time.
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