Abstract
Oxidation is one of the major pathways for the synthesis of chemical intermediates. 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. Of key importance is the use of oxygen as the oxidant, but in many cases more reactive, and less green, sources of oxygen are used. We report the solvent-free epoxidation of cyclooctene with air using supported gold catalysts with small amounts of a hydroperoxide. We identify the appropriate reaction conditions to maximize the selectivity of the epoxide. In the absence of a hydroperoxide initiator, using air at atmospheric pressure, no reaction is observed. Choice of the peroxide initiator is crucial and in the absence of a catalyst or a support the reaction of the alkene can be observed with di-t-butyl peroxide and t-butyl hydroperoxide (TBHP) only when high concentrations are used at high temperatures >= 80 degrees C, and TBHP was found to be the more selective to epoxide formation. In contrast, cumene hydroperoxide was highly reactive under all conditions evaluated. TBHP was selected for more detailed study. Use of graphite as a support was found to give the best combination of selectivity and conversion. In general the selectivity to the epoxide increased with reaction temperature from 60-80 degrees C and was highest at 80 degrees C. Other carbon supports, e. g. activated carbon, were found to be less effective. TiO(2)- and SiO(2)-supported Au catalysts were also selective for the epoxidation reaction and the general order of activity was: graphite > SiO(2) > TiO(2). The major by-product is the allylic alcohol and the reaction pathways to the epoxides and the allylic alcohol are discussed. Preparation of catalysts using a sol-immobilisation method significantly enhanced catalyst activity with retention of selectivity to the epoxide.