Epoxidation of cyclooctene catalyzed by dioxomolybdenum(VI) complexes in ionic liquids

Abstract

A series of room temperature ionic liquids (RTILs) were examined as solvents for dioxomolybdenum(VI) complexes in the catalytic epoxidation of cis -cyclooctene, using tert -butyl hydroperoxide (TBHP) as the oxidant. The best results in terms of catalyst/solvent recycling were obtained using a novel cationic Mo VI complex combined with the ionic liquid 1- n -butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM]NTf 2 ). A series of room temperature ionic liquids (RTILs) were tested as solvents for dioxomolybdenum(VI) complexes in the catalytic epoxidation of cis -cyclooctene, using tert -butyl hydroperoxide (TBHP) as the mono-oxygen source. In general, the best results were obtained using the RTIL 1- n -butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM]NTf 2 ). Upon addition of cyclooctene and TBHP (in decane) to solutions of MoO 2 X 2 ( p -tolyl-(CH 3 -DAB)) [X = Cl, Me; p -tolyl-(CH 3 -DAB) = N , N - p -tolyl-2,3-dimethyl-1,4-diazabutadiene] in [BMIM]NTf 2 , biphasic mixtures were obtained. The epoxidation reactions proceeded with 100% selectivity to cyclooctene oxide, but activities were lower than those observed for the same catalysts in the absence of any additional solvent (other than decane). By contrast, in the presence of the novel cationic complex [MoO 2 Cl(Bn 3 Me 3 -tame)]BF 4 [Bn 3 Me 3 -tame = N , N ′, N ″-tribenzyl-1,1,1-tris(methylaminomethyl)ethane] and [BMIM]NTf 2 , the catalytic activity was higher than that achieved with no additional solvent, although a minor amount of 1,2-cyclooctane diol was formed as a by-product. This system gave the best results in terms of catalyst/solvent recycling (carried out by extracting the reactants and products with n -hexane). There was a loss of catalytic activity from the first to the second run, but thereafter tended to stabilise. Interestingly, selectivity to the desired epoxide increased upon recycling, reaching 100% in the third run.