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چکیده
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Controlled epoxidation of alkenes with stable and efficient catalysts using environmental friendly oxidants is an important goal. Since the first epoxidation of allylic alcohols by Sharpless [1], various manganese Schiff base catalysts has been developed using different oxygen sources such as iodosylbenzene, sodium hypochlorite, hydrogen peroxide [2], alkyl hydroperoxide [3,4], molecular oxygen [5] and more recently urea-hydrogen peroxide (UHP) [6,7]. The catalytic epoxidation of styrene using urea–hydrogen peroxide and heterotrinuclear Cu(II) complexes with general formula (MnLn)2Cu(acac)2, where n = 1-3 is reported. Schiff base complexes MLn involving a 3,4-diaminopyridine bridge with free coordination site were used as the ligand, where (Ln)2- is [(5-x-Sal)2Py]2 and x = H, Br or NO2. The complexes were characterized by physico-chemical and spectroscopic methods. The electrochemical properties of Mn were modified upon trinuclear complex formation. The trinuclear complexes show high catalytic activity, with up to 86% conversion and 93% selectivity, while no catalytic properties were observed for the monomeric complexes. The catalyst could be reused with some loss of activity. In summary, while conversions using UHP and H2O2 as primary oxidant were similar, in the case of UHP the selectivity for styrene epoxide production was better. So, UHP as a water-free oxidant could be used as a stable, cheap, and environmentally friendly oxidant, capable of use in organic solvents and for water sensitive materials. The efficiency of these catalysts was strongly dependent on the structure of the dibasic Schiff-base ligands. These results could be due to several factors, such as the nature of the metals, their ability to coordinate the reactants, the solubility of the complexes, etc.
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