Abstract
The tetrahedral triphenylsiloxy complex MoO(2)(OSiPh(3))(2) (1) and its Lewis base adduct with 2,2'-bipyridine, MoO(2)(OSiPh(3))(2)(bpy) (2), were prepared and characterised by IR/Raman spectroscopy, and thermogravimetric analysis. Both compounds catalyse the epoxidation of cis-cyclooctene at 55 degrees C using tert-butylhydroperoxide (t-BuOOH) is decane as the oxidant, giving 1,2-epoxycyclooctane as the only product. The best results were obtained in the absence of a co-solvent (other than the decane) or in the presence of 1,2-dichloroethane, while much lower activities were obtained when hexane or acetonitrile were added. With no co-solvent, catalyst 1 (initial activity 272 mol x molMo(-1) x h(-1)for a catalyst:substrate: oxidant molar ratio of 1:100:150) is much more active than 2(initial activity 12 mol x molMo(-1) x h(-1)). The initial reaction rates showed first order dependence with respect to the initial concentration of olefin. With respect to the initial amount of oxidant, the rate order dependence for 1 (1.9) was higher than that for 2 (1.6).The dependence of the initial reaction rate on reaction temperature and initial amount of catalyst was also studied for both catalysts. The lower apparent activation energy of 1 (11 kcal x mol(-1)) as compared with 2 (20 kcal x mol(-1)) is in accordance with the higher activity of the former.
Highlights
Over the last twenty years or so, several tetracoordinate dioxomolybdenum(VI) complexes of the type MoO2(OR)2 (R = Me, Et, n-Pr, Ph [1], t-Bu, i-Pr, CH2t-Bu [2]) and MoO2(OSiR3)2 (R = t-Bu [3], Ot-Bu [4], Ph [5]) have been reported
The complexes with supporting siloxide ligands are of particular interest because they serve as models for isolated molybdenum atoms on a silica surface [6]
Some of us recently reported on the catalytic activity of the triphenylsiloxy complex MoO2(OSiPh3)2 (1) for the liquid phase epoxidation of cyclic olefins using tert-butylhydroperoxide (t-BuOOH) as the mono-oxygen source [9]
Summary
The performance of compounds 1 and 2 as epoxidation catalysts was studied using cis-cyclooctene as a model substrate and t-BuOOH as oxygen donor. Conversion after 24 h reaction followed the order: no co-solvent ≥ dichloroethane > hexane > CH3CN, and TOF (calculated at 30 min) of cyclooctene epoxidation was highest without a co-solvent or with dichloroethane (Table 2). The coordinating power of the solvent molecules does not solely explain the observed catalytic activity since when hexane, a non-coordinating solvent, is added to the reaction medium the epoxidation rate decreases significantly for both catalysts.
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