Kinetics of cyclooctene epoxidation with tert-butyl hydroperoxide in the presence of [MoO₂X₂L]-type catalysts (L = Bidentate Lewis base)

Mo^VI complexes with the general formula [MoO₂X ₂L] (X = Cl, Br; L = 4,4′-dimethyl-2,2′-bipyridine (dmbp) or 4-hexyl-4′-methyl-2,2′-bipyridlne (hmbp)] have been prepared and characterized by IR and solution NMR (¹H. ¹³ C, ¹⁷O and ⁹⁵Mo) spectroscopy. The complexes were applied as catalysts in the homogeneous phase for the epoxidation of cyclooctene, with tert-butyl hydroperoxide (TBHP) as the oxygen source. The desired epoxide was the only product and turnover frequencies of up to about 9000 h^-1 could be reached. The catalytic activities increased in the order [MoO ₂Br₂(dmbp)] < [MoO₂Br₂(hmbp)] < (MoO₂Cl₂(dmbp)] < [MoO₂Cl ₂(hmbp)]. A kinetic model was built up for a homogeneous batch reactor based on a simplified mechanism involving three steps: (i) reversible coordination of TBHP to the starting Mo^VI complex to give a Mo ^VI alkylperoxide; (ii) irreversible oxidation of cyclooctene to cyclooctene oxide by the species formed in step 1, with formation of the starting complex and tert-butyl alcohol; (iii) reversible coordination of fert-butyl alcohol to the starting complex. This model is consistent with the observed kinetics. The first step in this reaction mechanism was characterized in more detail by studying the kinetics of the reaction of the starting complexes with TBHP in the absence of any reductant by UV/Vis spectroscopy. Rate constants, equilibrium constants, and activation parameters were determined. All ΔS^‡ values were negative and therefore support an associative mechanism in which a seven-coordinate intermediate is formed. The results also suggest that the first step is not always the rate-limiting step of cyclooctene epoxidation with these complexes.