Dichloro and dimethyl dioxomolybdenum(VI)–diazabutadiene complexes as catalysts for the epoxidation of olefins

摘要

The dioxomolybdenum(VI) complex [MoO2Cl2{p-tolyl(CH3DAB)}] has been prepared in good yield by reactionof the solvent adduct MoO2Cl2(THF)2 with one equivalent of the bidentate ligand N,N-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH3MgCl gives the dimethylderivative [MoO2(CH3)2{p-tolyl(CH3DAB)}]. The complexes are highly active and selective catalysts for thehomogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant. In both cases,the initial activity is ca. 175 mol mol 1Mo h 1 and cyclooctene oxide is obtained quantitatively within 4 h. It waspossible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a smalldecrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as1-octene, 2-octene, cyclododecene and (R)-(þ)-limonene, with TBHP. The catalytic production of cycloocteneoxide was investigated in detail, varying either the reaction temperature or the initial concentrations ofsubstrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor–oxygen donorcomplex formation is first-order in TBHP and in the metal complex [MoO2Cl2{p-tolyl(CH3DAB)}]. A specificrate of 3.2 mol 1 dm3 s 1 was found for catalyst formation at 25 C. Activation parameters for this reactionhave also been measured (DH6¼ ¼ 48 3 kJ mol 1, DS6¼ ¼ 112 10 J mol 1 K 1).