Quantum chemical study of the mechanisms of oxidation of ethylene by Molybdyl and Tungstyl Chloride

Abstract

The mechanisms of oxidation of olefins with MoO2Cl2 and WO2Cl2 are studied with DFT. The formation of epoxide from these reactions is not very feasible by any of the postulated paths. If the epoxide precursor will form at all, it will arise via initial [3 + 2]O,Cl addition of ethene to MoO2Cl2 and WO2Cl2 to form an intermediate, followed by re-arrangement to form the precursor, from which the epoxide can be generated by hydrolysis. The chlorohydrin precursor was also found to originate from [3 + 2]O,Cl addition of ethene to MO2Cl2. The results also indicate that a dichloride is not a likely product in the oxidation of ethylene by molybdyl chloride. However, in the case of WO2Cl2, the formation of a dichloride may not be precluded. The formation of acetaldehyde and vinyl alcohol from the oxidation of ethylene does not appear energetically feasible with MoO2Cl2, but appears thermodynamically plausible with WO2Cl2. Thus, the oxidation of ethylene with MoO2Cl2 will most likely lead to the formation of chlorohydrins predominantly via [3 + 2]O,Cl addition; oxidation with WO2Cl2 may also form chlorohydrins, but only extremely slowly. The oxyhalides MO2Cl2 become weaker oxidants in the order CrO2Cl2>>MoO2Cl2 >WO2Cl2. Corresponding to this, reactions involving reduction of the metal [3 + 2] and [2 + 1] show a sharp increase in barrier going from Cr to W; reactions without a change in metal oxidation state ([2 + 2]) show much smaller variations, which are possibly mainly determined by sterics. [Figure not available: see fulltext.]