Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of alcohols and X-ray structures of octamolybdate [C4mim]4[Mo8O26] and tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(μ-O)] complexes

The direct formation of a β-alcoxyalcohol from the corresponding olefin was investigated through the study of the oxidation of cyclohexene with hydrogen peroxide, as test reaction, catalysed by commercially available MoO3 in the presence of alcohols. The formation of the corresponding β-alcoxycyclohexanol was favourable in the order Me > Et > iPr > tBu, reaching approximately no yield for tBuOH. In this reaction, the lack of selectivity was due to the simultaneous formation of cyclohexane-1,2-diol by epoxide hydrolysis, a reaction that it is competitive with respect to the epoxide ring-opening reaction by the alcohol. In order to decrease the cyclohexane-1,2-diol yield, by preventing the hydrolysis reaction, several strategies were analysed and discussed. In particular, 2-methoxycyclohexanol was obtained with high yields and 99% selectivity by using as catalyst the [C4mim]4[Mo8O26] complex (C4mim = 1-butyl-3-methylimidazolium). The structure of the latter octamolybdate and also that of the tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(μ-O)] (tmpy = 2,4,6-trimethylpyridine) complex were determined by X-ray crystallography. The latter complex shows a μ2-oxygen bridging atom and two μ2-κ2–κ1-peroxo bridging ligands. This overall coordination assembly is unprecedented for binuclear molybdenum complexes.

Graphical abstract2-Methoxycyclohexanol was obtained with high yields and 99% selectivity by reaction of cyclohexene with hydrogen peroxide in methanol using as catalyst the [C4mim]4[Mo8O26] complex.Figure optionsDownload full-size imageDownload as PowerPoint slide