The role of protons in cyclohexene oxidation with H 2O 2 catalysed by Ti(IV)-monosubstituted Keggin polyoxometalate

Abstract

The effect of the number of protons in the Ti(IV)-monosubstituted Keggin polyoxometalate Na 5 − n H n PTiW 11O 40 ( n = 1–5; Ti-POM) on its catalytic behaviour in cyclohexene (CyH) oxidation with aqueous H 2O 2 in MeCN is reported. It has been found that Ti-POMs with n = 2–5 catalyse efficiently CyH oxidation to yield trans-cyclohexane-1,2-diol as the main reaction product, while Ti-POM containing only one proton shows lower activity in CyH oxidation and produces allylic oxidation products, 2-cyclohexene-1-ol and 2-cyclohexene-1-one, along with comparable amounts of the corresponding epoxide and diol. The obtained results strongly support homolytic oxidation mechanism for CyH oxidation in the presence of the monoprotonated Ti-POM and heterolytic oxygen-transfer mechanism in the presence of Ti-POMs having two and more protons. The 31P and 183W NMR studies revealed that Ti-POMs are stable towards at least 100-fold excess of H 2O 2 and the high catalytic activity of Ti-POMs with n = 2–5 is not due to the formation of lower nuclearity species. The addition of one equivalent of H + to the monoprotonated peroxo complex [Bu 4N] 4[HPTi(O 2)W 11O 39] ( I, 31P NMR in MeCN: −12.40 ppm) results in the formation of the diprotonated titanium peroxo species [H 2PTi(O 2)W 11O 39] 3− ( II, 31P NMR in MeCN: −12.14 ppm). This peroxo species can also be obtained by adding an excess of H 2O 2 to Na 5 − n H n PTiW 11O 40 ( n = 2–5) in MeCN. The presence of the second proton in the peroxo species is a crucial factor determining the capability of II to oxidise alkenes via heterolytic oxygen transfer mechanism. Both 31P NMR and GC–MS studies corroborated that II reacts with CyH producing trans-cyclohexane-1,2-diol as the main reaction product, whereas I is not reactive towards CyH under stoichiometric conditions.