Oxidation of cyclohexene by t-butylhydroperoxide and dioxygen catalysed by polybenzimidazole-supported Cu, Mn, Fe, Ru and Ti complexes

Abstract

Polybenzimidazole (PBI) resin-supported complexes of Cu, Mn, Fe, Ru and Ti have been prepared by ligand exchange reactions using metal acetylacetonates or chlorides. Metal loading in the range 0.7–3.3 mmol metal g −1 resin has been achieved. The PBI metal complexes have been investigated as potential catalysts in the oxidation of cyclohexene by t-butylhydroperoxide (TBHP). PBI.Cu and PBI.Ti have been examined under both aerobic and anaerobic conditions whereas the others have been restricted to aerobic use only. When dioxygen is present it functions as a co-oxidant with TBHP. PBI.Cu and PBI.Fe behave very similarly and induce mainly allylic oxidation to yield cyclohexenol and cyclohexenone. Very significant levels of t-butylcyclohex-2-enyl peroxide are also formed, and PBI.Cu is a potentially useful heterogeneous catalyst for selective synthesis of this. PBI.Mn utilises TBHP in preference to O 2 as an oxidant, but again favours allylic oxidation. Interestingly the above peroxide is formed only transiently with PBI.Mn as the catalyst and so, although primarily free radical reactions are involved with both PBI.Cu and PBI.Mn catalysts, the balance of reactions differs. PBI.Ru induces allylic oxidation as well but also provides high conversion to cyclohexane-1,2-diol. This seems to occur by direct dioxygenation of cyclohexene and the reaction could be of synthetic utility under aerobic conditions. PBI.Ti induces allylic oxidation but also causes heterolytic fission of TBHP and formation of cyclohexene oxide. When O 2 is removed no allylic products are seen, and the epoxidation reaction is highly selective. The activity of PBI.Ti is, however, lower than our earlier described PBI.Mo catalyst.