Bismuth-substituted "sandwich" type polyoxometalate catalyst for activation of peroxide: Umpolung of the peroxo intermediate and change of chemoselectivity

The epoxidation of alkenes with peroxides by W^VI, Mo^VI, V^V, and Ti^IV compounds is well established, and it is well accepted that the active intermediate peroxo species are electrophilic toward nucleophilic substrates. Polyoxotungstates, for example, those of the "sandwich" structure, [WZn(TM-L)₂(ZnW₉O₃₄)²]^q- in which TM = transition metal and L = H₂O, have in the past been found to be excellent epoxidation catalysts. It has now been found that substituting the Lewis basic Bi^III into the terminal position of the "sandwich" polyoxometalate structure to yield [Zn₂Bi^III₂(ZnW₉O₃₄)₂]^14- leads to an apparent umpolung of the peroxo species and formation of a nucleophilic peroxo intermediate. There are two lines of evidence that support the formation of a reactive nucleophilic peroxo intermediate: (1) More electrophilic sulfoxides are more reactive than more nucleophilic sulfides, and (2) nonfunctionalized aliphatic alkenes and dienes showed ene type reactivity rather than epoxidation pointing toward "dark" formation of singlet oxygen from the nucleophilic intermediate peroxo species. Allylic alcohols reacted much faster than alkenes but showed chemoselectivity toward C-H bond activation of the alcohol and formation of aldehydes or ketones rather than epoxidation. This explained via alkoxide formation at the Bi^III center followed by oxidative β-elimination.