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 WVI, MoVI, VV, and TiIV 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 BiIII into the terminal position of the “sandwich” polyoxometalate
structure to yield [Zn2BiIII2(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 BiIII center followed
by oxidative β-elimination.