Sandwich anions [WZn{M(OH2)}2(B-ZnW9O34)2]12− (where M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) as oxygen transfer catalysts for H2O2 in the epoxidation of allylic alcohols under biphasic conditions

The heteropolyoxotungstate sandwich anions [WZn{M(OH2)}2(B-ZnW9O34)2]12−, where M = Mn(II), Co(II), Ni(II), Cu(II) or Zn(II) have been studied as epoxidation catalysts for oxygen transfer from 30% H2O2 to a range of allylic alcohols under biphasic conditions (1,2-dichloroethane/H2O) at 15 °C. The reactions have been carried out under conditions, whereby the substituted transition metals, M(II), in the waist of the anion were able to demonstrate their role in the reaction and establish the order: Zn(II) > Mn(II) ~ Cu(II) > Co(II) > Ni(II). This is related to the rate of exchange of aqua groups on the M(II) ions and implies the coordination of allylic alcohol at an M(II) site, prior to transfer of a peroxy oxygen from an adjacent W(O2) site, which is formed from a W=O unit by reaction with H2O2. Evidence of W(O2) formation was obtained through IR studies. The W(O2) group forms the epoxide by transfer of an oxygen atom to the C=C bond of the coordinated allylic alcohol. Kinetic studies have been carried out on the epoxidation of 3-methyl-2-buten-1-ol for the first time using the Mn(II)-substituted anion. The reaction mechanism uses only the key generic steps described above and models the observed induction period, which can be rationalized by an autocatalysis mechanism involving not only coordination of allylic alcohol to M(II), but also the product hydroxy epoxide (both through their –OH groups). The mechanism is supported by evaluation of the stability constant for coordination of allyl alcohol to [WZnCo2(ZnW9O34)2]12−, 8.6 (6) M−1, at 25 °C in 1,2-dichloroethane solution.