Exploring the catalytic activity of Lewis-acidic uranyl complexes in the nucleophilic acyl substitution of acid anhydrides

Abstract

The catalytic activities of several uranyl complexes, such as N,N′-disalicylidene-o-phenelyenediaminato(ethanol)dioxouranium(VI) (UO₂(salophen)EtOH), bis(dibenzoylmethanato)(ethanol)dioxouranium(VI) (UO₂(dbm)₂EtOH), pentakis(N,N-dimethylformamide)dioxouranium(VI) ([UO₂(DMF)₅]²⁺), and tetrakis(triphenylphosphine oxide)dioxouranium(VI) ([UO₂(OPPh₃)₄]²⁺), were examined in the nucleophilic acyl substitution of acid anhydrides. Among them, [UO₂(OPPh₃)₄]²⁺ was the most efficient to give ethyl acetate and acetic acid from acetic anhydride (Ac₂O) and ethanol, and was resistant towards decomposition during the catalytic reaction. Several nucleophiles were also subjected to the catalytic acylation reaction using acetic and pivalic anhydride. Kinetic and spectroscopic studies suggested that [UO₂(OPPh₃)₄]²⁺ interacts with Ac₂O to form [UO₂(Ac₂O)(OPPh₃)₃]²⁺. Interaction of this actual catalyst with additional Ac₂O determines the rate of the overall nucleophilic acyl substitution reaction.