Catalysis of the methanolysis of acetylimidazole by lanthanum triflate

Methanolysis of acetylimidazole (1) and N-acetylimidazolepentamine-Co(III) (2) was found to be markedly accelerated in the presence of La(OTf)(3). Pot entiometric titration of a solution of La3+(OTf-)(3) gave a pK(a) for the m etal bound CH3OH of 7.22. The kinetics of methanolysis of 1 and 2 were meas ured at 25degreesC at variouspH under buffered conditions as a function of increasing La3+. Analysis of both the kinetic and potentiometric data indic ates that the catalytically active species is a La3+-dimer, bridged by two methoxides, (CH3OH)(n)La3+(CH3O-)(2)La3+(CH3OH)(n). The maximum second-orde r rate constants for attack of the dimer on 1 and 2 are 1.50 x 10(3) M-1 s( -1) and 1.42 x 10(2) M-1 s(-1) respectively and both processes adhere to ti tration of a La3+(CH3OH) to generate the active form. The results are expla ined in terms of a mechanism where the methoxy-bridged La3+ dimer transient lybreaks a La3+-OCH3 bond to expose both a CH3O- nucleophile and a La3+ whi ch can act as a Lewis acid.Unlike the situation in water, the methanol resu lts indicate that the medium greatly stabilizes and solubilizes the active dimer without the necessity of creating specially designed ligands to stabi lize the dinuclear core.