Catalysis of the methanolysis of activated amides by divalent and trivalent metal ions.: The effect of Zn2+, Co2+, and La3+ on the methanolysis of acetylimidazole and its (NH3)5CoIII complex

The metal ions Zn2+, Co2+, and La3+ strongly catalyze the methanolysis of the activated amides acetylimidazole (1) and its ligand-exchange-inert Co-III complex, (NH3)(5)Co-III-AcIm (2). Studies of the kinetics of methanolysis are performed with (s)(s)pH measurement and control, and the metal ions are soluble in the medium throughout the (s)(s)pH regions where ionization of the Mx+(CH3OH)(y) occurs. Zn2+ and Co2+ act as Lewis acids toward 1, catalyzing attack of external methoxide on a 1:M2+ complex at values only 100-fold lower than the diffusion limit, the k(OR) values being 5.6 x 10(7) M-1 s(-1) and 2.5 x 10(7) M-1 s(-1), while that for CH3O- attack on 2 is 4.69 x 10(7) M-1 s(-1). Since neither Zn2+ nor Co2+ promotes the methanolysis of 2, these metals appear to be acting through transient binding to the distal N of 1, which activates the C=O of the complex to external CH3O- attack. La3+ catalyzes the methanolysis of both 1 and 2, which occurs by a mechanism that is fundamentally different from that exhibited by Zn2+ and Co2+ in that the active species appears to be a bis-methoxy-bridged dimer (La3+)(2)(CH3O-)(2)(CH3OH)(x) that interacts directly with the C=O unit of the substrate.