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)(5)Co-III complex

The metal ions Zn2+, Co2+, and La3+ strongly catalyze the methanolysis of t he activated amides acetylimidazole (1) and its ligand-exchange-inert Co-II I 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 ar e 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, catal yzing attack of external methoxide on a 1:M2+ complex at values only 100-fo ld 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. La 3+ 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 tha t the active species appears to be a bis-methoxy-bridged dimer (La3+)(2)(CH 3O-)(2)(CH3OH)(x) that interacts directly with the C=O unit of the substrat e.