Mild, rapid and selective alcoholysis of terpyridine-appended amide substrates by Cu2+-catalysis: protonation state and reactivity of the complex
摘要:
An amide bond of a terpyridine-appended substrate, 6-(L-phenylalanylamino)-2,2':6',2"-terpyridine (2a), was cleaved to yield phenylalanine methyl ester quantitatively in the presence of catalytic amounts of Cu2+ not only in methanol but also in aqueous methanol at 30 degrees C. The reaction proceeds via formation of an N3O (three terpyridine nitrogens and one carbonyl oxygen) type 1:1 metal complex 2a-Cu2+. From spectral titration, the structure of the 2a-Cu2+ complex was confirmed to have three different protonation states, i.e., A (non-deprotonated amide with alpha-ammonium), D1 (deprotonated-amide with alpha-ammonium) and D2 (deprotonated-amide with a-amino) states. Among them, the complex in the D2 state was exclusively responsible for the observed mild, rapid and selective alcoholysis, showing the first-order rate constant of 6 x 10(-3) s(-1) or half-decay time of 2 min in methanol at 30 degrees C. Acidity of the amide proton was found to be higher than that of the ammonium proton in the complex, allowing formation of the highly reactive amide-deprotonated D2 state in methanol even without addition of external bases. Factors contributing to the high reactivity of the complex were discussed. (c) 2004 Elsevier B.V. All rights reserved.