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Enzyme-Like Catalysis via Ternary Complex Mechanism: Alkoxy-Bridged Dinuclear Cobalt Complex Mediates Chemoselective O‑Esterification over N‑Amidation

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posted on 16.12.2015, 22:44 by Yukiko Hayashi, Stefano Santoro, Yuki Azuma, Fahmi Himo, Takashi Ohshima, Kazushi Mashima
Hydroxy group-selective acylation in the presence of more nucleophilic amines was achieved using acetates of first-row late transition metals, such as Mn, Fe, Co, Cu, and Zn. Among them, cobalt­(II) acetate was the best catalyst in terms of reactivity and selectivity. The combination of an octanuclear cobalt carboxylate cluster [Co4(OCOR)6O]2 (2a: R = CF3, 2b: R = CH3, 2c: R = tBu) with nitrogen-containing ligands, such as 2,2′-bipyridine, provided an efficient catalytic system for transesterification, in which an alkoxide-bridged dinuclear complex, Co2(OCOtBu)2­(bpy)22-OCH2-C6H4-4-CH3)2 (10), was successfully isolated as a key intermediate. Kinetic studies and density functional theory calculations revealed Michaelis–Menten behavior of the complex 10 through an ordered ternary complex mechanism similar to dinuclear metallo-enzymes, suggesting the formation of alkoxides followed by coordination of the ester.