Acid−Base Reactions of Methylnickel Hydroxo, Alkoxo, and Amide Complexes with Carbon Acids. Studies on the Reactivity of Noncyclic Nickel Enolates

Basic organonickel hydroxo, alkoxo, or amido complexes of composition Ni(Me)(X)(dippe) (X = OH, t-BuO, cyclo-NC₄H₈; dippe = i-Pr₂PCH₂CH₂Pi-Pr₂) react with enolizable ketones, esters, and nitriles, producing the corresponding enolate complexes. In the case of the hydroxo and alkoxo complexes, these reactions may lead to equilibrium mixtures, allowing a comparison of their respective basicities, while the amido compound reacts clean and quantitatively in all cases, allowing the isolation of the corresponding enolates. While the ketone derivatives display oxygen-bound enolate ligands, the ester and nitrile enolates bind the metal center through the carbon atom. The acetophenone enolate complex has strong nucleophilic properties and rapidly reacts with aldehydes (PhCHO) or CO₂, affording the corresponding aldolate and carboxylate addition products.