Nickel-on-Charcoal-Catalyzed Aromatic Aminations and Kumada Couplings: Mechanistic and Synthetic Aspects

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Abstract
Protocols for aromatic aminations and Kumada couplings catalyzed by ‘heterogeneous' nickel-on-charcoal (Ni/C) have been revised, making them simpler and more time efficient. For both types of reactions, reduction of the catalyst precursor Ni(II)/C using n-BuLi prior to addition of a substrate can be avoided. Instead, in amination reactions, the amine in combination with lithium tert-butoxide was found to convert Ni(II)/C to active Ni(0). For Kumada couplings, direct reduction of Ni(II)/C by the Grignard reagent is easily achieved. Reactions run in the presence of triarylphosphine ligands of varying substitution patterns and with differing electronic properties provided insight into the mechanism of these nickel-catalyzed transformations. Ligandless Kumada couplings were facile with aryl Grignards, which may be a consequence of π-complexation of nickel by the aryl group in the reagent. Larger scale reactions of both types of couplings have been successfully performed, suggesting that Ni/C-based processes can be scaled-up as needed.