Nickel-Catalyzed Reactions Directed toward the Formation of Heterocycles

Heterocycles have garnered significant attention because they are important functional building blocks in various useful molecules, such as pharmaceuticals, agricultural chemicals, pesticides, and materials. Several studies have been conducted regarding the preparation of heterocyclic skeletons with an emphasis on selectivity and efficiency. Three strategies are typically employed to construct cyclic molecules, namely, cyclization, cycloaddition, and ring-size alterations. Although each method has certain advantages, cycloaddition may be superior from the viewpoint of divergence. Specifically, cycloadditions enable the construction of rings from several pieces. However, the construction of heterocycles via cycloadditions is more challenging than the construction of carbocycles. For heterocycle construction, simple pericyclic reactions rarely work smoothly because of the large HOMO-LUMO gap unless well-designed combinations, such as electron-rich dienes and aldehydes, are utilized. Thus, a different approach should be employed to prepare heterocycles via cycloadditions. To this end, the use of metallacycles containing heteroatoms is expected to serve as a promising solution. In this study, we focused on the preparation of heteroatom-containing nickelacycles. Because nickel possesses a relatively high redox potential and an affinity for heteroatoms, several methods were developed to synthesize heteronickelacycles from various starting materials. The prepared nickelacycles were demonstrated to be reasonable intermediates in cycloaddition reactions, which were used to prepare various heterocycles. In this Account, we introduce the following four methods to prepare heterocycles via heteronickelacycles. (1) Direct oxidative insertion of Ni(0) to α,β-unsaturated enone derivatives: treatment of 3-ethoxycarbonyl-4-phenyl-3-buten-2-one with Ni(0) afforded an oxa-nickelacycle, which reacted with alkynes to give pyrans. (2) Substitution of a part of a cyclic compound with low-valent nickel, accompanied by elimination of small molecules such as CO, CO2, and acetophenone: treatment of phthalic anhydride with Ni(0) in the presence of ZnCl2 afforded the oxanickelacycle, which was formed via decarbonylative insertion of Ni(0) and reacted with alkynes to give isocumarins. (3) Cyclization to a nickelacycle, accompanied by two C-C σ-bond activations: insertion of Ni(0) into an arylnitrile, followed by aryl cyanation of an alkyne, gave alkenylnickel as an intermediate. The alkenylnickel species subsequently underwent an intramolecular nucleophilic attack with an arylcarbonyl group to form a cyclized product with concomitant cleavage of the C-C σ-bond between the carbonyl and aryl groups. (4) Assembly of several components to form a heteroatom-containing nickelacycle via cycloaddition: a new [2 + 2 + 1] cyclization reaction was carried out using an α,β-unsaturated ester, isocyanate, and alkyne via a nickelacycle. On the basis of these four strategies, we developed new methods to prepare heterocyclic compounds using nickelacycles as the key active species.