From the Development of Catalysts for Alkyne and Alkyne–Nitrile [2+2+2] Cycloaddition Reactions to Their Use in Polymerization Reactions

Several systems consisting of a ligand, a metal compound, and zinc have been developed as catalysts for alkyne cycloaddition reactions and alkyne–nitrile co-cycloaddition reactions. N-Heterocyclic carbene (NHC)–iron(III) chloride–zinc, NHC–cobalt(II) chloride-zinc, and 2-{[(2,6-diisopropylphenyl)imino]methyl}pyridine (dipimp)–iron(III) chloride hexahydrate–zinc systems catalyzed intramolecular cyclotrimerization reactions of alkynes. The dipimp–cobalt(II) chloride hexahydrate–zinc system catalyzed cycloaddition reactions of a variety of alkynes in intramolecular, partially intramolecular, and fully intermolecular fashions. The ethane-1,2-diylbis(diphenylphosphine)–cobalt(II) chloride hexahydrate–zinc system was effective in catalyzing the [2 + 2 + 2] co-cycloaddition of diynes with nitriles. Nickel complexes with an ionic liquid-tagged ligand converted 1,6-diynes into the corresponding cyclooctatetraenes in a toluene–ionic liquid biphasic system in the presence of zinc. The dipimp–nickel(II) chloride hexahydrate–zinc catalyst polymerized 1,6-diynes to form conjugated polyene cyclic polymers. These results and their applications in synthesis, including controlled polymerization reactions, are described. 1 Introduction 2 Alkyne [2+2+2] Cycloaddition 2.1 Development of N-Heterocyclic Carbene–Cobalt or Iron Compound–Zinc Catalyst Systems 2.2 Development of 2-{[(2,6-Diisopropylphenyl)imino]methyl}pyridine–Cobalt or Iron Salt–Zinc Catalyst Systems 2.2.1 Development of 2-{[(2,6-Diisopropylphenyl)imino]methyl}pyridine–Iron Chloride–Zinc Catalyst Systems 2.2.2 Development of the 2-{[(2,6-Diisopropylphenyl)imino]methyl}pyridine–Iron Chloride Hexahydrate–Zinc Catalyst System 2.3 The 2-{[(2,6-Diisopropylphenyl)imino]methyl}pyridine–Cobalt(II) Chloride Hexahydrate–Zinc Catalyst System 2.3.1 Reactivity and Functional Group Compatibility 2.3.2 Catalyst Activation 3 Alkyne–Nitrile [2+2+2] Co-cycloaddition 4 Applications 4.1 Uses in Organic Synthesis 4.2 Use as a Method for Polymer Synthesis 4.2.1 Preparation of Diverse Polymerizable Molecules (a Monomer Library) 4.2.2 Use in Polymer Functionalization 4.2.3 Use as a Polymerization Reaction 5 Other Reactions 5.1 Nickel-Catalyzed [2+2+2+2] Cycloaddition and Cycloaddition Polymerization of 1,6-Diynes 5.2 Hydroalkynylation of Internal Alkynes 6 Conclusion