Chemistry of 2-Arylhydrazonals

Approaches for the preparation of 2-arylhydrazonals as well asthe chemical reactivity profiles and structures of these substancesare reviewed. Emphasis is given to the use of these substances inthe synthesis of five- and six-membered heterocycles. 1 Introduction 2 Synthetic Approaches to 2-Arylhydrazonals 2.1 Synthesis via Coupling of Enamines with Aromatic DiazoniumSalts 2.2 Coupling Reactions of Active Methylenes with Aromatic DiazoniumSalts 2.3 Synthesis of Arylhydrazonals via Condensation Reactions withPhenylhydrazine 2.4 Synthesis of Arylhydrazonals via Oxidation of Methyl and HydroxymethylArylhydrazones 3 Structural Investigations 4 Reactions of Arylhydrazonals 4.1 Reactions Involving the Aldehyde Group 4.1.1 Formation of Formazans 4.1.2 Condensation Reactions with Amines, Hydrazines and [nl]Hydroxylamine 4.1.3 Condensation Reactions with Active Methylene Compounds 4.2 Reactions Resulting in the Formation of Heterocycles 4.2.1 Formation of Hydrazononitriles and 1,2,3-Triazoles 4.2.2 Synthesis of Pyrazoles and Fused Pyrazoles 4.2.3 Synthesis of Pyridazines and Fused Pyridazines 4.2.4 Miscellaneous 5 Conclusion