Transition-Metal-Free Reactions Between Boronic Acids and N-Sulfonylhydrazones or Diazo Compounds: Reductive Coupling Processes and Beyond

The metal-free reaction between diazo compounds and boronic acids has been established in recent years as a powerful C(sp3)–C bond-forming reaction. This account covers the recent advances in this area. First, the various synthetic applications of reactions with N-sulfonylhydrazones as a convenient source of diazo compounds is discussed. These transformations can be regarded as reductive couplings of carbonyl compounds. Also covered is the incorporation of other mild sources of diazo compounds in these reactions: diazotization of amines and oxidation of hydrazones. Finally, the development of sequential and cascade processes is presented.1 Introduction2 Early Work: Reactions Between Alkylboranes and Diazo Compounds or N-Sulfonylhydrazones2.1 Reactions Between Alkylboranes and Diazo Compounds2.2 Reactions Between Alkylboranes and N-Sulfonylhydrazones3 Reactions of N-Sulfonylhydrazones and Diazo Compounds with Aryl and Alkylboronic Acids3.1 Reactions of Arylboroxines with Diazo Compounds3.2 Reductive Couplings of N-Sulfonylhydrazones with Aryl- and Alkylboronic Acids3.3 Three-Component Reactions Between α-Halotosylhydrazones, Boronic Acids and Indoles4 Reactions of N-Tosylhydrazones with Alkenylboronic Acids5 Synthesis of Allenes by Reactions with Alkynyl N-Nosylhydrazones6 Reactions with Diazo Compounds Generated by Diazotization of Primary Amines7 Reactions with Diazo Compounds Generated by Oxidation of ­Hydrazones8 Reactions with Trimethylsilyldiazomethane9 Cascade Cyclization Reactions with γ- and δ-Cyano-N-tosylhydrazones10 Summary and Outlook