Diazo compounds: synthesis, carbene generation and reactivity

Diazo compounds are among the most popular intermediates in organic synthesis owing to the ease and versatility of generation of metal–carbenes. In this manuscript, we present an overview of recent methods for the synthesis of diazo compound precursors and the reactivity of metal–carbenes generated from diazo compounds. Synthetic methods, such as diazotization, nitrosoamide fragmentation, Bamford–Stevens reaction, oxidation of hydrazones, Förster reaction, Regitz diazo transfer, and retention of diazo functionality to synthesise complex diazo compounds are discussed. Reactivity of metal–carbenes with respect to X–H insertions, including C–H, O–H and N–H bonds are discussed with a focus on Cu, Rh and Pd catalysis. Furthermore, recent advances in the synthesis of ketenes by a direct carbonylation of diazo carbenes and cyclopropanation reactions are discussed. Finally, the expanding applications of diazo chemistry in various disciplines and future prospects that underscore its enduring relevance and transformative potential in synthetic methodology are discussed.