α-Diazo-λ 3 -iodanes and α-diazo sulfonium salts: the umpolung of diazo compounds

-iodanes and α-diazo sulfonium salts. Specifically, the routes available for their preparation and a classification of their unique reactivity patterns (either acting as equivalents of carbene-radical or carbene-carbocation synthons) are discussed. We also present a detailed overview of the synthetic utility of such species and, when possible, a critical comparation their reactivity and properties.