Reactivity of α-diazo sulfonium salts: rhodium-catalysed ring expansion of indenes to naphthalenes.

In the presence of catalytic amounts of the paddlewheel dirhodium complex Rh2(esp)2, α-diazo dibenzothiophenium salts generate highly electrophilic Rh-coordinated carbenes, which evolve differently depending on their substitution pattern. Keto-moieties directly attached to the azomethinic carbon promote carbene insertion into one of the adjacent C-S bonds, giving rise to highly electrophilic dibenzothiopyrilium salts. This intramolecular pathway is not operative when the carbene carbon bears ester or trifluoromethyl substituents; in fact, these species react with olefins delivering easy to handle cyclopropyl-substituted sulfonium salts. When indenes are the olefins of choice, the initially formed cyclopropyl rings smoothly open with concomitant departure of dibenzothiophene, enabling access to a series of 2-functionalized naphthalenes.

α-Diazo-λ3-iodanes and α-diazo sulfonium salts: the umpolung of diazo compounds.

This minireview aims to cover the developments of the last few years in the design of reagents able to accomplish the umpolung of the azomethine carbon in diazo compounds namely, α-diazo-λ3-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.