The Carbene Chemistry of N-Sulfonyl Hydrazones: The Past, Present, and Future

N-Sulfonyl hydrazones have been extensively used as operationally safe carbene precursors in modern organic synthesis due to their ready availability, facile functionalization, and environmental benignity. Over the past two decades, there has been tremendous progress in the carbene chemistry of N-sulfonyl hydrazones in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Many carbene transfer reactions of N-sulfonyl hydrazones are unique and cannot be achieved by any alternative methods. The discovery of novel N-sulfonyl hydrazones and the development of highly enantioselective new reactions and skeletal editing reactions represent the notable recent achievements in the carbene chemistry of N-sulfonyl hydrazones. This review describes the overall progress made in the carbene chemistry of N-sulfonyl hydrazones, organized based on reaction types, spotlighting the current state-of-the-art and remaining challenges to be addressed in the future. Special emphasis is devoted to identifying, describing, and comparing the scope and limitations of current methodologies, key mechanistic scenarios, and potential applications in the synthesis of complex molecules.

Palladium-catalyzed oxidative C-H activation/annulation of N-alkylanilines with bromoalkynes: access to functionalized 3-bromoindoles

A straightforward approach to the synthesis of 3-bromoindoles via palladium-catalyzed oxidative C-H activation/annulation of N-alkylanilines with bromoalkynes has been described. This protocol features high atom economy, excellent chemo- and regioselectivities, and good functional group tolerance. Moreover, the resultant 3-bromoindoles can be transformed to various functionalized indole derivatives, which demonstrates the practicability of this method in organic synthesis.

Metal-Free Cascade [4 + 1] Cyclization Access to 4-Aryl-NH-1,2,3-triazoles from N-Tosylhydrazones and Sodium Azide

A molecular iodine-mediated coupling cyclization reaction for the synthesis of 4-aryl-NH-1,2,3-triazoles has been developed from N-tosylhydrazones and sodium azide. This metal-free cascade [4 + 1] cyclization reaction could rapidly synthesize valuable compounds via a sequential C-N and N-N bond formation. Mechanistic studies demostrate that the nitrogen atoms of the 1,2,3-triazoles are not entirely from sodium azide.

Transition-Metal-Catalyzed Cross-Couplings through Carbene Migratory Insertion

Transition-metal-catalyzed cross-coupling reactions have been well-established as indispensable tools in modern organic synthesis. One of the major research goals in cross-coupling area is expanding the scope of the coupling partners. In the past decade, diazo compounds (or their precursors N-tosylhydrazones) have emerged as nucleophilic cross-coupling partners in C-C single bond or C═C double bond formations in transition-metal-catalyzed reactions. This type of coupling reaction involves the following general steps. First, the organometallic species is generated by various processes, including oxidative addition, transmetalation, cyclization, C-C bond cleavage, and C-H bond activation. Subsequently, the organometallic species reacts with the diazo substrate to generate metal carbene intermediate, which undergoes rapid migratory insertion to form a C-C bond. The new organometallic species generated from migratory insertion may undergo various transformations. This type of carbene-based coupling has proven to be general: various transition metals including Pd, Cu, Rh, Ni, Co, and Ir are effective catalysts; the scope of the reaction has also been extended to substrates other than diazo compounds; and various cascade processes have also been devised based on the carbene migratory insertion. This review will summarize the achievements made in this field since 2001.

N-Tosylhydrazones: versatile synthons in the construction of cyclic compounds

N-Tosylhydrazones have been extensively explored as versatile building blocks in the construction of various cyclic compounds.