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

-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 -sulfonyl hydrazones in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Many carbene transfer reactions of -sulfonyl hydrazones are unique and cannot be achieved by any alternative methods. The discovery of novel -sulfonyl hydrazones and the development of highly enantioselective new reactions and skeletal editing reactions represent the notable recent achievements in the carbene chemistry of -sulfonyl hydrazones. This review describes the overall progress made in the carbene chemistry of -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.