Iron‐Catalyzed Cyclopropanation of Allenes Using Unactivated Alkyl Sulfonium Salts

The catalytic generation and transfer of non‐stabilized carbenes represent a formidable challenge in synthetic chemistry, historically necessitating the use of hazardous diazoalkanes or relatively harsh reductive conditions. Herein, we describe a redox‐neutral, Fe‐catalyzed protocol that unlocks unactivated alkyl sulfonium salts as benign, bench‐stable non‐stabilized carbene precursors for the cyclopropanation of allenes. Diverging from the classical concerted mechanism in carbene transfer, this process proceeds via a stepwise radical‐involved pathway, delivering a broad range of synthetically valuable alkylidenecyclopropanes (ACPs) with exceptional regiocontrol and broad functional group compatibility. Moreover, this method can be further extended to the cyclopropanation of alkenes. Mechanistic studies, encompassing deuterium labeling, TEMPO inhibition, EPR study, stereochemical probe, thermal stability experiment, and time‐course analysis provide valuable insights into the reaction pathway. This work establishes unactivated alkyl sulfonium salts as a versatile and practical class of non‐stabilized carbene precursors for earth‐abundant metal catalysis.