Direct Access to Iron Carbenes from Aldehyde, Ketone, and Formamide Feedstocks.

Metallocarbenes are powerful intermediates for constructing complex molecular architectures, yet their generation typically relies on hazardous diazo or ylide precursors. We report photocatalytic and electrochemical strategies for direct carbene formation from unmodified carbonyl compounds using a low-valent iron system. The reaction proceeds through net oxidative addition of Fe(I) into the carbonyl bond, followed by α-protonation and α-elimination to generate a reactive Fe-carbene intermediate. This platform enables cyclopropanation of diverse alkenes─including complex, drug-like, and amino-acid-derived substrates─under mild conditions, delivering highly functionalized, sp3-rich products. Mechanistic studies reveal direct carbonyl activation rather than free radical intermediacy, with a concerted-asynchronous olefin addition pathway. These findings establish a general strategy for carbonyl-to-carbene conversion, expanding the scope of base-metal catalysis and providing a blueprint for redefining carbonyl activation in synthetic chemistry.