Metallaphotoredox Iron Catalysis Enables Direct Carbonyl-to-Carbene Conversion

Metallocarbenes are powerful intermediates for constructing complex molecular architectures, yet their generation typically relies on hazardous diazo or ylide precursors. We report a photocatalytic strategy 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 stereospecific cyclopropanation of diverse alkenes—including complex, drug-like, and peptide-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 insertion 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.