Metal-Free Catalytic N-Methylation of NH-Sulfoximines Using CO2

This study reports the catalytic N-methylation of NH-sulfoximines using carbon dioxide (CO₂) under metal-free conditions. A mesoionic N-heterocyclic olefin (mNHO) was used as a catalyst for the N-methylation of NH-sulfoximines in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN) under mild conditions. This process was used to convert various NH-sulfoximines into N-methylsulfoximines. This protocol was also applicable for the synthesis of ¹³C-labeled N-methylsulfoximines using ¹³CO₂. A mechanistic cycle was proposed by performing a series of control reactions and successfully characterizing active catalytic intermediates by either single-crystal X-ray or NMR spectroscopic studies.