Electron-withdrawing substitutions induce electron deficiency in the carbene carbon, resulting in a weakened electron-donating ability of the carbon and weaker π backbonding of the carbon's lone pair to the π * of N 2 . As a result, the strength of the connection between carbon and nitrogen weakens and needs less activation energy. We schematized a few compounds in figures S4-S6 to show how the most crucial characteristics affect the activation energy.