Switchable organocatalysis from N-heterocyclic carbene-carbodiimide adducts with tunable release temperature

N-heterocyclic carbenes (NHCs) are powerful organocatalysts, but their air- and moisture-sensitivity often require in situ generation of the free carbene from a stable precursor (i.e., “masked” NHC). Carboxylate adducts are the most well-studied thermally-labile NHC precursor, but catalyst activation is irreversible and the simplicity of the CO2 mask results in the NHC structure controlling both catalytic activity and activation temperature. Herein, we develop new NHC masks based on carbodiimides (CDIs), which have two nitrogen substituents that can tune catalyst release orthogonally from reactivity. Using a common imidazolylidene NHC, we show that CDI masks can be used to achieve switchable organocatalysis and that small electronic perturbations to the CDI substituents (i.e., phenyl versus para-tolyl) result in >10 °C difference in activation temperature without changing the NHC structure. This platform will enable more precise control over NHC organocatalysis than previous systems.