Redox reaction between N-heterocyclic carbenes and sulfonates: insights into unproductive catalytic paths

Unproductive consumption of catalysts is a common hurdle in developing efficient catalytic reactions. In the realm of N-heterocyclic carbene (NHC) organic catalysis, attention has mainly been focused on the involvement of air (oxygen) and water in the deactivation (unproductive consumption) of the catalysts. Here, we disclose that the carbene catalyst can react with sulfonates via a redox process that converts NHCs into the corresponding urea molecules. This process involves rather complicated steps that ultimately transfer the oxygen atom from the sulfonate to the carbene. NHCs with different structures exhibit varying degrees of tendency in the oxidation process. Since many substrates and reagents involved in NHC catalysis possess oxidizing abilities, our study suggests that investigating potential redox processes of NHC catalysts is crucial for developing future generations of effective NHC catalysts and reactions. Our study also offers new mechanistic insights into the possible reactivities of NHCs and their adducts with other molecules.