In other preliminary experiments, we found that dimethyl sulfoxide (DMSO) was a potent inhibitor of the RuAAC reaction. DMSO has previously been observed to be a ligand for several transition metal compounds and catalysts, including Cp ⋆ RuCl(cod). We also observed that preparing a stock solutions of the catalyst rather than adding solid catalyst material to the pre-mixed reactants, reduced RuAAC activity.