How Important are Impurities in Catalysis? An Example from Ring‐Closing Metathesis

The dramatic effect of substrate impurities on the performance of a specific ruthenium catalyst system is demonstrated in the benchmark metathesis reaction of diethyl diallylmalonate. Based on detailed two‐dimensional GC–time‐of‐flight MS measurements, the significant influence of small amounts of different contaminations, especially various organic halides, is shown. This work serves as an incisive example of the importance of impurities to catalyst performance, also in homogeneous catalysis, which is often ignored in academic research.