A Ruthenium(CAAC‐5) Olefin Metathesis Catalyst for Bioconjugation

ABSTRACT Olefin metathesis is a powerful transformation for carbon–carbon bond formation, yet its application in biological settings is challenged by the instability of most ruthenium catalysts in aqueous media. Herein, we report a Ru(CAAC‐5) olefin metathesis catalyst bearing a functional handle for bioconjugation. The resulting hybrid catalyst catalyzes ring‐closing metathesis (RCM) and displays extended lifetimes in aqueous media. Functionalized CAAC‐5 ligands bearing a protected aniline linker were prepared via enamine alkylation and coordinated to ruthenium to afford the corresponding Ru(CAAC‐5) complex. In a benchmark RCM reaction under biologically relevant conditions, the ruthenium complex exhibits enhanced catalytic robustness and reaches turnover numbers of up to 600. Deprotection of an amine‐functionalized handle enables the bioconjugation to proteins and peptides. Using this platform, the catalyst was anchored to biomolecules through supramolecular anchoring to human carbonic anhydrase II (hCA II), click conjugation to peptides (SS01), and covalent attachment to ubiquitin (Ubi). To the best of our knowledge, this aniline‐functionalized Ru(CAAC‐5) complex is the first CAAC‐bearing metathesis catalyst amenable to bioconjugation, while retaining the prolonged catalytic activity characteristic of CAAC‐based systems in aqueous media.