Comparative Investigation of Hoveyda–Grubbs Catalysts bearing Modified N ‐ Heterocyclic Carbene Ligands

This paper reports the structural modification of Hoveyda–Grubbs complexes through the introduction of either an N ‐alkyl‐ N′ ‐mesityl heterocyclic carbene, an N ‐alkyl‐ N′ ‐(2,6‐diisopropylphenyl) heterocyclic carbene, or an N ‐alkyl‐ N′ ‐alkyl heterocyclic carbene. The effect of the modified N‐heterocyclic carbene (NHC) ligand was investigated in representative ring‐opening metathesis polymerization (ROMP), ring‐closing metathesis (RCM) and cross metathesis (CM) reactions. A pronounced influence on both catalyst activity and selectivity was found to be exerted by the NHC amino substituents, which emphasizes that a rigorously selected steric environment is critical in olefin metathesis catalyst design.