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.