Hoveyda–Grubbs-Type Precatalysts with Unsymmetrical N-Heterocyclic Carbenes as Effective Catalysts in Olefin Metathesis

In our search for more-selective olefin metathesis catalysts, a series of Hoveyda–Grubbs-type second-generation complexes bearing unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized and tested in model reactions. It was found that the N-benzyl substituent in NHC has a positive influence on the selectivity of the newly obtained catalysts in the self-metathesis reaction of α-olefins. As expected, a typical relationship between activity and selectivity with respect to the N-aryl substituent used was observed. Dipp-containing complexes exhibited higher stability at elevated temperature, while Mes-bearing complexes typically gave better yields than their Dipp analogues.