Unsymmetrical NHC–Ruthenium Complexes: Effect of N-Substituents and Backbone Configuration on Olefin Metathesis Catalytic Performance

The design of unsymmetrical N-heterocyclic carbene (uNHC) ligands represents an effective strategy for tuning the catalytic performance of ruthenium olefin metathesis catalysts. In this work, two second-generation Hoveyda–Grubbs-type complexes bearing unsymmetrical NHC ligands with N-benzyl and N′-2-isopropylphenyl substituents and anti or syn backbone configurations were synthesized and fully characterized. Their catalytic performance was evaluated in representative olefin metathesis reactions, including cross-metathesis, ethenolysis of ethyl oleate, and ring-closing metathesis of substrates with increasing steric demand, and compared with that of the corresponding N-cyclohexyl analogues and the commercial second-generation Hoveyda–Grubbs catalyst. Replacing the N-cyclohexyl substituent with the more flexible N-benzyl group significantly influences catalyst performance, although this depends on both the backbone configuration and the type of metathesis transformation. Catalysts featuring an anti phenyl-substituted NHC backbone exhibited superior performance in the ethenolysis of ethyl oleate and in the sterically demanding ring-closing metathesis of linalool. The introduction of the benzyl substituent reduced the differences in catalytic behavior between the anti and syn isomers in RCM reactions compared with the corresponding cyclohexyl derivatives. Finally, the chiral anti catalyst was evaluated in a model asymmetric ring-opening cross-metathesis reaction, affording only low enantioselectivity.