Mixed N-Heterocyclic Carbene/Phosphite Ruthenium Complexes: The Effect of a Bulkier NHC.

The synthesis, characterization, and catalytic activity of two new complexes, 2a,b, featuring a sterically demanding NHC and two different phosphites are described. Complexes 2a,b display a mutually trans arrangement of the pi-acidic phosphite and the strong sigma-donor SIPr ligand (SIPr = N,N'-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene). The synergy between the phosphite and the NHC ligand was revealed in the RCM of challenging substrates leading to tetrasubstituted olefins where, in comparison to their phosphine-containing analogues, 2a,b allowed higher conversions of these challenging substrates.