Olefin Metathesis Catalysts Containing <i>N,N′</i>-Diamidocarbenes

Abstract

A series of Ru-based olefin metathesis catalysts containing N,N′-diamidocarbenes (DACs) were synthesized and studied. X-ray crystallographic analysis revealed that the Ru−Ccarbene distances (1.938(5)−1.984(4) Å) measured in the DAC-supported complexes were relatively short, particularly in comparison to the range of Ru−Ccarbene distances typically observed in analogous N-heterocyclic carbene (NHC) supported complexes (1.96−2.03 Å). While the Tolman electronic parameters (TEP) of various DACs (2056−2057 cm−1) were calculated to be similar to that of PCy3 (2056 cm−1), the ring-closing metathesis (RCM) of diethyl diallylmalonate facilitated by DAC-supported Ru complexes proceeded at a relatively slow rate. However, unlike the phosphine-containing complexes, the DAC analogues catalyzed the RCM of diethyl dimethallylmalonate to its respective tetrasubstituted olefin. A series of electrochemical experiments revealed that the Ru complexes bearing a DAC ligand underwent oxidation at significantly higher potentials (ΔEpa > 0.5 V) than analogous complexes containing phosphines and various N-heterocyclic carbenes (NHCs), including a tetrahydropyrimidinylidene, a saturated and strongly donating NHC analogue of the DAC. The relative catalytic activities observed were attributed to the steric properties of the aforementioned ligands.