Pursuing E-Selective Olefin Metathesis: Tuning Steric and Electronic Properties of S,N-Chelated Ruthenium Alkylidenes

Selective formation of E-olefins directly from 1-alkenes is still an unsolved problem more than 10 years after the first Z-selective metathesis catalysts were reported. Illustrating the challenge, the sulfidoindolide-based complex Ru5 (Ru(L1)(Me2IMes)(py)(=CHR); Me2IMes = 1,3-dimesityl-4,5-dimethylimidazol-2-ylidene; py = pyridine; R = 2-thienyl; L1 = 7-sulfidoindol-1-ide) was initially predicted, via density functional theory (DFT) calculations, to be E-selective. However, Ru5 turned out to have low catalytic activity and to be moderately Z-selective due to an unusually hindered product dissociation [Reim et al. Top. Catal. 2022, 65, 448]. To lower the dissociation barrier, we here investigated N-heterocyclic carbene (NHC) ligands that are less bulky than Me2IMes. We also hypothesized that a S,N ligand with a less nucleophilic nitrogen atom than that of indole might improve catalyst stability. However, catalyst Ru7, bearing 2-sulfidophenyltrifluoromethylsulfonylamide L2 instead of L1, was not significantly more productive or E-selective than Ru5. Whereas Ru nanoparticles formed on the addition of S,N ligand L2 to Ru complexes containing less bulky NHCs, L1 was found to be compatible with such NHCs. However, due to activation of o-C–H bonds of N-bound NHC phenyls, only the fluorinated 1,3-bis(2,6-difluorophenyl)-4,4-dimethyl-5,5-dihydro-imidazol-2-ylidene (Me2SIF2Ph) led to a stable sulfidoindolide-coordinated catalyst Ru12. Surprisingly, in Ru12, L1 bonds with the indolide positioned trans to pyridine, leading to low catalytic activity and E-selectivity in 1-alkene self-metathesis. DFT calculations show that the low activity originates in the weak trans influence and considerable π donation of the indolide. In contrast, the originally intended isomer Ru12′, with the thiolate positioned trans to pyridine, is predicted to be much more active and E-selective. Factors that might help achieve this isomer in future work are discussed.