Effects of alkyl group and NHC ligand variation with ruthenium-based olefin metathesis catalysts bearing chelating ortho-alkoxy benzylidenes

Abstract

A series of second-generation ruthenium olefin metathesis catalysts was investigated using a combination of reaction kinetics, X-ray crystallog., NMR spectroscopy, and DFT calcns. in order to det. the relationship between the structure of the chelating ortho-alkoxybenzylidene and the obsd. initiation rate. Included in this series were twenty new catalysts contg. an array of ortho-alkoxy benzylidenes and NHC ligands. The initiation rates of this series of catalysts were detd. using a UV/Vis assay. All new catalysts were obsd. to be faster-initiating than the corresponding isopropoxy control, and the dicyclohexylmethyl catalyst was found to be among the fastest-initiating Hoveyda-type catalysts reported to date. Anal. of the X-ray structures and computed energy-minimized structures of these catalysts revealed no correlation between Ru-O bond length and Ru-O bond strength. On the other hand, the initiation rate was found to correlate strongly with the computed Ru-O bond strength. This latter finding enables both rationalization and prediction of catalyst initiation through calcn. of a single thermodn. parameter, in which no assumptions about mechanism of the initiation step are made.