An Initiation Kinetics Prediction Model Enables Rational Design of Ruthenium Olefin Metathesis Catalysts Bearing Modified Chelating Benzylidenes

Shao-Xiong Luo, Keary M. Engle, Xiaofei Dong, A Hejl, Michael K. Takase, Lawrence M. Henling, Peng Liu, Kendall N. Houk, Robert H. Grubbs

Index: 10.1021/acscatal.8b00843

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Abstract

Rational design of second-generation ruthenium olefin metathesis catalysts with desired initiation rates can be enabled by a computational model that depends on a single thermodynamic parameter. Using a computational model with no assumption about the specific initiation mechanism, the initiation kinetics of a spectrum of second-generation ruthenium olefin metathesis catalysts bearing modified chelating ortho-alkoxy benzylidenes were predicted in this work. Experimental tests of the validity of the computational model were achieved by the synthesis of a series of ruthenium olefin metathesis catalysts and investigation of initiation rates by UV/Vis kinetics, NMR spectroscopy, and structural characterization by X-ray crystallography. Included in this series of catalysts were thirteen catalysts bearing alkoxy groups with varied steric bulk on the chelating benzylidene, ranging from ethoxy to dicyclohexylmethoxy groups. The experimentally observed initiation kinetics of the synthesized catalysts were in good accordance with computational predictions. Notably, the fast initiation rate of the dicyclohexylmethoxy catalyst was successfully predicted by the model, and this complex is believed to be among the fastest initiating Hoveyda–Grubbs-type catalysts reported to date. The compatibility of the predictive model with other catalyst families, including those bearing alternative NHC ligands or disubstituted alkoxy benzylidenes, was also examined.