Predicting initiation rates of Hoveyda-Grubbs complexes containing an electron-withdrawing group in four possible positions of the benzylidene ring

The initiation rate of ruthenium metathesis catalysts is one of their most important characteristics from the applicational point of view due to the fact, that initiation is often the rate-limiting step of the entire catalytic cycle. Such initiation rate can be adjusted by introducing various modifications to the commonly used Grubbs-like and Hoveyda-Grubbs-like catalysts. Using a DFT approach, we predicted the initiation rates of the 2nd generation Hoveyda-Grubbs catalyst analogues substituted with electron-withdrawing –NO, –NO2, and –SO2C4F9 groups in all positions of the phenyl ring in the benzylidene part. We show that some of the modifications should result in very fast-initiating catalysts. In particular, the –SO2C4F9-substitued derivatives are predicted to be the fastest-initiating in the series. We also found correlations between the selected computed parameters such as the Gibbs free energy barrier for initiation and ruthenium-oxygen bond strengths which, combined with distortion energy analysis, allowed us to provide an explanation of the main driving force behind fast initiation.