Evaluation of Steric and Electronic Parameters of NHCs on Vanadium Imido Alkylidenes for Olefin Metathesis

Among the first-row transition metals, vanadium stands out as a promising candidate for mediating olefin metathesis. Recent studies have shown that introducing electron-donating neutral ligands, such as N-heterocyclic carbenes (NHCs), significantly enhances the stability and efficiency of V catalysts in the ring-closing metathesis (RCM) of terminal alkenes. However, there is a lack of systematic studies examining how the properties of NHCs influence catalyst performance. To address this knowledge gap, we evaluated 13 different NHCs with varying steric and electronic parameters in the synthesis and activity of V-based NHC alkylidenes. Our research identified the necessary range of sizes and electron-donating properties of NHCs for efficient catalysts, which enabled us to achieve a high turnover number of 610 in the RCM of terminal dienes using one of the developed V NHC complexes. Furthermore, we discovered a correlation between the catalyst's performance and the ¹³C NMR chemical shift of the alkylidene carbon atom, which could be useful for future catalyst development.