Synthesis and Activity of Vanadium Oxo NHC Alkylidenes. Remarkable Preference for Degenerate Metathesis and Application for Carbon Isotope Exchange

V imido NHC phosphine alkylidenes are the most efficient V catalysts for ring-closing olefin metathesis of various terminal dienes. The presence of imido and phosphine ligands is responsible for catalyst decomposition. Therefore, the development of phosphine-free V oxo NHC alkylidenes is a logical next step to further improve V-based olefin metathesis. Herein we report V oxo NHC chloride and alkoxide alkylidenes and their reactivity in olefin metathesis. V oxo NHC chloride is readily involved in cycloaddition/cycloreversion steps with olefins. However, a remarkable preference for the formation of a 1,3-metallacyclobutane (MCB) leads to exclusive methylene group exchange (degenerate metathesis) utilized for carbon isotope exchange. DFT studies further support the preference for the 1,3-MCB formation.