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.