Cationic Tungsten-Oxo-Alkylidene-N-Heterocyclic Carbene Complexes: Highly Active Olefin Metathesis Catalysts

The
synthesis, structure, and olefin metathesis activity of the
first neutral and cationic W-oxo-alkylidene-N-heterocyclic carbene
(NHC) catalysts are reported. Neutral W-oxo-alkylidene-NHC catalysts
can be prepared in up to 90% isolated yield. Depending on the ligands
used, they possess either an octahedral (Oh) or trigonal bipyramidal ligand sphere. They can
be activated with excess AlCl3 to form cationic olefin
metathesis-active W-complexes; however, these readily convert into
neutral chloro-complexes. Well-defined, stable cationic species can
be prepared by stoichiometric substitution of one chloro ligand in
the parent, neutral W-oxo-alkylidene-NHC complexes with Ag­(MeCN)2B­(ArF)4 or NaB­(ArF)4; B­(ArF)4 = B­(3,5-(CF3)2-C6H3)4. They are highly active
olefin metathesis catalysts, allowing for turnover numbers up to 10,000
in various olefin metathesis reactions including alkenes bearing nitrile,
sec-amine, and thioether groups.