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Keywords:

  • alkene metathesis;
  • homogeneous catalysis;
  • N-heterocyclic carbenes;
  • phosphane ligands;
  • ruthenium;
  • zwitterions

Abstract

Five well-known ruthenium-N-heterocyclic carbene (NHC) catalysts for olefin metathesis and related reactions, including the second generation Grubbs and Hoveyda–Grubbs catalysts, were prepared by phosphane exchange between first generation ruthenium-benzylidene or indenylidene complexes and NHCs generated in situ upon thermolysis of imidazol(in)ium-2-carboxylates. Because N-heterocyclic carbene carbon dioxide adducts (NHC⋅CO2) are stable zwitterionic compounds that can be stored and handled with no particular precautions, this procedure is particularly attractive from a practical point of view. Reaction courses were conveniently monitored by 31P NMR spectroscopy and the experimental conditions were carefully adjusted to obtain high yields of pure products within short periods of time.