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Keywords:

  • chiral phosphines;
  • enantioselective desymmetrization;
  • enyne cycloisomerization;
  • N-heterocyclic carbenes;
  • platinacycles

Abstract

Platinum(II) complexes which combine six-membered N-heterocyclic carbene-containing metallacyclic units and monodentate chiral phosphines have been prepared. The key step of their synthesis is the intramolecular oxidative addition of N-2-iodobenzylimidazolylidene-platinum(0)-diene complexes in the presence of the chiral phosphorus ligands. The platinum(II) metallacycles have been used as well-defined pre-catalysts for the enantioselective cycloisomerization of nitrogen-tethered 1,6-enynes into 3-azabicyclo[4.1.0]hept-4-enes. High enantiomeric excesses have been obtained with either Monophos or phenyl-Binepine based catalysts (ees=82–96%), although phenyl-Binepine outperforms Monophos in these reactions. The absolute configuration of the final 3-azabicyclo[4.1.0.]heptenes has been established by X-ray diffraction studies. The method has been extended then to the cycloisomerization of dienynes with enantiotopic vinyl groups. An (S)-phenyl-Binepine-platinum(II) complex allows total diastereoselectivity and high enantioselectivity levels to be attained in these reactions (ees up to 95%) which represent the first enantioselective desymetrizations achieved via enyne cycloisomerizations.