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Keywords:

  • asymmetric catalysis;
  • enantioselectivity;
  • Heck reaction;
  • palladium;
  • phosphane ligands

Abstract

BINAP(O), rarely used as a chiral ligand, was found to induce significantly higher levels of enantioselection in representative desymmetrizing Mizoroki–Heck cyclizations where conventional BINAP produces essentially racemic material. BINAP(O) is a ligand with a stronger and weaker donor atom, and that hemilabile nature lends itself the ability to act as either a bi- or monodentate ligand. On that basis, we introduce mechanistic models where the weak donor, in one case, mediates the equilibration of diastereomeric alkene–palladium(II) complexes and, in another case, dissociates from the palladium(II) atom, thereby rendering BINAP(O) as a monodentate ligand. These new findings, along with the recently reported effects in intermolecular Mizoroki–Heck reactions, suggest that BINAP(O) ought to be included into ligand screenings.